Heteroleptic Tm(II) complexes: One more success for Trofimenko's scorpionates

Article abstract of DOI:10.1021/ja0776273

Reaction of TmI₂(THF)_x with the bulky scorpionate, KTp^tBu,Me, gave (Tp^tBu,Me)TmI(THF) (1). Complex 1 proved to be a useful starting material for a select number of heteroleptic Tm(II) compounds, (Tp^tBu,Me)TmER, including the first Tm(II)-hydrocarbyl derivative, (Tp^tBu,Me)Tm{CH(SiMe₃)₂} (2). Copyright

Full text of DOI:10.1021/ja0776273

Published on Web 01/16/2008
Heteroleptic Tm(II) Complexes: One More Success for Trofimenko’s
Scorpionates
Jianhua Cheng, Josef Takats,* Michael J. Ferguson, and Robert McDonald
Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta, Canada, T6G 2G2
Received October 3, 2007; E-mail: joe.takats@ualberta.ca
Scheme 1.
The emergence of new, and largely unexpected, divalent lan-
thanides for solution molecular chemistry has been one of the most
exciting recent developments in lanthanide chemistry. As pointed
out by Evans,1 since 1997 the number of divalent lanthanides
available for the synthetic chemist has doubled from the time tested
Eu(II), Yb(II), and Sm(II) to include the much more reducing Tm-
(II), Dy(II), and Nd(II).
Recent reviews by Bochkarev2 and Edelmann3 have catalogued
progress in this “new” divalent lanthanide field, including the
preparation of organometallic derivatives. However, heteroleptic
Tm(II) compounds, except for solvent adducts, have remained
elusive as has the synthesis of Tm(II)-hydrocarbyl species. It was
quickly established that the outcome of a reaction crucially
led to the isolation of Tm(III)-hydrocarbyl derivatives.8 The
depended on the choice of ligand, solvent, and even on the nature
of the inert atmosphere used.
successful preparation of 2 is yet another demonstration of the
remarkable kinetic stabilization conferred on otherwise highly
reactive metal centers by Trofimenko’s bulky scorpionates. The
structure of 2 is shown in Figure 1.
The bulky hydro-tris(3-tBu-5-Me-pyrazolyl)borate ligand
(TptBu,Me, Tp′), a second generation Trofimenko “scorpionate”,4 has
been used to prepare heteroleptic “(Tp′)LnER” (Ln ) Yb(II) and
Sm(II)) type complexes,5 including hydrocarbyl species. On the
basis of these observations, we attempted similar reactions with
TmI2. Here we report our preliminary results on the successful
synthesis of the first heteroleptic Tm(II) complexes, including the
Tm(II)-hydrocarbyl, (Tp′)Tm{CH(SiMe3)2}.
The Tm(II) center is bonded to a classical κ3 scorpionate and an
almost planar CH(SiMe3)2 hydrocarbyl ligand, with the sum of the
angles involving nonhydrogen atoms bonded to C1 being 351°. One
hydrocarbyl SiMe3 group (containing Si2) is wedged between two
pyrazolyl ligands (those containing N12 and N22) while the other
SiMe3 points toward the tBu group on the third pyrazole ring. Steric
repulsion between the latter two groups certainly contributes to the
observed distorted tetrahedral coordination geometry of Tm (the
Tm-C1 bond vector is 16.27(10)° off the B-Tm axis), but electronic
factors must also play a role. This is evidenced by the vastly
different Tm-C1-Si1 (132.18(18)°) and Tm-C1-Si2 (100.84-
(14)°) angles, the relatively short Tm---C7 distance (3.123(4) Å),
the small Tm-C1-H1 angle (100°) and the short Tm---H1 distance
(2.90 Å). All are indications of agostic Tm(II)---CH interactions, a
common feature with electron deficient lanthanide centers, as is
the case with complex 2. Although comparison to other Tm(II)-C
bond lengths is not possible, the Tm-C1 distance of 2.554(3) Å is
virtually identical to that of the immediate neighbor Yb(II) analogue,
2.552(5) Å. In view of the isoelectronic nature of the CH(SiMe3)2
and N(SiMe3)2 ligands, it is not surprising that the coordination
geometry of the metal center of (Tp′)Tm{N(SiMe3)2} (3) is a
similarly distorted tetrahedron (see Supporting Information). Com-
pounds 2 and 3 are also delicate. Further elaboration of the Tm-
(II)-C/N bonds is best carried out on freshly prepared materials.
