Tungsten-mediated activation of a PbII-N bond: A new route to tungsten-lead triple bonds

Article abstract of DOI:10.1002/anie.200801331

A W≡Pb bond is present in the hydrido plumbylidyne complex trans-[H-(PMe₃)₄W≡Pb(2,6-Trip₂C ₆H₃)], which is formed by an unprecedented Pb-N activation reaction of {Pb(2,6-Trip₂C₆H₃)-NMe ₂}₂, the first structurally characterized organolead(II) amide (see picture for molecular structure). Trip=2,4,6-iPr₃C ₆H₂. (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200801331

Angewandte
Chemie
DOI: 10.1002/anie.200801331
Plumbylidyne Complexes
II
À
Tungsten-Mediated Activation of a Pb N bond: A New Route to
Tungsten–Lead Triple Bonds**
Alexander C. Filippou,* Nils Weidemann, and Gregor Schnakenburg
Dedicated to Professor Wolfgang A. Herrmann on the occasion of his 60th birthday
Lead(II) amides are a rare class of reactive compounds which
have the profound tendency to undergo disproportionation in
the absence of sterically demanding substituents at the lead or
nitrogen atoms. The lead diamides Pb(NR1R2)2 (R1 = R2 =
SiMe3; R1 = SiMe3, R2 = tBu)[1] and [Pb(NR-kN)2(m-EMe2)]n
(R = tBu, E = Si, Sn, n = 1; R = iPr, E = Si, n = 2),[2] and PbII
porphyrins and phthalocyanines are the most familiar mem-
bers of this class of compounds.[3] In comparison, organo-
lead(II) amides of the general formula Pb(NR1 )(R2) are very
2
scarce, and the only known example is {Pb(2,6-
Trip2C6H3)NH2}2 (Trip = 2,4,6-iPr3C6H2).[4,5] The chemistry of
PbII amides has been little explored. In fact, only a few
reactions of the diamides Pb{N(SiMe3)2}2 and Pb(NtBu-
kN)2(m-SiMe2) have been reported to date, including coordi-
nation to a metal center,[1c] insertion into a metal–halogen
bond,[1c] Pb N bond protolysis,[6] nucleophilic displacement of
À
the amide ligands by silyl groups,[7] amide/chloride exchange
with Lewis acids,[8] and reduction to elemental lead.[9]
Figure 1. DIAMOND plot of the molecular structure of 1b in the solid
state. Thermal ellipsoids are set at 30%probability. Hydrogen atoms
are omitted for clarity. Selected bond lengths [Š] and angles [8]: Pb1-N1
2.329(3), Pb1-N2 2.426(3), Pb1-C1 2.399(4), Pb2-N1 2.427(3), Pb2-N2
2.321(3), Pb2-C39 2.395(3); C1-Pb1-N1 95.7(1), C1-Pb1-N2 118.2(1),
N1-Pb1-N2 78.7(1), N1-Pb2-N2 78.9(1), N1-Pb2-C39 116.5(1), N2-Pb2-
We report herein the first structural characterization of an
organolead(II) amide and its unprecedented tungsten-medi-
II
À
ated Pb N bond-cleavage reactions to form tungsten–lead
triple bonds.[10]
Treatment of {Pb(R)Br}2 (1a, R = 2,6-Trip2C6H3)[11] with
C39 96.1(1), Pb1-N1-Pb2 101.1(1), Pb1-N2-Pb2 101.3(1).
