The synthesis of monomeric terminal lead aryloxides: Dependence on reagents and conditions

Article abstract of DOI:10.1039/b707984b

The successful synthesis of terminal lead aryloxides is shown to be dependent upon reaction conditions, including choice of solvent and alkali metal aryloxide precursor. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b707984b

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The synthesis of monomeric terminal lead aryloxides: dependence on reagents
and conditions†‡
J. Robin Fulton,* Peter B. Hitchcock, Nick C. Johnstone and Eric C. Y. Tam
Received 25th May 2007, Accepted 14th June 2007
First published as an Advance Article on the web 26th June 2007
DOI: 10.1039/b707984b
The successful synthesis of terminal lead aryloxides is shown
to be dependent upon reaction conditions, including choice
of solvent and alkali metal aryloxide precursor.
1H NMR spectrum of the reaction mixture after five min revealed
new resonances corresponding to 1 : 1 mixture of lead-bound
b-diketiminate ligand and BHT. However, no precipitate other
than the slow forming yellow crystalline solid was observed, even
upon addition of pentane or other non-polar solvents. Confused
as to the identity of this new complex, an X-ray diffraction study
was performed on the crystalline solid, which has crystallized
about an inversion centre.18 The molecular structure revealed that
a lead-aryloxide complex had not formed, but instead heterote-
trametallic adduct, [{(BDI)PbCl}{LiO(2,6-di-tBu-4-MeC6H2)}]2
(2) had been generated in which a lithium-aryloxide [Li(OAr)]2
dimer is coordinated by two [(BDI)PbCl] moieties via a chloride–
lithium interaction (Fig. 1, Scheme 1). A pyramidal geometry is
observed around the lead metal centre, with the sum of the bond
angles around the metal centre equal to 272.3◦, or a degree of
pyramidalization (DP) of 98%.14,19 This is similar to the parent
chloride complex 1 (DP = 103%); however, as would be expected
There is a large literature base detailing the coordination chemistry
of lead(II) and trying to address its bonding properties, with an
emphasis on how divalent lead might interact with biological
molecules.1,2 In addition, there has been considerable interest in the
lead(II) lone pair with regards to both its stability and its purported
stereochemical activity.3–6 However, the chemistry of the ligands
bonded to lead, especially in mimicking the reactivity of transition
metal counterparts, has been widely ignored. This is partially due
to the instability of lead complexes and their tendency to form
insoluble precipitates which are difficult to characterize and use in
further transformations. For instance, only a handful of terminal
lead alkoxide and aryloxides have been reported to date,7–10 and
only one of these is monometallic.7 This is in sharp contrast with
the vast array of transition metal counterparts.11–13
We have recently reported the synthesis of a series of b-diketi-
minate lead(II) halide complexes in which we employed the bulky
b-diketiminate anion, [{N(2,6-iPr2C6H3)C(Me)}2CH], (BDI) to
support monomeric, 3-coordinate lead centres.14 These ligands
have been previously utilised to stabilise low-coordinate transition
metal complexes as the isopropyl groups on the ligand aryl
group limit the number of coordinated ligands around the metal
center.15–17 Both the BDI–lead chloride and bromide complexes are
relatively thermally and photolytically stable and the [(BDI)PbCl]
(1) can be generated in sufficient quantities for use as a starting
reagent for the development of the chemistry of low-coordinate
lead.
by such a complex, both the Pb–Cl and Pb–N bond distances
14
˚
˚
are elongated by 0.073 A and 0.035 A, respectively. The Li2O2
dimer is a common binding motif for bulky lithium-aryloxide
complexes, with the THF adduct of LiBHT crystallographically
characterized.20
To minimize the risk of forming aggregates in solution, our
initial focus was on generating bulky lead-aryloxide complexes.
Synthesis of a lead-aryloxide containing the 2,6-di-tert-butyl-4-
methylphenoxide (BHT) group was attempted by treatment of
chloride 1 with LiBHT in THF. An insoluble white precipitate
was formed, however, the major species remaining in solution was
neutral BDI-H. Interestingly, when a solution of chloride 1 was
added to a suspension of unsolvated LiBHT in toluene or benzene,
the mixture became homogeneous, and formation of a yellow
crystalline solid was observed upon standing for 30 min. The
Fig. 1 ORTEP diagram of compound 2 with H atoms omitted and C
atoms minimized for clarity. Ellipsoid probably shown at 30%. Atoms
with a prime (ꢀ) in the atoms labels are at equivalent position (−x, −y, −z).
