Agostic-type B - H...Pb interactions stabilize a dialkylplumbylene. Structure of and bonding in [{nPr2P(BH3)}(Me 3Si)C(CH2)]2Pb

Article abstract of DOI:10.1021/om800598b

The reaction between Cp₂Pb and 1 equiv of [[{nPr ₂P(BH₃)] (Me₃Si)C(CH₂)]Li(THF) ₂]₂ in toluene affords the dialkylplumbylene [{nPr ₂P(BH₃)}(Me₃Si)C(CH₂)] ₂Pb (10) in excellent yield; compound 10 may readily be separated into its two diastereomeric forms by a simple crystallization procedure. X-ray crystallography shows that rac- and meso-10 crystallize as discrete dialkylplumbylenes in which there are either two (rac-10) or one (meso-10) short agostic-type B - H...Pb contacts; for meso-10 there is a secondary, weak B - H...Pb contact, which affords this diastereomer a further, weak stabilization. Multielement and variable-temperature NMR studies indicate that, while rac-10 is essentially static in solution, meso10 undergoes dynamic exchange between the free and bound BH₃ groups. Unusually, the methylene backbone protons in both diastereomers (at low temperature for meso-10) lic within the range 2.5-6.5 ppm, possibly due to the close proximity of the 6s lone pair on the lead center. DFT studies indicate that there is significant delocalization of B - H σ-bonding electron density into the vacant lead 6p orbital, furnishing an overall stabilization of 40.6 and 30.3 kcal mol^-1 for rac- and meso-10, respectively.

Full text of DOI:10.1021/om800598b

4386
Organometallics 2008, 27, 4386–4394
Agostic-Type B-H · · · Pb Interactions Stabilize a Dialkylplumbylene.
Structure of and Bonding in [{nPr2P(BH3)}(Me3Si)C(CH2)]2Pb
Keith Izod,* William McFarlane, Corinne Wills, William Clegg, and Ross W. Harrington
Main Group Chemistry Laboratory and Crystallography Laboratory, School of Chemistry,
Bedson Building, UniVersity of Newcastle, Newcastle upon Tyne, NE1 7RU, U.K.
ReceiVed June 27, 2008
The reaction between Cp2Pb and 1 equiv of [[{nPr2P(BH3)}(Me3Si)C(CH2)]Li(THF)2]2 in toluene affords
the dialkylplumbylene [{nPr2P(BH3)}(Me3Si)C(CH2)]2Pb (10) in excellent yield; compound 10 may readily
be separated into its two diastereomeric forms by a simple crystallization procedure. X-ray crystallography
shows that rac- and meso-10 crystallize as discrete dialkylplumbylenes in which there are either two
(rac-10) or one (meso-10) short agostic-type B-H · · · Pb contacts; for meso-10 there is a secondary,
weak B-H · · · Pb contact, which affords this diastereomer a further, weak stabilization. Multielement
and variable-temperature NMR studies indicate that, while rac-10 is essentially static in solution, meso-
10 undergoes dynamic exchange between the free and bound BH3 groups. Unusually, the methylene
backbone protons in both diastereomers (at low temperature for meso-10) lie within the range 2.5-6.5
ppm, possibly due to the close proximity of the 6s lone pair on the lead center. DFT studies indicate that
there is significant delocalization of B-H σ-bonding electron density into the vacant lead 6p orbital,
furnishing an overall stabilization of 40.6 and 30.3 kcal mol-1 for rac- and meso-10, respectively.
Pb) have been reported;5 there have been few reports of the
corresponding dialkyltetrylenes (R3C)2E. Until recently, crys-
Introduction
The synthesis of stable heavier group 14 carbene analogues
(diorganotetrylenes) remains a fascinating and challenging area
of contemporary main group chemistry. Over the last three
decades enormous strides have been made in the synthesis of
heteroatom-stabilized tetrylenes, analogues of the Arduengo-
type N-heterocyclic carbenes and their acyclic counterparts, in
which the electron deficiency of the tetrel center is mitigated
by pπ-pπ interactions between the lone pairs on the heteroatoms
and the vacant p orbital on the tetrel center.1-4 By contrast, the
chemistry of hydrocarbyl-substituted diorganotetrylenes, in
which such stabilizing interactions are absent, has been much
less well explored, although, in recent years, several examples
of sterically hindered diaryltetrylenes Ar2E (E ) Si, Ge, Sn,
tallographically characterized dialkyltetrylenes were limited to
the cyclic species reported by Kira and co-workers (1-3),6-8
the acyclic dialkylgermylene reported by Jutzi and co-workers
(4),9 and the highly sterically hindered tin and lead metallacycles
reported by Eaborn, Smith, and co-workers (5, 6) (Chart 1);10,11
of these compounds, only one is a dialkylplumbylene. In
addition to the foregoing, a range of sterically hindered
(5) (a) Yang, X.-J.; Wang, Y.; Wei, P.; Quillan, B.; Robinson, G. H.
Chem. Commun. 2006, 403. (b) Phillips, A. D.; Hino, S.; Power, P. P. J. Am.
Chem. Soc. 2003, 125, 7520. (c) Pu, L.; Olmstead, M. M.; Power, P. P.;
Schiemenz, B. Organometallics 1998, 17, 5602. (d) Stu¨rmann, M.;
Weidenbruch, M.; Klinkhammer, K. W.; Lissner, F.; Marsmann, H.
Organometallics 1998, 17, 4425. (e) Lay, U.; Pritzkow, H.; Gru¨tzmacher,
H. Chem. Commun. 1992, 260. (f) Simons, R. S.; Pu, L.; Olmstead, M. M.;
Power, P. P. Organometallics 1997, 16, 1920. (g) Gru¨tzmacher, H.;
Pritzkow, H.; Edelmann, F. T. Organometallics 1991, 10, 23. (h) Brooker,
S.; Buijink, J.-K.; Edelmann, F. T. Organometallics 1991, 10, 25. (i) Tajima,
T.; Takeda, N.; Sasamori, T.; Tokitoh, N. Organometallics 2006, 25, 3552.
(j) Spikes, G. H.; Peng, Y.; Fettinger, J. C.; Power, P. P. Z. Anorg. Allg.
Chem. 2006, 632, 1005. (k) Wegner, G. L.; Berger, R. J. F.; Schier, A.;
Schmidbaur, H. Organometallics 2001, 20, 418. (l) Weidenbruch, M.;
Schlaefke, J.; Schafer, A.; Peters, K.; von Schnering, H. G.; Marsmann, H.
Angew. Chem., Int. Ed. Engl. 1994, 33, 1846. (m) Braunschweig, H.; Drost,
C.; Hitchcock, P. B.; Lappert, M. F.; Pierssens, L. J.-M. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 261. (n) Bender, J. E.; Banaszak Holl, M. K.; Kampf,
J. W. Organometallics 1997, 16, 2743. (o) Kano, N.; Shibita, K.; Tokitoh,
N.; Okazaki, R. Organometallics 1999, 18, 2999. (p) Jutzi, P.; Schmidt,
H.; Neumann, B.; Stammler, H.-G. Organometallics 1996, 15, 741. (q) Pu,
L.; Twamley, B.; Power, P. P. Organometallics 2000, 19, 2874.
(6) Kira, M.; Yauchibara, R.; Hirano, R.; Kabuto, C.; Sakurai, H. J. Am.
Chem. Soc. 1991, 113, 7785.
* Corresponding author. E-mail: k.j.izod@ncl.ac.uk.
(1) For recent reviews of heavier tetrylene chemistry see:(a) Barrau, J.;
Rima, G. Coord. Chem. ReV. 1998, 178-180, 593. (b) Tokitoh, N.; Okazaki,
R. Coord. Chem. ReV. 2000, 210, 251. (c) Kira, M. J. Organomet. Chem.
2004, 689, 4475. (d) Weidenbruch, M. Eur. J. Inorg. Chem. 1999, 373. (e)
Klinkhammer, K. W. In Chemistry of Organic Germanium, Tin and Lead
Compounds; Rappoport, Z., Ed.; Wiley: New York, 2002; Vol. 2, pp 283-
357. (f) Veith, M. Angew. Chem., Int. Ed. Engl. 1987, 26, 1.
