Heavy metal boryl chemistry: Complexes of cadmium, mercury and lead

Article abstract of DOI:10.1039/c4cc00697f

Synthetic routes to the first boryl complexes of cadmium and mercury are reported via transmetallation from boryllithium; the syntheses of related group 14 systems highlight the additional factors associated with extension to more redox-active post-transi

Full text of DOI:10.1039/c4cc00697f

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Heavy metal boryl chemistry: complexes of
cadmium, mercury and lead†
Cite this: Chem. Commun., 2014,
50, 3841
Andrey V. Protchenko, Deepak Dange, Andrew D. Schwarz, Christina Y. Tang,
Nicholas Phillips, Philip Mountford,* Cameron Jones* and Simon Aldridge*
Received 26th January 2014,
Accepted 25th February 2014
DOI: 10.1039/c4cc00697f
www.rsc.org/chemcomm
Synthetic routes to the first boryl complexes of cadmium and mercury
are reported via transmetallation from boryllithium; the syntheses of
related group 14 systems highlight the additional factors associated
with extension to more redox-active post-transition elements.
Fig. 1 Previously reported group 12 metal boryl complexes.10a
Transition metal boryl complexes have attracted much recent
attention,1 in part due to their implication in unique C–H
functionalization chemistry.2,3 More recently, the synthesis of reagents
possessing nucleophilic character at boron4,5 has enabled the exten-
sion of such chemistry to the f- and early d-block elements.6,7 In
addition, the large steric profile and strong s-donor properties
of the {(HCNDipp)2B} ligand has led to its use as a bespoke
ancillary substituent in the synthesis of main group carbenoid
systems capable of facile E–H bond activation (E = H, C).8
While reagents such as {(HCNDipp)2B}Li(thf)2 are known to
be strong reducing agents as well as powerful nucleophiles, the
modulation of reactivity achieved for related alkyl/aryl systems by
employing a less polar M–C bond,9 suggests that post transition
metal boryl complexes represent synthetically valuable targets.
Thus, for example, zinc boryl complexes have been reported and
their conjugate addition to a, b unsaturated ketones explored.10
While related systems have been reported for the B-metal triads Cu,
Ag, Au,10a and Ga, In, Tl,11 well-defined group 12 boryl complexes
are confined to two zinc species (Fig. 1).10a
ligands, free from complications involving p back bonding.
Existing examples of Hg–B single bonds are confined to exo-
polyhedral linkages associated with borane clusters;12 simple
2-centre 2-electron Cd–B and Pb–B bonds are hitherto unknown.13
The reactions of {(HCNDipp)2B}Li(thf)2 with HgBr2 in non-polar
solvents such as benzene provide a versatile access point to a range of
donor-free mercury boryl systems (Scheme 1). Thus, reactions in 1 : 1
and 2 : 1 stoichiometries generate complexes of the composition
{(HCNDipp)2B}HgBr (1) and {(HCNDipp)2B}2Hg (2), respectively. Con-
version of 1 into 2 can be effected by the addition of a second
equivalent of {(HCNDipp)2B}Li(thf)2 to 1, and the reverse transforma-
tion by mixing equimolar quantities of 2 and HgBr2. The composition
of both mercury boryl complexes is suggested by multinuclear
In the current study we targeted the first examples of cadmium
and mercury boryls using a salt metathesis approach, and sought
to explore whether this synthetic methodology could be extended
to the more redox active metal lead. Moreover, in addition to
homoleptic systems, routes to unsymmetrical (linear) complexes
of the type (boryl)MX were sought, as these offer a convenient
platform on which to study the s donor properties of boryl
Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford,
South Parks Rd, Oxford, UK OX1 3QR. E-mail: Simon.Aldridge@chem.ox.ac.uk;
Tel: +44 (0)1865 285201
Scheme 1 Syntheses of borylmercury complexes 1–3. Key reagents/
conditions: (i) {(HCNDipp)2B}Li(thf)2 (0.85 equiv.), benzene, RT, 5 min,
36%; (ii) {(HCNDipp)2B}Li(thf)2 (2.00 equiv.), benzene-d6, RT, 5 min, 43%;
(iii) HgBr2 (1.0 equiv.), benzene-d6, RT, 5 min, 96%; (iv) [{(HCNDipp)2Ga}-
K(OEt2)]2 (1.0 equiv.), benzene-d6, RT, 5 min, 35%.