◦
˚
Selected bond lengths (A) and angles ( ): Pb–N(1), 2.315(3); Pb–N(2),
2.315(3); Pb–Cl, 2.6381(10); Li–Oꢀ, 1.849(7); Li–O, 1.885(7); Li–Cl,
2.358(7); O–C(30), 1.337(4); Oꢀ–Li, 1.849(7); N(1)–Pb–N(2), 82.86(11);
N(1)–Pb–Cl, 94.14(3); N(2)–Pb–Cl, 94.33(8); Oꢀ–Li–O, 96.8(3); Oꢀ–Li–Cl,
140.1(4); O–Li–Cl, 123.2(3); Li–Cl–Pb, 102.19(17); C(30)–O–Liꢀ, 145.3(3);
C(30)–O–Li, 131.0(3); Liꢀ–O–Li, 83.2(3).
Department of Chemistry, University of Sussex, Brighton, UK BN1 9QJ.
E-mail: j.r.fulton@sussex.ac.uk; Fax: +44 (0)1273 677196; Tel: +44
(0)1273 873170
† CCDC reference numbers 641614–648823. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b707984b
‡ Electronic supplementary information (ESI) available: Fig. S1. Temper-
ature dependent NMR spectra of 3 showing the change in resonances
corresponding to the (2,6-di-tBu-4-MeC6H2) protons (Tc = 278 K) and
the N-(2,6-di-iPrC6H3) protons (Tc = 248 K). See DOI: 10.1039/b707984b
Heterotetrametallic complex 2 is both light and temperature
sensitive and will decompose to a black insoluble precipitate over
the course of a day at ambient temperatures. We were unable
to force complex 2 to eliminate lithium chloride by heating,
sonicating, or addition of N,N,Nꢀ,Nꢀ-tetramethylethylenediamine
or AgOTf, the latter of which resulted in the immediate
3360 | Dalton Trans., 2007, 3360–3362
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Fig. 2 ORTEP diagram of compound 4 with H atoms omitted and C
atoms minimized for clarity. E◦llipsoid probably shown at 30%. Selected
˚
bond lengths (A) and angles ( ): Pb–N(1), 2.270(5); Pb–N(2), 2.288(5);
Pb–O, 2.182(4); O–C(30), 1.345(8); N(1)–Pb–N(2), 82.34(19); N(1)–Pb–O,
83.32(18); N(2)–Pb–O, 88.97(18); Pb–O–C(30), 127.3(4).
bound chloride.21–24 As there are significantly fewer examples of
sodium-chloride adducts of transition metals potentially due to the
weaker Na–Cl bond,25 the BHT–aryloxide reagent was changed
to NaBHT. This route was indeed successful as treatment of a
toluene solution of chloride 1 with NaBHT resulted in clean
formation of the aryloxide 3 after stirring overnight (Scheme 1).
This complex is stable in the solid state but, similar to aryloxide 4,
slowly decomposes in solution.
Scheme 1 Reactivity of 1 with Li- and NaBHT (BHT = 2,6-di-tert-butyl-
4-methylphenoxide).