(2) For leading references to diamidogermylenes, -stannylenes, and
-plumbylenes see:(a) Fjeldberg, T.; Hope, H.; Lappert, M. F.; Power, P. P.;
Thorne, A. J. J. Chem. Soc., Chem. Commun. 1983, 639. (b) Olmstead,
M. M.; Power, P. P. Inorg. Chem. 1984, 23, 413. (c) Tang, Y.; Felix, A. M.;
Zakharov, L. N.; Rheingold, A. L.; Kemp, R. A. Inorg. Chem. 2004, 43,
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M. F. Z. Anorg. Allg. Chem. 2004, 630, 2090. (e) Gans-Eichler, T.; Gudat,
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Hitchcock, P. B.; Lappert, M. F.; Leung, W.-P.; Power, P. P.; Olmstead,
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P. B.; Lappert, M. F.; Pierssens, L. J.-M. Angew. Chem., Int. Ed. Engl.
1994, 33, 1156. (h) Lappert, M. F.; Slade, M. J.; Atwood, J. L.; Zaworotko,
M. J. J. Chem. Soc., Chem. Commun. 1980, 621. (i) Bazinet, P.; Yap,
G. P. A.; Richeson, D. S. J. Am. Chem. Soc. 2001, 123, 11162.
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(8) Kira, M.; Ishida, S.; Iwamoto, T.; Ichinohe, M.; Kabuto, C.;
Ignatovich, L.; Sakurai, H. Chem. Lett. 1999, 263.
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1991, 10, 1647.
(10) Eaborn, C.; Hill, M. S.; Hitchcock, P. B.; Patel, D.; Smith, J. D.;
Zhang, S. Organometallics 2000, 19, 49.
(4) For recent reviews of N-heterocyclic silylenes see:(a) Hill, N. J.;
West, R. J. Organomet. Chem. 2004, 689, 4165. (b) Gehrhus, B.; Lappert,
M. F. J. Organomet. Chem. 2001, 617, 209.
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10.1021/om800598b CCC: $40.75
2008 American Chemical Society
Publication on Web 08/13/2008

Agostic-Type B-H · · · Pb Interactions
Organometallics, Vol. 27, No. 17, 2008 4387
Chart 1
Scheme 1
heteroleptic aryl/alkyl-tetrylenes5d,q,12 and both homoleptic and
heteroleptic, intramolecularly base-stabilized diorganotetrylenes
have been reported.13
Since hydrocarbyl-substituted diorganotetrylenes lack π-donor
substituents, they are typically stabilized solely by the use of
sterically demanding groups. However, even with very bulky
substituents, the heavier dialkyltetrylenes may be subject to
dimerization to the corresponding tetraalkylditetrenes R2EdER2.
For example, the archetypal dialkylstannylene {(Me3Si)2CH}2Sn
(7), first reported by Lappert and co-workers in the 1970s, is
monomeric in the gas phase, but dimerizes to the distannene
{(Me3Si)2CH}2SndSn{CH(SiMe3)2}2 in the solid state; in
solution 7 is subject to a stannylene-distannene equilibrium.14
The corresponding dialkylplumbylene {(Me3Si)2CH}2Pb (8) also
forms weakly bonded tetraalkyldiplumbene dimers in the solid
state.1e
Results and Discussion
Synthesis and Solid State Structures. Whereas the reaction
between the dilithium salt [[{nPr2P(BH3)}(Me3Si)C(CH2)]-
Li(THF)2]2 (11) and SnCl2 gives the dialkylstannylene 9 in
excellent yield,15 similar reactions between either PbCl2 or PbI2
and 1 equiv of 11, in a variety of solvents and under a variety
of conditions, lead to extensive reduction and the deposition of
elemental lead; the only hydrocarbon-soluble product from these
reactions is the free phosphine-borane [{nPr2P(BH3)}(Me3Si)-
CH(CH2)]2. Such reduction side-reactions may potentially be
inhibited through the use of more robust starting materials such
as lead amides or aryloxides. However, while reactions between
11 and lead aryloxides such as Pb(OC6H2-2,6-tBu2-4-Me)2 yield
yellow-orange solutions that have 31P NMR spectra consistent
with the formation of 10, we were unable to separate this
compound cleanly from the lithium aryloxide side-products. In
contrast, the reaction between Cp2Pb and 1 equiv of 11 in cold
toluene cleanly gives the dialkylplumbylene [{nPr2P(BH3)}-
(Me3Si)C(CH2)]2Pb (10) in excellent yield after a simple workup
as a yellow-orange air-sensitive solid (Scheme 1). Crystallization
of this solid from cold diethyl ether over 12 h yields the rac
isomer (rac-10) exclusively; after removal of all of the rac
isomer by successive crystallizations from diethyl ether in this
way, a clean sample of the meso isomer (meso-10) may be
obtained by recrystallization of the residue from cold n-hexane.
1H and 31P NMR spectra of the crude product indicate that rac-
and meso-10 are formed in an approximately 1.5:1 ratio.
Compound 10 is sensitive to both heat and light, decomposing
to give elemental lead above ca. 80 °C in the solid state or on
exposure of a solution in toluene to ambient light at room
temperature for several days.
We recently reported a new strategy for the stabilization of
dialkyltetrylenes involving agostic-type B-H · · · E interactions,
which alleviate the electron deficiency of the tetrel center in
these compounds: the dialkylstannylene [{nPr2P(BH3)}-
(Me3Si)C(CH2)]2Sn (9) crystallizes with either one (meso-9) or
two (rac-9) short B-H · · · Sn contacts in the solid state.15
Spectroscopic and theoretical studies indicate that these short
contacts are preserved in solution and are associated with
significant delocalization of electron density from the B-H
σ-bond into the vacant 5p orbital on tin and so may be classified
as genuine B-H · · · Sn agostic interactions. In order to prove
the generality of this strategy for diorganotetrylene stabilization,
we have embarked on an extended program to investigate this
phenomenon. In this contribution we describe the synthesis of
the lead analogue of 9, [{nPr2P(BH3)}(Me3Si)C(CH2)]2Pb (10)
(only the second dialkylplumbylene to be structurally character-
ized), its convenient separation into its two diastereomeric forms,
its highly unusual spectroscopic properties, and theoretical
studies of the bonding in this compound.
(12) (a) Eichler, B. E.; Power, P. P. Inorg. Chem. 2000, 39, 5444. (b)
Setaka, W.; Hirai, K.; Sakamoto, K.; Kira, M. J. Am. Chem. Soc. 2004,
126, 2696. (c) Hino, S.; Olmstead, M.; Phillips, A. D.; Wright, R. J.; Power,
P. P. Inorg. Chem. 2004, 43, 7346.
(13) For examples see:(a) Drost, C.; Hitchcock, P. B.; Lappert, M. F.;
Pierssens, L. J.-M. Chem. Commun. 1997, 1141. (b) Drost, C.; Hitchcock,
P. B.; Lappert, M. F. Organometallics 1998, 17, 3838. (c) Wingerter, S.;
Gornitzka, H.; Bertermann, R.; Pandey, S. K.; Rocha, J.; Stalke, D.
Organometallics 2000, 19, 3890. (d) Engelhardt, L. M.; Jolly, B. S.; Lappert,
M. F.; Raston, C. L.; White, A. H. J. Chem. Soc., Chem. Commun. 1988,
336. (e) Jolly, B. S.; Lappert, M. F.; Engelhardt, L. M.; White, A. H.; Raston,
C. L. J. Chem. Soc., Dalton Trans. 1993, 2653. (f) Cardin, C. J.; Cardin,
D. J.; Constantine, S. P.; Drew, M. G. B.; Rashid, H.; Convery, M. A.;
Fenske, D. J. Chem. Soc., Dalton Trans. 1998, 2749. (g) Benet, S.; Cardin,
C. J.; Cardin, D. J.; Constantine, S. P.; Heath, P.; Rashid, H.; Teixeira, S.;
Thorpe, J. H.; Todd, A. K. Organometallics 1999, 18, 389.
