Lanthanide(II) complexes of a phosphine-borane-stabilised carbanion

Article abstract of DOI:10.1039/c001468k

The reaction between two equivalents of the potassium salt [(Me ₃Si)₂{Me₂P(BH₃)}C]K (4) and SmI ₂(THF)₂ in refluxing THF yields the dialkylsamarium(ii) compounds [(Me₃Si)₂{Me₂P(BH₃)}C] ₂Sm(THF) (5a) or [(Me₃Si)₂{Me ₂P(BH₃)}C]₂Sm(THF)₃ (5b), depending on the crystallisation conditions, in good yield as air- and moisture-sensitive crystalline solids. X-ray crystallography shows that, whereas both alkyl ligands chelate the samarium(ii) ion in 5a, in 5b one alkyl ligand chelates the metal centre and one binds the metal only through its borane hydrogen atoms. The reaction between YbI₂ and two equivalents of 4 in refluxing benzene yields the solvent-free dialkylytterbium(ii) compound [(Me₃Si) ₂{Me₂P(BH₃)}C]₂Yb (8). In contrast to 5a and 5b, compound 8 reacts rapidly with THF to give the free phosphine-borane (Me₃Si)₂{Me₂P(BH ₃)}CH as the only identifiable product. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c001468k

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www.rsc.org/dalton | Dalton Transactions
Lanthanide(II) complexes of a phosphine-borane-stabilised carbanion†
Keith Izod,* William Clegg and Ross W. Harrington
Received 25th January 2010, Accepted 4th May 2010
First published as an Advance Article on the web 17th May 2010
DOI: 10.1039/c001468k
The reaction between two equivalents of the potassium salt [(Me3Si)2{Me2P(BH3)}C]K (4) and
SmI2(THF)2 in refluxing THF yields the dialkylsamarium(II) compounds
[(Me3Si)2{Me2P(BH3)}C]2Sm(THF) (5a) or [(Me3Si)2{Me2P(BH3)}C]2Sm(THF)3 (5b), depending on
the crystallisation conditions, in good yield as air- and moisture-sensitive crystalline solids. X-ray
crystallography shows that, whereas both alkyl ligands chelate the samarium(II) ion in 5a, in 5b one
alkyl ligand chelates the metal centre and one binds the metal only through its borane hydrogen atoms.
The reaction between YbI2 and two equivalents of 4 in refluxing benzene yields the solvent-free
dialkylytterbium(II) compound [(Me3Si)2{Me2P(BH3)}C]2Yb (8). In contrast to 5a and 5b, compound 8
reacts rapidly with THF to give the free phosphine-borane (Me3Si)2{Me2P(BH3)}CH as the only
identifiable product.
often exhibit reactivities and undergo reactions which are dif-
ferent from, but which complement, those of their metallocene
cousins. In this regard, compound 1 undergoes reactions which
have no precedent in lanthanide metallocene chemistry; for
Introduction
Over the last three decades the organometallic chemistry of
the lanthanide(II) ions has undergone a period of rapid and
sustained expansion, due largely to the unique structures and
reactions exhibited by these species.1,2 The unique reactivity of
these compounds is exemplified by the bent metallocene Cp*2Sm,
first isolated by Evans and co-workers in 1984, which exhibits
remarkable reactivity towards a wide variety of inorganic and
organic substrates.2
example, 1 reacts with ethyl ethers according to Scheme 1
to give the corresponding heteroleptic alkyl/alkoxyytterbium(II)
species.3,4,6 Neither the europium nor samarium analogues 2 and
3,6,7 nor the less sterically hindered dialkylytterbium complex
{(Me3Si)2CH}2Yb(OEt2)2 exhibit this behaviour;4b indeed, this lat-
ter complex is isolated as a stable diethyl ether adduct. Compound
Whilst the chemistry of lanthanide(II) metallocenes is
1 also exhibits unusual reactivity towards halocarbons. Typically,
now relatively well developed, surprisingly few s-bonded
organolanthanide(II) species are known and, amongst these,
alkyl derivatives are particularly rare.3–20 The first crys-
lanthanide(II) compounds undergo oxidation to the corresponding
lanthanide(III) halide on treatment with an alkyl bromide or
iodide. In contrast, the reaction between 1 and either methyl
iodide or 1,2-diiodoethane yields the corresponding alkylytter-
bium iodide [{(Me3Si)3C}Yb(m-I)(OEt2)]2, without oxidation of
the ytterbium(II) centre.3,6
We have recently developed a number of phosphine-borane-
stabilised carbanions and dicarbanions and have shown that these
are excellent ligands for the synthesis of complexes of the main
group elements from groups 1, 2 and 14.21 We now show that one
of these ligands enables the stabilisation of novel Yb(II) and Sm(II)
complexes and that the binding mode of this ligand is dependent
on the exact crystallisation conditions.