Complex 4 joins the relatively short list of electropositive metal
complexes, including lanthanides, which contain an intact alkyl-
borohydride ligand.9 Because of the higher precision of the present
structure, compared to the Yb(II)/Sm(II) analogues,5a,c the nature
of the bonding between the Tm(II) and the (HBEt3)- ligand could
be fully established. As shown in Figure 2, in addition to the
bridging hydride, the (HBEt3)- ligand has one of its ethyl groups
oriented toward thulium and takes part in agostic interactions as
evidenced by the severely bent Tm-H2-B2 bridge (113(6)°) and
the close contacts between Tm and methylene protons H5A and
H5B (2.71 Å and 2.55 Å, respectively).10 The coordination mode
of the (HBEt3)- ligand is thus similar to that observed in (C5Me5)2-
La(HBEt3)(THF).9 Compound 4 is the most stable of the heteroleptic
Addition of KTp′ to a THF solution of TmI2 at room temperature
(rt) in a glovebox (He/N2 atmosphere), followed by simple work
up afforded dark green (Tp′)TmI(THF) (1) in moderate yield.
THF
KTp′ + TmI2(THF)x
8 (Tp′)TmI(THF)
(1)
-KI
Complex 1 was fully characterized, including the solid-state
X-ray structure (Scheme 1). The complex is isostructural with the
Yb(II)/Sm(II) analogues,5 in particular the metrical parameters are
very close to those of the slightly smaller Yb(II) with distances Ln
(Tm/Yb) of Ln-Nave ) 2.462(3)/2.453(6) Å, Ln-I ) 3.0595(4)/
3.0536(8) Å, and Ln-O ) 2.454(2)/2.447(6) Å. The coordination
geometry is distorted trigonal bipyramidal with the THF oxygen
and N22 occupying axial sites (O-Tm-N22 ) 145.15(9)°) and I,
N12, and N32 the equatorial positions.
Although isolable, complex 1 undergoes slow decomposition both
in solution and in the solid state,6,7 when kept at -30 °C. Pure 1
can be recovered from small amounts of decomposed material
(appearance of white solid) by recrystallization from OEt2.
Despite its delicate nature, complex 1 is a useful starting material
for the synthesis of a select number of heteroleptic Tm(II)
complexes (Scheme 1). Thus simple salt metathesis gave the dark
brown Tm(II)-hydrocarbyl, (Tp′)Tm{CH(SiMe3)2} (2), dark brown
(Tp′)Tm{N(SiMe3)2} (3), and the dark green triethylborohydride
complex (Tp′)Tm(µ-HBEt3)(THF) (4). The complexes were fully
characterized, including EA, and the solid-state structures were
established by single-crystal X-ray studies.
The most remarkable compound is (Tp′)Tm{CH(SiMe3)2} (2),
with a Tm(II)-hydrocarbyl bond, the first time such σ-alkyl species
could be isolated. Although Izod has provided evidence for the
likely presence of a Tm(II)-σ-carbon bond, these previous attempts
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C O M M U N I C A T I O N S
Yb(CH2SiMe3)(THF), which allowed the isolation of the unique
Ln(II)-hydride, [(Tp′)Yb(µ-H)]2.11 Unexpectedly, the reaction was
accompanied by ligand redistribution and the only Tm(II) species
that could be isolated was (Tp′)2Tm (5), eq 2. The structure of 5 is
identical to that of the Yb(II)/Sm(II) analogues,12 with a κ3-N3 and
κ3-N2H scorpionate ligands.
OEt2
(Tp′)TmI(THF) + KCH2SiMe3
8 (Tp′)2Tm + “others” (2)
-KI
Ligand redistribution was also observed when preparation of
primary amido derivatives, (Tp′)Tm(NHR), was attempted via salt
metathesis. However the outcome of a protonolysis protocol proved
very different. Addition of a 2,5-(tBu)2-aniline to a hexane solution
of 2 in the drybox (He/N2 atmosphere) initially gave a dark brown
solution, which bleached in 1 h, indicating formation of a Tm(III)
complex. Our initial hopes that internal redox produced a Tm(III)-
imido species were quickly dashed when the solid-state structure
showed that the yellow compound 6 was [(Tp′)Tm{NHC6H3-
(tBu)2}]2(µ-η2:η2-N2),13 (Figure 3) with similar inner core as the
[(C5Me5)2Ln(THF)1/0]2(µ-η2:η2-N2) complexes of Evans.14
In summary, we have shown that the sterically very demanding
scorpionate, TptBu,Me, is capable of stabilizing heteroleptic Tm(II)
derivatives, including the first Tm(II)-hydrocarbyl species, and this,
for complexes 1-5, even in the presence of dinitrogen. Future
studies are aimed at expanding the range of Tm(II) complexes, to
study their reactivities and to reinvestigate the synthesis in the
absence of dinitrogen with a view to harness possible internal redox
behavior to generate Tm(III)-multiple-bonded species, like (Tp′)-
TmNR. Extension of the work to the more reducing Dy(II) and
Nd(II) is also planned.