LiNMe2 in diethyl ether afforded selectively aryl lead(II)
amide {Pb(R)NMe2}2 (1b), which was isolated as a pale
yellow, very air sensitive,[12] microcrystalline solid in 77%
yield.[13] Compound 1b decomposes on heating at 1358C and
is photolabile; its yellow solutions in pentane or toluene
slowly deposit elemental lead on prolonged exposure to
daylight. The thermal stability of 1b is remarkable given the
thermal lability of {Pb(NMe2)2}2, which decomposes below
room temperature.[14]
Compound 1b is the first organolead(II) amide to be
structurally characterized (Figure 1).[13,15] In the solid state it
forms nearly centrosymmetric dimers in which the NMe2
groups bridge the lead atoms to form a planar[16] Pb2N2
À
parallelogram with two pairs of quite different Pb N bond
À
À
À
lengths (Pb1 N1 2.329(3), Pb2 N2 2.321(3); Pb1 N2
À
2.426(3), Pb2 N1 2.427(3) Š). The endocyclic bond angles
at the lead atoms are considerably more acute (78.7(1) and
78.9(1)8) than those at the nitrogen atoms (101.1(1) and
101.3(1)8; Figure 1). Expectedly, the bridging Pb Namide
bonds of 1b (2.32–2.43 Š) are longer than terminal Pb
II
À
II
À
Namide bonds (2.23–2.28 Š).[1c,5] This leads to a transannular
Pb···Pb distance of 3.6723(7) Š that is considerably shorter
than that in {Pb(R)X}2 (X = Br (1a): 4.257(8) Š; X = I (1c):
4.603(1) Š).[10b,11] However, the separation of the lead centers
in 1b is longer than the interatomic distance in elemental lead
[*] Prof. Dr. A. C. Filippou, N. Weidemann, G. Schnakenburg
Institut für Anorganische Chemie
Universität Bonn
(3.494 Š),[17] and lies outside the range of distances attributed
[18]
À
to direct Pb Pb bonding (2.84–3.53 Š). The three-coordi-
Gerhard-Domagk Strasse 1, 53121 Bonn (Germany)
Fax: (+49)30-735-327
E-mail: filippou@uni-bonn.de
nate lead centers have trigonal-pyramidal coordination, as
evidenced by the sum of angles at Pb1 (292.68) and Pb2
(291.58), which are similar to those of 1a (292.3 and 293.08)
and 1c (297.28).[10b,11] Both the NMe2 groups and the trans-
arranged aryl substituents are tilted with respect to the Pb2N2
[**] We thank the Deutsche Forschungsgemeinschaft (project FI 445/6-
1) for the generous financial support of this work. We also thank U.
Kätel, W.-D. Bloedorn, and A. Thiesies at the Humboldt University of
Berlin for the elemental analyses and the 2D NMR spectra.
core.[19] The Pb Caryl bonds (2.399(4) and 2.395(3) Š) are
longer than those of 1a (2.33(1) and 2.31(1) Š) and 1c
À
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801331.
(2.326(6) Š), and suggest increased intramolecular steric
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repulsion of the substituents in 1b resulting from the shorter
Pb···Pb distance.
The NMR spectra of 1b corroborate the solid-state
structure.[13] Thus, two singlet resonance signals of equal
intensity are observed in the 1H NMR spectrum (C6D6, 298 K)
for the methyl protons of the NMe2 groups (d = 1.70 and
4.17 ppm),[19] that is, the dimeric structure of 1b is retained in
solution.[20] The 13C{1H} NMR spectrum of 1b displays a
characteristic low-field signal for the lead-bonded Caryl atom
at d = 244.8 ppm, which appears at higher field than the
corresponding signals of the aryl lead(II) halides 1a (dC =
287.9 ppm) and 1c (dC = 276.7 ppm).[10b,11]
Treatment of lead(II) amide 1b with two equivalents of
[W(h2-CH2PMe2)(H)(PMe3)4] (2)[21] in toluene at 808C
afforded the hydrido plumbylidyne complex trans-[H-
ꢀ
(PMe3)4W Pb(2,6-Trip2C6H3)] (4a, Scheme 1). The same
Figure 2. DIAMOND plot of the molecular structure of 4a·2(n-C5H12)
in the solid state. Thermal ellipsoids are set at 30%probability.
Hydrogen atoms except H59 and those of the solvent molecules are
omitted for clarity. Selected bond lengths [Š] and angles [8]: W-Pb
2.5525(3), W-H59 1.71(2), W-P1 2.436(2), W-P2 2.421(2), W-P3
2.451(2), W-P4 2.424(2), Pb-C1 2.229(6); W-Pb-C1 178.7(2), Pb-W-H59
173(2), Pb-W-P1 91.27(4), Pb-W-P2 107.11(5), Pb-W-P3 90.77(5), Pb-
W-P4 108.30(5).
À
PbC6H3(2,6-Trip2C6H3)] (X = Br (4b), W Pb 2.5464(5) Š;
[10b,25]
À
X = I (4c), W Pb 2.5477(3) Š).