(14) (a) Davidson, P. J.; Lappert, M. F. J. Chem. Soc., Chem. Commun.
1973, 317. (b) Goldberg, D. E.; Harris, D. H.; Lappert, M. F.; Thomas,
K. M. J. Chem. Soc., Chem. Commun. 1976, 261. (c) Zilm, K. W.; Lawless,
G. A.; Merrill, R. M.; Millar, J. M.; Webb, G. G. J. Am. Chem. Soc. 1987,
109, 7236.
Both rac- and meso-10 crystallize as discrete dialkylplum-
bylene species and represent only the second such species to
be crystallographically characterized; the closest Pb · · · Pb
distances in the solid state are in excess of 6.8 Å. The molecular
structures of rac- and meso-10 are shown in Figure 1, along
with selected bond lengths and angles. Both rac- and meso-10
crystallize as five-membered metallacycles; for meso-10 there
are two crystallographically independent molecules in the
asymmetric unit, which differ only trivially in their bond lengths
(15) Izod, K.; McFarlane, W.; Tyson, B. V.; Carr, I.; Clegg, W.;
Harrington, R. W. Organometallics 2006, 25, 1135.

4388 Organometallics, Vol. 27, No. 17, 2008
Izod et al.
observed in 9. In rac-10 the Pb · · · H(1B) and Pb · · · H(2A)
distances are 2.46(6) and 2.43(6) Å, respectively, whereas in
meso-10 the single, short Pb(1) · · · H(1B) distance is 2.25(4) Å
[Pb(2) · · · H(4B) 2.29(4) Å]; these distances are significantly
shorter than the sum of the van der Waals radii of Pb and H
(3.22 Å). In addition, meso-10 exhibits a second, somewhat
longer Pb · · · H contact [Pb(1) · · · H(2C) 2.95(5) Å; Pb(2) · · · H(3A)
3.04(4) Å in the second molecule in the asymmetric unit], which
is, once again, within the sum of the van der Waals radii of Pb
and H and which appears to be associated with a much weaker
Pb · · · H interaction (see DFT studies below). For meso-9 no
weaker Sn · · · H interactions were observed, and so we attribute
the presence of the second Pb · · · H interaction in meso-10 to
the increased size of the metal atom in this case, which enables
the closer approach of a second BH3 group.
The short, agostic-type B-H · · · Pb contacts observed in 10
are reminiscent of the agostic-type C-H · · · Pb contacts proposed
for the mixed alkyl/arylplumbylene (2,4,6-tBu3C6H2)(3,5-
tBu2C6H3CMe2CH2)Pb (12),5d derived from the spontaneous
isomerization of the homoleptic compound (2,4,6-tBu3C6H2)2Pb.
However, the Pb · · · B distances in 10 [3.233 and 3.227 Å (rac-
10); 3.210 Å (for both molecules of meso-10)] are substantially
longer than the Pb · · · C distances in 12 (2.796 and 2.826 Å),
possibly as a consequence of the more rigid nature of the aryl
ligand in the latter compound. To the best of our knowledge,
the only previous report of a short B-H · · · Pb contact is in the
tris(2-mercaptoimidazolyl)borate complex (TmPh)2Pb (13),16 in
which one of the TmPh ligands adopts an inverted η4-coordina-
tion mode, binding the lead atom through its three S-donors
and the central B-H group [TmPh ) HB(2-S,3-PhC3N2)3]. In
this complex the Pb · · · H distance is 2.39 Å, similar to the
corresponding distances in rac- and meso-10.
Figure 1. Molecular structures of (a) rac-10 and (b) meso-10 with
40% probability ellipsoids and with H atoms bonded to carbon
omitted for clarity. Selected bond lengths (Å) and angles (deg):
rac-10: Pb-C(1) 2.390(4), Pb-C(4) 2.402(4), C(1)-P(1) 1.815(4),
C(1)-Si(1) 1.889(4), C(4)-P(2) 1.818(5), C(4)-Si(2) 1.895(4),
P(1)-B(1) 1.939(6), P(2)-B(2) 1.945(5), Pb · · · H(1B) 2.46(6),
Pb · · · H(2A) 2.43(6), C(1)-Pb-C(4) 83.16(15). meso-10 (values
for the second molecule in the asymmetric unit in square brackets):
Pb(1)-C(1) 2.422(3) [Pb(2)-C(23) 2.372(3)], Pb(1)-C(4) 2.369(3)
[Pb(2)-C(26) 2.402(3)], C(1)-P(1) 1.807(3) [C(23)-P(3) 1.810(3)],
C(1)-Si(1) 1.885(3) [C(23)-Si(3) 1.895(3)], C(4)-P(2) 1.806(3)
[C(26)-P(4) 1.801(3)], C(4)-Si(2) 1.902(3) [C(26)-Si(4) 1.892(3)],
P(1)-B(1) 1.932(4) [P(3)-B(3) 1.928(4)], P(2)-B(2) 1.927(4)
[P(4)-B(4) 1.941(4)], Pb(1) · · · H(1B) 2.25(4) [Pb(2) · · · H(4B)
2.29(4)], C(1)-Pb(1)-C(4) 81.81(10) [C(23)-Pb(2)-C(26)
82.42(10)].
and angles. The Pb-C distances of 2.390(4) and 2.402(4) Å
(rac-10) and 2.369(3) and 2.422(3) Å (meso-10; 2.372(3) and
2.402(3) Å in the second molecule in the asymmetric unit) are
similar to the Pb-C distances in 6 [2.397(6) and 2.411(5) Å];10
however, the C-Pb-C angles in rac- and meso-10 [83.16(15)°
and 81.81(10)/82.42(10)°, respectively] are substantially nar-
rower than the corresponding angle in 6 [117.1(2)°] and are
slightly narrower than the C-Sn-C angles in rac- and meso-9
[85.32(5)° and 84.53(15)°, respectively].15 The C-Pb-C angles
are consistent with (i) the presence of a five-membered
metallacycle in rac- and meso-10 compared to the seven-
membered metallacycle in 6, (ii) the larger size of the lead atom
in 10 versus the tin atom in 9, and (iii) substantial Pb 6p
character in the Pb-C bonds (i.e., substantial 6s character in
the lead lone pair). The C-Pb-C angles in 10 are the smallest
such angles to be reported to date; typical C-Pb-C angles in
acyclic diorganoplumbylenes span the range 91.8(2)° to 121.5(3)°,
the narrowest of these C-Pb-C angles being observed in the
acyclic heteroleptic diorganoplumbylene {2,6-(C6H3-i-
Pr2)2C6H3}PbMe.5q
Spectroscopic Characterization. Both rac- and meso-10
have been characterized by multielement (1H, 13C{1H},
11B{1H}, 29Si{1H}, 31P{1H}, and 207Pb{1H}) and variable-
temperature NMR spectroscopy. The 1H NMR spectrum of rac-
10 in d8-toluene is largely as expected; the SiMe3 protons give
rise to a singlet at 0.29 ppm, while the BH3 protons give rise to
an extremely broad quartet centered at 0.58 ppm, which
collapses to a doublet (JPH ) 7.8 Hz) on selective decoupling
of the 11B nucleus, and the diastereotopic propyl groups yield
overlapping multiplets in the range 0.88-1.95 ppm. However,
somewhat surprisingly, the methylene protons from the back-
bone of the five-membered metallacycle give rise to multiplets
at 2.62 and 6.25 ppm (Figure 2), which collapse to doublets on
selective decoupling of the 31P nucleus; this assignment was
confirmed by 1H-1H COSY, HSQC, DEPT, and selective
1
1H{31P} and H{1H} experiments. The 13C{1H} spectrum of
rac-10 is also largely as expected; the central quaternary carbon
atoms of the ligand are observed as a broad doublet at 79.34
Perhaps the most striking features of the structures of rac-
and meso-10 are the short B-H · · · Pb contacts, two for rac-10
and one for meso-10; similar short B-H · · · Sn distances were
(16) Bridgewater, B. M.; Parkin, G. Inorg. Chem. Commun. 2000, 3,
534.