tallographically characterised alkyllanthanide(II) compounds
{(Me3Si)3C}2Yb (1),3 [{(Me3Si)3C}Yb(m-OEt)(OEt2)]2
and
3,4
(TptBu,Me)Yb{CH(SiMe3)2} were reported as recently as 1994
[TptBu,Me = hydrotris(3-tert-butyl-5-methypyrazolyl)borate]. The
first alkyleuropium(II) compound {(Me3Si)3C}2Eu (2) was
reported in 1996,6 and in 1997 we reported the first
crystallographically characterised alkylsamarium(II) compound
{(Me3Si)2(Me2MeOSi)C}2Sm(THF) (3).7 Very recently Takats
and co-workers reported the synthesis of the remarkable s-bonded
organothulium(II) compound (TptBu,Me)Tm{CH(SiMe3)2}.C6H14.19
The reactions of alkyllanthanide(II) species have yet to be
studied in depth, but early results suggest that these compounds
5
Results and discussion
We have previously shown that the samarium(II) dialkyl
{(Me3Si)2(Me2MeOSi)C}2Sm(THF) (3) is readily prepared by
a metathesis reaction between SmI2(THF)2 and two equiva-
lents of {(Me3Si)2(Me2MeOSi)C}K.7 It is perhaps somewhat
surprising, therefore, that the corresponding reaction between
Main Group Chemistry Laboratories, School of Chemistry, Bedson Building,
University of Newcastle, Newcastle upon Tyne, UK NE1 7RU. E-mail:
k.j.izod@ncl.ac.uk
† CCDC reference numbers 763178 and 763179. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/c001468k
Scheme 1
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6705–6710 | 6705
©

Scheme 2 [R = SiMe3].
SmI2(THF)2 and two equivalents of the potassium alkyl [(Me3Si)2-
{Me2P(BH3)}C]K (4)21d in THF at room temperature is rather
sluggish, much of the potassium salt remaining after 48 h; however,
this reaction may be forced to completion by heating under reflux
for 2 h in the same solvent (Scheme 2). Removal of the solvent
and extraction of the residue into methylcyclohexane gives a deep
purple-brown solution from which dark purple-black blocks of
the mono-THF adduct [(Me3Si)2{Me2P(BH3)}C]2Sm(THF) (5a)
◦
deposit as a methylcyclohexane solvate on cooling to -30 C for
several days. The 1H NMR spectrum of 5a is consistent with
this formulation, although the solvent of crystallisation is readily
lost under vacuum and so is not observed: singlets are observed
at -1.87 and 0.53 ppm due to the PMe2 and SiMe3 protons,
respectively, whilst signals due to the THF co-ligand are observed
at 4.13 and 14.10 ppm. However, we were unable to observe a signal
1
11
due to the BH3 protons, even in the H{ B} spectrum, probably
due to a combination of the quadrupolar nature of the 11B nucleus
and the proximity of these protons to the paramagnetic Sm(II)
Fig. 1 Molecular structure of 5a wit◦h H atoms omitted for clarity.
1
1
centre. The 31P{ H} and 11B{ H} NMR spectra of 5a consist of
broad singlets at -1.9 and -41.6 ppm, respectively; 31P-11B coupling
is not resolved on either signal.
˚
Selected bond lengths (A) and angles ( ): Sm–C(1) 2.853(8), Sm–C(10)
2.827(9), Sm–O 2.565(6), Sm ◊ ◊ ◊ B(1) 2.895(11), Sm ◊ ◊ ◊ B(2) 2.861(11),
C(1)–P(1) 1.760(9), C(1)–Si(1) 1.865(9), C(1)–Si(2) 1.832(9), C(10)–P(2)
1.752(9), C(10)–Si(3) 1.850(10), C(10)–Si(4) 1.862(9), P(1)–B(1) 1.962(11),
P(2)–B(2) 1.943(12), C(1)–Sm–B(1) 66.2(3), C(10)–Sm–B(2) 66.2(3),
C(1)–Sm–C(10) 135.0(3), B(1)–Sm–B(2) 165.2(4).