† Electronic supplementary information available: Synthetic and characterizing
data for all new compounds; CIFs for 1–4, 7 and 8. CCDC 950737, 950743, 950744
and 968284–968286. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4cc00697f
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Scheme 2 Syntheses of Cd complexes 4–6. Key reagents/conditions:
(i) {(HCNDipp)2B}Li(thf)2 (2.00 equiv.), benzene-d6, RT, 12 h, 90%;
(ii) {(HCNDipp)2B}Li(thf)2 (1.00 equiv.), thf, ꢁ78 1C to 0 1C, 5 h, 46%;
(iii) [{(HCNDipp)2Ga}K(tmeda)]2 (0.50 equiv.), thf, ꢁ78 1C to 20 1C, 12 h, 70%.
systems, based on analyses of 195Pt–31P coupling constants.18
The Hg–Ga linkage – the first of its kind involving three-coordinate
gallium – features a bond length [2.476(1) Å] reflective of the difference
in the covalent radii of gallium and boron (Dd = 0.38 Å). The only
other example of a Hg–Ga bond, found in [{HC(CMeNDipp)2}-
Ga(SC6F5)]2Hg, is markedly longer [2.534(1) Å], presumably due to
the tetra-coordinate nature of the gallium centres.19
Fig. 2 Molecular structures of 1 (upper left), 2 (upper right), one of the
components of the asymmetric unit of 3 (lower left) and 4 (lower right). Here
and elsewhere: H atoms omitted and iPr groups shown in wireframe format for
clarity; thermal ellipsoids set at the 40% probability level. Key bond lengths (Å) and
angles (1) for 1: Hg(1)–B(3) 2.116(5), Hg(1)–Br(2) 2.486(1), Hg(1)–Br(20) 3.295(1),
B(3)–Hg(1)–Br(2) 169.0(1). Key metrical parameters for 2–4 are listed in Table 1.
While Hg–B bonds have some precedent (albeit not for simple
mono-boron ligand systems),12 unsupported Cd–B and Pb–B bonds
are hitherto unknown.13 As such, we hypothesized that salt metathesis
chemistry utilizing {(HCNDipp)2B}Li(thf)2 and appropriate cadmium
or lead electrophiles might allow access to a range of novel bond
types. In the case of cadmium, both bromide and iodide precursors
prove to be amenable to such chemistry (Scheme 2). Moreover, the
presence of an additional neutral donor – such as tmeda (N,N,N0,N0-
tetramethylethylene-diamine) modulates the observed reactivity,
providing control of product stoichiometry. Thus, in the absence
of such a donor, the reaction of cadmium dibromide with
{(HCNDipp)2B}Li(thf)2 generates bis(boryl)cadmium complex 4,
irrespective of stoichiometry, with the bromo(boryl) system
analogous to 1 presumably being more labile to substitution
than (sparingly soluble) CdBr2. In the case of the more soluble
precursor (tmeda)CdI2, the use of one equiv. of boryllithium allows
for the controlled introduction of a single boryl ligand yielding
a heteroleptic system of the type (tmeda)Cd(boryl)I (i.e. 5).17
Further substitution can then be effected, for example using a
gallylpotassium reagent to give 6.
