The synthesis of half-sandwich bis(pentafluorophenyl)boryl-substituted cyclopentadienyl zirconium, niobium and tantalum complexes and the isolation and molecular structure of a zwitterionic niobocene

Article abstract of DOI:10.1039/b300552f

Reaction of B(C₅H₄SiMe₃)(C ₆F₅)₂ with MCl₅ (M = Nb, Ta) leads to the first group 5 borylcyclopentadienyl half-sandwich complexes MCl ₄{C₅H₄B(C₆F₅) ₂} (1 and 2). In contrast, the reaction with ZrCl₄ gives the metallocene ZrCl₂{C₅H₄B(C₆F ₅)₂}₂ (3). The use of ZrCl₄(SMe ₂)₂ instead of ZrCl₄ as starting material allows the isolation of the monocyclopentadienyl zirconium complex ZrCl ₃{C₅H₄B(C₆F₅) ₂(SMe₂)} (4). The utility of LiCp′ as a general route to zirconocenes is demonstrated by the synthesis of ZrCl ₂(Cp){C₅H₄B (C₆F₅) ₂(SMe₂)} (5) and ZrCl₂(Ind){C₅H ₄B(C₆F₅)₂-(SMe₂)} (6) (Ind = η⁵-indenyl). Boron-substituted niobocenes are prepared through the dehalostannylation reaction between the half-sandwich complexes and tin-substituted cyclopentadienes. They adopt zwitterionic structures in which a chloride ligand is transferred to boron, for example Nb⁽⁺⁾Cl ₂(C₅H₄SiMe₃){C₅H ₄B^(-)(Cl)(C₆F₅)₂} (9). The crystal structure of 9 has been determined by X-ray crystallography. Reaction of the strong base, pyridine, with the borylcyclopentadienyl complexes TiCl₃{C₅H₄B(C₆F₅) ₂}, TiCl₂(Cp){C₅H₄B(C ₆F₅)₂} and 1-6 leads to the generation of a series of pyridine adducts (10-17) in which the pyridine is bound to boron. The solid-state structures of the four coordinate adducts TiCl₂(Cp) {C₅H₄B(C₆F₅)₂(py)} (10) and TiCl₃{C₅H₄B(C₆F ₅)₂(py)} (14) are described. The half-sandwich zirconium and niobium complexes 15 and 16 are shown by spectroscopic and structural methods to coordinate a further one (Nb) or two (Zr) equivalents of pyridine to attain an octahedral geometry at the metal centre. The zwitterionic complexes 8 and 9 do not react with pyridine. The Royal Society of Chemistry 2003.

Full text of DOI:10.1039/b300552f

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The synthesis of half-sandwich bis(pentafluorophenyl)boryl-
substituted cyclopentadienyl zirconium, niobium and tantalum
complexes and the isolation and molecular structure of a
zwitterionic niobocene
Simon J. Lancaster* and David L. Hughes
Wolfson Materials and Catalysis Centre, School of Chemical Sciences and Pharmacy,
University of East Anglia, Norwich, UK NR4 7TJ. E-mail: S.Lancaster@uea.ac.uk
Received 14th January 2003, Accepted 24th March 2003
First published as an Advance Article on the web 8th April 2003
Reaction of B(C5H4SiMe3)(C6F5)2 with MCl5 (M = Nb, Ta) leads to the first group 5 borylcyclopentadienyl half-
sandwich complexes MCl4{C5H4B(C6F5)2} (1 and 2). In contrast, the reaction with ZrCl4 gives the metallocene
ZrCl2{C5H4B(C6F5)2}2 (3). The use of ZrCl4(SMe2)2 instead of ZrCl4 as starting material allows the isolation of the
monocyclopentadienyl zirconium complex ZrCl3{C5H4B(C6F5)2(SMe2)} (4). The utility of LiCpЈ as a general route to
zirconocenes is demonstrated by the synthesis of ZrCl2(Cp){C5H4B (C6F5)2(SMe2)} (5) and ZrCl2(Ind){C5H4B(C6F5)2-
(SMe2)} (6) (Ind = η5-indenyl). Boron-substituted niobocenes are prepared through the dehalostannylation reaction
between the half-sandwich complexes and tin-substituted cyclopentadienes. They adopt zwitterionic structures in
which a chloride ligand is transferred to boron, for example Nb(ϩ)Cl2(C5H4SiMe3){C5H4B(Ϫ)(Cl)(C6F5)2} (9). The
crystal structure of 9 has been determined by X-ray crystallography. Reaction of the strong base, pyridine, with the
borylcyclopentadienyl complexes TiCl3{C5H4B(C6F5)2}, TiCl2(Cp){C5H4B(C6F5)2} and 1–6 leads to the generation
of a series of pyridine adducts (10–17) in which the pyridine is bound to boron. The solid-state structures of the
four coordinate adducts TiCl2(Cp){C5H4B(C6F5)2(py)} (10) and TiCl3{C5H4B(C6F5)2(py)} (14) are described. The
half-sandwich zirconium and niobium complexes 15 and 16 are shown by spectroscopic and structural methods to
coordinate a further one (Nb) or two (Zr) equivalents of pyridine to attain an octahedral geometry at the metal
centre. The zwitterionic complexes 8 and 9 do not react with pyridine.
Other metallocene complexes with Lewis acidic substituents
Introduction
have been made by the hydroboration of allyl-Cp complexes
Early transition metal complexes with boron Lewis acid-
substituted cyclopentadienyl ligands are of interest because
of their unique structural and chemical properties. The first
boryl half-sandwich titanium complexes TiCl3{C5H3RBX2}
with HB(C6F5)2.6,7 A number of related boron-bridged ansa-
titanocenes and ansa-zirconocenes have been reported where
the Lewis acidity is attenuated by a π-bonding substituent
(IV)8–10 or coordination of a base, L (V).11,12 There are also
(R = H, Me; X = Cl, Br, OEt, Me) (Structure I, Chart 1) were
examples of anionic borato-substituted complexes, introduced
prepared in 1979 by a dehalosilylation route.1 More recently
as borato-substituted cyclopentadienyl ligands13,14 or formed
a similar approach was used to prepare TiCl3{C5H4BX2}
through the electrophilic substitution reaction of a metallocene
(X = C6H5, C6F5).2,3 A dehalosilylation route was also employed
complex (VI).15–17 Most recently borato-bridged ansa-zircono-
by Reetz to synthesise a series of boryl-substituted zircono-
cenes ZrCl2{C5H4BX2}2 (II) and ZrCl2(Cp){C5H4BX2} (X =
cenes of type (VII) have been prepared.18,19
Cyclopentadienyl complexes with Lewis acidic bis(penta-
Me, Et, OEt, C6F5).4 We reported the first synthesis of
fluorophenyl)boryl-substituents were initially of interest as self-
boryl-titanocenes, TiCl2(CpЈ){C5H4B(C6F5)2} (CpЈ = C5H5
(III), C9H7, C5H4SiMe3), by reacting TiCl3{C5H4B(C6F5)2} with
LiCpЈ.5
activating alkene polymerisation catalysts.3,4 However, both
boron-based Lewis acids20 and early transition metal cyclo-
pentadienyl complexes are employed in an ever-increasing array
of synthetic and catalytic reactions. There is increasing interest
in using a combination of organometallic compounds with
Lewis acids.21 The attraction of borylcyclopentadienyl com-
plexes stems from the intramolecular combination of metal and
Lewis acid reactivity. One of the goals of our research is to
utilise interactions between groups bound to the metal centre
and the boron substituent. We have demonstrated that intra-
molecular Ti–Cl activation takes place in boryl-substituted
titanocene complexes.5 The object of the present study is to
learn more about the prevalence of these interactions and how
they depend upon both electronic (Lewis acidity of boron and
metal) and steric (interligand distances and angles) parameters,
in particular whether such interactions are possible for com-
plexes other than titanocenes.
Here we present the synthesis of the first half-sandwich and
metallocene borylcyclopentadienyl complexes of niobium,
tantalum and zirconium. A general method for the prepar-
ation of borylcyclopentadienyl zirconocenes and the first
niobocenes are described. The reactivity of borylcyclopenta-
dienyl complexes towards Lewis bases is explored and a
Chart 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 1 7 7 9 – 1 7 8 9
1779

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number of pyridine adducts have been isolated and structurally
characterised.
facilitated the preparation of the representative zirconocene
complexes ZrCl2(Cp){C5H4B(C6F5)2(SMe2)} (5) and ZrCl2-
(Ind){C5H4B(C6F5)2(SMe2)} (6) (Scheme 2). The presence of
SMe2 in the 1H NMR spectra indicates that the complexes crys-
tallise as the SMe2 adducts. However, 11B NMR (CDCl3, 20 ЊC)
chemical shifts at δ 45.0 and 39.7 for 5 and 6 respectively lie
between the regions expected for three- and four-coordination
and indicate partial dissociation in solution. Attempts to obtain
a SMe2-free sample by removing the SMe2 under high vacuum
or by repeated dissolution/solvent removal were not successful.
The use of B(C6F5)3 as a scavenger for SMe2 did lead to the
formation of B(C6F5)3ؒSMe2 (7), which was confirmed by the
preparation of an authentic sample (11B NMR: δ 0.83). How-
ever, this approach, too, was unsatisfactory due to the similar
solubility of 7 and the desired product.