decomposition of the complex. The inability to convert 2 to our
desired lead aryloxide was initially attributed to the bulky aryl
substituent, that is, we postulated the desired lead–BHT complex
(3) would be destabilized by interaction between the tert-butyl
groups on the aryloxide and the isopropyl groups on the b-
diketiminate ligand. We reasoned that decreasing the steric bulk
around the aryloxide ligand would make the synthesis of a lead-
aryloxide viable, indeed, treatment of complex 1 with lithio-2,4-di-
tert-butylphenoxide resulted in the formation of the lead aryloxide
4 after 30 min at room temperature (eqn (1)). This compound is
stable indefinitely in the solid state, but will slowly decompose
in solution. The X-ray crystal structure (Fig 2) revealed that the
angles between the ligands around the metal centre to be even
more acute than the chloride 1 (sum of the bond angles = 254.6◦,
DP = 118%). The Pb–N bond lengths are similar to other BDI–
The X-ray crystal structure of 3 revealed slightly longer Pb–
˚
O (2.212(2) A) and Pb–N bond lengths compared to 4 (Fig 3);
however, the geometry around both the metal centre as well as
the aryloxide ligand are considerably distorted as compared to
complex 4. For instance, in the latter, the bond angles around
the lead metal centre ranged from 82–89◦. In contrast, one of
the N–Pb–O bond angles of the BHT complex 3 is significantly
more obtuse (103.00(7)◦), leading to a larger sum of bond angles
(273.4◦) and a 96.2% DP. In addition, the ipso carbon atom
˚
of the BHT ligand lies 0.198 A out of the plane generated by
the other ring carbons (note, in the solid state structure of 4,
˚
this distance is only 0.073 A). The solid-state structure also
˚
Pb complexes and the Pb–O bond length of 2.182(4) A is slightly
revealed the BHT aryloxide ligand to be canted towards one
of the 2,6-isopropyl aryl rings of the BHT ligand. Although
factors governing all of these distortions are unclear, some can be
shorter than Van Zandt’s lead aryloxide dimer [Pb(OArꢀ)2]2 (Arꢀ =
9
˚
2,6-Ph2C6H3) of 2.229–2.257 A.
(1)
The viability of a [(BDI)Pb(BHT)] complex 3 appeared feasible
from the structural data of 4. The 2-tert-butyl group on the
aryloxide ligand of 4 lies directly below the BDI-plane, leaving
a void in front of the Pb–O bond that is potentially large enough
to accommodate another tertiary butyl group. Space filling models
of the postulated BHT complex 3 revealed that this species,
although sterically crowded, is potentially viable. Lithium-chloride
adducts of transition metals are not unusual, and, in a few
notable examples, lithium chloride adducts have been isolated
as apparent intermediates in salt metathesis reactions in which
the lithium ion is bound to both the leaving group, such as an
amido or alkoxide functional group, as well as the transition-metal
Fig. 3 ORTEP diagram of compound 3 with H atoms omitted and C
atoms minimized for clarity. E◦llipsoid probably shown at 30%. Selected
˚
bond lengths (A) and angles ( ): Pb–N(1), 2.309(2); Pb–N(2), 2.302(2);
Pb–O, 2.212(2); O–C(30), 1.352(3); N(1)–Pb–N(2), 83.06(8); N(1)–Pb–O,
87.32(7); N(2)–Pb–O, 103.00(7); C(30)–O–Pb, 125.91(16).
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attributed to solid-state packing forces. The canting of the BHT
ligand was not observed in the solution phase as only two different
environments were detected for the isopropyl groups, even at
−78 ◦C. However, broadening of the resonances corresponding
to the BHT ligand was observed at room temperature, indicative
of hindered rotation around the Pb–O bond (DG‡ = 13 kcal mol−1),
similar to that observed in the [(BDI)Pb–N(SiMe3)2] system.14 In
addition, broadening of the N-aryl resonances was observed at RT,
indicating a hindered rotation of the Naryl-C bond (DG‡ = 16 kcal
mol−1). This restricted rotation has not been observed in other
BDI–Pb systems and may be a function of the bulky aryloxide
substituent.14
In conclusion, we have developed a reliable synthetic route
towards the synthesis of rare terminal lead aryloxide complexes via
salt metathesis from lead chloride complex 1. We have isolated a
lithium-lead heterotetrametallic complex, which initially appeared
to be an intermediate in the salt metathesis pathway. However,
as we were unable to force this complex to lose LiCl and form
the desired BHT-aryloxide 3 and instead generated 3 from an
alternative pathway, the validity of the complex as an actual
intermediate in the salt metathesis pathway is in question. Reasons
behind the solvent effect on the reaction outcome as well as factors
governing the relative stability of the heterotetrametallic complex
2 compared to the lead-aryloxide complexes 3 and 4, including
DFT investigations, are currently in progress.
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Acknowledgements
J. R. F. gratefully acknowledges the Leverhulme Trust and
University of Sussex start-up funds for financial support.
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3362 | Dalton Trans., 2007, 3360–3362
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