Figure 1. ORTEP plot of (Tp′)Tm{CH(SiMe3)2} (2). Ellipsoids are drawn
at the 20% probability level.
Acknowledgment. We are grateful to the National Sciences and
Engineering Research Council of Canada and the University of
Alberta for financial support. We thank Dr. Andrea Sella (UCL)
for helpful comments on the manuscript and the staff of the
Department’s High Field NMR Laboratory for technical assistance.
Figure 2. ORTEP plot of (Tp′)Tm(µ-HBEt3)(THF) (4). Ellipsoids are drawn
at the 20% probability level.
Supporting Information Available: Synthesis and spectroscopic
and X-ray diffraction details. This material is available free of charge
via the Internet at http://pubs.acs.org.
References
(1) Evans, W. J. Inorg. Chem. 2007, 46, 3435-3449.
(2) Bochkarev, M. N. Coord. Chem. ReV. 2004, 248, 835-851.
(3) Gottfriedsen, J.; Edelmann, F. T. Coord. Chem. ReV. 2007, 251, 142-
202.
(4) Trofimenko, S. Scorpionates: The Coordination Chemistry of Polypyra-
zolyborate Ligands; Imperial College Press: London, 1999; p 20.
(5) (a) Hasinoff, L.; Takats, J.; Zhang, X. W.; Bond, A. H.; Rogers, R. D. J.
Am. Chem. Soc. 1994, 116, 8833-8834. (b) Maunder, G. H.; Sella, A.;
Tocher, D. A. J. Chem. Soc., Chem. Commun. 1994, 2689-2690. (c)
Takats, J. J. Alloys Cmpds. 1997, 249, 52-55.
(6) Similar behavior has been observed with homoleptic Tm(II) cyclopenta-
dienyl and phospholyl compounds, where only the bulkiest ligand
prevented gradually decomposition (see reference 7).
(7) Jaroschik, F.; Nief, F.; Le Goff, X.-F.; Richard, L. Organometallics 2007,
26, 3552-3558.
Figure 3. ORTEP plot of 6. Ellipsoids are drawn at the 20% probability
level.
(8) Bowman, L. J.; Izod, K.; Clegg, W.; Harrington, R. W. Organometallics
2007, 26, 2646-2651.
(9) Evans, W. J.; Perotti, J. M.; Ziller, J. W. Inorg. Chem. 2005, 44, 5820-
5825 and references therein.
Tm(II) derivatives isolated in this study, showing no sign of
decomposition after one week at ambient temperature. Important
for enhanced stability, the ethyl groups appeared equivalent in the
(10) Both the hydride and agostic hydrogens were located, but only the former
could be freely refined. Thus the agostic Tm---H distances are not as
reliable as the Tm-H2 bond length.
(11) (a) Ferrence, G. M.; McDonald, R.; Takats, J. Angew. Chem., Int. Ed.
1999, 38, 2233-2237. (b) Ferrence, G. M.; Takats, J. J. Organomet. Chem.
2002, 647, 84-93.
1
room temperature H NMR spectrum, indicating that the Tm-Et
agostic interactions are insufficiently strong to prevent ligand re-
arrangement. Lowering the temperature was accompanied by
spectral changes and the emergence of a complex spectrum at
-80 °C, with widely separated signals (-90 to 140 ppm) due to
the paramagnetic Tm2+ center. Unfortunately, the broad signals
could not be reliably assigned and hence we were unable to extract
the energetics of the Tm---Et agostic interactions.
(12) Zhang, X. W.; McDonald, R.; Takats, J. New J. Chem. 1995, 19, 573-
585.
(13) The possibility that 6 contains µ-peroxo moiety was considered, however
the structure refinement is consistent with the µ-η2:η2-N2 formulation.
The N1-N2 distance (1.215(10) Å) is in excellent agreement with other
similar species14 and considerably shorter than the normal µ-peroxo
distance of ca. 1.50
Å (1.543(4) Å in [Yb2{N(SiMe3)2}4(THF)2
(µ-O2)] 15).
(14) Evans, W. J.; Rego, D. B.; Ziller, J. W. Inorg. Chem. 2006, 45, 10790-
Wishing to expand the number of Tm(II) hydrocarbyl species,
complex 1 was treated with KCH2SiMe3, hoping to obtain the Tm-
(II) analogue of the versatile and synthetically very useful (Tp′)-
10798 and references therein.
(15) Niemeyer, M. Z. Anorg. Allg. Chem. 2002, 628, 647-657.
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