Results and discussion
Synthesis and characterisation of new borylcyclopentadienyl
complexes
We have previously reported that the reaction of B(C5H4-
SiMe3)(C6F5)2 with one equivalent of TiCl4 leads to dehalosilyl-
ation and the formation of TiCl3{C5H4B(C6F5)2}.3 The reaction
with NbCl5 or TaCl5 (Scheme 1) in dichloromethane similarly
leads to the orange–red NbCl4{C5H4B(C6F5)2} (1) and yellow
TaCl4{C5H4B(C6F5)2} (2). These complexes are isolated follow-
ing precipitation from the reaction medium and are sparingly
soluble in hydrocarbon and halocarbon solvents. Solution
NMR studies required solvation in THF-d8. Spectroscopic data
of all new compounds are collected in Table 1. The 11B NMR
signals are observed at δ 2.7 and 4.2 for 1 and 2 respectively,
which is consistent with 4-coordinate boron following the
formation of adducts with THF-d8.
Scheme 2
Unfortunately crystals suitable for X-ray diffraction were not
obtained but the structure of these zirconocenes (Scheme 2) is
clearly significantly different from that of the related titano-
cenes. It appears that the SMe2 coordinated to boron is symp-
tomatic of a reduced tendency to form Cp–B–Cl–M linkages of
the sort exhibited by TiCl2(Cp){C5H4B(C6F5)2} (III). Indeed III
does not form a SMe2 adduct and the 1H and 11B NMR spectra
remain largely unchanged on addition of several equivalents to
a C6D6 solution.
Scheme 1
Since the Cp–B–Cl–M interaction is not favourable in the
boryl zirconocene dichlorides24 we were interested in establish-
ing whether such bridging chloride interactions were indeed
unique to the boryl titanocenes. Having successfully prepared
half-sandwich borylcyclopentadienyl complexes of niobium
and tantalum, the corresponding metallocenes were an obvious
target. It is well established that the reaction of monocyclo-
pentadienyl complexes with LiCp, although successful for
the group 4 metal complexes, is not suitable as a method of
preparing niobium(V) metallocenes.25
The reaction with ZrCl4 is not selective for the half-sandwich
compound and leads to consecutive dehalosilylation reactions
and formation of ZrCl2{C5H4B(C6F5)2}2 (3). Although this
compound has been described by Reetz,4 full multinuclear
NMR data were not given and are presented here in order to
provide a comparison to the novel compounds. Purification of 3
proved difficult and a spectroscopically pure sample could only
be obtained by recrystallisation from diethyl ether. No 11B
NMR signal for 3 was apparent at 20 ЊC in CDCl3. At 80 ЊC in
toluene-d8 a broad peak at δ 60 was observed.
However, treatment
of a dichloromethane suspension of 1, with the non-reducing
SnMe3
(C H ) (Scheme 3) gave a clear solution from which 8
5
5
Dimethylsulfide has been demonstrated to modify the
reactivity of ZrCl4 towards dehalosilylation allowing the prep-
aration of ZrCl3(Cp)ؒL complexes.22 The reaction of ZrCl4-
(SMe2)2 with B(C5H4SiMe3)(C6F5)2 leads to the isolation of
ZrCl3{C5H4B(SMe2)(C6F5)2}(SMe2) (4). It appears that 4 slowly
1
loses SMe2 under vacuum, and integration of the H NMR
spectrum and elemental analysis indicates between 1 and 2
equivalents. Only one SMe2 resonance is observed at 20 ЊC,
implying rapid exchange between free SMe2 and SMe2 coordin-
ated to the boron and zirconium Lewis acids. The 11B NMR
resonance is observed at δ 45, which is intermediate between
three- and four-coordinate boron, again suggesting a rapid
exchange.
Reetz prepared ZrCl2(Cp){C5H4B(C6F5)2} by the dehalosilyl-
ation reaction between ZrCl3Cp and B(C5H4SiMe3)(C6F5)2. The
application of this approach is restricted by the reduced ten-
dency of substituted monocyclopentadienyl zirconium trichlor-
ide complexes to undergo dehalosilylation and the lack of
accessibility of the base-free trichlorides.23 Reaction of the half-
sandwich complex 4 with LiCpЈ (CpЈ = Cp or Ind) in a fashion
analogous to that employed for boryl-substituted titanocenes
Scheme 3
D a l t o n T r a n s . , 2 0 0 3 , 1 7 7 9 – 1 7 8 9
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Table 1 NMR data
Complex
NMR data
NbCl4{C5H4B(C6F5)2} (1)
TaCl4{C5H4B(C6F5)2} (2)
ZrCl2{C5H4B(C6F5)2}2 (3)
ZrCl3{C5H4B(C6F5)2(SMe2)} (4)
δH (300 MHz, THF-d8, 20 ЊC) 7.72 (br, 2H, C5H4), 7.30 (br, 2H, C5H4)
δC (75.47 MHz; THF-d8, 20 ЊC) 139.09 (C5H4), 129.62 (C5H4)
δB (96.29 MHz, THF-d8, 20 ЊC) 2.7
δF (282.40 MHz, THF-d8, 20 ЊC) Ϫ131.64 (br, 4F, o-F), Ϫ158.61 (d, 2F, J(FF) 20 Hz, p-F),
Ϫ165.32 (t, 4F, J(FF) 18 Hz, m-F)
δH (300 MHz, THF-d8, 20 ЊC) 7.34 (br, 2H, C5H4), 6.90 (tr, 2H, J 2.5 Hz, C5H4)
δC (75.47 MHz; THF-d8; 20 ЊC) 134.85 (C5H4), 125.53 (C5H4)
δB (96.29 MHz, THF-d8, 20 ЊC) 4.2
δF (282.40 MHz, THF-d8, 20 ЊC) Ϫ133.45 (t, 4F, J(FF) 19 Hz, o-F), Ϫ160.35 (d, 2F, J(FF)
20 Hz, p-F), Ϫ167.20 (t, 4F, J(FF) 21 Hz, m-F)
δH (300 MHz, CDCl3, 20 ЊC) 6.40 (br, 4H, C5H4), 6.08 (tr, 2H, J 2.6 Hz, C5H4)
δC (75.47 MHz; CDCl3; 20 ЊC) 126.93 (C5H4), 121.29 (C5H4)
δB (96.29 MHz, CDCl3, 20 ЊC) not observed
δF (282.40 MHz, CDCl3, 20 ЊC) Ϫ128.17 (d, 4F, J(FF) 20 Hz, o-F), Ϫ148.07 (t, 2F, J(FF)
21 Hz, p-F), Ϫ160.69 (d, 4F, J(FF) 20 Hz, m-F)
δH (300 MHz, CDCl3, 20 ЊC) 6.91 (br, 2H, C5H4), 6.72 (tr, 2H, J 2.6 Hz, C5H4), 2.31 (s, 6H,
S(CH3)2)
δC (75.47 MHz; CDCl3; 20 ЊC) 126.69 (C5H4), 122.33 (C5H4), 21.05 (S(CH3)2)
δB (96.29 MHz, CDCl3, 20 ЊC) not observed
δF (282.40 MHz, CDCl3, 20 ЊC) Ϫ128.4 (br, 4F, o-F), Ϫ153.86 (br, 2F, p-F), Ϫ162.25 (br, 4F,
m-F)
ZrCl2(Cp){C5H4B(C6F5)2(SMe2)} (5)
ZrCl2(Ind){C5H4B(C6F5)2(SMe2)} (6)
δH (300 MHz, CDCl3, 20 ЊC) 6.92 (br, 2H, C5H4), 6.74 (tr, 2H, J 2.6 Hz, C5H4), 6.50 (s, 5H,
Cp), 2.14 (s, 6H, S(CH3)2)
δC (75.47 MHz; CDCl3; 20 ЊC) 125.50 (C5H4), 120.70 (C5H4), 116.92 (Cp), 18.90 (S(CH3)2)
δB (96.29 MHz, CDCl3, 20 ЊC) 45.0
δF (282.40 MHz, CDCl3, 20 ЊC) Ϫ129.47 (d, 4F, J(FF) 23 Hz, o-F), Ϫ156.9 (t, 2F, J(FF) 20 Hz,
p-F), Ϫ163.5 (d, 4F, J(FF) 21 Hz, m-F)
δH (300 MHz, CDCl3, 20 ЊC) 7.66–7.63 (m, 2H, Ind), 7.35–7.32 (m, 2H, Ind) 6.80 (tr, 1H,
J 3.4 Hz, Ind) 6.67 (br, 2H, C5H4), 6.51 (d, 2H, J 3.4 Hz, Ind) 6.32 (tr, 2H, J 2.5 Hz C5H4), 2.16
(s, 6H, S(CH3)2)
δC (75.47 MHz; CDCl3; 20 ЊC 126.93 (Ind), 125.44, 124.80 (C5H4), 122.02 (Ind), 119.79 (C5H4),
103.80 (Ind), 19.08 (S(CH3)2)
δB (96.29 MHz, CDCl3, 20 ЊC) 39.7
δF (282.40 MHz, CDCl3, 20 ЊC) Ϫ127.74 (d, 4F, J(FF) 22 Hz, o-F), Ϫ151.66 (t, 2F, J(FF)
21 Hz, p-F), Ϫ161.41 (d, 4F, J(FF) 21 Hz, m-F)
B(C6F5)3ؒSMe2 (7)
δH (300 MHz, CDCl3, 20 ЊC) 2.17
δC (75.47 MHz; CDCl3; 20 ЊC) 20.78
δB (96.29 MHz, CDCl3, 20 ЊC) 0.83
δF (282.40 MHz, CDCl3, 20 ЊC) Ϫ130.15 (d, 6F, J(FF) 20 Hz, o-F), Ϫ154.41 (t, 3F, J(FF)
20 Hz, p-F), Ϫ162.45 (d, 6F, J(FF) 20 Hz, m-F)
Nb(ϩ)Cl2(Cp){C5H4B(Ϫ)(Cl)(C6F5)2} (8)
δH (300 MHz, CDCl3, 20 ЊC) 7.59 (br, 2H, C5H4), 7.04 (s, 5H, Cp), 6.76 (br, 2H, C5H4)
δC (75.47 MHz; CDCl3; 20 ЊC) 147.52 (d, J(CF) 235 Hz, o-C, C6F5), 139.74 (d, J(CF) 256 Hz,
p-C, C6F5), 137.67 (d, J(CF) 254 Hz, m-C, C6F5), 130.96 (C5H4), 126.75 (C5H4), 123.10 (Cp).