NMR and micro-analytical measurements and is confirmed by
X-ray crystallography (Fig. 2). The monomeric structure of 2
features the linear two-coordinate geometry commonly associated
with Hg(II) [+(B–Hg–B) = 179.0(1)1], and comparison of the Hg–B
bond lengths [2.150(3), 2.151(3) Å] with the sum of the respective
covalent radii (2.16 Å),14 is consistent with descriptions as 2-centre
2-electron single bonds. Similar bond lengths have been reported
by Hawthorne and co-workers for the Hg–B bonds associated with
cyclic mercura-carborand systems.12c
In the case of 1, by contrast, the solid state structure is based
around loosely bound centro-symmetric [{(HCNDipp)2B}HgBr]2
dimers, featuring a planar Hg2Br2 core [+(Br–Hg–Br) = 80.3(1)1;
+(Hg–Br–Hg) = 99.7(1)1] and (superficially at least) a T-shaped
mercury centre. Comparison of the two different Hg–Br distances
[d(Hg(1)–Br(2)) = 2.486(1) Å; d(Hg(1)–Br(20)) = 3.295(1) Å] is consistent
with weak association of the (boryl)HgBr units (cf. 2.52/3.40 Å for
the sum of the respective covalent/van der Waals radii).14 The
B(3)–Hg(1)–Br(2) angle [169.0(1)1] is, however, 101 more acute
than the B–Hg–B angle determined for the strictly two-
coordinate 2, consistent with a small but significant distortion
from linearity imposed by the secondary Hgꢀ ꢀ ꢀBr contacts.
The synthetic utility of 1 in giving access to further examples
of heteroleptic mercury boryl systems can be demonstrated by
its reaction with the anionic gallyl equivalent [{(HCNDipp)2Ga}-
K(OEt2)]2,15,16 which yields the mixed group 13 donor species
{(HCNDipp)2B}Hg{Ga(NDippCH)2} (3) via salt metathesis chemistry.
The synthesis of 3 is significant in that reactions of mercury(II)
dihalides with the same gallyl anion lead to the reductive formation
of mercury metal, rather than metathesis.17 In common with 2, the
structure of 3 features a linear co-ordination geometry at mercury
[+(B–Hg–Ga) = 179.1(1)1], with the shorter Hg–B distance [2.116(5) Å]
measured for 3 being consistent with the weaker trans influence of
the gallyl ligand compared to its boryl counterpart. Such an assertion
By contrast, in the case of lead dihalide precursors (and to
some extent their tin analogues), redox chemistry provides a
competing reaction pathway to straightforward salt metathesis
(Scheme 3). Thus, although the primary product of the reaction
of SnCl2 is {(HCNDipp)2B}2Sn (as reported previously),8a a small
Scheme 3 Syntheses of Sn and Pb complexes 7 and 8. Key reagents/
conditions: (i) {(HCNDipp)2B}Li(thf)2 (2.00 equiv.), Et2O, RT, 1 h, ca. 4% [+51%
is consistent with observations made previously for square planar Sn(boryl)2];8a (ii) {(HCNDipp)2B}Li(thf)2 (2.02 equiv.), benzene, RT, 5 min, 46%.
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substrates, while the more accessible lead centre in 8 is extremely
labile, and its tin counterpart shows a wide range of controlled
reactivity involving both insertion and addition processes. Studies
of this chemistry will be reported shortly.
We acknowledge financial support from the Leverhulme
Trust, OUP John Fell Fund, the ARC and the EPSRC.
Notes and references
Fig. 3 Molecular structures of 7 (left), 8 (right). Key bond lengths (Å) and
angles (1) for 7: Sn(2)–B 2.290(5), Sn(5)–B 2.278(6), Sn(4)–B 2.595(7)/
2.526(7), Sn(1)–Sn(2) 2.796(1), Sn(1)–Sn(4) 2.883(1), Sn(1)–Sn(5) 2.865(1),
Sn(3)–Sn(2) 2.852(1), Sn(3)–Sn(4) 2.888(1), Sn(3)–Sn(5) 2.799(1). Key metrical
parameters for 8 are listed in Table 1.
1 For reviews of metal boryl chemistry see, for example: (a) G. J. Irvine,
M. J. G. Lesley, T. B. Marder, N. C. Norman, C. R. Rice, E. G. Robins,
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Chem. Rev., 2010, 110, 3924.
quantity of a pentanuclear tin cluster species (7) is also obtained. 7
features four boryl ligands and consequently a mean metal oxidation
state of 0.8; three different tin environments are revealed crystallo-
graphically featuring either zero [Sn(1) and Sn(3)], one [Sn(2) and
Sn(5)] or two metal-bound boryl ligands [Sn(4)] (Fig. 3).20 In the
case of lead(II) bromide, by contrast, the only isolable product is
the bis(boryl)lead complex, 8, but the propensity for reduction
is signalled by the formation of metallic lead and the relatively
low yield of 8 (46%).