The hydride ligand
could be localized in the difference Fourier map at a distance
of 1.71(2) Š from the tungsten center, and is trans-arranged to
the plumbylidyne ligand (Pb-W-H59 173(2)8; Figure 2).[26]
À
À
The W P bonds (average length 2.43 Š) and Pb Caryl bond
À
(2.229(6) Š) of 4a are shorter than those of 4b (W P 2.48 Š,
À
À
À
Pb Caryl 2.254(6) Š) and 4c (W P 2.48 Š, Pb Caryl
2.258(3) Š), probably due to the reduced steric demand of
the hydride ligand.
Scheme 1. Syntheses of hydrido plumbylidyne complex 4a.
Composition and structure of 4a were confirmed by the
spectroscopic data. Thus, the IR spectrum of 4a displays a
À1
À
complex was selectively formed on heating of 1b and two
equivalents of cis-[W(N2)2(PMe3)4] (3)[22] in refluxing toluene
(Scheme 1). No intermediates were observed in these unpre-
characteristic n(W H) band at 1678 cm , which appears at
ꢀ
higher energy than that of trans-[H(PMe3)4W Ge(2,6-
À1 [21c]
À
Trip2C6H3)]
(n(W H) = 1658 cm )
(dmpe)2W CMes]
and
;
trans-[H-
dmpe =
À1
ꢀ
À
cedented reactions, which probably involve tungsten-medi-
(n(W H) = 1600 cm
ated Pb Namide bond heterolysis.[23] Alternatively, plumbyli-
Me2PCH2CH2PMe2).[27] This observation suggests that the
II
À
dyne complex 4a can be selectively obtained in two steps
starting from 2. The first step involves thermal activation of
the lead(II) bromide 1a by complex 2 to yield selectively
trans influence of the ylidyne ligand decreases in the series
CR > GeR > PbR. In the H NMR spectrum of 4a a distinc-
1
tive quintet signal is observed for the hydride ligand at d =
À6.52ppm ( 2J(P,H) = 30.7 Hz, 1J(W,H) = 63 Hz). The
31P{1H} NMR spectrum of 4a shows one singlet resonance
for the PMe3 ligands at d = À34.5 ppm, which appears at
lower field than those of 4b (dP = À43.6 ppm) and 4c (dP =
À53.9 ppm),[10b] and confirms the trans configuration of the
complex. The 31P NMR signal is flanked by a pair of tungsten
satellites,[28] for which the 1J(W,P) coupling constant of 258 Hz
compares well with those of plumbylidyne complexes 4b
(1J(W,P) = 257 Hz) and 4c (1J(W,P) = 258 Hz).[10b] Finally, the
13C{1H} NMR spectrum of 4a features a low-field-shifted
signal for the lead-bonded Caryl atom at d = 267.2 ppm, which
has a similar chemical shift to those of complexes 4b (dC =
270.2 ppm) and 4c (dC = 267.9 ppm).[10b]
ꢀ
plumbylidyne
complex
trans-[Br(PMe3)4W Pb(2,6-
Trip2C6H3)] (4b), which is then treated with an excess of
LiNMe2 in THF at ambient temperature to afford the desired
product (Scheme 1). Hydrido plumbylidyne complex 4a was
isolated as a brown, extremely air-sensitive solid, which
decomposes on melting at 1868C and is very soluble in
pentane.
The molecular structure of 4a was determined by single-
crystal X-ray diffraction (Figure 2).[13] The trans-configured
octahedral complex features a nearly linearly coordinated
À
lead atom (W-Pb-C1 178.7(2)8) and a W Pb triple bond
length of 2.5525(3) Š, which compares well with those of the
isostructural[24] plumbylidyne complexes trans-[X(PMe3)4W
ꢀ
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Chemie
[13] All syntheses and analytical and spectroscopic data are included
Compounds featuring a triple bond to lead are of
immense significance for the understanding of chemical
in the Supporting Information. Suitable yellow prismatic single
crystals of 1b were grown by slow evaporation of a concentrated
pentane solution at ambient temperature in a glove box, and
dark brown single crystals of 4a·2(n-C5H12) were obtained by
slow cooling of a concentrated pentane solution from ambient
temperature to À308C. CCDC 681781 (1b) and 681780 (4a·2(n-
C5H12)) 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.
bonding.[10,29] The unprecedented Pb N bond heterolysis of
À
organolead(II) amide 1b reported herein suggests that other
lead–element s bonds also could be activated by electron-rich
metal centers to provide access to a plethora of exciting
compounds featuring triply bonded lead.