δB (96.29 MHz, CDCl3, 20 ЊC) Ϫ2.9
δF (282.40 MHz, CDCl3, 20 ЊC) Ϫ131.4 (br, 4F, o-F), Ϫ158.9 (br, 2F, p-F), Ϫ164.0 (br, 4F,
m-F)
Nb(ϩ)Cl2(C5H4SiMe3){C5H4B(Ϫ)(Cl)(C6F5)2} (9)
δH (300 MHz, CDCl3, 20 ЊC) 7.50 (m, 4H, C5H4B & C5H4Si), 6.95 (br, 2H, C5H4Si), 6.80
(br, 2H, C5H4B), 0.35 (s, 9H, Si(CH3)3)
δC (75.47 MHz; CDCl3; 20 ЊC) 136.13 (C5H4Si), 129.74 (C5H4Si), Ϫ1.11 (Si(CH3)3)b
δB (96.29 MHz, CDCl3, 20 ЊC) Ϫ2.9
δF (282.40 MHz, CDCl3, 20 ЊC) Ϫ131.39 (d, 4F, J(FF) 17 Hz, o-F), Ϫ159.20 (tr, 2F, J(FF)
20 Hz, p-F), Ϫ164.34 (tr, 4F, J(FF) 23 Hz, m-F)
TiCl2(Cp){C5H4B(C6F5)2(py)} (10)
δH (300 MHz, CD2Cl2, 20 ЊC) 9.00 (d, 2H, J 5.6 Hz, o-H, C5H5N), 8.19 (tr, 1H, J 7.7 Hz, p-H,
C5H5N), 7.71 (tr, 2H, J 7.2 Hz, m-H, C5H5N), 6.51 (tr,2H J 2.6 Hz, C5H4), 6.46 (s, 5H, Cp),
6.36 (br, 2H, C5H4)
δC (75.47 MHz; CD2Cl2; 20 ЊC) 147.86 o-C, C5H5N), 142. 77 (p-C, C5H5N), 127.83 (C5H4),
126.26 (m-C, C5H5N), 120.34 (Cp), 119.00 (C5H4)
δB (96.29 MHz, CD2Cl2, 20 ЊC) 1.0
δF (282.40 MHz, CD2Cl2, 20 ЊC) Ϫ129.59 (d, 4F, J(FF) 23 Hz, o-F), Ϫ158.20 (t, 2F, J(FF)
20 Hz, p-F), Ϫ164.74 (d, 4F, J(FF) 20 Hz, m-F)
D a l t o n T r a n s . , 2 0 0 3 , 1 7 7 9 – 1 7 8 9
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Table 1 (Contd.)
Complex
NMR data
ZrCl2(Cp){C5H4B(C6F5)2(py)} (11)
δH (300 MHz, CDCl3, 20 ЊC) 8.78 (d, 2H, J 5.9 Hz, o-H, C5H5N), 8.18 (tr, 1H, J 7.7 Hz, p-H,
C5H5N), 7.68 (tr, 2H, J 7.2 Hz, m-H, C5H5N), 6.49 (s, 5H, Cp), 6.38 (tr, 2H, J 2.6 Hz, C5H4),
6.21 (br, 2H, C5H4)
δC (75.47 MHz; CDCl3; 20 ЊC) 147.86 o-C, C5H5N), 142.07 (p-C, C5H5N), 125.8 (m-C,
C5H5N), 122.52 (C5H4)), 116.31 (C5H4), 115.99 (Cp)
δB (96.29 MHz, CDCl3, 20 ЊC) 1.3
δF (282.40 MHz, CDCl3, 20 ЊC) Ϫ129.47 (d, 4F, J(FF) 23 Hz, o-F), Ϫ156.9 (t, 2F, J(FF) 20 Hz,
p-F), Ϫ163.5 (d, 4F, J(FF) 21 Hz, m-F)
ZrCl2(Ind){C5H4B(C6F5)2(py)} (12)
δH (300 MHz, CDCl3, 20 ЊC) 8.77 (d, 2H, J 5.7 Hz, o-H, C5H5N), 8.15 (tr, 1H, J 7.7 Hz, p-H,
C5H5N), 7.65 (tr, 2H, J 7.2 Hz, m-H, C5H5N), 7.64–7.60 (m, 2H, Ind), 7.30–7.25 (m, 2H, Ind),
7.19 (br, 2H, C5H4), 6.81 (tr, 1H, J 3.4 Hz, Ind), 6.42 (d, 2H, J 3.4 Hz, Ind) 6.08 (br, 2H, C5H4)
δC (75.47 MHz; CDCl3; 20 ЊC) 147.85 o-C, C5H5N), 142.09 (p-C, C5H5N), 129.03 (C5H4B),
126.31 (Ind), 124.50 (m-C, C5H5N), 124.34 (Ind), 121.62 (Ind) 116.79 (C5H4B), 103.16 (Ind)
δB (96.29 MHz, CDCl3, 20 ЊC) 1.4
δF (282.40 MHz, CDCl3, 20 ЊC) Ϫ129.30 (d, 4F, J(FF) 24 Hz, o-F), Ϫ156.71 (t, 2F, J(FF)
21Hz, p-F), Ϫ163.46 (d, 4F, J(FF) 21 Hz, m-F)
ZrCl2{C5H4B(C6F5)2(py)} (13)
δH (300 MHz, CDCl3, 20 ЊC) 8.78 (d, 2H, J 5.9 Hz, o-H, C5H5N), 8.17 (tr, 1H, J 7.7 Hz, p-H,
C5H5N), 7.68 (tr, 2H, J 7.2 Hz, m-H, C5H5N), 6.39 (tr, 2H, J 2.6 Hz, C5H4), 6.21 (br, 2H, C5H4)
δC (75.47 MHz; CDCl3; 20 ЊC) 147.85 o-C, C5H5N), 142.07 (p-C, C5H5N), 125.8 (m-C,
C5H5N), 122.56 (C5H4), 116.32 (C5H4)
δB (96.29 MHz, CDCl3, 20 ЊC) 1.2
δF (282.40 MHz, CDCl3, 20 ЊC) Ϫ129.50 (d, 4F, J(FF) 23 Hz, o-F), Ϫ156.9 (t, 2F, J(FF) 20 Hz,
p-F), Ϫ163.58 (d, 4F, J(FF) 21 Hz, m-F)
TiCl3{C5H4B(C6F5)2(py)} (14)
δH (300 MHz, CD2Cl2, 20 ЊC) 8.74 (d, 2H, J 5.8 Hz, o-H, C5H5N), 8.30 (tr, 1H, J 7.7 Hz, p-H,
C5H5N), 7.79 (tr, 2H, J 7.7 Hz, m-H, C5H5N), 7.12 (tr, 2H, J 2.2 Hz, C5H4), 6.97 (tr, 2H,
J 2.2 Hz, C5H4)
δC (75.47 MHz; CD2Cl2; 20 ЊC) 147.65 o-C, C5H5N), 143.69 (p-C, C5H5N), 129.87 (C5H4),
127.71 (C5H4), 126.80 (m-C, C5H5N)
δB (96.29 MHz, CD2Cl2, 20 ЊC) 0.80
δF (282.40 MHz, CD2Cl2, 20 ЊC) Ϫ127.85 (d, 4F, J(FF) 22 Hz, o-F), Ϫ150.65 (t, 2F, J(FF)
20 Hz, p-F), Ϫ161.03 (d, 4F, J(FF) 23 Hz, m-F)
ZrCl3{C5H4B(C6F5)2(py)}ؒpy2 (15)
δH (300 MHz, CDCl3, 50 ЊC) 9.15 (d, 2H, J 4.7 Hz, o-H, C5H5N–B), 8.94 (d, 4H, J 4.5 Hz, o-H,
C5H5N–Zr), 8.20 (tr, 1H, J 7.3 Hz, p-H, C5H5N–B), 7.75 (br, 4H, p-H, C5H5N–Zr, m-H,
C5H5N–B), 7.30–7.24 (m, 4H, m-H, C5H5N–Zr), 6.59 (tr, 2H, J 2.4 Hz, C5H4), 6.38 (br, 2H,
C5H4)
δC (75.47 MHz; CDCl3; 50 ЊC) 151.42 o-C, C5H5N–Zr) 148.63 o-C, C5H5N–B), 142.1 (p-C,
C5H5N–B), 137.99 (p-C, C5H5N–Zr), 127.0 (C5H4), 125.66 (m-C, C5H5N–B), 123.84 (m-C,
C5H5N–Zr), 121.04 (C5H4)
δB (96.29 MHz, CDCl3, 20 ЊC) 1.6
δF (282.40 MHz, CDCl3, 50 ЊC) Ϫ129.99 (d, 4F, J(FF) 21 Hz, o-F), Ϫ157.67 (t, 2F, J(FF)
21 Hz, p-F), Ϫ163.90 (d, 4F, J(FF) 21 Hz, m-F)
NbCl4{C5H4B(C6F5)2(py)}ؒpy (16)
δH (300 MHz, THF-d8, 20 ЊC) 9.56 (br, 2H, o-H, C5H5N–Nb), 9.09 (br, 2H, o-H, C5H5N–B),
8.44 (tr, 1H, J 7.6 Hz, p-H, C5H5N–B), 7.93–7.85 (m, 3H, p-H, C5H5N–Nb, m-H, C5H5N–B),
7.46 (br, 2H, m-H, C5H5N–Nb), 7.20 (br, 2H, C5H4), 6.98 (br, 2H, C5H4)
δC (75.47 MHz; THF-d8, 20 ЊC) 150.85 br, o-C, C5H5N–Nb) 149.62 o-C, C5H5N–B), 144.81 (p-
C, C5H5N–B), 139.47 (br, p-C, C5H5N–Nb), 129.01 (br, C5H4), 127.50 (m-C, C5H5N–B),
123.88 (br, m-C, C5H5N–Nb), 122 (v br, C5H4)
δB (96.29 MHz, THF-d8, 20 ЊC) 1.0
δF (282.40 MHz, THF-d8, 20 ЊC) Ϫ129.72 (br, 4F, o-F), Ϫ159.12 (t, 2F, J(FF) 24 Hz, p-F),
Ϫ165.61 (t, 4F, J(FF) 20 Hz, m-F)
TaCl4{C5H4B(C6F5)2(py)}ؒpy (17)
δH (300 MHz, THF-d8, 20 ЊC) 9.61 (v br, 2H, o-H, C5H5N–Ta), 9.09 (d, 2H, J 7.7 Hz, o-H,
C5H5N–B), 8.45 (tr, 1H, J 7.7 Hz, p-H, C5H5N–B), 7.97 (br, 1H, p-H, C5H5N–Ta), 7.87 (tr, 2H,
J 7.0 Hz, m-H, C5H5N–B), 7.51 (br, 2H, m-H, C5H5N–Ta), 6.82 (tr, 2H, J 2.1 Hz C5H4), 6.98
(tr, 2H, J 2.4 Hz, C5H4)
δC (75.47 MHz; THF-d8, 20 ЊC) 149.65 o-C, C5H5N–B), 144.74 (p-C, C5H5N–B), 131.59
(C5H4), 129.68 (m-C, C5H5N–B), 124.76(C5H4)b
δB (96.29 MHz, THF-d8, 20 ЊC) 0.6
δF (282.40 MHz, THF-d8, 20 ЊC) Ϫ129.75 (d, 4F, J(FF) 23 Hz, o-F), Ϫ159.33 (t, 2F, J(FF)
20 Hz, p-F), Ϫ165.71 (t, 4F, J(FF) 24 Hz, m-F)
a The 1H, 13C and 11B NMR data of complex 3 were first published in ref. 4 but are included here for comparison. b Other resonances not observed.
was isolated as a red oil, which solidified on drying to give a
glass. Although this material could not be induced to crystal-
lise, satisfactory elemental analyses were obtained, and the