The molecular structure of 5a is shown in Fig. 1, along with
selected bond lengths and angles. Compound 5a crystallises as
a discrete molecular species in which the phosphine-borane-
substituted alkyl ligands bind the samarium(II) ion through their
carbanion centres and the borane groups (for which the hydrogen
atoms were not located), generating two pseudo-four-membered
chelate rings [the C–Sm–B “bite angle” is 66.2(3)◦ for both
ligands]; the coordination of the samarium ion is completed by
a molecule of THF.
bond lengths to the samarium ion in 5a and the corresponding
distances in comparable Sm(III) compounds. Unfortunately, the
crystallographic data for 5a are not of sufficient quality for the
precise location of the borane hydrogen atoms in this compound;
however, the Sm(II) ◊ ◊ ◊ B distances are 2.895(11) and 2.861(11)
˚
˚
The Sm–C distances of 2.853(8) and 2.827(9) A are simi-
A. For comparison, typical Sm ◊ ◊ ◊ B distances in samarium(III)
˚
lar to the corresponding distances in the few previously re-
ported alkylsamarium(II) compounds; for example, the Sm–C
distances in {(Me3Si)2(Me2MeOSi)C}2Sm(THF) are 2.787(5) and
borohydride complexes lie in the range 2.58–3.03 A; for example,
the Sm(III) ◊ ◊ ◊ B distances in the dimer [(1,3-But2C5H3)2Sm(BH4)]2
23
˚
are 2.833(6) and 2.882(6) A, whereas the Sm ◊ ◊ ◊ B distances
7
˚
˚
2.845(5) A, whereas the Sm–C distances in {(Me3Si)(C6H4-2-
in Cp*2Sm(BH4)(THF) are 2.62(2) and 2.58(2) A (for the two
8
independent molecules in the asymmetric unit),24 and the Sm ◊ ◊ ◊ B
˚
NMe2)CH}2Sm(THF)2 are 2.768(3) and 2.789(3) A. To the best
5
distances in the cluster [(h -Me4PrnC5)Sm(BH4)2]6 range from 2.64
of our knowledge, there has been no previous report of a crys-
tallographically characterised Sm(II) ◊ ◊ ◊ BHn contact, although
there are several examples of Sm(III) borohydride complexes.
to 3.03 A.25 The similarity between the Sm(II) ◊ ◊ ◊ B distances in 5a
˚
and typical Sm(III) ◊ ◊ ◊ B distances suggests that the Sm ◊ ◊ ◊ BH3
interactions in 5a are relatively strong.
According to Shannon the ionic radii of Sm(II) and Sm(III) are
22
˚
1.22 and 1.02 A, respectively, for eight-coordination and so a
The C(1)–Sm–C(10) and B(1)–Sm–B(2) angles are 135.0(3) and
165.2(4)◦, respectively, and so the geometry at the samarium(II) ion
˚
difference of approximately 0.2 A might be expected between
6706 | Dalton Trans., 2010, 39, 6705–6710
This journal is
The Royal Society of Chemistry 2010
©

is best described as lying somewhere between square pyramidal,
with an apical THF, and trigonal bipyramidal, with axial borane
groups.
useful comparator for the Sm(II) ◊ ◊ ◊ H contacts in 5b. The
Sm ◊ ◊ ◊ H contacts fall in the range 2.53(3)–2.68(3) and these
compare with Eu ◊ ◊ ◊ H distances ranging from 2.30(6) to 2.57(4)
˚
Somewhat unexpectedly, a second batch of crystals of this
compound, obtained by an identical synthetic route, was
found by X-ray crystallography to have the composition
[(Me3Si)2{Me2P(BH3)}C]2Sm(THF)3 (5b), also as a methylcy-
clohexane solvate. These crystals have an essentially identical
appearance to those of 5a, but rapidly lose crystallinity on
exposure to vacuum. This is associated with rapid loss of both the
solvent of crystallisation (methylcyclohexane) and two of the three
molecules of THF, as demonstrated by 1H NMR spectroscopy. The
molecular structure of 5b is shown in Fig. 2, along with selected
bond lengths and angles.