The structures of the bis(boryl) complexes of both cadmium
(4) and lead (8) have been determined crystallographically,
confirming the expected linear [177.5(1)1] and bent geometries
[118.3(1)1] at the respective metal centres (Fig. 2 and 3). Salient
structural parameters are included in Table 1 (along with those
of related systems) allowing the following general trends to be
identified for post-transition metal boryl complexes: (i) the
stronger s-donor properties of the boryl ligand over gallyl and (less
surprisingly) bromide counterparts; (ii) M–B bond lengths within
individual groups (i.e. 12 and 14) which conform to the trends
predicted on the basis of covalent radii, as expected for 2-centre
2-electron single bonds; and (iii) the over-arching influence of the
steric demands of the boryl substituent. The latter factor leads to
shorter M–B bonds for linear vs. bent bis(boryl) derivatives,
even to the extent of out-weighing the influence of covalent radius
(e.g. for Cd, In, Sn and Hg, Tl, Pb), and presumably results from
enhanced steric ‘buttressing’ between pendant Dipp groups as the
B–M–B angle narrows.
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Chem., 1987, 59, 879; (b) K. Burgess and M. J. Ohlmeyer, Chem. Rev.,
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DOI: 10.1038/nchem.1870. For a Ga(III) boryl see: (b) N. Dettenrieder,
Steric considerations also appear to be a factor in modulating
the reactivity of heavier p-block bis(boryl) systems, thus for example,
mercury system 2 is surprisingly unreactive towards a range of
¨
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Table 1 M–B bond lengths and B–M–B angles for bis(boryl) and related
complexes of the heavier group 12, 13 and 14 metals
14
r(M–B)/Å
+(B–M–X)/1
rcov (M)
10a
Zn(boryl)2
2.052(3), 2.053(3)
2.205(2), 2.207(2)
2.150(3), 2.151(3)
2.116(5)
178.5(1)
177.5(1)
179.0(1)
169.0(1)
179.1(1)
145.4(1)
177.6(2)
118.8(3)
118.3(1)
1.22
1.44
1.32
1.32
1.32
1.42
1.45
1.39
1.46
Cd(boryl)2 (4)
Hg(boryl)2 (2)
BrHg(boryl) (1)
(Gallyl)Hg(boryl) (3)
2.116(5)
11a
In(boryl)211a
2.242(3), 2.246(3)
2.167(5), 2.173(6)
2.285(8), 2.294(8)
2.363(4), 2.372(4)
Tl(boryl)2
Sn(boryl)2
8a
Pb(boryl)2 (8)
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V. I. Meshcheryakov, O. L. Tok, P. V. Petrovskii and Z. A. Starikova, 16 Attempts to synthesize Hg(I) systems of the type {(HCNDipp)2B}Hg–
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13 Compounds have been structurally characterized featuring M–H–B
Hg{B(NDippCH)2} via reduction of 1 with KC8 lead instead to the
predominant formation of 2 and elemental mercury.
interactions (M = Cd or Pb) or incorporating the metal in a 17 For related chemistry leading to the formation of a system of the
delocalized cluster bonding framework, but direct metal–boron
interactions are limited to a single plumbylene–B(C6F5)3 complex:
type (tmeda)Cd(gallyl)I see: O. Bonello, C. Jones, A. Stasch and
W. Woodul, Organometallics, 2010, 29, 4914.
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´
´
´
Chem.–Eur. J., 2012, 16, 6041.
´
E. Cremandes, F. Barrigan and S. Alvarez, Dalton Trans., 2008, 2832. 20 For a recent example of a metal-rich tin cluster featuring bulky terphenyl
15 R. J. Baker, R. D. Farley, C. Jones, M. Kloth and D. M. Murphy,
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