Air-stable helical bis(cyclopentadienylphosphazene) complexes of divalent ytterbium

Article abstract of DOI:10.1016/j.mencom.2010.06.004

Two homoleptic Yb^II constrained geometry complexes [Yb{η⁵,η¹-R₂P(C₅Me ₄)NAd}₂] [R = Me (3), Ph (4); Ad = 1-adamantyl] derived from P-amino(cyclopentadienylidene)phosphoranes (CpP

Full text of DOI:10.1016/j.mencom.2010.06.004

Mendeleev
Communications
Mendeleev Commun., 2010, 20, 197–199
Air-stable helical bis(cyclopentadienylphosphazene)
complexes of divalent ytterbium†
Alex R. Petrov,a Konstantin A. Rufanov,*b Noa K. Hangaly,a
Michael Elfferding,a Klaus Harmsa and Jörg Sundermeyer*a
a Fachbereich Chemie der Philipps-Universität Marburg, 35032 Marburg, Germany.
Fax: +49 6421 28 25711; e-mail: jsu@staff.uni-marburg.de
b Chemical Diversity Research Institute, 114401 Khimki, Moscow Region, Russian Federation.
Fax: +7 495 626 9780; e-mail: kruf@iihr.ru
DOI: 10.1016/j.mencom.2010.06.004
Two homoleptic YbII constrained geometry complexes [Yb{η5,η1-R2P(C5Me4)NAd}2] [R = Me (3), Ph (4); Ad = 1-adamantyl]
derived from P-amino(cyclopentadienylidene)phosphoranes (CpPN) R2P(C5Me4)NHAd [R = Me (1), Ph (2)] were found to be
remarkably stable towards dioxygen, as shown by NMR spectroscopy and further confirmed by the crystal structure analysis of 4
and discussed in terms of efficient steric shielding and electronic stabilization of the YbII metal centre by this CpPN ligand type.
In the course of systematic studies of ambident organophospho-
rus(V) donor ligands of the general type [R2P(X)Z]– (X, Z = S, NR',
Ph
AdN3
CH2, CHR', Cp, Ind, Flu; as for X = Z and X ¹ Z), we recently
focused on the coordination chemistry of P-amino(cyclopenta-
dienylidene)phosphoranes R2P(C5R'4)NHR'' (CpPN ligand type).1
Taking into consideration the general concept that three-
valent complexes on the basis of monoanionic CpPN type ligands
are isolobal, in case of group III metals even isoelectronic, with
tetravalent group IV transition metal complexes of dianionic
CpSiN type ligands (Figure 1), which became one of the best
developed classes of complexes bearing chelating cyclopenta-
dienyl(amido) ligands of the early transition metals,2 the so-called
‘constrained geometry catalysts’ (CGC).3
P
– N2
Ph
+
Ph
Ph
Ph
Ph
Ph
Ph
P
P
P
+
NH
N
N
Ad
Ad
Ad
2a 79%
2b 9%
2c 12%
Scheme 1 Synthesis of the CpPN ligand 2.
R4'
R4'
cipated, the reactivity of the diphenylphosphane towards AdN3
is considerably lower than that of the dimethylphosphane. After
the optimisation of reaction conditions (toluene, 110 °C, 14 h),
bright yellow crystalline compound 2 was isolated in 50% yield.‡
Unlike 1, compound 2 is only slightly air-sensitive, and it
can be stored under aerobic conditions in a matter of days.
Furthermore, the 31P NMR characterization of compound 2
showed three isomeric forms 2a–c, which are not observed for
ligand 1.1(d) The (cyclopentadienylidene)phosphorane 2a form
is the predominant one (79% estimated by 31P NMR); it crys-
tallized from the isomeric mixture and was characterized by
X-ray structure analysis (Figure 2).§ Minor tautomers 2b and 2c
are assigned to P-allylic and P-vinylic bonded cyclopentadienyl-
(imino)phosphoranes, respectively.1(d) All these isomers lead to
only one 31P NMR signal upon metalation and back again to the
three 31P NMR signals in the thermodynamic equilibrium upon
reprotonation.
R
R
R
R
Si
P
R4'
R4'
N
N
R''
CpSiN
R''
CpPN
R
R
R
R
Si
P
ML2
ML2
N
N
R''
M = Ti, Zr, Hf
L = Hal, Alk
R''
M = Sc, Y, La–Lu
Figure 1 Isolobal relationship between complex families.
