A new type of heteroleptic complex of divalent lead and synthesis of the P-plumbyleniophosphasilene, R2Si=P-Pb(L): (L = β-diketiminate)

Article abstract of DOI:10.1039/b710888e

The synthesis and structures of the first heteroleptic β-diketiminato complexes of lead(ii) with terminal phenolato, bis(trimethylsilyl)amido, bis(trimethylsilyl)phosphanido and silylidenephosphanido ligands are reported; ²⁰⁷Pb NMR spectroscopic data indicate that lead(ii) can serve as a σ-donor or acceptor centre, depending on the electronegativity of the terminal ligand. The Royal Society of Chemistry.

Full text of DOI:10.1039/b710888e

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A new type of heteroleptic complex of divalent lead and synthesis of the
P-plumbyleniophosphasilene, R2SiLP–Pb(L): (L = b-diketiminate){
Shenglai Yao, Stefan Block, Markus Brym and Matthias Driess*
Received (in Cambridge, UK) 16th July 2007, Accepted 10th August 2007
First published as an Advance Article on the web 23rd August 2007
DOI: 10.1039/b710888e
analogous reaction of [LLi(OEt2)]7 with PbCl2 in ethereal solvents
The synthesis and structures of the first heteroleptic b-diketi-
leads exclusively to redox reactions, with formation of elemental
lead. In order to circumvent redox processes, the reactions were
performed in hydrocarbons as solvents, using the sterically
encumbered Pb(II) diphenolate 18 instead of PbCl2. Thus, the
latter conversion yielded the desired plumbylene 2a in the form of
yellow crystals in 83% yield (Scheme 1). According to an X-ray
diffraction analysis, the asymmetric unit contains two independent
molecules of 2a.{
The molecules of 2a consist of slightly puckered six-membered
C3N2Pb rings in which the lead atoms adopt a trigonal pyramidal
geometry with angles of 83.6–97.9u between the nitrogen donor
and phenoxy-oxygen atoms (Fig. 1). The folding angle between the
planes defined by the N1, N2, C2, C3, C4 atoms (plane 1) and the
N1, Pb1, N2 atoms (plane 2) is 14.6u (14.9u in molecule 2).
The sum of bond angles at lead (274.8 and 286.9u) illustrates the
high 6p-character of the lead s-orbitals in the Pb–X (X = N, O)
bonds due to a relativistic contraction of the lead 6s orbital which
prevents mixing (hybridisation) of the lead-based valence orbitals.9
In other words, the lone pair of electrons at lead possess high
s-character. The Pb–N and Pb–O distances are similar to values
observed in related divalent Pb–N(amido)10 and Pb–O(phenolato)
complexes.11 The electronic situation of the Pb(II) atom in 2a is
also reflected in its 207Pb NMR spectrum which reveals a singlet at
d 1040 ppm. This value is close to that observed for a related
donor-supported bis(b-diketiminato)plumbylene (d 1060 ppm).12
Since the phenolato ligand is a good nucleophilic leaving group,
the reaction of 2a with LiN(SiMe3)2 in a 1 : 1 molar stoichiometry
minato complexes of lead(II) with terminal phenolato,
bis(trimethylsilyl)amido, bis(trimethylsilyl)phosphanido and
silylidenephosphanido ligands are reported; 207Pb NMR
spectroscopic data indicate that lead(II) can serve as a s-donor
or acceptor centre, depending on the electronegativity of the
terminal ligand.
The b-diketiminate ligand is a particularly versatile ligand for the
stabilisation of single-site metal centres in unusual low valence
states.1 Their coordination feature resembles those of related
N-heterocyclic ligands which were used for the synthesis of unusual
low-coordinate metal complexes. The low-valent (or low-coordi-
nate) metal centres in such complexes benefit from intramolecular
NAM donor–acceptor bonds and the presence of sterically
demanding substituents at the nitrogen atoms. Diketiminato metal
complexes were employed as pre-catalysts for polymerisation and
C–X coupling processes in homogeneous catalysis.2 In line with
that, main-group metal complexes can be employed for a better
understanding and a rational design of particular molecular
catalysts.
Recently, it was shown that appropriate tin(II) complexes can
serve as single-site initiators for the polymerisation of lactides.3 We
are particularly interested in the structure–reactivity relationship of
N-heterocyclic divalent group 14 element complexes L(X)E: A (L =
b-diketiminate; X = alkoxide, amide, halide, phosphanide; E = Si,
Ge, Sn, Pb) (Scheme 1). These complexes can serve as Lewis-acid
catalysts for ring-opening polymerisations and represent new
ligands for the synthesis of isolable carbene-analogue transition
metal complexes (E = M) capable of olefin metathesis reactions.
However, while a series of appropriate tin(II)4 and germanium(II)
complexes A5 were already reported, the respective isolable
divalent silicon and lead complexes of type A are still rare or
currently unknown. Recently, we reported the synthesis of the first
N-heterocyclic silylene of type A.6 Here we describe the synthesis
and structural features of the novel isolable heteroleptic lead(II)
complexes of type A, compounds 2a–c, including the first
plumbyleno-substituted phosphasilene R2SiLP–PbL, compound 3.
