New reactions of β-diketiminatolanthanoid complexes: Sterically induced self-deprotonation of β-diketiminato ligands

Article abstract of DOI:10.1039/b416549g

Attempted synthesis of sterically demanding bis- or tris-β- diketiminato complexes of lanthanoids resulted in ligand deprotonation and the formation of complexes containing both a normal and a deprotonated ligand; one of these on protonation gave the first cationic β-diketiminato- Ln complex. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b416549g

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
New reactions of b-diketiminatolanthanoid complexes: sterically
induced self-deprotonation of b-diketiminato ligands{
Peter B. Hitchcock, Michael F. Lappert* and Andrey V. Protchenko
Received (in Cambridge, UK) 28th October 2004, Accepted 26th November 2004
First published as an Advance Article on the web 6th January 2005
DOI: 10.1039/b416549g
The preparation of the complexes 1, 2a and 2b is outlined in
Scheme 1; the yields of the crystalline compounds were not
optimised. A Tm(III) iodide–KL reaction was initially carried out
in a 1 : 2 molar stoichiometry, in an attempt to make TmL2I, but 1
was the only crystalline product. It may be that successive transient
intermediates along the pathway to 1 were [TmL2]I and [TmL2][L].
It is noted that a bis(b-diketiminato)ytterbium(III) complex was
Attempted synthesis of sterically demanding bis- or tris-
b-diketiminato complexes of lanthanoids resulted in ligand
deprotonation and the formation of complexes containing both
a ‘‘normal’’ and a deprotonated ligand; one of these on
protonation gave the first cationic b-diketiminato–Ln complex.
The use of b-diketiminates as important monoanionic spectator
ligands is well documented.1 However, recent observations have
shown that in selected instances such coordinated ligands may
themselves undergo transformations. Examples include reduction
of certain Li or Yb b-diketiminates to produce di- or trianionic
analogues,2 and deprotonation, resulting in the formation of
dianionic ligand–metal complexes.3 The latter process occurred
either (a) thermally, by alkane elimination from a Ca3c or Sc3d
b-diketiminate containing an adjacent alkyl ligand (examples of a
complex-induced proximity effect, CIPE4); or (b) for a Ge,3a Sc,3b
or Ti3e complex, by use of an external strong base—a carbanion3a,e
or 2N(SiMe3)2.3b
not accessible from YbCl3 + 2LiL in thf, [YbCl2(L)(thf)2] being
3+
isolated;5 the radii of the Tm3+ (0.880 A) and the Yb (0.868 A)
are almost identical. Complex 1 was also isolated in a modest yield
upon the work up of a reaction mixture of [TmI2(dme)3] and 2KL
in thf. The CH2 group of 1 was readily protonated, using an
appropriate [HNR2R9][BAr94]; the salt 2a was thermally unstable
and its formation was confirmed by 1H NMR spectroscopy, while
the salt 2b was isolated in a good yield. Complexes 2a and 2b are
rare examples of salts containing a homoleptic bis-b-diketimina-
tometal cation. A bis(b-diketiminato)thulium iodide Tm(L9)2I has
been reported.6
˚
˚
We now report the new self-deprotonation reaction of
the b-diketiminato ligand in sterically hindered lanthanoid (Ln)
complexes, which resulted in formation of new compounds: (i) the
thulium(III) complex [TmL(Ldep)] (1), containing both the mono-
anionic b-diketiminato ligand [{N(C6H3Pri2-2,6)C(Me)}2CH]2
The molecular formulae of complexes 1, 2a and 2b, as shown in
Scheme 1, were consistent with their elemental analyses and
1H-NMR spectra (19 CH3 signals, in the wide range d 241 to
d 2245 ppm, for 1; but only 10 such signals for 2a or 2b). Single
crystal X-ray data were obtained for 1{ (Fig. 1) and 2b.{
(5 L2) as well as its deprotonated derivative [Ldep 22
] ; (ii) the
salts [TmL2]X (2a, 2b) via protonation of 1; (iii) the unprecedented
cyclometallated ytterbium(III) complex [YbL9(L9dep)] (4)
(L9 5 [{N(SiMe3)C(Ph)}2CH]2) where L9dep is the C,N,N9-
tridentate bicyclic ligand.
Fig. 1 ORTEP drawing and atom numbering scheme for complex 1
(20% ellipsoids; non-coordinated LH and toluene solvate molecules are
˚
Scheme 1 Synthesis of 1 and 2 (Ar 5 C6H3Pri2-2,6).
not shown). Selected bond lengths (A) and angles (u): Tm–N1 2.273(2),
Tm–N2 2.301(2), Tm–N3 2.207(2), Tm–N4 2.161(2), N1–C1 1.317(3), N2–
C3 1.345(3), C1–C2 1.430(4), C2–C3 1.384(4), N3–C30 1.401(3), N4–C32
1.400(3), C30–C31 1.350(4), C31–C32 1.493(3), C30–C33 1.524(3), C32–
C34 1.330(4), Tm–C30 2.717(3), Tm–C31 2.826(3), N1–Tm–N2 87.25(7),
N3–Tm–N4 96.35(7).