Agostic-Type B-H · · · Pb Interactions
Organometallics, Vol. 27, No. 17, 2008 4389
Scheme 2
bond of the free phosphine-borane group,15 no such behavior
was observed for meso-10. The lack of a second observable
dynamic process for meso-10 may possibly be attributed to the
somewhat reduced steric hindrance about the P-C bond in this
compound due to the longer Pb-C distance. Line shape analysis
of the variable-temperature 31P{1H} spectra of meso-10 yields
values of ∆Hq ) 26 ( 2 kJ mol-1 and ∆Sq ) -64 ( 5 J K-1
mol-1 for the phosphine-borane exchange process, consistent
with intramolecular exchange.
We were unable to distinguish the unique B-H · · · Pb proton
1
resonances in the H{11B} spectra for either diastereomer of
10, even at -90 °C. This is consistent with rapid exchange
among the hydrogen atoms within each BH3 group on the NMR
time scale; a similarly rapid exchange process was observed
for the tin analogue 9.
Figure 2. 1H NMR spectrum of rac-10 in d8-toluene at room
temperature.
Perhaps the most striking features of the 1H NMR spectra of
rac- and meso-10 are the unusual low-field chemical shifts of
one set of methylene protons from each metallacycle backbone
(Figure 2). Of the two principal conformers of rac-10 it is to
be expected that conformer i will predominate due to the lack
of stabilizing B-H · · · Pb contacts in conformer ii (Chart 2);
the 1H, 31P{1H}, and 11B{1H} spectra of rac-10 do not provide
any evidence for conformer ii. One possible explanation for the
unusually large deshielding of one set of backbone protons in
i, i.e., either Ha or Hb, is their greater exposure to the magnetic
effect of diamagnetic circulation of the large, diffuse, essentially
6s-type lone pair on the lead atom. Since Ha is in closer
proximity to the lead atom, this would suggest that the signal
at 6.25 ppm in rac-10 is due to these protons; however, see
below for an alternative explanation.
ppm (JPC ) 4.6 Hz) exhibiting broad satellites due to coupling
to 207Pb (JPbC ) 660 Hz). The room-temperature 31P{1H} and
11B{1H} spectra of rac-10 consist of a broad quartet and doublet
at 23.1 and -43.1 ppm, respectively [JPB ) 86 Hz; ∆ν1/2 (31P)
) 240 Hz, ∆ν1/2 (11B) ) 160 Hz].
1
For rac-10 the H, 13C{1H}, 11B{1H}, and 31P{1H} spectra
are invariant over the temperature range 20 to -90 °C. In
1
contrast, the variable-temperature H, 31P{1H}, and 11B{1H}
spectra of meso-10 indicate that this diastereomer is subject to
dynamic behavior in solution. At room temperature the 1H NMR
spectrum of meso-10 consists of a singlet at 0.13 and a broad
quartet at 0.80 ppm, which collapses to a doublet (JPH ) 7.8
Hz) on decoupling the 11B nuclei, due to the SiMe3 and BH3
protons, respectively, a series of overlapping multiplets between
0.88 and 1.95 ppm, due to the diastereotopic propyl groups,
and a pair of extremely broad multiplets at 4.54 ppm due to the
metallacycle backbone protons. As the temperature is reduced,
these signals broaden and decoalesce, until, at -90 °C, the
lowest temperature that we were able to attain, the spectrum
consists of two singlets at 0.02 and 0.28 ppm, due to the
inequivalent SiMe3 groups; a series of overlapping multiplets
between 0.85 and 1.83 ppm, due to the BH3 protons and the
propyl groups; and two pairs of broad, poorly resolved multiplets
at 2.45 and 2.80 ppm and at 6.59 and 6.76 ppm, due to the
backbone methylene protons. The broad quartet observed at 22.9
ppm (JPB ) 71 Hz; ∆ν1/2 ) 230 Hz) in the room-temperature
31P{1H} spectrum of meso-10 also broadens and decoalesces
as the temperature is reduced, until, at -90 °C, the spectrum
consists of two equal intensity, poorly resolved quartets at 18.0
and 24.2 ppm; similarly, the broad doublet observed at -39.0
ppm (∆ν1/2 ) 170 Hz) in the room-temperature 11B{1H}
spectrum of meso-10 also decoalesces at low temperatures such
that, at -90 °C, the spectrum consists of two equal intensity,
broad, featureless signals at -37.9 and -40.0 ppm. The low-
temperature spectra of meso-10 are consistent with the structure
obtained in the solid state, in which only one phosphine-borane
group is associated with the lead center.
In contrast to rac-10, the two principal conformers, iii and
iv, of meso-10 are mirror image forms of equal energy. For
this isomer all four backbone protons are inequivalent in any
one conformer, but Ha and Hc, and similarly Hb and Hd, may
interconvert due to conformational flipping associated with
exchange of the free and bound phosphine-borane groups.
Therefore, at low temperatures these protons should give rise
to four separate signals, with Ha and Hc at similar positions
(similarly for Hb and Hd), and these will coalesce at higher
temperatures to give two signals at averaged positions, as
observed. Once again, due to the proximity of Ha and Hc to the
lead(II) center, we tentatively assign the signals at 6.59 and 6.76
1
ppm in the low-temperature H spectrum of meso-10 to these
protons.
The 207Pb spectra of rac- and meso-10 consist of extremely
broad, featureless signals at 4580 (∆ν1/2 ) 720 Hz) and 5430
ppm (∆ν1/2 ) 505 Hz), respectively. These values are to
substantially higher field than typical chemical shifts for diaryl-
or dialkylplumbylenes; for example, the 207Pb chemical shifts
of 610 and {2,6-(2,4,6-Me3C6H2)2C6H3}2Pb (14)5f are 10500 and
8844 ppm, respectively. The higher field chemical shift of rac-
10 in comparison to that of meso-10 is consistent with the
presence of two short, agostic-type interactions in the former,
which mitigate the electron deficiency of the lead(II) center,
rather than the one such interaction in the latter diastereomer.
The chemical shifts observed for 10 are closer to those observed
for 125d and for the fluorine-containing diarylplumbylene {2,4,6-
(CF3)3C6H2}2Pb (15)5h (5067 and 4878 ppm, respectively), in
which the electron deficiency of the lead center is mitigated by
Thus meso-10 exhibits dynamic exchange between the free
and bound phosphine-borane groups, which is rapid on the NMR
time scale at room temperature (Scheme 2), but which may be
frozen out at low temperatures. Similar dynamic behavior was
observed for meso-9; however, whereas meso-9 was subject to
a second dynamic process, observed only at very low temper-
atures, which we attributed to restricted rotation about the P-C

4390 Organometallics, Vol. 27, No. 17, 2008
Izod et al.
Chart 2
either putative C-H · · · Pb or C-F · · · Pb contacts, respectively.
The 207Pb chemical shift of rac-10 decreases by 68 ppm at -50
°C, which we attribute to a reduction in the proportion of
conformer ii. In conformity with this, the 207Pb chemical shift
of meso-10 was essentially unchanged over a wide temperature
range since the amounts of conformers iii and iv will be the
same at all temperatures.