In contrast to the procedure for the synthesis of the
N-heterocyclic chloro complexes L(Cl)E: A (E = Ge, Sn), the
Technische Universita¨t Berlin, Institute of Chemistry: Metalorganics
and Inorganic Materials, Sekr. C2, Strasse des 17. Juni 135,
D-10623 Berlin, Germany. E-mail: matthias.driess@tu-berlin.de;
Fax: +49 (0)30-314-29732; Tel: +49 (0)30-314-29731
{ Electronic supplementary information (ESI) available: Synthesis,
characterisation and crystallographic data, in CIF or other electronic
format, of 2a, 2b, 2c and 3. CCDC 654313–654316. See DOI: 10.1039/
b710888e
Scheme 1 Group 14 metal b-diketiminato complexes of type A and
synthesis of 2a–2c and 3 (Ar = C6H3Pri2-2,6; R = C6H2Pri3-2,4,6; Ar9 =
C6H3But2-2,6).
3844 | Chem. Commun., 2007, 3844–3846
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Fig. 1 Molecular structure of 2a with thermal ellipsoids for C2–C4, N1,
N2, Pb1, O1 drawn at the 50% probability level (one of the two molecules
in the asymmetric unit; hydrogen atoms are not shown). Selected distances
(pm) and angles (u): Pb1–O1 221.6(5), Pb1–N1 230.3(6), Pb1–N2 228.9(6);
N1–Pb1–N2 83.6(2), O1–Pb1–N1 97.9(2), O1–Pb1–N2 93.3(2).
Fig. 3 Molecular structure of 2c with thermal ellipsoids for C2–C4, N1,
N2, Pb1, P1, Si1, Si2 drawn at the 50% probability level (hydrogen atoms
are not shown). Selected distances (pm) and angles (u): Pb1–P1 271.5(2),
Pb1–N1 235.9(6), Pb1–N2 232.5(6), Si1–P1 226.0(3), Si2–P1 224.8(3);
N1–Pb1–N2 81.2(2), P1–Pb1–N1 97.3(2), P1–Pb1–N2 103.2(2).
furnishes solely the desired amido complex 2b which was isolated
in 88% yield in the form of pale yellow crystals. Its composition is
proven by multinuclear NMR spectroscopy and elemental
analysis. Single crystal X-ray data were obtained for 2b which
revealed similar geometric parameters of the almost planar six-
membered C3N2Pb ring with those in 2a (Fig. 2).{ The sum of
bond angles around lead is 289u. The N1 atom of the terminal
N(SiMe3)2 group is almost trigonal planar coordinated (sum of
bond angles 358.4u) due to the intrinsically low inversion barrier of
nitrogen in silyl-substituted amines.13 Owing to the stronger
s-acceptor character of the N atom in the N(SiMe3)2 group, the
exocyclic Pb–N distance (227.1(8) pm) is smaller than those of the
endocyclic ones (Pb1–N3 231.5(8) and Pb1–N2 233.4(8) pm).
Interestingly, replacement of the phenolato ligand in 2a by the
N(SiMe3)2 group in 2b causes a significant deshielding of the 207Pb
nucleus of Dd 784 ppm in the 207Pb NMR spectrum (d 1824 ppm);
this suggests that the N(SiMe3)2 group is a stronger Pb(II)AX (X =
O, N) s-acceptor than the phenolato ligand. Furthermore, we
investigated the electronic changes at Pb by using the more
electropositive phosphanido ligand P(SiMe3)2. The synthesis of the
analogous phosphanido complex 2c is as simple and straightfor-
ward as the access to 2b. Thus, conversion of 2a with LiP(SiMe3)2
affords the desired compound 2c in the form of orange crystals in
79% yield (Scheme 1). Compound 2c is the second isolated
monomeric phosphanyl-substituted plumbylene.14 It is remarkably
stable in non-protic organic solvents and remains unchanged even
in boiling toluene for 12 h. Cryoscopic measurements prove that 2c
remains monomeric in hydrocarbon solutions. The structure of 2c,
determined from X-ray diffraction analysis,{ shows that the
phosphanido group at lead causes a drastic geometrical change in
the ring conformation to give a boat-shaped six-membered
C3N2Pb ring (Fig. 3). The folding angle between the planes
defined by the N1, C2, C3, C4, N2 atoms and the N1, Pb1, N2
atoms is 43.9u and thus much larger than those in 2a (14.6u) and 2b
(11u). However, this does not influence the degree of pyramidalisa-
tion of the Pb(II) atom (sum of bond angles at Pb 281.7u) which is
only slightly different from that in 2a and 2b. The same is true for
the endocyclic Pb–N distances which are practically unchanged.