complex could be characterised by multinuclear NMR. The 1H
NMR spectrum consists of a single resonance for the Cp ligand
and broad peaks integrating to 2 protons for the 2,5 and 3,4
positions of the boron-substituted cyclopentadienyl ligand,
confirming the expected composition. The more informative
indicator in determining whether there is any B–Cl interaction
is the 11B chemical shift. At δ Ϫ2.9 this is clearly in the region
consistent with four-coordinate boron, at even higher field than
the 11B resonance of III (δ 7.0).
D a l t o n T r a n s . , 2 0 0 3 , 1 7 7 9 – 1 7 8 9
1782

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In order to unequivocally demonstrate the presence and
nature of a B–Cl interaction a molecular structure determin-
ation was required. Since crystallisation of 8 proved difficult we
prepared the trimethylsilyl substituted analogue, 9, from 1 and
(SnMe3)(SiMe3)C5H4. Complex 9 exhibits the expected 1H
NMR spectrum for a metallocene with two different mono-
substituted cyclopentadienyl rings and the 11B chemical shift is
identical to that for 8, indicating the same basic structure. X-ray
quality crystals were grown from toluene solution. The bonding
and geometry in 9 is discussed in detail later but the crystallo-
graphic study reveals a zwitterionic structure, Nb(ϩ)Cl2(C5H4-
SiMe3){(C5H4)B(Ϫ)(Cl)(C6F5)2}, where one chloride ion has
been abstracted to give a formally anionic –B(Cl)(C6F5)2 substi-
tuent and a cationic niobium centre, Fig. 1. In the light of the
strong spectroscopic similarities, 8 is therefore formulated as
Nb(ϩ)Cl2(Cp){(C5H4)B(Ϫ)(Cl)(C6F5)2} (Scheme 3).
Scheme 4
Fig. 2 View of a molecule of TiCl2(Cp){C5H4B(C6F5)2(py)} (10).
Fig. 1 View of a molecule of Nb(ϩ)Cl2(C5H4SiMe3){C5H4B(Ϫ)(Cl)-
(C6F5)2} (9), indicating the atom numbering scheme. In all the figures,
atoms labelled ‘n’ represent the carbon atoms C(n); hydrogen atoms are
omitted for clarity; and thermal ellipsoids are drawn at the 50%
probability level.
the X-ray structure of TiCl2(Cp){C5H4B(C6F5)2(py)} (10),
Fig. 2.
The high basicity and modest steric hindrance of the pyridine
ligand coupled with good crystallinity and stability of the
resulting complexes encouraged us to prepare a series of pyrid-
ine adducts of the borylcyclopentadienyl complexes. Not sur-
prisingly, pyridine displaces SMe2 in the zirconocenes 5 and 6 to
give the pyridine adducts ZrCl2(Cp){C5H4B(C6F5)2(py)} (11)
and ZrCl2(Ind){C5H4B(C6F5)2(py)} (12). While 11 was obtained
as pale yellow crystals, 12 separates from solution as a dark
yellow oil and as a consequence the elemental analysis result is
relatively poor. Both Lewis acidic boron sites in 3 accept a pyri-
dine donor to give ZrCl2{C5H4B(C6F5)2(py)}2 (13). In each of
the complexes 10–13, pyridine coordination is obvious from the
downfield shifts in the 1H NMR spectra of the pyridine ligand
and the 11B chemical shifts which are remarkably similar and
occur within a narrow range from δ 1.0 to 1.4.
One can envisage the zwitterionic niobocene 9 reacting with a
Lewis base either to reverse the zwitterion formation and form
a –B(C6F5)2(py) adduct, or with coordination to the cationic
niobium. In practice neither reaction occurs, and 9 can be
recrystallised unchanged from a toluene solution containing an
excess of pyridine.27
The half-sandwich complex, TiCl3{C5H4B(C6F5)2}, reacts
with an excess of pyridine to give exclusively the mono-adduct,
TiCl3{C5H4B(C6F5)2(py)} (14). The coordination of pyridine to
the boron centre was indicated by a 11B chemical shift of 0.8
versus 59.8 for the free compound. The presence of only one
Having found that boryl niobocenes were capable of quanti-
tative self-ionisation, we wished to determine whether a similar
reaction could be promoted for TiCl2(Cp){C5H4B(C6F5)2} (III,
Chart 1). Examination of the bond lengths in the crystallo-
graphically characterised indenyl analogue suggests that the
bonding is best described as Cp–B–Cl Ti.5 Reaction with a
base was expected to break the linkage between B–Cl or Ti–Cl
to give either a neutral titanocene with a {C5H4B(py)(C6F5)2}
ligand or a zwitterion similar to 9, with a basic ligand occupy-
ing the vacated coordination site at titanium (Scheme 4). How-
ever, the addition of excess tetrahydrofuran to III did not
produce a significant change in the solution 1H and 11B solution
NMR.26 In contrast, addition of a stoichiometric amount of
pyridine to a toluene suspension of III caused the metallocene
to dissolve to give a very dark red solution from which red
crystals were obtained. The 1H NMR of these crystals (Table 1)
shows one equivalent of pyridine downfield shifted versus free
pyridine, which is consistent with coordination to a Lewis acid.
The borylcyclopentadienyl resonances adopt the expected
AAЈBBЈ pseudotriplets for a freely rotating mono-substituted
cyclopentadienyl ligand. The 11B NMR signal was observed at
1.0 vs. 7.0 ppm for TiCl2(Cp){C5H4B(C6F5)2}. The latter value
is consistent with formation of a neutral adduct 10A, rather
than an ionic product 10B (Scheme 4). This was confirmed by
D a l t o n T r a n s . , 2 0 0 3 , 1 7 7 9 – 1 7 8 9
1783

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1
1
molecule of pyridine indicated by the H NMR spectrum and
coordination which is maintained in THF solution. The H
elemental analysis was confirmed by X-ray crystallography,
Fig. 3. Presumably the titanium remains uncomplexed due to
steric congestion. By contrast, the zirconium complex, 4, reacts
with excess pyridine in toluene to give, ZrCl3{C5H4B(C6F5)2-
NMR spectra show two inequivalent pyridines suggesting one
coordinated to the boron centre and the second to the metal.
The octahedral structure of the niobium complex was con-
firmed by X-ray diffraction, Fig. 5.