We were interested in investigating the ability of CpPN ligands
to stabilize divalent lanthanide ions in a solid state for further
applications as the components of luminescent materials.4
Recently, we have reported the synthesis and crystal struc-
ture of the constrained geometry CpPN lutetium complex
The orange crystals of 2 suitable for X-ray structure deter-
mination were obtained by crystallization from a saturated
[{η5,η1-Me2P(C5Me4)NAd}Lu(CH2SiMe3)2]
(Ad = 1-ada-
hexane solution at room temperature. Ligand 2 crystallizes in
–
mantyl).1(c) The ligand Me2P(C5Me4)NHAd 1 was synthesized
by the Staudinger reaction of AdN3 and Me2P–C5Me4H accord-
ing to the procedure reported previously.1(c),(d) Analogously, the
new CpPN ligand Ph2P(C5Me4)NHAd 2 was synthesized from
the phosphane precursor Ph2P–C5Me4H (Scheme 1). As anti-
the triclinic space group P1. The asymmetric unit contains two
almost identical independent molecules (I and II). For one of
them (II), a disordered model for the N–Ad group has been
refined as two sites with 62÷38 occupancy. The observed small
differences in bond lengths and angles in the molecules are
†
‡
Part 6 in the series ‘Organophosphorus(V) ligands for coordination and
For procedures and characteristics of products, see Online Supplementary
organometallic chemistry’. For part 5, see ref. 1(e).
Materials.
– 197 –
© 2010 Mendeleev Communications. All rights reserved.

Mendeleev Commun., 2010, 20, 197–199
sparingly soluble in Et2O or DME and insoluble in alkanes.
Whereas complex 3 can be purified by crystallization from Et2O
solutions, complex 4 crystallizes from the same solvent with
half a molecule of non-coordinated diethyl ether as 4·1/2Et2O.
Prolonged drying in a vacuum (10–3 mbar) or heating in toluene
leads to the formation of the solvent-free powdery compound.
The 1H NMR spectra of complexes 3 and 4 are similar. In the
aliphatic region, four sharp resonances of the methyl groups
at the Cp ring were observed. Moreover, the presence of two
doublets assigned to the Me2P group in 3 and two sets of
resonances of o-Ph protons in 4 point out the asymmetric
coordination mode of the ligands at the metal centre. The
asymmetric arrangement was also verified by the presence of
five coupled resonances for the Cp ring carbon atoms and two
sets of resonances for R2P groups in the 13C NMR spectra. The
31P NMR resonances were found at 0.5 (3) and 3.1 ppm (4),
respectively, in the characteristic region of P-cyclopentadienyl-
iminophosphoranes [cf. Me2P(=NR)C5Me4H, R = SiMe3, 2,6-di-
isopropylphenyl].1(d)
The multinuclear NMR spectroscopy of 3 and 4 clearly
indicates that both complexes of the bidentate CpPN ligands
are best described as derivates of divalent ytterbocene and the
bonding between nitrogen and the metal centre is weak. A further
insight into the bonding situation was gained by X-ray diffrac-
tion analysis.
Large deep red crystals of 4 suitable for X-ray structure
determination were grown by cooling a saturated diethyl ether
solution to –10 °C. Complex 4 crystallizes with 1/2 molecule
of Et2O per ytterbium complex (4·1/2Et2O) in the monoclinic
space group P2/n.§ Two independent molecules in the unit cell
can be best described as a distorted tetrahedron with ytterbium
atoms located on the crystallographic C2 axis. Both ligands in
each molecule adopt a chiral helical arrangement around the
ytterbium atom. The two independent molecules are the rota-
tional Δ- and Λ-enantiomers. The molecular structure of the
Δ-enantiomer is presented in Figure 3. The selected bond lengths
and angles are given in Table 1. The observed small differences
in bond lengths and angles in the molecules predominantly result
from packing effects.
C(1)
P(1)
N(1)
H(1)
Figure 2 Molecular structure of ligand 2. The thermal ellipsoids are drawn
at the 50% probability level. Molecule I is presented. All hydrogen atoms,
except for H(1) have been omitted for clarity. Selected bond lengths (Å)
and angles (°): N–P 1.667(4), P–CCp 1.706(3), P–CPh 1.815(4) and 1.824(3),
N–CAd 1.489(5); CCp–P–N 118.3(2), CPh–P–CPh 102.1(2), P–N–CAd 125.9(3).
predominantly a result of packing effects. Therefore, only mole-
cule I is shown in Figure 2.