The P atom is pyramidally coordinated, with a sum of bond angles
of 305.7u owing to the pronounced s-donor character of the silyl
groups to phosphorus. As expected, the Pb–P distance of
271.5(2) pm is similar to those in the dimeric plumbylene
{Pb[P(SiMe3)2]2}2 with a puckered Pb2P2 and three coordinate
Pb(II) atoms (terminal Pb–P 269.6(7) pm)15 but longer than that in
a
homoleptic diphosphanyl-substituted plumbylene with di-
coordinate lead (265.4(4) pm).14
What factors are responsible for the drastic change in the
C3N2Pb ring conformation? Doubtless, the s-donor ability of
phosphorus (intrinsic Pd+–Pbd2 bond polarity) plays a major role,
whereas a PbLP p-bond can be excluded because of the
unfavourable P and Pb configurations and the inert pair effect
of the 6s-electrons at Pb. The proposed Pd+–Pbd2 bond polarity is
in accordance with the 31P and 207Pb NMR spectra of 2c: the 31
P
nucleus of the P(SiMe3)2 ligand resonates at much lower field
(d 2116.6 ppm) than that of the terminal P(SiMe3)2 group in the
related dimer {Pb[P(SiMe3)2]2}2 (d 2217.3 ppm). Correspondingly,
the 207Pb nucleus in 2c shows in the 207Pb NMR spectrum
(d 21737 ppm) a remarkably strong shielding of Dd 23561 ppm in
comparison to 2b. Also intriguing is the unprecedentedly large
magnitude of the 1J(31P, 207Pb) coupling constant of 2852 Hz,
which is much larger than that of {Pb[P(SiMe3)2]2}2 (1264 and
1183 Hz) and even exceeds the value observed for related
Fig. 2 Molecular structure of 2b with thermal ellipsoids for C2–C4, N1–
N3, Pb1, Si1, Si2 drawn at the 50% probability level (hydrogen atoms are
not shown). Selected distances (pm) and angles (u): Pb1–N1 227.1(8), Pb1–
N2 233.4(8), Pb1–N3 231.5(8), Si1–N1 171.5(8), Si2–N1 174.6(8); N1–
Pb1–N3 99.2(3), N1–Pb1–N2 106.6(3), N3–Pb1–N2 83.2(3).
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The folding angle between the planes defined by the N1, N19,
C2, C3, C29 atoms (plane 1) and the N1, Pb1, N19 atoms (plane 2)
is 7.7u. The single-crystal X-ray diffraction analysis{ of 3 (Fig. 4)
revealed an almost planar C3N2Pb ring in which the lead atom
adopts a trigonal pyramidal geometry similar to that in 2a and 2b.
The Pb–P distance of 267.1(1) pm is ca. 4 pm shorter than that
in 2c owing to the lower coordination number of phosphorus.
As expected, the low-valent silicon atom is trigonal planar
coordinated and the SiLP and Si–Si distances are marginally
longer than those in the related P-zinciophosphasilene.16
The novel plumbylene complexes 2a–c and 3 are surprisingly
robust under anaerobic conditions in non-protic organic solvents
even at elevated temperatures (,110 uC). Thus, they represent
valuable building blocks for the synthesis of other heteroleptic
plumbylene complexes with Pb–X bonds (X = non-metal, metal
atom) and tunable electrophilic or nucleophilic properties.
Preliminary results show that the new compounds are very active
initiators for the synthesis of polylactides and its copolymers.
We gratefully acknowledge financial support from the Deutsche
Forschungsgemeinschaft.
Fig. 4 Molecular structure of 3 with thermal ellipsoids for C2, C3, C29,
N1, N19, Pb1, P1, Si1, Si2 drawn at the 50% probability level (hydrogen
atoms are not shown). The molecule lies about a mirror plane. The atom
labels with the additional prime character (9) indicate that these atoms are
at equivalent positions (x, 1/2 2 y, z). Selected distances (pm) and angles
(u): Pb1–P1 267.1(1), Pb1–N1 236.8(3), Pb1–N19 236.8(3), P1–Si1 208.5(2),
Si1–Si2 241.4(2); N1–Pb1–N19 82.0(1), N1–Pb1–P1 97.2(3), Si1–P1–Pb1
97.10(5), P1–Si1–Si2 117.05(7).
monomeric homoleptic diphosphanyl plumbylenes (1995 and
1979 Hz) with di-coordinate Pb(II) atoms.14 The large magnitude
Notes and references
1
of the J(31P, 207Pb) coupling constant in 2c clearly suggests a
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moiety should result in a re-planarisation of the six-membered
C3N2Pb ring and, in turn, deshielding of the 207Pb nucleus in the
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electronegative PLSiR2 group, bearing an sp2-hybridised P atom.
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(‘‘P-phosphasilene’’ group), But3Si(R)SiLP– (R
=
C6H2Pri3-
2,4,6), was chosen in order to examine the outcome of the reduced
donor ability of phosphorus towards lead. The compound
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(Scheme 1). Compound 3 is the first isolable phosphasilene
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low-field 207Pb chemical shift indicates that the P atom serves as a
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electronic situation in 2c and analogous to that in 2b. In turn, the
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hyperconjugation as shown for
a
P-zinciophosphasilene.16
Accordingly, the electronic difference in the P atom in 3 vs. 2c
causes also a drastic change in the C3N2Pb ring conformation.
3846 | Chem. Commun., 2007, 3844–3846
This journal is ß The Royal Society of Chemistry 2007