Received: March 19, 2008
Published online: June 23, 2008
[14] {Pb(NMe2)2}2 is the only other lead(II) dimethylamide presently
known: M. M. Olmstead, P. P. Power, Inorg. Chem. 1984, 23, 413.
[15] Only two base-stabilized organolead(II) amides have been
structurally characterized so far: a) W. Leung, Q. W. Ip, S.
Yong, T. C. W. Mak, Organometallics 2003, 22, 4604; b) W.
Leung, K. Wong, Z. Wang, T. C. W. Mak, Organometallics 2006,
25, 2037.
[16] The maximum deviation of the atoms N1, N2, Pb1, and Pb2 from
the least-squares plane is 0.003(3) Š.
[17] a) H. P. Klug, J. Am. Chem. Soc. 1946, 68, 1493; b) A. F. Wells,
Structural Inorganic Chemistry, 5th ed., Clarendon, Oxford,
1984, p. 1288.
À
Keywords: amides · hydride ligands · lead · Pb N activation ·
tungsten
.
[1] a) D. H. Harris, M. F. Lappert, J. Chem. Soc. Chem. Commun.
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[19] The interplane angles between the amino groups (C37/N1/C38
and C75/N2/C76) and the Pb2N2 core of 80.1(4) and 81.9(3)8,
respectively, lead to two different pairs of Pb-N-C angles (Pb1-
N1-C37 107.3(3), Pb2-N2-C75 109.5(2)8; Pb1-N1-C38 113.4(2),
Pb2-N2-C76 112.1(2)8). Thereby, short contacts result between
the methyl hydrogen atoms at C37 and C75 and the center of
gravity (Cg) of the peripheral phenyl rings C60–C65 and C22–
C27, respectively (H(C37/C75)···Cg 2.64/2.65 Š), which may
explain the considerable upfield 1H NMR shift of one of the
signals of the N-bonded methyl groups (C(37/75)H3: d =
1.70 ppm, C(38/76)H3: d = 4.17 ppm). Similarly, the terphenyl
substituents are tilted, as evidenced by the dihedral angles of
71.9 and 73.68 between the Pb2N2 ring plane and the plane
defined by the atoms Pb1/Pb2/C1 and Pb1/Pb2/C39, respectively.
[20] Dissociation of 1b in solution to form monomers should lead to
exchange of the methyl positions through rapid rotation about
[4] C. Stanciu, S. S. Hino, M. Stender, A. F. Richards, M. M.
Olmstead, P. P. Power, Inorg. Chem. 2005, 44, 2774.
[5] For a few recent examples of heteroleptic lead(II) amides
bearing a pendant donor atom, see: a) A. Murso, M. Straka, M.
Kaupp, R. Bertermann, D. Stalke, Organometallics 2005, 24,
3576; b) M. Chen, J. R. Fulton, P. B. Hitchcock, N. C. Johnstone,
M. F. Lappert, A. V. Protchenko, Dalton Trans. 2007, 2770.
[6] a) B. Cetinkaya, I. Gümrükcü, M. F. Lappert, J. L. Atwood,
R. D. Rogers, M. J. Zaworotko, J. Am. Chem. Soc. 1980, 102,
2088; b) P. B. Hitchcock, M. F. Lappert, B. J. Samways, E. L.
Weinberg, J. Chem. Soc. Chem. Commun. 1983, 1492; c) S. Suh,
D. M. Hoffman, Inorg. Chem. 1996, 35, 6164; d) R. Papiernik,
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[7] a) K. W. Klinkhammer, W. Schwarz, Angew. Chem. 1995, 107,
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K. W. Klinkhammer, B. Tumanskii, H. Krüger, H. Kelm, Angew.
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[8] a) M. Veith, M. Grosser, V. Huch, Z. Anorg. Allg. Chem. 1984,
513, 89; b) B. Wrackmeyer, J. Weidinger, W. Milius, Z. Anorg.