The divalent ytterbium complexes were synthesized via the
salt metathesis route starting from [YbI2(thf)2] and the potas-
sium salts of CpPN ligands 1 and 2, which were obtained by
deprotonation with benzyl potassium [BnK] in THF (Scheme 2).
Because of the low solubility of starting [YbI2(thf)2] and
[BnK] in other common organic solvents, the syntheses were
performed in THF. After filtration of precipitated KI and sub-
sequent crystallization from diethyl ether solutions, deep red crys-
talline solids were obtained in 82 and 60% yields, respectively.‡
Unexpectedly, the complexes are stable towards dioxygen:
they can be handled in air and, in the solid state, they do not
change for months.¶ Both 3 and 4 show high solubility in aromatic
and polar solvents such as THF and pyridine, but they are
Me4
Me4
Ad
R
P
N
i, BnK / THF
ii, [YbI2(thf)2]
R
Yb
R
R
R
R
N
P
P
Ad
NH
The average bond lengths of P–CCp [1.758(3) and 1.763(3) Å]
are longer than those in the free ligand [1.706(3) Å], while the
P–N bonds are essentially shorter [for 4, 1.597(2) and 1.596(2) Å;
for 2, 1.667(4) Å].
Ad
Me4
1 R = Me
2 R = Ph
3 R = Me, 82%
4 R = Ph, 60%
The average dihedral N–Yb–N' and Z–Yb–Z' angles in 4
(Z = centroid of Cp ring) are 125.0(1) and 132.6(1)°, respectively.
Although both Cp rings are carrying four methyl groups, the
structures are less strained than in a related sterically hindered
molecule, [{η5-Bu2t C5H3}2Yb(dme)],5 with a Z–Yb–Z' angle of
143.2(1)°. This can be explained by the constraints within our
Scheme 2 Synthesis of the ytterbium(II) complexes.
§
X-ray diffraction data were collected on a STOE IPDS2 diffractometer
using graphite monochromated MoKα radiation (l = 0.71073 Å) at 173(2) K.
The structures were solved by direct methods and refined by difference
Fourier syntheses using SIR9216 and SHELXL-9717 software package.
Programme Diamond 3.0a18 was used for structure representations.
–
Crystal data for 2: C31H38NP, M = 455.59, triclinic, space group: P1,
C(1)
N(1)
P(1)
a = 11.696(1), b = 13.415(1) and c = 17.197(1) Å, a = 69.970(7)°, b =
= 78.667(7)°, g = 80.750(8)°, V = 2472.5(4) Å3, Z = 4, dcalc = 1.224 g cm–3,
m(MoKα) = 0.131 mm–1
. Crystal dimensions: 0.22×0.08×0.06 mm.
P(1)
P(1')
19230 reflections collected, 8679 independent reflections and 2956 with
I > 2s(I). Final agreements factors were R1 = 0.0473 (observed reflec-
tions) and wR2 = 0.0729 (all data). For one of the two independent mole-
cules a disordered model for the N-adamantyl group has been refined.
Crystal data for 4: C66H84N2OP2Yb2, M = 1156.33, monoclinic, space
group P2/n, a = 21.486(1), b = 11.545(1) and c = 23.604(1) Å, b =
= 106.194(3)°, V = 5622.8(4) Å3, Z = 2, dcalc = 1.366 g cm–3, m(MoKα) =
= 1.764 mm–1. Crystal dimensions: 0.24×0.24×0.15 mm. 78030 reflec-
tions collected, 10932 independent reflections and 8915 with I > 2s(I).
Final agreements factors were R1 = 0.0258 and wR2 = 0.0598 (all data).
CCDC 764288 and 721138 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
For details, see ‘Notice to Authors’, Mendeleev Commun., Issue 1, 2010.
Yb(1)
N(1)
N(1)
N(1')
C(1)
P(1)
Figure 3 Molecular structure of complex 4·1/2Et2O. The thermal ellipsoids
are drawn at the 50% probability level. The Δ-enantiomer is presented; the
Et2O molecules in the unit cell are not depicted. View along the C2-axis
(left) and the side-view (right); all hydrogen atoms and Ph groups, except
for their ipso-carbon atoms, have been omitted for clarity.
¶
No changes have been observed after storing complexes 3 and 4 at room
temperature without any precaution over period of more than 6 months,
that was confirmed by repeated NMR and elemental analyses.
– 198 –

Mendeleev Commun., 2010, 20, 197–199
electron-rich YbII central atom, thus, to compounds that are
less susceptible toward protolytic and oxidative degradation.