An inversion-recovery experiment showed that in rac-10 the
207Pb spin-lattice relaxation time, T1, was 4.1 ms at room
temperature. This corresponds to a contribution to line broaden-
ing of 78 Hz, so it is clear that the observed line-width of 720
Hz arises mainly from a T2 mechanism. It has been noted
previously that related diorganostannylenes are subject to large
119Sn chemical shift anisotropies (CSA),17 and this effect will
probably be even greater in our lead compounds. However, this
mechanism affects T1 and T2 to similar extents18 and so cannot
be the major contributor to the latter, although it may dominate
T1. Furthermore, at a reduced measuring field strength (6.9
instead of 11.5 T, corresponding to 0.36 CSA) the 207Pb line-
width was essentially unaltered. A contribution to T2 from
modulation of 3JPbB by 11B quadrupolar relaxation would require
an unreasonably large value of this coupling to give the observed
effect, and the line-widths in the 11B and 31P spectra also show
that the 11B relaxation is too slow for this.
An ordinary chemical exchange process can be ruled out on
the basis that no corresponding effects are observed in the
spectra of any of the other nuclei present in the species.
Additionally, the half-height line-widths of the 207Pb spectra
of rac- and meso-10 increase to 935 and 905 Hz, respectively,
at -50 °C, and it is likely that much more profound changes
would be observed if chemical exchange were the explanation.
We therefore tentatively ascribe the broad nature of these signals
to the presence of a low concentration, possibly lead-centered,
radical species in rapid equilibrium with the dominant diamag-
netic molecules; a similar, although less pronounced, effect was
observed in the 119Sn spectra of 9. This interpretation also
provides an alternative explanation for the anomalous shieldings
for certain of the backbone protons in rac- and meso-10, namely,
that they arise from a pseudocontact interaction with the
unpaired electron spin density centered on the lead atom and
that the differences between the axial and equatorial protons
are due mainly to angular rather than radial dependence.
The solid state infrared spectrum of rac-10 contains bands
at 2383, 2181, and 2116 cm-1, which may be attributed to B-H
stretching vibrations; the corresponding absorptions for meso-
10 occur at 2375 and 2089 cm-1. Comparisons with the
vibrational frequencies obtained from DFT calculations (see
below) indicate that the red-shifted absorptions at 2181 and 2116
cm-1 (rac-10) and at 2089 cm-1 (meso-10) may be assigned to
B-H stretching vibrations for the H atom(s) involved in the
B-H · · · Pb contacts. In solution it is not possible to distinguish
the unique B-H( · · · Pb) stretching vibrations, and only bands
at averaged positions are observed.
The UV-visible spectrum of rac-10 contains a principal
absorption at 382 nm along with a weaker, broad absorption at
approximately 315 nm (ε ) 941 and 510 dm3 mol-1 cm-1
,
respectively), whereas meso-10 exhibits weak to moderate
absorptions at 406 and 335 nm (ε ) 728 and 1128 dm3 mol-1
cm-1, respectively). These absorptions are independent of
solvent polarity (for example, the spectrum of rac-10 in
dichloromethane has a principal absorption at 389 nm (ε ) 1060
dm3 mol-1 cm-1)), excluding charge transfer as their source,
and so the lowest energy absorptions may confidently be
attributed to a transition between the lone pair and the vacant
6p orbital on the lead atom in each case (see TD-DFT studies
below). The higher energy absorptions at 315 and 335 nm for
rac- and meso-10, respectively, may be attributed to a transition
from the HOMO-1 (C-Pb) orbital to the vacant 6p orbital on
the lead atom [σ(C-Pb) f 6p(Pb)]. The yellow color of rac-
and meso-10 contrasts markedly with the deep red-purple colors
of other hydrocarbyl-substituted diorganoplumbylenes. For
example, in 6, the only other crystallographically characterized
dialkylplumbylene, the corresponding n f p transition occurs
at 610 nm, whereas this transition occurs at 526 nm in the
diarylplumbylene 14. The n f p transition in 10 is much closer
to the corresponding transition in 12 (406 nm), and the pale
yellow color of 10 is mirrored in the yellow color of 15; for
compounds 12 and 15 either agostic-type C-H · · · Pb or dative
C-F · · · Pb contacts have been proposed to account for their
unusual spectroscopic properties. The blue shift observed for
the n f p absorptions in rac- and meso-10 may be attributed
to perturbation of these orbitals by the B-H · · · Pb interactions.
DFT Calculations. In order to gain greater insight into the
nature of the bonding in 10, we have undertaken a DFT study
of both the rac and meso diastereomers (rac- and meso-10a,
respectively). Although somewhat computationally expensive,
calculations were carried out on the complete molecules in order
to replicate as completely as possible the B-H · · · Pb interac-
(17) Eichler, B. E.; Phillips, B. L.; Power, P. P.; Augustine, M. P. Inorg.
Chem. 2000, 39, 5450.
(18) Abragam, A. The Principles of Nuclear Magnetism; Oxford
University Press: Oxford, UK, 1961; p 316.

Agostic-Type B-H · · · Pb Interactions
Organometallics, Vol. 27, No. 17, 2008 4391
Table 1. Comparison of Calculated and Crystallographically Determined Bond Lengths (Å) and Angles (deg) for 10 and 10a
rac-10
2.390(4)
rac-10a
2.426
2.405
1.922
1.922
1.836
1.841
1.957
1.954
1.236
meso-10a
meso-10a
Pb-C
2.422(3)
2.377
2.462
1.929
1.916
1.844
1.828
1.950
1.951
1.248
2.402(4)
1.889(4)
1.895(4)
1.815(4)
1.818(5)
1.939(6)
1.945(5)
1.01(6)
2.369(3)
1.885(3)
1.902(3)
1.807(3)
1.806(3)
1.932(4)
1.927(4)
1.16(4)
C-Si
C-P
P-B
B-H( · · · Pb)
1.15(6)
0.94(7)-1.13(5)
2.43(6)
1.240
1.206-1.208
2.385
B-Hb
H · · · Pb
1.05(5)-1.15(5)
2.25(4)
1.205-1.223
2.265
2.46(6)
2.431
C-Pb-C
83.16(15)
83.58
81.81(10)
82.24
a Values for one of the two crystallographically independent molecules in the asymmetric unit. b Range of B-H distances to hydrogen atoms not
involved in B-H · · · Pb interactions.
Table 2. Crystallographic Data for rac-10 and meso-10
The short B-H · · · Pb contacts observed in the crystal
structures of rac- and meso-10 are reproduced extremely well
in the calculated structures; two such contacts are observed for
rac-10a (one from each BH3 group) and one for meso-10a. The
Pb · · · H distances for rac-10a are 2.39 and 2.43 Å, whereas
the Pb · · · H distance in meso-10a is 2.27 Å; these distances are
very similar to those found crystallographically [Pb · · · H 2.46(6)
and 2.43(6) Å (rac-10); 2.25(4) Å [2.29(4) Å for the second
molecule in the asymmetric unit] (meso-10)]. As expected, the
single Pb · · · H distance in meso-10a is significantly shorter than
the two Pb · · · H distances in rac-10a, consistent with weaker
B-H · · · Pb agostic-type interactions in the latter due to com-
petition for the vacant 6p orbital on lead. In addition to the
short Pb · · · H contact in meso-10a, a somewhat longer contact
of 2.828 Å is observed between the lead center and a hydrogen
atom in the second BH3 group; this mirrors the second, longer
Pb · · · H contact observed crystallographically for meso-10
[Pb(1) · · · H(1B) 2.95(5) Å, Pb(2) · · · H(3A) 3.04(4) Å]. The rac
diastereomer is calculated to be 2.8 kcal mol-1 more stable than
the meso diastereomer, consistent with the observation by NMR
spectroscopy that rac- and meso-10 are formed in an ap-
proximately 1.5:1.0 ratio.