1
(py)}ؒpy2 (15), as colourless crystals. The H NMR spectrum
clearly indicates three equivalents of pyridine, one of which
shows the strong down-field shift associated with coordination
to the boron centre, while the other two pyridine molecules give
rise to only one slightly broad signal in the 1H NMR. The struc-
ture of 15 was determined by single crystal X-ray diffraction,
Fig. 4, and the bonding and geometry is described below.
Fig. 5 View of a molecule of NbCl4{C5H4B(C6F5)2(py)}ؒpy (16).
Fig. 3 View of a molecule of TiCl3{C5H4B(C6F5)2(py)} (14).
X-Ray crystallographic analysis of complexes 9, 10 and 14–16
Complexes 9, 10 and 14 were shown by X-ray methods to have a
number of structural similarities. In each, the metal atom (Nb,
Ti and Ti respectively) are tetrahedral, assuming that the both
the CpЈ and CpB ligands (where CpB is a boron-substituted
cyclopentadienyl ligand) are considered to occupy a single
coordination site. The boron atom in each is also tetrahedral.
Selected geometric parameters are collated in Table 2.
The Nb–Cl(5) and Nb–Cl(6) distances in complex 9 at
2.3367(9) and 2.3615(11) Å are typical for cationic niobocene
dichlorides.28 Cl(2) is bonded to the boron atom at a distance
of 1.929(4) Å,29,30 the bond is directed away from the
Nb atom [torsion angle Cl(2)–B–C(11)–Nb is Ϫ43.8(3)Њ]
and the Nb ؒ ؒ ؒ Cl(2) distance is 3.996(8) Å; hence there is
no bridging B–Cl–Nb contact in this molecule. A similar
arrangement has been noted in [W(H)3{C5H4B(Cl)(C6F5)2}]
where the B–Cl distance is 1.915(5).16 This contrasts with
the arrangement in [TiCl(Ind){C5H4B(Cl)(C6F5)2}]5 where the
boron atom is displaced to the same side of the C5 ring as the
metal atom and the chloride ligand forms a B–Cl–Ti bridge,
with a B–Cl distance of 2.007(4) and a Ti–Cl distance of
2.4641(9) Å.
The chloride group on the boron atom in the zwitterion 9 is
replaced in the neutral adducts 10 and 14 by a pyridyl group,
but the orientation of the C6F5 rings is similar in all three mole-
cules. About the metal atoms, the ligands, viz. (CpB)Cl3, (CpB)-
Cl2(Cp) and (CpB)Cl2(CpЈ), have very similar orientations. In
the CpB groups, the boron atom in each complex is displaced
slightly from the C5 mean-plane away from the metal atom. As
one would have expected from the consistency of the 11B NMR
data, the B–N distances in 10 and 14 are similar.
The Ti–C(11) bond in complex 10 at 2.546(2) Å is signifi-
cantly longer than those to the other cyclopentadienyl carbons,
which are otherwise comparable to the average Ti–Cp distance
of 2.31 Å in TiCl2Cp2.31 This is in sharp contrast with the short-
ening of the Ti–C(B) bond in TiCl2(Ind){C5H4B(C6F5)2}. The
Fig. 4 View of a molecule of ZrCl3{C5H4B(C6F5)2(py)}ؒpy2 (15).
Complexes 1 and 2 are poorly soluble in toluene but when
suspensions are treated with excess pyridine they dissolve before
crystallising as the adducts NbCl4{C5H4B(C6F5)2(py)}ؒpy
(16) and TaCl4{C5H4B(C6F5)2(py)}ؒpy (17) from solution. The
spectroscopic details for the two complexes are similar
and closely related to 15. The 11B signals indicate pyridine
D a l t o n T r a n s . , 2 0 0 3 , 1 7 7 9 – 1 7 8 9
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Table 2 Selected dimensions in the four-coordinate complexes 9, 10 and 14. Bond lengths are in Ångströms, angles in degrees. E.s.d.s are in
parenthesesa
9
10
14
Nb–C(11)
Nb–C(12)
Nb–C(13)
Nb–C(14)
Nb–C(15)
Nb–C(71)
Nb–C(72)
Nb–C(73)
Nb–C(74)
Nb–C(75)
Nb–C(1x)
Nb–Cl(5)
Nb–Cl(6)
Nb–C(7x)
B–Cl(2)
2.501(3)
2.455(3)
2.442(3)
2.374(3)
2.402(3)
2.470(3)
2.440(3)
2.389(3)
2.405(3)
2.433(3)
2.115
Ti–C(11)
Ti–C(12)
Ti–C(13)
Ti–C(14)
Ti–C(15)
Ti–C(61)
Ti–C(62)
Ti–C(63)
Ti–C(64)
Ti–C(65)
Ti–C(1x)
Ti–Cl(5)
Ti–C(6x)
Ti–Cl(7)
B–N(21)
2.546(2)
2.403(2)
2.351(2)
2.340(2)
2.444(2)
2.393(2)
2.372(2)
2.362(2)
2.409(2)
2.401(2)
2.097
Ti–C(11)
Ti–C(12)
Ti–C(13)
Ti–C(14)
Ti–C(15)
2.4182(19)
2.348(2)
2.332(2)
2.324(2)
2.360(2)
Ti–C(1x)
Ti–Cl(5)
Ti–Cl(6)
Ti–Cl(7)
B–N(21)
2.024
2.3367(9)
2.3615(11)
2.109
2.3629(6)
2.069
2.3733(7)
1.632(3)
2.2483(8)
2.2399(7)
2.2426(7)
1.623(3)
1.929(4)
C(1x)–Nb–Cl(5)
C(1x)–Nb–Cl(6)
C(1x)–Nb–C(7x)
Cl(5)–Nb–Cl(6)
Cl(5)–Nb–C(7x)
Cl(6)–Nb–C(7x)
105.9
106.1
130.0
97.90(4)
107.8
104.7
C(1x)–Ti–Cl(5)
C(1x)–Ti–C(6x)
C(1x)–Ti–Cl(7)
C(6x)–Ti–Cl(5)
Cl(5)–Ti–Cl(7)
C(6x)–Ti–Cl(7)
107.2
131.4
107.4
105.7
94.78(2)
104.4
C(1x)–Ti–Cl(5)
C(1x)–Ti–Cl(6)
C(1x)–Ti–Cl(7)
Cl(6)–Ti–Cl(5)
Cl(7)–Ti–Cl(5)
Cl(6)–Ti–Cl(7)
115.37
116.46
115.96
101.51(3)
103.11(3)
102.32(3)
a N.B. C(1x), C(6x) and C(7x) are the centroids of the CpB and Cp rings.
Ti–Cl distances in 10 are very similar to those observed for
TiCl2Cp2.
preparation of niobocenes requires a dehalostannylation
route, and they relieve the steric congestion by chloride transfer
to boron to become zwitterions. The monocyclopentadienyl
complexes react with excess pyridine to form adducts. The
titanium complex coordinates pyridine only at the boron,
whereas the zirconium, niobium and tantalum complexes co-
ordinate pyridine both at boron and at the metal centre to
attain octahedral geometries. Pyridine displaces boron-bound
SMe2 in the borylcyclopentadienyl zirconocenes and splits the
Ti–Cl–B bridge in borylcyclopentadienyl titanocenes, to give
group 4 metallocenes with the {C5H4B(C6F5)2(py)} ligand,
whereas the zwitterionic niobocenes do not react with pyridine.
Although the longest of the Ti–C bonds in 14, the Ti–C(11)
bond length, at 2.4182(19) is much shorter than that in 10 and
only a little longer than that determined for the pyridine-free
TiCl3{C5H4B(C6F5)2}.3 The Ti–Cl distances are also slightly
longer in 14 than in TiCl3{C5H4B(C6F5)2}. This is consistent
with a reduction in electron-deficiency at the titanium centre on
base coordination to the electron-withdrawing substituent on
the cyclopentadienyl ligand.
The two six-coordinate complexes, 15 and 16, also share
many geometrical features (Fig. 4 and 5).32 With respect to the
B–C(11) bond, the orientations of the C6F5 and pyridine groups
bound to boron are almost identical in both complexes. The tilt
of the CpB ring, and the arrangements of the other five ligands,
about the Zr or Nb atoms are also very similar. The metal
atoms are displaced by ca. 0.61 Å (Zr) or 0.53 Å (Nb) from the
mean-planes through the chlorine atoms which form the ‘equa-
torial’ planes in each complex; in both molecules, the CpB
ligand is trans to a pyridine. Selected geometric parameters are
collated in Table 3.
To the best of our knowledge pyridine complexes of the type
NbCl4(Cp)ؒpy or ZrCl3(Cp)ؒpy2 have not been crystallo-
graphically characterised. The octahedral geometry around
zirconium and tetrahedral environment of the boron atom is
reminiscent of [NEt4]2[Zr(µ-Cl)Cl2{C5H4B(C6F5)3}]2, where the
terminal Zr–Cl distances were slightly shorter than those of 15
at 2.4200(8) and 2.4603(8) Å.14 In contrast, a very similar Zr–Cl
distance, 2.504(1) was observed for monocyclopentadienyl
imido complex ZrCl(C5Me5)(N-2,6-iPrC6H3)(py)2. The Zr–N
distances 2.435(4) and 2.414(4) Å for the pyridine ligands in this
complex are inside the range defined by the equatorial and axial
Zr–N distances in 15.33
Experimental
General experimental
All manipulations were performed under a dinitrogen atmos-
phere using Schlenk techniques. Solvents were distilled under
N2 over sodium (toluene), Na/K alloy (light petroleum (bp 40–
60 ЊC)), CaH2 (dichloromethane) or sodium/benzophenone
(diethyl ether). NMR solvents were dried over 4 Å molecular
sieves and degassed by several freeze–thaw cycles. NMR spectra
were recorded on a Bruker Avance DPX300 spectrometer.