A in the europium(II) bis((cyclooctane-1,5-diyl)dihydroborate)
26
˚
(THF)4Eu{(m-H)2BC8H14}2 and from 2.41(4) to 2.56(3) A in the
related salt [NMe4]2[Ln{(m-H)2BC8H14}4].27
The carbanion centre in the chelating alkyl ligand has a dis-
tinctly pyramidal CSi2P skeleton [sum of angles at C(1) = 343.54◦;
˚
displacement of C(1) from the Si(1)–Si(2)–P(1) plane = 0.433 A].
In contrast, the carbanion centre in the monodentate, borane-
donor ligand is much closer to planar [sum of angles at C(10) =
354.65◦; displacement of C(10) from the Si(3)–Si(4)–P(2) plane =
˚
0.241 A]. Consistent with this the C(1)–P(1), C(1)–Si(1) and C(1)–
Si(2) distances [1.756(3), 1.855(3) and 1.855(3), respectively] are
significantly longer than the C(10)–P(2), C(10)–Si(3) and C(10)–
Si(4) distances [1.719(3), 1.820(3) and 1.827(3), respectively]. This
is in accord with a greater degree of charge localisation when
the carbanion is coordinated to the samarium ion compared to
the essentially sp2-hybridised monodentate ligand, where charge
is extensively delocalised from the carbanion centre onto the
adjacent heteroatoms.
The difference in structure between 5a and 5b clearly suggests
that there is a delicate balance between solvation and ligand
coordination mode in these systems. In this regard, it is instructive
to compare the structures of 5a and 5b with that of the corre-
sponding strontium derivative. According to Shannon, the ionic
2+
radii of Sr2+ and Sm2+ differ by just 0.01 A [the ionic radii of Sr
˚
and Sm2+ are 1.21 and 1.22 A for a seven-coordinate ion] and
22
˚
these ions often display very similar coordination chemistries.8 It
is, therefore, notable that the strontium analogue of 5 adopts a
different structure again in the solid state from those of 5a and
5b. This compound crystallises from methylcyclohexane/THF as
the adduct [(Me3Si)2{Me2P(BH3)}C]2Sr(THF)5 (6), in which the
strontium is bound by both alkyl ligands through their borane
hydrogen atoms only, with no short Sr–C contacts, and by five
molecules of THF.21e Interestingly, the THF ligands in 6 appear to
be more tightly held than those in 5b and are not lost when this
compound is exposed to vacuum.
Fig. 2 Molecular structure of 5b with H atoms bonded to carbon
◦
˚
omitted for clarity. Selected bond lengths (A) and angles ( ): Sm–C(1)
2.873(3), Sm–O(1) 2.5743(18), Sm–O(2) 2.608(2), Sm–O(3) 2.6020(17),
Sm–H(1C) 2.53(3), Sm–H(1B) 2.68(3), Sm–H(2A) 2.60(3), Sm–H(2B)
2.55(2), Sm–H(2C) 2.61(3), Sm ◊ ◊ ◊ B(1) 2.959(4), Sm ◊ ◊ ◊ B(2) 2.783(3),
C(1)–P(1) 1.756(3), C(1)–Si(1) 1.855(3), C(1)–Si(2) 1.855(3), C(10)–P(2)
1.719(3), C(10)–Si(3) 1.820(3), C(10)–Si(4) 1.827(3), P(1)–B(1) 1.939(4),
P(2)–B(2) 1.928(3), C(1)–Sm–B(1) 64.68(10), Si(1)–C(1)–Si(2) 112.80(16),
Si(1)–C(1)–P(1) 114.70(15), Si(2)–C(1)–P(1) 116.04(16), Si(3)–C(10)–Si(4)
118.58(17), Si(3)–C(10)–P(2) 120.52(17), Si(4)–C(10)–P(2) 115.55(16).