rac-10
meso-10
formula
fw
cryst size, mm
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
ꢀ, deg
γ, deg
V, Å3
C22H56B2P2PbSi2
667.6
C22H56B2P2PbSi2
667.6
0.10 × 0.04 × 0.04 0.34 × 0.30 × 0.20
monoclinic
P21/n
triclinic
j
P1
11.4175(8)
17.6207(13)
16.6197(12)
11.4623(8)
14.0466(12)
22.4174(15)
101.861(1)
93.528(1)
114.037(1)
3183.1(4)
4
106.796(1)
3201.0(4)
4
Z
T/K
120
1.385
5.45
150
1.393
5.48
F
calcd, g cm-3
µ, mm-1
no. reflns measd
no. unique reflns, Rint
33 323
9462, 0.060
28 521
14842, 0.022
12686
0.257-0.407
0.025, 0.058
0.033, 0.062
1.020
no. reflns with F2 > 2σ(F2) 8084
trans coeff range
0.612-0.811
a
R, Rw (F2 > 2σ)
0.044, 0.090
0.051, 0.093
1.050
a
R, Rw (all data)
Sa
no. of refined params
297
609
max. min. diff map, e Å-3 1.56, -1.43
1.64, -0.79
Natural bond orbital (NBO) analysis enables an interpretation
of the bonding in these molecules based on a set of occupied
Lewis-type and unoccupied non-Lewis-type localized orbitals.22
An NBO analysis of rac- and meso-10a reveals that the HOMO
in both diastereomers consists of an essentially pure 6s lone
pair [percentage 6s character: 86.7% for both rac- and meso-
10a]; the LUMO in each case comprises a pure 6p orbital on
the lead atoms which lies orthogonal to the plane of the
metallacycle [percentage 6p character: 100% (rac-10a), 99%
(meso-10a)]. Analysis of the donor-acceptor interactions in 10a
confirms that there is significant delocalization of electron
density from the B-H σ-bond(s) into the vacant lead 6p orbital
in each case; that is, the short H · · · Pb contacts are associated
with an agostic-type interaction. The E(2) energy, calculated
by second-order perturbation theory, provides an estimate of
the energies of these interactions; for rac-10a the E(2) energies
a Conventional R ) ∑|Fo| - |Fc|/∑|Fo|; Rw ) [∑w(Fo - Fc ) /
2
2 2
∑w(Fo2)2]1/2; S ) [∑w(Fo - Fc ) /(no. data - no. params)]1/2 for all
2
2 2
data.
tions. Geometry optimizations were performed with the B3LYP
hybrid functional19 with a Lanl2dz effective core potential basis
set20 on the lead atoms and an all-electron 6-31G(d,p) basis
set21 on the remaining atoms; minima were confirmed by the
absence of imaginary vibrational frequencies in each case.
The calculated gas-phase structures of rac- and meso-10a
closely resemble the structures obtained by X-ray crystal-
lography (Table 1). For both diastereomers the calculated bond
lengths are approximately 0.01 to 0.05 Å longer than those
determined crystallographically; however, the calculated C-Pb-C
angles [83.58° (rac-10a), 82.24° (meso-10a)] are very close to
those observed in the corresponding crystal structures [83.16(15)°
and 81.81(10)/82.42(11)°, respectively].
for the B-H · · · Pb interactions are 16.9 and 18.1 kcal mol-1
,
whereas the E(2) energy for the B-H · · · Pb interaction in meso-
10a is 24.0 kcal mol-1 [the E(2) energy for the second, weaker
(19) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Stephens, P. J.;
Devlin, F. J.; Chablowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98,
11623. (c) Hertwig, R. H.; Koch, W. Chem. Phys. Lett. 1997, 268, 345.
(20) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt,
W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt, W. R.
J. Chem. Phys. 1985, 82, 299.
(21) (a) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
(b) Francl, M. M.; Petro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.;
DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654.
(22) (a) Carpenter, J. E.; Weinhold, F. J. Mol. Struct. (Theochem) 1988,
169, 41. (b) Carpenter, J. E. Ph.D. Thesis, University of Wisconsin, Madison,
WI, 1987. (c) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102,
7211. (d) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 78, 4066. (e)
Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 1736. (f) Reed, A. E.;
Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735. (g) Reed,
A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899.

4392 Organometallics, Vol. 27, No. 17, 2008
Izod et al.
1
B-H · · · Pb interaction is just 3.4 kcal mol-1]. Selective deletion
of the principal agostic-type interactions (using the NBODel
routine in NBO 3.0)23 suggests that these interactions afford
an overall stabilization energy of approximately 40.6 (for the
two B-H · · · Pb contacts) and 26.5 kcal mol-1 for rac- and
meso-10a, respectively. For meso-10a deletion of both the
principal and the weaker, secondary B-H · · · Pb interactions
suggests a combined stabilization energy of 30.3 kcal mol-1
for this diastereomer. As expected, the B-H · · · Pb interactions
in 10a are somewhat less stabilizing than the corresponding
B-H · · · Sn interactions in 9, consistent with the larger, more
diffuse 6p orbitals on the lead center; using the above methodol-
ogy, we calculate that the corresponding stabilization energies
associated with the B-H · · · Sn agostic interactions in rac- and
meso-9 are 45.1 (for the two B-H · · · Sn contacts) and 33.8 kcal
mol-1, respectively.
Attempts to calculate the H NMR spectra of 10 using the
GIAO method27 were unsuccessful. While the chemical shifts
of the SiMe3 and propyl protons were satisfactorily modeled,
the chemical shifts of the backbone protons were calculated to
lie between 1.91 and 2.24 ppm for rac-10a and between 0.65
and 2.14 ppm for meso-10a. Clearly, these results are not in
agreement with the highly unusual chemical shifts observed
experimentally for the methylene backbone protons, although
this may be an artifact of the ECP basis set used for the lead
atoms in these calculations.
Conclusions
Although the synthesis of dialkylplumbylenes is often ham-
pered by reduction of lead(II) starting materials such as PbCl2
or PbI2 to give elemental lead, we find that use of the more
robust precursor Cp2Pb enables the synthesis of a novel cyclic
dialkylplumbylene (10). This compound may readily be sepa-
rated into its constituent diastereomers via a straightforward
fractional crystallization process, and both diastereomers have
been characterized by X-ray crystallography; this represents only
the second structural characterization of a dialkylplumbylene.
The solid state structures of rac- and meso-10 reveal that the
lead(II) center forms part of a five-membered plumbacycle in
each case and that there are either one (meso-10) or two (rac-
10) short, agostic-type Pb · · · H contacts; in addition, the large
Pb center enables a second, weaker Pb · · · H contact in meso-
10 that is not observed in its tin(II) analogue. The agostic nature
of these contacts is evident from the infrared, UV-visible, and
multielement NMR spectra of both rac- and meso-10. DFT
studies also confirm that there is substantial delocalization of
B-H σ-bonding electron density into the vacant 6p orbital of
the lead center in each diastereomer. This delocalization
Frequency calculations reveal that the B-H stretching
vibrations in 10a occur in the ranges 2168-2449 cm-1 (rac-
10a) and 2128-2468 cm-1 (meso-10a), after scaling24 [cf.
2116-2383 and 2089-2374 cm-1 for the observed B-H
stretching vibrations in rac- and meso-10, respectively]. The
B-H stretching vibrations associated with the B-H · · · Pb
interactions are clearly identified as those occurring at 2168 and
2221 cm-1 (rac-10a; cf. 2116 and 2181 cm-1 observed in rac-
10) and 2128 cm-1 (meso-10a; cf. 2089 observed in meso-10).
The calculations predict a red shift for these absorptions
compared to the remaining B-H stretching vibrations of
between 188 and 281 cm-1 for rac-10a and between 173 and
340 cm-1 for meso-10a; these compare well with the observed
red shifts of between 202 and 267 cm-1 for rac-10 and of 285
cm-1 for meso-10.