{ Electronic supplementary information (ESI) available: synthesis and
characterisation of 1, 2b, 3, 4 and 4a. See http://www.rsc.org/suppdata/cc/
b4/b416549g/
*m.f.lappert@sussex.ac.uk
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 951–953 | 951

View Article Online
The thulium atom in molecule 1 is at the spiro-junction of two
N,N9-chelating ligands: N1 and N2 of L2 and N3 and N4 of the
conversion of [Yb{N(SiMe3)2}3] into [Yb{N(SiMe3)2}2{N(SiMe3)-
Si(Me)2CH2}Na(thf)3] (5).11
deprotonated [L ]
dep 22. The TmL ring has the boat conformation
The structure of the crystalline compound 4, determined by
single crystal X-ray diffraction,{ is illustrated in Fig. 2. The central
Yb atom is joined through N3 and N4 to form a boat-shaped
b-diketiminatoytterbium moiety; and via N1, N2 and C19 to the
(k2-ligand-to-metal bonding mode) and is significantly, but far
from completely, p-delocalised (e.g., the endocyclic N–C bond
˚
lengths are unequal, with N1–C1 almost 0.03 A shorter than
N2–C3). The Tm(Ldep) moiety is best described as implicating
bicyclic ligand [L9dep 22
] . The endocyclic bond distances and angles
g3-1-azaallyl (N3–C30–C31) and amido(N4)-centred bonds to the
of the 4-membered ring of 4 are similar to those in the
corresponding ring of 5.11 The Yb–N3(N4) distances are shorter
…
…
Tm atom; thus, the Tm C30 and Tm C31 contacts are near the
upper range of the Tm–C(g5-Cp) distances (2.598–2.829, av.
7
2.670 A) of [TmCp3], whereas the Tm C32 contact in 1 is longer
than in [Yb(L9)2] (av. 2.410 A),12 in agreement with the difference
˚
in Yb3+ and Yb2+ ionic radii. The bonding mode in the Yb(L9)
moiety of 4 is close to g5, the dihedral angle between the N3–C22–
C24–N4 and N3–Yb–N4 planes being 63.7u (cf.,12 10.8u in the
k2-bonded [Yb(L9)2] and 67.5u in the g5-bonded [Yb{N(SiMe3)C-
(C6H4Me-4)CHC(adamantyl-1)N(SiMe3)}2]).
…
˚
˚
at 2.908(3) A. The Tm–N4 bond length is ca. 0.026 A shorter than
˚
˚
the average Tm–N distance of 2.187 A in the tetracoordinated
Tm(III) amide [Tm{N(SiMe2CH2)2}3(m-Cl)Li(OEt2)3] (2.179(2)–
2.189(2) A). The ligand [Ldep 22
8
˚
]
has previously featured in
[Ge(Ldep)(H)B(H)(m-H)2Li(OEt2)3]3a and [Ti(Ldep)(NC6H3Pri2-
2,6)(OEt2)],3e but only the latter has closely similar M(Ldep
The structure of crystalline 4a showed that the ligand L9 in the
molecule of Yb(L9)2 adopts the same g5-bonding mode as in 4, in
contrast to the k2-bonding found in the crystal of isolated
[Yb(L9)2].12
)
geometric parameters (apart from the shorter M–N bond lengths)
to those in 1. The ligand bite angle for Ldep is 9.1u wider than for L,
thus facilitating the Tm C(30 or 31) close contacts in Tm(Ldep).
…
In conclusion, the following observations are noteworthy. (1) A
b-diketiminato ligand in a highly encumbered Ln complex may
The crystalline salt 2b comprises a well-separated ion pair; there
are two independent pairs. The geometric parameters of the cation
are closely similar to those in the Tm(L) moiety of 1, with Tm–N
undergo a facile deprotonation of the type 2[A]2 A [Adep 22
+
]
[AH] (A 5 L9 or L). (2) Protonation of the thulium complex
[TmL(Ldep)] (1), using an appropriate ammonium tetraarylborate,
is a convenient route to the first homoleptic cationic lanthanoid
salt 2a or 2b, notable for containing a potentially highly
electrophilic cation. (3) The compound [Yb(L9)(L9dep)] (4) is
˚
˚
bond lengths of 2.232 A (range 2.214(7)–2.250(7) A) and ligand
bite angles of 89.2(3)u and 85.2(2)u (molecule A) or 88.8(2)u and
89.1(2)u (molecule B).