In contrast to the ready synthesis of 5, the reaction between
YbI2 and two equivalents of 4 in THF at room temperature
does not proceed cleanly, yielding a mixture of unreacted 4 and
the free phosphine-borane (Me3Si)2{Me2P(BH3)}CH (7) as the
major products, even after extended reaction times. Heating this
reaction mixture under reflux leads to complete decomposition,
yielding 7 as the only identifiable product. Nevertheless, the
desired dialkylytterbium compound [(Me3Si)2{Me2P(BH3)}C]2Yb
(8) may be accessed in good yield from the reaction between YbI2
and two equivalents of 4 after heating under reflux in benzene
for 16 h. Deep orange crystals of 8 may be obtained from cold
n-hexane, although, despite repeated efforts, we were unable to
obtain single crystals suitable for X-ray crystallography. However,
the composition of this compound was confirmed unambiguously
by elemental analysis and multi-element NMR spectroscopy. The
1H NMR spectrum of 8 consists of a singlet at 0.32 ppm due to
the SiMe3 protons, a doublet at 1.20 ppm due to the PMe2 protons
and a broad 1 : 1 : 1 : 1 quartet at 1.96 ppm (JBH = 88.4 Hz) due
to the BH3 protons, the latter of which collapses to a doublet on
The samarium(II) ion in 5b is coordinated by the carbanion
centre and the borane hydrogen atoms of one alkyl ligand,
generating a pseudo-four-membered chelate ring [C–Sm–B “bite
angle” 64.68(10)◦], by the borane hydrogen atoms of the second
alkyl ligand, and by three molecules of THF; the carbanion
centre of the second alkyl ligand has no short contact with the
samarium ion. This gives the samarium ion a highly distorted
pseudo-octahedral geometry, in which the THF ligands adopt a
meridional disposition.
˚
The Sm–C distance [2.873(3) A] is similar to the corresponding
distances in 5a and related compounds. For compound 5b the
borane hydrogen positions were located and freely refined; the
3
borane group of the monodentate alkyl ligand binds in an h -
fashion whereas the borane group of the chelating alkyl ligand
2
binds the Sm(II) ion in an h -fashion. Although there are no
previous examples of Sm(II) ◊ ◊ ◊ H–B contacts, a small number
of crystallographically characterised europium(II) borohydrides
have been reported; since the ionic radii of Eu(II) and Sm(II)
1
decoupling of the 11B nucleus (JPH = 9.2 Hz). The 31P{ H} and
22
1
˚
differ by just 0.02 A, Eu(II) ◊ ◊ ◊ H distances may act as a
11B{ H} NMR spectra consist of a broad quartet at -11.7 ppm
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6705–6710 | 6707
©

and a broad doublet at -24.8 ppm, respectively; the 171Yb spectrum
consists of a singlet at 794 ppm.
deoxygenated by three freeze-pump-thaw cycles and stored over
21d
˚
activated 4A molecular sieves. [(Me3Si)2{Me2P(BH3)}C]K (4),
YbI2,31 and SmI2(THF)2 were prepared by previously published
32
Unfortunately, the solution NMR data for 8 do not provide un-
ambiguous evidence for the coordination modes of the carbanions
in this compound. Previously reported 171Yb chemical shifts span
the range from -500 to 2000 ppm and the chemical shifts of even
very similar Yb(II) compounds often vary dramatically; for exam-
procedures. All other compounds were used as supplied by the
manufacturer.
NMR spectra were recorded on a JEOL ECS500 spectrometer
operating at 500.16 (1H), 125.65 (13C), 160.47 (11B), 202.47 (31P),
and 87.52 (171Yb) MHz, respectively; chemical shifts are quoted in
ppm relative to tetramethylsilane (1H and 13C), external BF3(OEt2)
(11B), external 85% H3PO4 (31P), and external Cp*2Yb(THF)2
5
ple, the 171Yb chemical shifts of the metallocenes (h -C5Me5)2YbL2
are 0 (L = THF),28 98 (L = 1/2DME)29 and 949 ppm (L = py).28
However, the 171Yb chemical shift of 8 is in the same region as
the shifts of related dialkylytterbium compounds; for example the
171Yb chemical shift of formally two-coordinate 1 is 812 ppm,3
whereas the chemical shift of the five-coordinate complex
{(Me3Si)(Me2MeOSi)C(SiMe2CH2)}2Yb(THF) is 1049 ppm13
and the chemical shift of the four-coordinate complex
{(Me3Si)2(Me2(EtOCH2CH2)Si)C}2Yb is 544 ppm.6
(
171Yb). Elemental analyses were obtained by the Elemental
Analysis Service of London Metropolitan University.
[(Me3Si)2{Me2P(BH3)}C]2Sm(THF) (5)
To a solution of SmI2(THF)2 (0.76 g, 1.38 mmol) in THF (15
ml) was added a solution of 4 (0.96 g, 3.54 mmol) in THF
(15 ml). The resulting solution was heated under reflux for 2 h.
Solvent was removed in vacuo and the sticky residue was extracted
into methylcyclohexane (15 ml) and filtered. The filtrate was
concentrated to ca 5 ml and was cooled to -30 ◦C for 2 days
to give purple-black crystals of the mono-THF adduct 5a as its
methylcyclohexane solvate. The solvent of crystallisation is rapidly
lost under vacuum and is not observed in the following analyses.