TD-DFT calculations were performed on rac- and meso-10a,
both in the gas phase and with solvation by heptane (using the
EIF-PCM model as implemented in Gaussian03).25 Little
difference was observed between these two sets of calculations,
and so the following discussion relates only to the results that
explicitly include solvation. The calculations reveal that the
lowest energy absorptions observed in the UV-visible spectra
of rac- and meso-10 are associated with n f p transitions at
the lead center in each case. The lowest energy transitions to
singlet excited states are calculated to occur at 333 and 366 nm
for rac- and meso-10a, respectively. However, for both rac-
and meso-10a, there is a low-lying triplet state just 12.5 (rac-
10a) and 11.5 kcal (meso-10a) above the ground state.
stabilizes rac- and meso-10 by 40.6 and 30.3 kcal mol-1
respectively. Thus, it appears that such agostic-type interactions
may provide a new, general method for the stabilization of
diorganotetrylenes.
,
Experimental Section
All manipulations were carried out using standard Schlenk
techniques under an atmosphere of dry nitrogen or argon. Diethyl
ether, n-hexane, methylcyclohexane, and toluene were distilled
under nitrogen from sodium/potassium alloy, whereas dichlo-
romethane was distilled from CaH2 under nitrogen; all solvents were
stored over a potassium film, except dichloromethane, which was
stored over activated 4 Å molecular sieves. Deuterated toluene was
distilled from potassium, deoxygenated by three freeze-pump-thaw
cycles, and stored over activated 4 Å molecular sieves. All
compounds were used as supplied by the manufacturer, except
[[{nPr2P(BH3)}(Me3Si)C(CH2)]Li(THF)2]2 (11),28 which was pre-
pared by a previously published method, and Cp2Pb, which was
prepared by a modification of a literature method (see below).29
NMR spectra were recorded on a JEOL Lambda500 spectrometer
operating at 500.16 (1H), 125.65 (13C), 160.47 (11B), 99.37 (29Si),
202.47 (31P), and 104.32 (207Pb) MHz, respectively; chemical shifts
are quoted in ppm relative to tetramethylsilane (1H, 13C, and 29Si),
The low-lying triplet states of rac- and meso-10 can be
observed experimentally. Irradiation of frozen solutions of rac-
10 in methylcyclohexane at either of the absorption maxima
(315 or 382 nm) yields essentially identical phosphorescence
spectra consisting of two distinct maxima at 462 and 523 nm
(τ ) 9.7 µs); similarly, irradiation of meso-10 at either 335 or
406 nm yields essentially identical phosphorescence spectra with
two distinct maxima at 454 and 528 nm (τ ) 9.9 µs).
Phosphorescence from s2 heavy metal centers such as Pb(II)
has been observed previously and has been assigned to the 3P1
1
f S0 transition, consistent with our observations.26
(23) Wong, M. W. Chem. Phys. Lett. 1996, 256, 391.
(27) (a) McWeeny, R. Phys. ReV. 1962, 126, 1028. (b) Ditchfield, R.
Mol. Phys. 1974, 27, 789. (c) Dodds, J. L.; McWeeny, R.; Sadlej, A. J.
Mol. Phys. 1980, 41, 1419. (d) Wolinski, K.; Hilton, J. F.; Pulay, P. J. Am.
Chem. Soc. 1990, 112, 8251.
(28) Izod, K.; McFarlane, W.; Tyson, B. V.; Clegg, W.; Harrington,
R. W. Chem. Commun. 2004, 570.
(29) (a) Fischer, E. O.; Grubert, H. Z. Anorg. Allg. Chem. 1956, 286,
237. (b) Armstrong, D. R.; Davidson, M. G.; Moncrieff, D.; Russell, C. A.;
Stourton, C.; Steiner, A.; Stalke, D.; Wright, D. S. Organometallics 1997,
16, 3340.
(24) NIST Computational Chemistry Comparison and Benchmark
Database, NIST Standard Reference Database Number 101, Release 14,
Sept 2006, Editor: Russell D. Johnson III, http://srdata.nist.gov/cccbdb.
(25) (a) Cance`s, M. T.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997,
107, 3032. (b) Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 106, 5151.
(c) Mennucci, B.; Cance`s, E.; Tomasi, J. J. Phys. Chem. B 1997, 101, 10506.
(d) Tomasi, J.; Mennucci, B.; Cance`s, E. J. Mol. Struct. (THEOCHEM)
1999, 464, 211.
(26) Nikol, H.; Becht, A.; Vogler, A. Inorg. Chem. 1992, 31, 3277.

Agostic-Type B-H · · · Pb Interactions
Organometallics, Vol. 27, No. 17, 2008 4393
external BF3(OEt2) (11B), external 85% H3PO4 (31P), and external
Me4Pb (207Pb), as appropriate. The positions of the BH3 signals in
the 1H NMR spectra of rac- and meso-10 and JPH for these signals
(m), 883 (s), 847 (s), 831 (s), 772 (m), 755 (s), 712 (m), 685 (m),
675 (m), 643 (s), 602 (m), 581 (s), 456 (m), 434 (s), 421 (m), 410
(m), 408 (m).
1
Spectroscopic Data for meso-10. H{11B} NMR (d8-toluene,
1
were determined using a selective H{11B} experiment; spectral
assignments in the 1H and 13C{1H} spectra were aided by the use
of 1H-1H COSY, HSQC, DEPT, and selective 1H{1H} and
1H{31P} experiments. The spin-lattice relaxation time in rac-10
was determined by a standard inversion recovery experiment.
UV-visible spectra were recorded in matched quartz cells as 1.0
mM solutions in methylcyclohexane or dichloromethane on a
Hitachi U-3010 spectrometer; low-temperature phosphorescence
spectra were recorded using optically dilute solutions in methyl-
cyclohexane on a Hitachi F-4500 spectrophotometer; phosphores-
cence lifetimes were measured on a PTI Easy Life spectrometer
using the Xeno flash system and fitted using the included software.
Infrared spectra were recorded either as powders or as solutions in
methylcyclohexane on a Nicolet Avatar 370DTGS spectrometer.
Elemental analyses were obtained by the Elemental Analysis Service
of London Metropolitan University.
Preparation of Cp2Pb. This is a modification of the procedure
previously reported by Fischer and Grubert.29 To a slurry of PbI2
(14.10 g, 30.6 mmol) in THF (20 mL) was added a solution of
CpNa (5.39 g, 61.2 mmol) in THF (20 mL). This solution was
stirred for 1 h and solvent was removed in Vacuo. Pure Cp2Pb was
obtained by sublimation of the solid residue at 150 °C/10-7 mmHg.
Yield: 3.92 g, 38%. [N.B. the previously reported difficulty in
dealing with the sublimation residue, where spontaneous ignition
has been observed, is avoided by the use of PbI2 as a starting
material.]
20 °C): δ 0.13 (s, 18H, SiMe3), 0.80 (d, JPH ) 9.2 Hz, 6H, BH3),
0.94 (t, 3JHH ) 6.9 Hz, 6H, CH2CH2CH3), 0.96 (t of d, 3JHH ) 6.7
4
Hz, JPH
) 0.9 Hz, CH2CH2CH3), 1.30-1.97 (m, 16H,
CH2CH2CH3), 4.40 (br m, 2H, CH2CH2), 4.67 (br m, 2H, CH2CH2).
13C{1H} NMR (d8-toluene, 20 °C): δ 2.01 (SiMe3), 16.09 (d, JPC
) 13.4 Hz, CH2CH3), 16.24 (d, JPC ) 13.4 Hz, CH2CH3), 16.77
(CH2CH3), 18.10 (d, JPC ) 1.9 Hz, CH2CH3), 32.08 (d, JPC ) 27.8
Hz, CH2CH2CH3), 33.77 (d, JPC ) 31.7 Hz, CH2CH2CH3), 42.85
(d, JPC ) 7.7 Hz, CH2CH2), 89.84 (br, CPb). 11B{1H} (d8-toluene,
20 °C): δ -39.0 (br d, JPB ) 71 Hz). 29Si NMR (d8-toluene, 20
°C): δ -2.80 (d, JSiP ) 2.42 Hz). 31P{1H} NMR (d8-toluene, 20
°C): δ 22.9 (br q, JPB ) 71 Hz). 207Pb{1H} NMR (d8-toluene, 20
°C): δ 5430 (br s). UV/vis (1.0 mM in methylcyclohexane): λmax
406 nm (ε ) 728 dm3 mol-1 cm-1), 335 nm (ε ) 1128 dm3 mol-1
cm-1). IR (cm-1): 2959 (m), 2931 (m), 2896 (m), 2871 (m), 2375
(m), 2299 (w), 2089 (m), 1462 (m), 1454 (m), 1403 (s), 1376 (w),
1337 (w), 1245 (s), 1228 (m), 1127 (w), 1075 (m), 1065 (m), 1039
(m), 1021 (m), 984 (m), 909 (m), 889 (m), 834 (s), 802 (s), 741
(m), 684 (m), 676 (m), 652 (m), 636 (m), 618 (m), 581 (m), 455
(m), 444 (m), 431 (m).