These properties render the CpPN building unit an ideal spectator
ligand for functionalized lanthanide complexes such as hetero-
leptic amides and alkyls.
Table 1 Selected bond lengths (Å) and angles (°) for the Δ- and Λ-enantio-
mers of 4·1/2Et2O.
Δ-enantiomer
Λ-enantiomer
Yb(2)–N(2)
N(2)–P(2)
P(2)–C(32)
Yb(2)–Z(2)
Yb(1)–N(1)
N(1)–P(1)
P(1)–C(1)
Yb(1)–Z(1)
2.537(2)
1.597(2)
1.758(3)
2.622(1)
2.618(2)
1.596(2)
1.763(3)
2.563(1)
K.A.R. akcnowledges Deutsche Akademischer Austausch
Dienst (DAAD) for funding a research fellowship at Philipps-
Universität Marburg within the Ostpatnerschaftsprogramm. This
study was supported by DFG (SPP-1166).
N(1)–Yb(1)–C(1)
N(1)–Yb(1)–Z(1)
N(1)–P(1)–C(1)
N(1)–Yb(1)–N(1)'
Z(1)–Yb(1)–Z(1)'
60.2(1)
85.2(1)
103.1(1)
122.1(1)
133.6(1)
N(2)–Yb(2)–C(32)
N(2)–Yb(2)–Z(2)
N(2)–P(2)–C(32)
N(2)–Yb(2)–N(2)'
Z(2)–Yb(2)–Z(2)'
59.6(1)
85.2(1)
102.7(1)
127.8(1)
131.5(1)
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.mencom.2010.06.004.
CpPN chelate ligand and by the helical configuration of both.
The molecular bonding distances and angles of the CpPN
ligand are closely related to those found in the structure of the
[{η5,η1-Me2P(C5Me4)NAd}Lu(CH2SiMe3)2] complex.1(c) How-
ever, a further investigation into the ligand bonding mode, by
comparison of Lu3+ and Yb2+ complexes, reveals characteristic
differences. Although the ionic radius of ytterbium(II) (0.159 Å)6
is larger than that of lutetium(III), the average Yb–N bond
length is significantly greater [2.578(4) Å] than that of the
corresponding Lu–N bond in the lutetium complex [{η5,η1-
Me2P(C5Me4)NAd}Lu(CH2SiMe3)2] [2.278(3) Å]. On the one
hand, the value is better comparable with those found in closely
related ytterbocenes with σ-donating nitrogen ligands (e.g.,
2.486, 2.514 Å in [{η5-1,3-(Me3Si)2C5H3}2Yb(Phen)],7 2.544,
2.586 Å in [{η5-C5Me5}2Yb(py)2]8 and av. 2.579 Å in [{η5,η1-
C5H4(CH2)2NMe2}2Yb]9). On the other hand, the average Yb–Z
distance of 2.593(2) Å is unexpectedly long and falls out of the
range typical of ytterbocenes. For instance, in ytterbocenes
with the polysubstituted cyclopentadienyl ligands, the Yb–Z
distance of av. 2.466(3) Å was found in [{η5-C5Me5}2Yb(py)2]8
and 2.443(2) Å in [{η5-C5Me5}2Yb(terpy)].10 Note that the Yb–Z
distances in complexes with a non-substituted Cp moiety are,
in general, significantly shorter; the distance of only 2.408(1) Å
was found in [{η5-C5H5}2Yb(dme)]11 and av. 2.376(2) Å in
[{η5-C5H5}2Yb(thf)].12 The elongation of the Yb-Z distance
in the structure of complex 4 can be attributed to the significant
electron withdrawing –I and/or –M effects of the iminophos-
phorane moiety. The comparison of the average P–CPh bond length
[av. 1.835(3) Å] in 4 with the closely related Ph2P(C5H4)Me
(av. 1.808 Å)13 and Ph3P(C5H4) (av. 1.806 Å)14 is in accordance
with this assumption.
References
1
(a) K. A. Rufanov, B. Ziemer, M. Hummert and S. Schutte, Eur. J.