Isolated yield: 0.71 g, 75%. Anal. Calcd for C22H62B2OP2Si4Sm:
Although 8 appears to be indefinitely stable in benzene solution,
treatment of a crystalline sample of 8 with THF results in rapid
decomposition, again yielding the free phosphine-borane 7 as
the only identifiable product, suggesting that the failure of the
attempted synthesis of 8 in THF may be attributed to its instability
towards this solvent and its consequent decomposition immedi-
ately it is formed in the reaction. This behaviour is reminiscent
of that of the isoelectronic dialkylytterbium compound 1, which
reacts rapidly with THF to give (Me3Si)3CH as the only identifiable
product.3,6
1
11
C 38.35, H 9.07. Found C 38.27, H 9.11. H{ B} NMR (C6D6,
◦
25 C): d -1.87 (s, 6H, PMe2), 0.53 (s, 18H, SiMe3),◦4.13 (s, 4H,
There is often a strong correlation between the chemistries
of Yb(II) and Ca(II)30 [the ionic radii of Ca2+ and Yb2+
are 1.00 and 1.02 A, respectively]22 and the decomposi-
˚
1
THF), 14.10 (s, 4H, THF). 11B{ H} NMR (C6D6, 25 C): d -41.6
◦
1
(br s). 31P{ H} NMR (C6D6, 25 C): d -1.9 (br s).
Crystallisation of a second batch of 5, prepared by the same
route, from methylcyclohexane at -30 ◦C gave crystals of the
tris-THF adduct [(Me3Si)2{Me2P(BH3)}C]2Sm(THF)3 (5b) as a
methylcyclohexane solvate. The extra two molecules of THF and
the solvent of crystallisation are rapidly lost under vacuum and all
analyses of this material are identical with those of 5a.
tion chemistry of 8 has parallels with the behaviour of the
calcium analogue [(Me3Si)2{Me2P(BH3)}C]2Ca (9): although
this latter compound may be isolated as the THF adduct
[(Me3Si)2{Me2P(BH3)}C]2Ca(THF)4 (9a), it rapidly decomposes
in the presence of excess THF to give 7 as the sole phosphorus-
containing product.20e The corresponding strontium and barium
compounds [(Me3Si)2{Me2P(BH3)}C]2M(THF)5 [M = Sr, Ba] are
stable for long periods in THF solution and show no evidence for
decomposition. This parallels the behaviour of 5, which is prepared
in refluxing THF and is clearly stable towards this solvent, and
suggests that the stability of dialkyllanthanide(II) compounds is
dependent upon both the size of the metal ion and the steric
demands of the ligands.
In summary, we have isolated new alkyl complexes of samar-
ium(II) (5a/5b) and ytterbium(II) (8), the former of which may be
crystallised in two different modifications, which differ both in the
number of THF co-ligands bound to the metal centre and in the
binding mode of the alkyl ligands. While the ytterbium compound
8 is unstable towards THF, compound 5 is isolated as one of two
THF adducts, suggesting that the reactivity of these compounds
is subtly affected by the size of the metal ion.
[(Me3Si)2{Me2P(BH3)}C]2Yb (8)
To a suspension of YbI2 (0.46 g, 1.08 mmol) in benzene (20 ml)
was added a solution of 4 (0.59 g, 2.17 mmol) in warm benzene
(20 ml). The resulting mixture was heated under reflux for 16 h,
and then solvent was removed in vacuo. The sticky orange residue
was extracted into n-hexane (20 ml) and filtered.◦The filtrate was
concentrated to ca 5 ml and was cooled to -30 C for 3 days to
give 8 as poor quality orange crystals. Isolated yield: 0.39 g, 57%.
Anal. Calcd for C18H54B2P2Si4Yb:◦C 33.80, H 8.51. Found C 33.88,
11
H 8.47. 1H{ B} NMR (C6D6, 25 C): d 0.32 (s, 18H, SiMe3), 1.20
(d, 2JPH = 9.2 Hz, 6H, PMe2), 1.96 (d, 2JPH = 8.3 Hz, 3H, BH3).