Crystal Structure Determinations of rac- and meso-10.
Measurements were made on Bruker APEX2 and SMART 1K CCD
diffractometers using synchrotron radiation (λ ) 0.6895 Å) for rac-
10 and graphite-monochromated Mo KR radiation (λ ) 0.71073
Å) for meso-10. For both compounds cell parameters were refined
from the observed positions of all strong reflections in each data
set. Intensities were corrected semiempirically for absorption, based
on symmetry-equivalent and repeated reflections, and for incident
beam decay for rac-10. The structures were solved by direct
methods and refined on F2 values for all unique data. Table 2 gives
further details. The hydrogen atoms of the BH3 groups were located
in difference maps and were freely refined. Non-hydrogen atoms
were refined anisotropically, and all remaining H atoms were
constrained with a riding model; U(H) was set at 1.2 (1.5 for methyl
groups) times Ueq for the parent atom. Minor disorder was resolved
for one propyl group in the second molecule of meso-10. Programs
were Bruker AXS APEX2, SMART (control) and SAINT (integra-
tion), and SHELXTL for structure solution, refinement, and
molecular graphics.30
DFT Calculations. Geometry optimizations on the gas-phase
molecules were performed with the Gaussian03 suite of programs
(revision C.02)31 on a 224-core Silicon Graphics Altix 4700
computer with 1.6 GHz Montecito Itanium2 processors and 896
Gb of memory, via the EPSRC National Service for Computational
Chemistry Software (http://www.nsccs.ac.uk). Optimizations were
performed using the B3LYP hybrid functional19 with a Lanl2dz
effective core potential basis set20 for Pb and a 6-31G(d,p) all-
electron basis set on the remaining atoms21 [default parameters were
used throughout]. Minima were confirmed by the absence of
imaginary vibrational frequencies; vibrational frequencies were
corrected using a scaling factor of 0.961.24 Natural bond orbital
analyses were performed using the NBO 3.0 module of Gaussi-
an03;22 the stabilization energy associated with the B-H · · · Pb
interactions was calculated using the NBODel routine in which the
elements affording this interaction were selectively deleted.23 NMR
shielding tensors were calculated using the GIAO method at the
B3LYP/Lanl2dz,6-311++G(2d,p)//Lanl2dz,6-31G(d,p) level of
theory; chemical shifts are quoted in ppm relative to TMS calculated
Preparation of [{nPr2P(BH3)}(Me3Si)C(CH2)]2Pb (10). To a
cold (-78 °C) solution of freshly sublimed Cp2Pb (0.51 g, 1.52
mmol) in toluene (20 mL) was added, dropwise, a cold (-78 °C)
solution of [{nPr2P(BH3)}(Me3Si)C(CH2)]2Li2(THF)4 (1.16 g, 1.52
mmol) in toluene (20 mL), excluding light as much as possible.
This mixture was allowed to attain room temperature and was stirred
for 18 h. Solvent was removed in Vacuo, and the product was
extracted into diethyl ether (3 × 15 mL) and filtered. The yellow
solution was concentrated to 10 mL and cooled to -30 °C for 1
week. The yellow plates of rac-10 that had deposited after this
time were isolated by filtration; this process was repeated twice
more to give two more batches of rac-10 (combined yield 0.49 g).
After this process the solvent was removed in Vacuo from the
filtrate, and the orange oil was dissolved in n-hexane (10 mL) and
cooled to -30 °C for 24 h. The orange blocks of meso-10 that
deposited were isolated by filtration (yield 0.40 g). Combined yield
of rac- and meso-10: 0.89 g, 88%. Anal. Calcd for C22H56B2P2PbSi2:
C, 39.58; H 8.45. Found: C, 39.72; H 8.32. Mp: 88 °C [dec (rac-
10)], 81 °C [dec (meso-10)].
Spectroscopic Data for rac-10. 1H{11B} NMR (d8-toluene, 20
°C): δ 0.29 (s, 18H, SiMe3), 0.58 (d, JPH ) 7.8 Hz, 6H, BH3), 0.88
3
3
(t, JHH ) 6.9 Hz, 6H, CH2CH2CH3), 0.94 (t, JHH ) 6.7 Hz, 6H,
CH2CH2CH3), 1.33-1.95 (m, 16H, CH2CH2CH3), 2.62 (m, 2H,
CH2CH2), 6.25 (m, 2H, CH2CH2). 13C{1H} NMR (d8-toluene, 20
°C): δ 3.73 (d, JPC ) 1.9 Hz, SiMe3), 16.04 (d, JPC ) 14.4 Hz, 2 x
CH2CH3), 16.78 (CH2CH2CH3), 18.40 (CH2CH2CH3), 30.72 (d, JPC
) 39.3 Hz, CH2CH2CH3), 36.06 (d, JPC ) 22.1 Hz, CH2CH2CH3),
41.16 (d, JPC ) 18.2 Hz, CH2CH2), 79.34 (d, JPC ) 4.6 Hz, JPbC
)
)
660 Hz, CPb). 11B{1H} (d8-toluene, 20 °C): δ -43.1 (br d, JPB
86 Hz). 29Si (d8-toluene, 20 °C): δ 0.57 (d, JSiP ) 8.5 Hz). 31P{1H}
(d8-toluene, 20 °C): δ 23.1 (br q, JPB ) 86 Hz). 207Pb{1H} (d8-
toluene, 20 °C): δ 4580 (br s). UV/vis (1.0 mM in methylcyclo-
hexane): λmax 382 nm (ε ) 941 dm3 mol-1 cm-1), 315 nm (ε )
510 dm3 mol-1 cm-1). IR (cm-1): 2962 (m), 2931 (m), 2871 (m),
2826 (m), 2810 (m), 2383 (m), 2189 (m), 2116 (m), 1461 (m),
1454 (m), 1246 (s), 1076 (m), 1048 (m), 1034 (m), 978 (m), 902
(30) (a) APEX2, SMART, and SAINT software for CCD diffractometers;
Bruker AXS Inc.: Madison, WI, 2004 and 1997. (b) Sheldrick, G. M. Acta
Crystallogr., Sect. A 2008, 64, 112.
(31) Frisch, M. J.; et al. Gaussian 03, Revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004.

4394 Organometallics, Vol. 27, No. 17, 2008
Izod et al.
at the same level of theory.27 TD-DFT studies were carried out at
the B3LYP/Lanl2dz,6-31G(d,p) level of theory, both in the gas
phase and with solvation by heptane, the latter using the EIF-
polarizable continuum model implemented in Gaussian03.25
Supporting Information Available: For rac- and meso-10
details of structure determination, atomic coordinates, bond lengths
and angles, and displacement parameters in CIF format. For rac-
and meso-10a details of DFT calculations, final atomic coordinates,
and energies. Complete details of ref 31. This material is available
free of charge via the Internet at http://pubs.acs.org. Observed and
calculated structure factor details are available from the authors
upon request.
Acknowledgment. The authors are grateful to the EPSRC
for financial support and to the EPSRC-funded National
Crystallography Service and STFC for access to synchrotron
facilities; Mr. K. Elliott and Prof. A. Harriman are thanked
for the phosphorescence measurements on rac- and meso-
10.
OM800598B