Allg. Chem. 1998, 624, 98; c) B. Wrackmeyer, A. Pedall, J.
Weidinger, Z. Naturforsch. B 2001, 56, 1009.
À
the Pb N bond.
[21] a) V. C. Gibson, C. E. Graimann, P. M. Hare, M. L. H. Green,
J. A. Bandy, P. D. Grebenik, K. Prout, J. Chem. Soc. Dalton
Trans. 1985, 2025; b) M. L. H. Green, G. Parkin, M. Chen, K.
Prout, J. Chem. Soc. Dalton Trans. 1986, 2227; c) A. C. Filippou,
N. Weidemann, A. I. Philippopoulos, G. Schnakenburg, Angew.
Chem. 2006, 118, 6133; Angew. Chem. Int. Ed. 2006, 45, 5987.
[22] E. Carmona, A. Galindo, M. L. Poveda, R. D. Rogers, Inorg.
Chem. 1985, 24, 4033.
[9] M. Veith, J. Fr›res, P. König, O. Schütt, V. Huch, J. Blin, Eur. J.
Inorg. Chem. 2005, 3699.
[10] For compounds featuring triple bonds to lead, see: a) A. C.
Filippou, H. Rohde, G. Schnakenburg, Angew. Chem. 2004, 116,
2293; Angew. Chem. Int. Ed. 2004, 43, 2243; b) A. C. Filippou, N.
Weidemann, G. Schnakenburg, H. Rohde, A. I. Philippopoulos,
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6512.
[11] L. Pu, B. Twamley, P. P. Power, Organometallics 2000, 19, 2874.
[12] Yellow solutions of 1b in pentane or diethyl ether turn
immediately cloudy and colorless upon exposure to air.
[23] The reaction pathways leading from 2 or 3 to complex 4a are
presently unknown. A possible sequence of reaction steps
involves metal coordination of 1b to give the aminoplumbyl-
idene complex intermediate [WL(PMe3)4(Pb(R)NMe2)] (L =
PMe3, N2), followed by rapid ligand dissociation and migration
of the NMe2 group to the tungsten center to give the
plumbylidyne complex intermediate [W(NMe2)(PMe3)4(PbR)],
which finally undergoes b-hydride elimination to give the final
Angew. Chem. Int. Ed. 2008, 47, 5799 –5802
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5801

Communications
ꢀ
product. The latter intermediate is probably also formed in the
reaction of 4b with LiNMe2 to give 4a.
of the germylidyne complexes trans-[X(PMe3)4W Ge(2,6-
Trip2C6H3)] (X = H, Cl, I) and suggests that the strong p-
À
[24] Complexes 4a–c have very similar structures. Thus, the m-
terphenyl substituents always adopt an eclipsed conformation, as
indicated by respective angles of 0.7(2), 5.4(2), and 6.3(1)8
between the central aryl ring plane and the least-squares plane
passing through the atoms Pb, W, P1, and P3. In addition,
complex 4a shows a similar geometric distortion of the WP4 core
to 4b and 4c, such that two trans-disposed PMe3 ligands (P2and
P4 in Figure 2) are pushed further away from the plumbylidyne
ligand than the other two (P1 and P3).
bonding part of the W E bond (E = Ge, Pb), which is not
expected to be distinctly effected by the trans-disposed ligand X,
À
has a major influence on the W E triple bond length.
À
À
[26] The W H distance of 4a lies in the range of W H bond lengths
determined by neutron diffraction (1.71–1.78 Š): R. Bau, M. H.
Drabnis, Inorg. Chim. Acta 1997, 259, 2 7.
[27] F. Furno, T. Fox, H. W. Schmalle, H. Berke, Organometallics
2000, 19, 3620.
[28] No 207Pb satellites were observed in the 31P{1H} NMR spectrum
of 4a at room temperature.
À
[25] The W Pb triple bond length of 4a does not differ from those of
4b and 4c, despite the presence of a hydride ligand exerting a
strong trans influence. The same effect was observed in the case
[29] a) G. Balꢁzs, L. J. Gregoriades, M. Scheer, Organometallics 2007,
26, 3058; b) P. P. Power, Organometallics 2007, 26, 4362.
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