◦
1
1
13C{ H} NMR (C6D6, 25 C): d 6.61 (SiMe3), 18.65 (d, JPC
=
=
◦
35.5 Hz, PMe). 11B{ H} NMR (C6D6, 25 C): d -24.8 (d, 1JPB
1
◦
85.6 Hz). 31P{ H} NMR (C6D6, 25 C): d -11.7 (q, JPB = 85.6
1
1
Hz). 171Yb NMR (C6D6, 25 ◦C): d 794 (s)
Experimental
Crystal structure determinations of 5a and 5b
All manipulations were carried out using standard Schlenk tech-
niques under an atmosphere of dry nitrogen. Methylcyclohexane,
n-hexane, THF, benzene and light petroleum (b.p. 40–60 ◦C) were
dried prior to use by distillation under nitrogen from sodium
or sodium/potassium alloy and were stored over a potassium
film. Deuterated benzene was distilled from potassium and was
Measurements were made at 150 K on an Oxford Diffraction
Gemini A Ultra diffractometer using graphite-monochromated
˚
Mo-Ka radiation (l = 0.71073 A). Cell parameters were refined
from the observed positions of all strong reflections. Intensi-
ties were corrected semi-empirically for absorption, based on
6708 | Dalton Trans., 2010, 39, 6705–6710
This journal is
The Royal Society of Chemistry 2010
©

Table 1 Crystallographic data for 5a and 5b
3 C. Eaborn, P. B. Hitchcock, K. Izod and J. D. Smith, J. Am. Chem.
Soc., 1994, 116, 12071.
4 (a) P. B. Hitchcock, S. A. Holmes, M. F. Lappert and S. Tian, J. Chem.
Soc., Chem. Commun., 1994, 2691; (b) J. van den Hende, P. B. Hitchcock,
S. A. Holmes, M. F. Lappert and S. Tian, J. Chem. Soc., Dalton Trans.,
1995, 3933.
5 L. Hasinoff, J. Takats, X. W. Zhang, A. H. Bond and R. D. Rogers,
J. Am. Chem. Soc., 1994, 116, 8833.
6 C. Eaborn, P. B. Hitchcock, K. Izod, Z.-R. Lu and J. D. Smith,
Organometallics, 1996, 15, 4783.
7 W. Clegg, C. Eaborn, K. Izod, P. O’Shaughnessy and J. D. Smith,
Angew. Chem., Int. Ed. Engl., 1997, 36, 2815.
8 S. Harder, Angew. Chem., Int. Ed., 2004, 43, 2714.
Compound
5a
5b
Formula
M
C22H62B2OP2Si4Sm
689.0
0.32 ¥ 0.20 ¥ 0.20
Monoclinic
P21/c
11.9950(2)
29.7059(5)
12.4072(2)
C30H78B2O3P2Si4Sm
833.2
Crystal size/mm
Crystal system
Space group
0.25 ¥ 0.20 ¥ 0.20
Triclinic
¯
P1
˚
a/A
13.7414(8)
13.7832(7)
15.9703(8)
66.697(5)
88.844(5)
70.297(5)
2592.9(2)
2
1.31
27058
10171
0.037
7815
419
0.028
0.062
0.921
0.45, -0.32
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
9 M. Niemeyer, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2001, 57,
m578.
10 G. Qi, Y. Nitto, A. Saiki, T. Tomohiro, Y. Nakamaya and H. Yasuda,
Tetrahedron, 2003, 59, 10409.
93.442(2)
3
˚
V/A
4412.99(13)
4
1.52
41414
7755
0.043
7166
Z
11 G. B. Deacon and C. M. Forsyth, Chem.–Eur. J., 2004, 10, 1798.
12 G. B. Deacon and C. M. Forsyth, Organometallics, 2003, 22, 1349.
13 L. J. Bowman, K. Izod, W. Clegg and R. W. Harrington,
Organometallics, 2007, 26, 2646.
14 G. Heckmann and M. Niemeyer, J. Am. Chem. Soc., 2000, 122, 4227.
15 M. N. Bochkarev, V. V. Khramenkov, Yu. F. Rad’kov, L. N. Zakharov
and Yu. T. Struchkov, J. Organomet. Chem., 1991, 421, 29.
16 M. N. Bochkarev, V. V. Khramenkov, Yu. F. Rad’kov, L. N. Zakharov
and Yu. T. Struchkov, J. Organomet. Chem., 1992, 429, 27.