Inorg. Chem., 2004, 4759; (b) K. A. Rufanov, B. Ziemer and M. Meisel,
Dalton Trans., 2004, 3808; (c) K. A. Rufanov, A. R. Petrov, V. V. Kotov,
F. Laquai and J. Sundermeyer, Eur. J. Inorg. Chem., 2005, 3805;
(d) A. R. Petrov, K. A. Rufanov, B. Ziemer, P. Neubauer, V. V. Kotov
and J. Sundermeyer, Dalton Trans., 2008, 909; (e) K. A. Rufanov and
A. Spannenberg, Mendeleev Commun., 2008, 18, 32.
2
3
(a) P. J. Shapiro, E. E. Bunel, W. P. Schaefer and J. E. Bercaw, Organo-
metallics, 1990, 9, 867; (b) P. J. Shapiro, W. D. Cotter, W. P. Schaefer,
J. A. Labinger and J. E. Bercaw, J. Am. Chem. Soc., 1994, 116, 4623;
(c) W. E. Piers, P. J. Shapiro, E. E. Bunel and J. E. Bercaw, Synlett,
1990, 2, 74; (d) J. Okuda, Chem. Ber., 1990, 123, 1649; (e) J. Okuda,
Top. Curr. Chem., 1991, 160, 97.
(a) J. C. Stevens, F. J. Timmers, G. W. Rosen, G. W. Knight and
S. Y. Lai, Eur. Pat. Appl. EP 0 416 815 A2, 1991 (filed August 30,
1990); (b) J. A. Canich, Eur. Pat. Appl. EP 0 420 436 A1, 1991 (filed
September 10, 1990); (c) J. C. Stevens, Stud. Surf. Sci. Cat., 1996, 101,
11; (d) A. L. McKnight and R. M. Waymouth, Chem. Rev., 1998, 98,
2587.
4
5
G. Blasse and B. C. Grabmaier, Luminescent Materials, Spriger-Verlag,
Berlin, Heidelberg, 1994.
A. V. Khvostov, A. I. Sizov, B. M. Bulychev, S. Ya. Knjazhanski and
V. K. Belsky, J. Organomet. Chem., 1998, 559, 97.
6
7
R. D. Shannon, Acta Crystallogr., Sect. A, 1976, 32, 751.
M. Schultz, J. M. Boncella, D. J. Berg, T. D. Tilley and R. A. Andersen,
Organometallics, 2002, 21, 460.
8
9
T. D. Tilley, R. A. Andersen, B. Spencer and A. Zalkin, Inorg. Chem.,
1982, 21, 2647.
T. D. Tilley, R. A. Andersen, B. Spencer and A. Zalkin, Inorg. Chim.
Acta, 1998, 280, 138.
10 C. J. Kuehl, R. E. Da Re, B. L. Scott, D. E. Morris and K. D. John,
Chem. Commun., 2003, 2336.
Thus, not only steric but also electronic effects of the CpPN
ligands are responsible for the unique stability of the above
compounds with respect to dioxygen. The ligands reveal an
ambident character; the monoanionic charge is partially localized
on the Cp ring, as well as on the nitrogen atom of the phos-
phazene unit (Scheme 3). Due to the negative-hyperconjugative
stabilizing effect15 of the phosphorus(V) σ* orbitals and the
characteristic charge distribution, CpPN ligands can be con-
sidered as much weaker donors than classical ligands [C5R5]–
or [NPR3]–, which are well established in lanthanide chemistry.
Both the weaker donor character and charge distribution lead
to less polar metal carbon and metal nitrogen bonds and a less
11 J. Jin, S. Jin and W. Chen, J. Organomet. Chem., 1991, 412, 71.
12 T. D. Tilley, R. A. Andersen, B. Spencer, H. Ruben, D. H. Templeton
and A. Zalkin, Inorg. Chem., 1980, 19, 2999.
13 J. H. Brownie, H. Schmider and M. C. Baird, Organometallics, 2007,
26, 1433.
14 P. Laavanya, B. S. Krishnamoorthy, K. Panchanatheswaran and
M. Manoharan, J. Mol. Struct. THEOCHEM, 2005, 716, 149.
15 T. Leyssens and D. Peeters, J. Org. Chem., 2008, 73, 2725 and
references cited therein.
16 C. Giacovazzo, Z. Kristallogr., 1993, 206, 161.
17 G. M. Sheldrick, SHELXS 97, Program for Solution of Crystal Structures,
Universität Göttingen, 1997.
18 K. Brandenburg, Diamond 3.0a, Crystal Impact GbR, Bonn, Germany,
1997–2004.
Me4
Me4
Me4
Ad
Ad
Ad
R
R
R
R
R
R
P
P
P
N
N
N
Yb
Yb
Yb
R
R
R
R
R
R
N
N
N
P
P
P
Ad
Ad
Ad
Me4
Me4
Me4
Scheme 3 Valence bond description of the title complexes.
Received: 10th February 2010; Com. 10/3464
– 199 –