Chemical shifts are reported in ppm and referenced to residual
solvent resonances (1H, 13C NMR) or external BF3ؒOEt2 (11B)
and CFCl3 (19F). Titanium tetrachloride, niobium penta-
chloride, tantalum pentachloride, and anhydrous SMe2
were
purchased from Aldrich and used as supplied. Zirconium tetra-
chloride (Acros) was sublimed immediately prior to use. Pyrid-
ine (Aldrich) was dried over activated 4 Å molecular sieves.
TiCl3{C5H4B(C6F5)2},3 and TiCl2(Cp){C5H4B(C6F5)2}5 were
prepared according to literature procedures.
Synthesis
Conclusion
NbCl4{C5H4B(C6F5)2} (1). To a dichloromethane solution of
B(C5H4SiMe3)(C6F5)2 (51.7 cm3, 0.45 M, 32.3 mmol) was added
NbCl5 (8.72 g, 32.3 mmol). The solid dissolved quickly, and
within minutes a fine orange–red solid precipitated from the
deep red solution. The reaction was stirred at room temperature
for 3 h. The product was allowed to settle overnight, separated
by filtration, washed with light petroleum and dried under
vacuum to give 1 as a very fine orange–red powder (15 g, 23.3
Half-sandwich complexes of zirconium, niobium and tantalum
with the {C5H4B(C6F5)2} ligand are readily accessible from
the metal chlorides and B(C5H4SiMe3)(C6F5)2 by selective
dehalosilylation. Borylcyclopentadienyl zirconocenes can be
prepared by reaction of the half-sandwich complex with LiCpЈ,
but unlike the analogous titanocenes there is no interaction
between the boron and the chloride ligands. In contrast, the
D a l t o n T r a n s . , 2 0 0 3 , 1 7 7 9 – 1 7 8 9
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Table 3 Selected dimensions in the six-coordinate complexes 15 and 16. Bond lengths are in Ångströms, angles in degrees. E.s.d.s are in
parenthesesa
16 (major component molecules)
15
Mol. A
Mol. B
Mol. A
Mol. B
Zr(1a)–C(11a)
Zr(1a)–C(12a)
Zr(1a)–C(13a)
Zr(1a)–C(14a)
Zr(1a)–C(15a)
Zr(1a)–C(1a)
Zr(1a)–N(51a)
Zr(1a)–Cl(6a)
Zr(1a)–Cl(7a)
Zr(1a)–Cl(8a)
Zr(1a)–N(91a)
B(1a)–N(21a)
2.651(2)
2.547(2)
2.514(2)
2.497(2)
2.546(2)
2.250
2.412(2)
2.5067(7)
2.4994(7)
2.4855(7)
2.492(2)
1.631(3)
2.647(2)
2.547(2)
2.504(3)
2.484(3)
2.544(2)
2.243
2.418(2)
2.5260(7)
2.4893(7)
2.4852(8)
2.494(2)
1.635(3)
Nb(1a)–C(11a)
Nb(1a)–C(12a)
Nb(1a)–C(13a)
Nb(1a)–C(14a)
Nb(1a)–C(15a)
Nb(1a)–C(1a)
Nb(1a)–Cl(5a)
Nb(1a)–Cl(6a)
Nb(1a)–Cl(7a)
Nb(1a)–Cl(8a)
Nb(1a)–N(91a)
B(1a)–N(21a)
2.576(5)
2.489(5)
2.427(5)
2.396(6)
2.447(5)
2.157
2.4140(12)
2.419(2)
2.427(2)
2.4209(15)
2.391(5)
1.620(7)
2.588(5)
2.493(5)
2.440(6)
2.398(5)
2.448(5)
2.162
2.4384(16)
2.4169(15)
2.4168(14)
2.4263(14)
2.388(5)
1.644(7)
N(51a)–Zr(1a)–C(1a)
Cl(6a)–Zr(1a)–C(1a)
Cl(7a)–Zr(1a)–C(1a)
Cl(8a)–Zr(1a)–C(1a)
N(91a)–Zr(1a)–C(1a)
N(51a)–Zr(1a)–Cl(6a)
N(51a)–Zr(1a)–Cl(7a)
N(51a)–Zr(1a)–Cl(8a)
N(51a)–Zr(1a)–N(91a)
Cl(7a)–Zr(1a)–Cl(6a)
Cl(8a)–Zr(1a)–Cl(6a)
N(91a)–Zr(1a)–Cl(6a)
Cl(8a)–Zr(1a)–Cl(7a)
N(91a)–Zr(1a)–Cl(7a)
Cl(8a)–Zr(1a)–N(91a)
102.10
102.95
102.57
105.76
100.76
101.73
103.51
106.01
Cl(5a)–Nb(1a)–C(1a)
Cl(6a)–Nb(1a)–C(1a)
Cl(7a)–Nb(1a)–C(1a)
Cl(8a)–Nb(1a)–C(1a)
N(91a)–Nb(1a)–C(1a)
Cl(5a)–Nb(1a)–Cl(6a)
Cl(5a)–Nb(1a)–Cl(7a)
Cl(5a)–Nb(1a)–Cl(8a)
N(91a)–Nb(1a)–Cl(5a)
Cl(6a)–Nb(1a)–Cl(7a)
Cl(6a)–Nb(1a)–Cl(8a)
N(91a)–Nb(1a)–Cl(6a)
Cl(8a)–Nb(1a)–Cl(7a)
N(91a)–Nb(1a)–Cl(7a)
N(91a)–Nb(1a)–Cl(8a)
101.74
103.28
101.96
103.90
101.53
103.37
101.78
103.83
175.91
174.49
179.98
179.56
85.09(5)
155.26(5)
85.49(5)
74.50(7)
87.59(2)
151.06(2)
74.68(5)
89.64(3)
80.77(5)
76.44(5)
86.15(5)
155.66(5)
85.92(5)
74.55(7)
87.24(2)
152.10(2)
75.29(5)
89.07(3)
81.11(5)
76.82(5)
87.60(6)
156.29(6)
88.35(5)
78.26(10)
86.82(7)
152.78(7)
76.73(11)
86.16(6)
78.03(11)
76.10(11)
86.94(6)
156.69(6)
88.01(5)
78.03(11)
88.18(5)
152.80(6)
76.61(12)
85.98(5)
78.66(11)
76.19(12)
Deviation of metal from ‘equatorial’ plane
0.6169(2)
0.6003(2)
0.5331(10)
0.5321(9)
a N.B. C(1a) is the centroid of the Cpb ring. The metals of Mol. B in both complexes are Zr(2b) and Nb(2b); all other atoms have labels corresponding
to those in Mol. A, viz. C(11b), C(12b), etc.
mmol, 72%). (Found: C, 30.76; H, 0.75; Cl, 22.85. C17H4BCl4-
F10Nb requires C, 31.72; H 0.63; Cl, 22.03%.)
solid LiCp (0.30 g, 4.2 mmol) in one portion. A yellow solution
resulted, together with and a small amount of fine white pre-
cipitate. The reaction was stirred for a further 2 h and the solids
were filtered off. The filtrate was concentrated to 15 cm3 and
cooled to Ϫ25 ЊC overnight to afford 5 as small off-white crys-
tals (1.5 g, 1.98 mmol, 47%). (Found: C, 45.47; H, 2.68; Cl, 9.53.
C28.62H20.28BCl2F10SZr requires C, 45.28; H 2.69; Cl, 9.34%.)
TaCl4{C5H4B(C6F5)2} (2). Following a very similar procedure
to 1, the reaction of B(C5H4SiMe3)(C6F5)2 (14.8 cm3, 0.45 M,
6.6 mmol) with TaCl5 (2.38 g, 6.6 mmol) gave 2 as a fine pale
yellow solid (3 g, 4.1 mmol, 62%). (Found: C, 28.36; H, 0.57; Cl,
19.77. C17H4BCl4F10Ta requires C, 27.90; H 0.55; Cl, 19.38%.)
ZrCl2(Ind){C5H4B(C6F5)2(SMe2)} (6). Following the pro-
cedure given for compound 5: 6 was made from 2.6 g (3.98
mmol) of 4 and 0.49 g (3.98 mmol) LiInd in toluene (20 cm3).
Removal of the solvent gave as an orange solid (2.5 g, 3.3
mmol, 83%). An analytically pure sample was obtained by
addition of SMe2 to a saturated solution of the crude material
and cooling to Ϫ25 ЊC overnight (Found: C, 45.23; H, 2.45; Cl,
9.33. C28H17BCl2F10SZr requires C, 44.94; H 2.29; Cl, 9.47%.)
ZrCl2{C5H4B(C6F5)2}2ؒ0.5(Et2O) (3). A solution of B(C5H4-
SiMe3)(C6F5)2 (20 g, 41.4 mmol) in toluene (100 cm3) was added
to a suspension of ZrCl4 (4.66 g, 20 mmol) in toluene (40 cm3).