17 G. B. Deacon and C. M. Forsyth, Organometallics, 2000, 19, 1713.
18 P. B. Hitchcock, A. V. Khvostov and M. F. Lappert, J. Organomet.
Chem., 2002, 663, 263.
m/mm-1
Data collected
Unique data
Rint
Data with F2>2s
Refined parameters
R (on F, F2>2s)
Rw (on F2, all data)
Goodness of fit on F2
305
0.076
0.177
1.351
1.23, -1.65
Min, max electron
-3
˚
density/e A
19 J. Cheng, J. Takats, M. J. Ferguson and R. McDonald, J. Am. Chem.
Soc., 2008, 130, 1544.
20 (a) M. S. Hill and P. B. Hitchcock, Dalton Trans., 2003, 4570; (b) M.
Wiecko, P. W. Roesky, V. V. Burlakov and A. Spannenberg, Eur. J. Inorg.
Chem., 2007, 876; (c) T. K. Panda, A. Zulys, M. T. Garner and P. W.
Roesky, J. Organomet. Chem., 2005, 690, 5078.
symmetry-equivalent and repeated reflections. The structures were
solved by direct methods and refined on F2 values for all unique
data; Table 1 gives further details. All non-hydrogen atoms were
refined anisotropically, and all H atoms in 5a and the C-bound H
atoms in 5b were constrained with a riding model, while B-bound
H atoms in 5b were freely refined; U(H) was set at 1.2 (1.5 for
methyl groups) times Ueq for the parent C atom. Difference maps
revealed methylcyclohexane molecules present in both structures,
but these could not be satisfactorily refined due to apparent
disorder and partial occupancy, so they were treated with the
SQUEEZE procedure of PLATON33 and omitted subsequently
from the refined structure models. These solvent molecules are
not included in the chemical formulae and derived properties,
but the unplaced BH3 atoms of 5a are included. Other programs
were Oxford Diffraction CrysAlisPro and SHELXTL for structure
solution, refinement, and molecular graphics.34 CCDC reference
numbers 763178 and 763179.†
21 (a) K. Izod, W. McFarlane, B. V. Tyson, W. Clegg and R. W. Harrington,
Chem. Commun., 2004, 570; (b) K. Izod, W. McFarlane, B. V. Tyson, I.
Carr, W. Clegg and R. W. Harrington, Organometallics, 2006, 25, 1135;
(c) K. Izod, C. Wills, W. Clegg and R. W. Harrington, Organometallics,
2006, 25, 38; (d) K. Izod, C. Wills, W. Clegg and R. W. Harrington,
Organometallics, 2006, 25, 5326; (e) K. Izod, C. Wills, W. Clegg and
R. W. Harrington, Inorg. Chem., 2007, 46, 4320; (f) K. Izod, C. Wills,
W. Clegg and R. W. Harrington, Organometallics, 2007, 26, 2861;
(g) K. Izod, C. Wills, W. Clegg and R. W. Harrington, Dalton Trans.,
2007, 3669; (h) K. Izod, C. Wills, W. Clegg and R. W. Harrington,
J. Organomet. Chem., 2007, 692, 5060; (i) K. Izod, W. McFarlane, C.
Wills, W. Clegg and R. W. Harrington, Organometallics, 2008, 27, 4386;
(j) K. Izod, C. Wills, W. Clegg and R. W. Harrington, Organometallics,
2009, 28, 2211; (k) K. Izod, L. J. Bowman, C. Wills, W. Clegg and R. W.
Harrington, Dalton Trans., 2009, 3340; (l) K. Izod, C. Wills, W. Clegg
and R. W. Harrington, Organometallics, 2009, 28, 5661.
22 R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor.
Gen. Crystallogr., 1976, 32, 751.
23 Y. K. Gun’ko, B. M. Bulychev, G. L. Soloveichik and V. K. Belsky,
J. Organomet. Chem., 1992, 424, 289.
24 H. Schumann, M. Keitsch, J. Demtschuk and S. Muhle, Z. Anorg. Allg.
Chem., 1998, 624, 1811.
Acknowledgements
25 F. Bonnet, M. Visseaux, D. Barbier-Baudry, A. Hafid, E. Vigier and
M. M. Kubicki, Inorg. Chem., 2004, 43, 3682.
The authors are grateful to the EPSRC for support.
26 X. Chen, S. Lim, C. E. Plecˇnik, S. Liu, B. Du, E. A. Myers and S. G.
Shore, Inorg. Chem., 2004, 43, 692.
27 X. Chen, S. Lim, C. E. Plecˇnik, S. Liu, B. Du, E. A. Myers and S. G.
Shore, Inorg. Chem., 2005, 44, 6052.
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The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6705–6710 | 6709
©