Homoleptic Ln(III) b-diketiminates are rare, doubtless for steric
reasons.9 Attempts to make a bis(b-diketiminato)cerium alkyl
from Ce(L9)2Cl and LiCH(SiMe3)2 yielded [Ce(L9){CH-
(SiMe3)2}2].6 Thus we sought an alternative oxidative approach
to an Ln(L9)3 complex, based on an analogy with a strategy which
had successfully been employed to make sterically encumbered
tricyclopentadienides of Sm(III) and U(IV).10 The first step was to
prepare [Pb(L9)2] (3) (see supplementary data{), which is the first
structurally characterised Pb b-diketiminate and an unprecedented
group 14 metal(II) bis(k2-b-diketiminate).1
significant for possessing the new bicyclic ligand [L9dep 22
] , which
may well be found more widely. (4) The oxidative route, based on
a Pb(II) reagent, is likely to find more general application,
particularly for complexes of metals having available adjacent
oxidation states.
We thank EPSRC for the award of a fellowship to A.V.P.
The preparation of the cyclometallated ytterbium(III)
b-diketiminate [Yb(L9)(L9dep)] (4) is illustrated in Scheme 2. In
one experiment, a product 4a, a co-crystal of 4 (76%) and Yb(L9)2
(24%), was isolated in place of 4, as revealed by crystallography.{
It is possible that a transient intermediate in the [YbL2]–3 system
was [Yb(L9)2][L9], in which the third loosely attached ligand
…
deprotonates one of the Me groups activated by the Yb Me
agostic interaction. Examples of base-induced cyclometallation of
bis(trimethylsilyl)amidometal complexes are known, including the
Fig. 2 ORTEP drawing and atom numbering scheme for complex 4
˚
(20% ellipsoids). Selected bond lengths (A) and angles (u): Yb–N1 2.265(2),
Yb–N2 2.289(2), Yb–N3 2.328(2), Yb–N4 2.272(2), Yb–C19 2.406(3), N1–
C1 1.348(3), N2–C3 1.316(3), N3–C22 1.338(3), N4–C24 1.328(3), C1–C2
1.398(4), C2–C3 1.417(4), C22–C23 1.414(4), C23–C24 1.420(3), N1–Yb–
N2 79.93(7), N3–Yb–N4 82.37(7), N2–Yb–C19 70.48(9).
Scheme 2 Syntheses of 4.
952 | Chem. Commun., 2005, 951–953
This journal is ß The Royal Society of Chemistry 2005

View Article Online
Peter B. Hitchcock, Michael F. Lappert* and Andrey V. Protchenko
The Chemistry Department, University of Sussex, Brighton, UK
BN1 9QJ. E-mail: m.f.lappert@sussex.ac.uk; Fax: +44 1273 677196;
Tel: +44 1273 678316
atoms with H atoms omitted. Data collection Kappa CCD. Refinement
using SHELXL-97. CCDC numbers 254749–254753 for complexes 1, 2b, 3,
4 and 4a, respectively. See http://www.rsc.org/suppdata/cc/b4/b416549g/ for
crystallographic data in .cif or other electronic format.
1 L. Bourget-Merle, M. F. Lappert and J. R. Severn, Chem. Rev., 2002,
102, 3031; W. E. Piers and D. J. H. Emslie, Coord. Chem. Rev., 2002,
233, 131.
2 (a) A. G. Avent, A. V. Khvostov, P. B. Hitchcock and M. F. Lappert,
Chem. Commun., 2002, 1410; (b) O. Eisenstein, P. B. Hitchcock,
A. V. Khvostov, M. F. Lappert, L. Maron, L. Perrin and
A. V. Protchenko, J. Am. Chem. Soc., 2003, 125, 10790; (c)
A. G. Avent, P. B. Hitchcock, A. V. Khvostov, M. F. Lappert and
A. V. Protchenko, Dalton Trans., 2004, 2272.
Notes and references
{ Crystal data: for 1?LH?0.5PhMe (yellow prism 0.25 6 0.20 6 0.10 mm3):
[C58H81N4Tm]?(C29H42N2)?0.5(C7H8), M 5 1467.91, triclinic, space group
¯
˚
P1, a 5 10.9404(2), b 5 16.9293(2), c 5 24.5836(3) A, a 5 70.187(1),
3
˚
b 5 83.689(1), c 5 75.243(1)u, V 5 4140.99(10) A , Z 5 2, T 5 173(2) K,
m 5 1.12 mm21, 14519 independent reflections [Rint 5 0.058], final
R1 5 0.034 [for 12932 reflections with I . 2s(I)], wR2 5 0.082 (all data).