The mixture was warmed to 60 ЊC for 4 h and stirred at room
temperature overnight. Removal of the volatiles under vacuum
gave a viscous brown oil. Tituration with light petroleum
yielded a tan-coloured solid which was recrystallised from
diethyl ether to give very small off-white crystals (4.5 g, 4.4
mmol, 22%). (Found: C, 42.54; H, 1.33; Cl, 7.56. C36H13B2-
Cl2F20O0.5Zr requires C, 42.51; H 1.29; Cl, 6.97%.)
B(C6F5)3ؒSMe2 (7). Tris(pentafluorophenyl)boron (2.3 g, 4.5
mmol) was dissolved in toluene (30 cm3) at 60 ЊC. The solution
was then treated with dimethylsulfide (1.0 cm3, 0.85 g, 13.7
mmol) and the mixture was stirred to ensure uniform mixing.
Cooling the solution overnight (Ϫ25 ЊC) afforded cubic colour-
less crystals of B(C6F5)3ؒSMe2 (2.0 g, 3.5 mmol, 78%). (Found:
C, 41.77; H, 0.97. C20H6BF15S requires C, 41.84; H, 1.05%.)
ZrCl3{C5H4B(C6F5)2(SMe2)}ؒ0.25(C7H8) (4). To a dichloro-
methane solution of B(C5H4SiMe3)(C6F5)2 (56 cm3, 0.45 M,
25.2 mmol) was added ZrCl4(SMe2)2 (8.9 g, 25 mmol). The pale
orange solution was stirred overnight. Concentration of the
solution and cooling to Ϫ25 ЊC overnight gave small yellow
crystals, which were filtered off and washed with light petrol-
eum. The material was recrystallised from toluene to give 4
(10.5 g, 15.2 mmol, 61%). The crystals slowly lose solvent of
crystallisation under vacuum. (Found: C, 35.82; H, 1.91; Cl,
14.06. C20.75H12BCl3F10SZr requires C, 36.03; H 1.75; Cl,
15.38%.)
Nb(؉)Cl2(Cp){C5H4B(؊)(Cl)(C6F5)2} (8). To a stirred suspen-
sion of 4.78 g (7.43 mmol) 1 in dichloromethane (30 cm3) was
added Me3SnC5H5 (1.83 g, 8.0 mmol). The suspended solids
dissolved within 1 h to give a very dark red solution. After a
further 2 h the solution was filtered to separate a small amount
of dark solid. The product was isolated by concentrating to
10 cm3 and precipitating with 40 cm3 of light petroleum to give
a dark red oil that dried to a glassy solid, yield 3.5 g (5.2 mmol,
ZrCl2(Cp){C5H4B(C6F5)2(SMe2)}ؒ0.66(C7H8) (5). To a sus-
pension of 4 (2.8 g, 4.2 mmol) in toluene (30 cm3) was added
1
70%). The H NMR indicated that the material retains some
D a l t o n T r a n s . , 2 0 0 3 , 1 7 7 9 – 1 7 8 9
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solvents even after extended periods of drying under high
vacuum. Repeated attempts to purify by recrystallisation from
toluene and dichloromethane/light petroleum mixtures were
unsuccessful. (Found: C, 40.25; H, 2.06. C22H9BCl3F10Nb
requires C, 39.24; H 1.35%.)
N, 4.49; Cl, 11.36%.) Crystals suitable for X-ray analysis were
obtained by recrystallisation from CDCl3.
NbCl4{C5H4B(C6F5)2(py)}ؒpy (16). Pyridine (1 cm3) was
added to a suspension of 1 (2.8 g, 4.3 mmol) in dichloro-
methane (20 cm3) to produce a dark red solution. A small
amount of insoluble residue was removed by filtration. On
standing at room temperature the solution afforded dark red
crystals (3.3 g, 4.1 mmol, 95%). (Found: C, 40.11; H, 1.77; N,
3.55; Cl, 17.89. C27H14BF10N2Nb requires C, 40.44; H, 1.76; N,
3.49; Cl, 17.68%.)
Nb(؉)Cl2(C5H4SiMe3){C5H4B(؊)(Cl)(C6F5)2} (9). Following a
similar procedure to that outlined for 8, a stirred suspension of
1 (3.08 g, 4.8 mmol) was treated with 1.5 g (5 mmol)
(SnMe3)(SiMe3)C5H4. The product was isolated by removing
the volatiles and washing the solid with light petroleum to give
9 as a dark red solid, yield 2 g (2.7 mmol, 56%). (Found: C,
39.65; H, 2.59; Cl, 14.62. C25H17BCl3F10NbSi requires C, 40.28;
H, 2.30; Cl, 14.27%.) Dark red crystals suitable for X-ray dif-
fraction were obtained by recrystallisation from 20 cm3 toluene
at Ϫ25 ЊC overnight.
TaCl4{C5H4B(C6F5)2(py)}ؒpy (17). Following the procedure
described for 16 reaction of 2 (4.4 g, 6.6 mmol) with pyridine
(2 cm3) gave pale yellow crystals (4 g, 4.5 mmol, 68%).
Although this sample did not give a satisfactory elemental
analysis, the compound was identified spectroscopically and by
comparison with the Nb analogue 16.
TiCl2(Cp){C5H4B(C6F5)2(py)}ؒ0.5(C7H8) (10). To a solution
of TiCl3{C5H4B(C6F5)2}, (1.62 g, 2.74 mmol) in toluene (20
cm3) was added pyridine (0.24 g, 3.0 mmol). There was an
appreciable exotherm but no significant colour change. The
solution was left to stand at room temperature for 30 min. A
microcrystalline red solid precipitated which was filtered off
and dried to give 10 (1.2 g, 1.7 mmol, 62%). (Found: C, 50.44;
H, 2.34; N, 2.07; Cl, 10.40. C30.5H18BCl2F10NTi requires C,
51.02; H, 2.53; N, 1.95; Cl, 9.87%.) Crystals suitable for X-ray
analysis were obtained by cooling a saturated toluene solution
to Ϫ25 ЊC overnight.
Crystal structure analyses
The X-ray analysis of [Nb(ϩ)Cl2(C5H4SiMe3){C5H4B(Ϫ)(Cl)-
(C6F5)2}]ؒ1.5(toluene), complex 9, is described here; the analy-
ses of the other complexes followed similar procedures, except
where stated below. Crystal data are collated in Table 4.
Crystals of 9 are orange–brown, diamond-shaped plates.
From a sample under oil, one, ca. 0.55 × 0.32 (across diagonals)
and 0.15 mm thick, was mounted on a glass fibre and fixed in
the cold nitrogen stream (140 K) on a Rigaku R-Axis IIc image
plate diffractometer equipped with a rotating anode X-ray
source (Mo–Kα radiation λ = 0.71069 Å) and graphite mono-
chromator. Using 4Њ oscillations, 48 exposures of 54 min each
were made.
ZrCl2(Cp){C5H4B(C6F5)2(py)}ؒ0.5(C7H8) (11). Following the
procedure given for 10, compound 11 was made from
ZrCl2(Cp){C5H4B (C6F5)2(SMe2)} (1.0 g, 1.43 mmol) and pyrid-
ine (1.0 cm3) as pale yellow crystals (0.5 g, 0.66 mmol, 46%).
(Found: C, 48.05; H, 2.20; N, 2.24; Cl, 9.90. C30.5H18BCl2F10-
NZr requires C, 48.11; H, 2.38; N, 1.84; Cl, 9.31%.)
Data were processed using the DENZO/SCALEPACK pro-
grams.34 The structure was determined by the direct methods
routines in the SHELXS program35 and refined by full-matrix
least-squares methods, on F 2s with SHELXL.35 In addition to
the Nb complex molecule in the asymmetric unit, there is one
ordered toluene molecule plus a further toluene molecule which
is disordered about a centre of symmetry. The disordered sol-
vent molecule was refined with geometrical restraints. The non-
hydrogen atoms (except those of partial occupancy in the dis-
ordered toluene) were refined with anisotropic thermal param-
eters. Hydrogen atoms in the complex and ordered toluene were
included in idealised positions and their Uiso values were set to
ride on the Ueq values of the parent carbon atoms; H atoms
were not included in the disordered toluene molecule. At the
conclusion of the refinement, wR2 = 0.112 and R1 = 0.047 35 for
ZrCl2(Ind){C5H4B(C6F5)2(py)}ؒ(C7H8)(C5H5N) (12). Follow-
ing the procedure outlined for 10, compound 12 was made from
ZrCl2(Ind){C5H4B(C6F5)2(SMe2)} (0.5 g, 0.67 mmol) and pyrid-
ine (1 cm3) as a dark yellow oil. Decanting the supernatant and
drying under vacuum produced a dry yellow foam (0.5 g, 0.53
mmol, 80%). (Found: C, 53.49; H, 2.96; N, 3.17; Cl, 6.39.
C43H29BCl2F10N2Zr requires C, 55.14; H, 3.12; N, 2.99; Cl,
7.57%.)
ZrCl2{C5H4B(C6F5)2(py)}2ؒ(C7H8) (13). Following a similar
procedure as for 10, compound 13 was prepared from
ZrCl2{C5H4B(C6F5)2}2 (0.5 g, 0.51 mmol) and pyridine (0.5
cm3). A fine colourless solid precipitate formed almost immedi-
ately. This was redissolved by adding further pyridine (1 cm3)
followed by gentle heating. Almost colourless crystals were
obtained after cooling the solution to 5 ЊC overnight (0.5 g, 0.44
mmol, 86%). (Found: C, 49.60; H, 2.17; N, 2.63; Cl, 6.32.
C51H26B2Cl2F20N2Zr requires C, 49.78; H, 2.13; N, 2.28; Cl,
5.76%.)
2
all 6318 reflections, weighted w = [σ2(Fo ) ϩ (0.0656P)2]Ϫ1 with
2
P = (Fo ϩ 2Fc2)/3; for the ‘observed’ data only, R1 = 0.039. In
the final difference map, the highest peaks (to ca. 0.60 e ÅϪ3
)
were in the disordered solvent region.
In complex 10, the carbon atoms of the toluene solvent
molecule, which is disordered about a twofold symmetry axis,
were refined isotropically. Hydrogen atoms in this solvent mole-
cule were included in idealised positions.
TiCl3{C5H4B(C6F5)2(py)}ؒ0.5(C7H8) (14). To a solution of
TiCl3{C5H4B(C6F5)2} (2.24 g, 3.9 mmol) in toluene (20 cm3)
was added pyridine (1 cm3). Allowing the solution to stand at
room temperature afforded needle-like red crystals, fragments
of which were suitable for X-ray analysis (1.7 g, 2.5 mmol,
63%). (Found: C, 43.76; H, 1.68; N, 2.07; Cl, 16.36. C25.5H13-
BCl3F10NTi requires C, 44.49; H 1.90; N, 2.03; Cl, 15.45%.)
There are two independent Zr-complex molecules in the
asymmetric unit of the cell of complex 15, together with three
solvent CDCl3 molecules (one of which is disordered).
During the refinement of two independent molecules of
complex 16, the principal difference peaks (ca. 12 e ÅϪ3) were
identified as alternative molecules and were included with a site
occupancy which was adjusted to 0.15 (and that of the major
molecules to 0.85). In the minor component molecules, only the
Nb and Cl atoms were refined anisotropically and no hydrogen
atoms were included.
ZrCl3{C5H4B(C6F5)2(py)}ؒpy2ؒ(C7H8) (15). Following a simi-
lar procedure as for 10, compound 15 was made from
ZrCl3{(C5H4)B(SMe2)(C6F5)2} (1.2 g, 1.8 mmol) and pyridine
(1 cm3) and obtained as very small colourless needle-shaped
crystals (1 g, 1.1 mmol, 61%). (Found: C, 49.21; H, 2.95; N,
4.84; Cl, 10.74. C29H27BCl3F10N3Zr requires C, 50.04; H, 2.91;
Scattering factors for neutral atoms were taken from ref. 36.
Computer programs used in these analyses include those noted
above, those in Table 4 of ref. 37, and the SHELXTL suite,38
and were run on a Silicon Graphics Indy at the University
D a l t o n T r a n s . , 2 0 0 3 , 1 7 7 9 – 1 7 8 9
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of East Anglia, or a DEC-AlphaStation 200 4/100 in the
Biological Chemistry Department, John Innes Centre.
CCDC reference numbers 201466–201470.
See http://www.rsc.org/suppdata/dt/b3/b300552f/ for crystal-
lographic data in CIF or other electronic format.
Acknowledgements
The authors are grateful to the EPSRC and the University of
East Anglia for financial support. We would like to thank Prof
Manfred Bochmann and Dr Tim Woodman for helpful
discussions.
References and notes
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3 R. Duchateau, S. J. Lancaster, M. Thornton-Pett and M.
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4 M. T. Reetz, H. Brümmer, M. Kessler and J. Kuhnigk, Chimia, 1995,
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5 S. J. Lancaster, S. Al-Benna, M. Thornton-Pett and M. Bochmann,
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6 R. E. v. H. Spence and W. E. Piers, Organometallics, 1995, 14, 4617.
7 W. E. Piers, Y. Sun and L. W. M. Lee, Top. Catal., 1999, 7, 133.
8 H. Braunschweig, R. Dirk, M. Müller, P. Nguyen, R. Resendes,
D. P. Gates and I. Manners, Angew. Chem., Int. Ed. Engl., 1997, 36,
2338.
9 H. Braunschweig, C. von Koblinski and R. Wang, Eur. J. Inorg.
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10 A. J. Ashe, X. Fang and J. W. Kampf, Organometallics, 1999, 18,
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11 D. S. Stelck, P. J. Shapiro, N. Basickes and A. L. Rheingold,
Organometallics, 1997, 16, 4546.
12 P. J. Shapiro, Eur. J. Inorg. Chem., 2001, 321.
13 S. J. Lancaster, M. Bochmann and O. B. Robinson, J. Chem. Soc.,
Chem. Commun., 1995, 2081.
14 S. J. Lancaster, M. Thornton-Pett, D. M. Dawson and M.
Bochmann, Organometallics, 1998, 17, 3829.
15 J. Ruwwe, G. Erker and R. Fröhlich, Angew. Chem., Int. Ed. Engl.,
35, 80.
16 L. H. Doerrer, A. J. Graham, D. Haussinger and M. L. H. Green,
J. Chem. Soc., Dalton Trans., 2000, 813.
17 P. Sinnema, P. J. Shapiro, D. M. J. Foo and B. Twamley, J. Am.
Chem. Soc., 2002, 124, 10996.
18 C. T. Burns, D. S. Stelck, P. J. Shapiro, V. Ashwani, K. Kunz,
G. Kehr, T. Concolino and A. L. Rheingold, Organometallics, 1999,
18, 5432.
19 S. J. Lancaster and M. Bochmann, Organometallics, 2001, 20, 2093.
20 W. E. Piers and T. Chivers, Chem. Soc. Rev., 1997, 26, 345.
21 C. Zhao, P. Li, X. Cao and Z. Xi, Chem. Eur. J., 2002, 8, 4292.
22 T. Livinghouse and E. C. Lund, Organometallics, 1990, 9, 2426.
23 C. H. Winter, X.-X. Zhou, D. A. Dobbs and M. J. Heeg,
Organometallics, 1991, 10, 210.
24 In zirconium complexes where Cl is replaced by –ER2 (E = N, P), we
have observed Cp–B–E–Zr interactions: S. J. Lancaster, A. J.
Mountford, D. L. Hughes, M. Schormann and M. Bochmann,
J. Organomet. Chem., 2003, in press.
25 P. Royo and E. Ryan, in Metallocenes, ed. A. Togni and R. L.
Halterman, Wiley-VCH, Chichester, 1998, vol. 1, p. 321.
26 There is a typographical error in ref. 5, the NMR data for complex 2
in Table 1 should read ‘6.31 (br, 2H) C5H4B’, not ‘6.13 (br, 2H)
C5H4B’.
27 Indeed X-ray quality crystals were obtained in such a fashion and
the structure determined. The molecular structure is almost
identical to that described for 9 but there is only one molecule of
toluene solvent per niobium molecule. Crystal data: C25H17-
BCl3F10NbSiؒC7H8, M = 837.7. Monoclinic, space group P21/c (no.
14), a = 13.801(1), b = 15.892(1), c = 15.892(1) Å, β = 110.45(1)Њ,
V = 3265.9(4) Å3. Z = 4, Dc = 1.704 g cmϪ3, F(000) = 1672, T = 140(1)
K, µ(Mo-Kα) = 7.3 cmϪ1, λ(Mo-Kα) = 0.71069 Å. Intensity data
from a Rigaku R-Axis IIc image plate diffractometer. Refinement
35
gave final wR2 = 0.089 and R1 = 0.036
for all 5959 unique
reflections; for the 5347 ‘observed’ data, R1 = 0.033.
28 R. J. Harrison, M. G. H. Wallbridge, E. DeBoer, D. J. Duncalf and
A. McCamley, J. Organomet. Chem., 1998, 568, 271.
29 The B–Cl bond length in the [Cl(B(C6F5)3]Ϫ anion is 1.907(8) Å.
D a l t o n T r a n s . , 2 0 0 3 , 1 7 7 9 – 1 7 8 9
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30 B. E. Bosch, G. Erker, R. Fröhlich and O. Meyer, Organometallics,
1997, 16, 5449; J. Zhou, S. J. Lancaster, D. A. Walker, S. Beck,
M. Thornton-Pett and M. Bochmann, J. Am. Chem. Soc., 2001, 123,
223.
31 A. Clearfield, D. K. Warner, C. H. Saldarriaga-Molina and
R. Ropal, Can. J. Chem., 1975, 53, 1622.
33 D. J. Arney, M. A. Bruck, S. R. Huber and D. E. Wigley,
Inorg. Chem., 1992, 31, 3749.
34 Z. Otwinowski and W. Minor, Methods Enzymol., 1997, 276, 307.
35 G. M. Sheldrick, in SHELX-97, Program for crystal structure
determination (SHELXS) and refinement (SHELXL), University of
Göttingen, Germany, 1997.
32 The unit cell of 15 contains two independent molecules, essentially
identical and with similar orientations in the cell. Complex 16 not
only has two independent molecules, which form the major
components of a disordered cell, but also two minor component
molecules with essentially the same orientations as the major
molecules but which are shifted by almost precisely (0, ½, 0) from
the major molecules.
36 International Tables for X-ray Crystallography, Kluwer Academic
Publishers, Dordrecht, 1992, vol. C, pp. 500, 219 and 193.
37 S. N. Anderson, R. L. Richards and D. L. Hughes, J. Chem. Soc.,
Dalton Trans., 1986, 245.
38 G. M. Sheldrick, in SHELXTL Package, including XS for structure
determination, XL for refinement and XP for molecular graphics,
Siemens Analytical Inc., Madison, WI, USA, 1995.
D a l t o n T r a n s . , 2 0 0 3 , 1 7 7 9 – 1 7 8 9
1789