For 2b (yellow prism 0.20 6 0.15 6 0.10 mm3): [C82H82BF20N4Tm],
3 (a) Y. Ding, H. Hao, H. W. Roesky, M. Noltemeyer and H.-G. Schmidt,
Organometallics, 2001, 20, 4806; (b) A. M. Neculai, H. W. Roesky,
D. Neculai and J. Magull, Organometallics, 2001, 20, 5501; (c) S. Harder,
Angew. Chem., Int. Ed., 2003, 42, 3430; (d) P. G. Hayes, W. E. Piers,
L. W. M. Lee, L. K. Knight, M. Parvez, M. R. J. Elsegood and
W. Clegg, Organometallics, 2001, 20, 2533; (e) F. Bazuli, J. C. Huffman
and D. J. Mindiola, Inorg. Chem., 2003, 42, 8003.
4 M. C. Whisler, S. MacNeil, V. Snieckus and P. Beak, Angew. Chem.,
Int. Ed., 2004, 43, 2206.
5 Y. Yao, Y. Zhang, Q. Shen and K. Yu, Organometallics, 2002, 21,
819.
6 P. B. Hitchcock, M. F. Lappert and S. Tian, Dalton Trans., 1997, 1945.
7 S. H. Eggers, W. Hinrichs, J. Kopf, W. Jahn and R. D. Fischer,
J. Organomet. Chem., 1986, 311, 313.
8 O. Just and W. S. Rees, Jr., Inorg. Chem., 2001, 40, 1751.
9 D. Drees and J. Magull, Z. Anorg. Allg. Chem., 1994, 620, 814.
10 W. J. Evans, K. J. Forrestal, J. T. Leman and J. W. Ziller,
Organometallics, 1996, 15, 527; W. J. Evans, G. W. Nyce,
M. A. Johnston and J. W. Ziller, J. Am. Chem. Soc., 2000, 122, 12019.
11 M. Karl, K. Harms, G. Seybert, W. Massa, S. Fau, G. Frenking and
K. Dehnicke, Z. Anorg. Allg. Chem., 1999, 625, 2055.
M 5 1683.26, monoclinic, space group P21/c, a 5 27.9702(3),
3
˚
˚
b 5 19.9944(3), c 5 27.9170(4) A, b 5 98.971(1), V 5 15421.5(4) A ,
Z 5 8, T 5 173(2) K, m 5 1.25 mm21, 26992 independent reflections
[Rint 5 0.075], final R1 5 0.070 [for 20129 reflections with I . 2s(I)],
wR2 5 0.155 (all data). For 3 (yellow plate 0.20 6 0.20 6 0.05 mm3):
¯
[C42H58N4Si4Pb], M 5 938.47, triclinic, space group P1, a 5 11.3492(2),
˚
b 5 12.1384 (2), c 5 16.5846(3) A, a 5 89.975(1), b 5 97.414(1),
3
21
˚
c 5 93.140(1)u, V 5 2262.16(7) A , Z 5 2, T 5 173(2) K, m 5 3.87 mm
,
8131 independent reflections [Rint 5 0.055], final R1 5 0.031 [for 7692
reflections with I . 2s(I)], wR2 5 0.079 (all data). For 4 (orange hexagonal
prism 0.40 6 0.35 6 0.30 mm3): [C42H57N4Si4Yb], M 5 903.32, triclinic,
¯
˚
space group P1, a 5 11.2608(1), b 5 12.8457(2), c 5 16.0891(3) A,
3
˚
a 5 89.620(1), b 5 86.087(1), c 5 79.121(1)u, V 5 2280.13(6) A , Z 5 2,
T 5 173(2) K, m 5 2.19 mm21, 7714 independent reflections [Rint 5 0.036],
final R1 5 0.022 [for 7714 reflections with I . 2s(I)], wR2 5 0.053 (all
data). For 4a (brown prism 0.30 6 0.30 6 0.30 mm3): [C42H57N4Si4Yb],
¯
M 5 903.32, triclinic, space group P1, a 5 11.2433(2), b 5 12.9389(2),
˚
˚
c 5 16.2117(3) A, a 5 89.319(1), b 5 86.149(1), c 5 79.253(1)u,
V 5 2311.81(6) A , Z 5 2, T 5 173(2) K, m 5 2.16 mm21, 10926
3
independent reflections [Rint 5 0.037], final R1 5 0.041 [for 10399
reflections with I . 2s(I)], wR2 5 0.094 (all data). The structure is
disordered, with 74% as shown and 26% as [YbL92]. The lower occupancy
atom sites for the alternative N3Si3Me3 moiety were included as isotropic
12 A. G. Avent, P. B. Hitchcock, A. V. Khvostov, M. F. Lappert and
A. V. Protchenko, Dalton Trans., 2003, 1070.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 951–953 | 953