Some 2,6-bis(dimethylamino)phenyl-mercury(II) and -boron complexes

Article abstract of DOI:10.1016/S0277-5387(01)01037-3

As a continuation of our studies on the 2,6-bis(dimethylamino)phenyl ligand (R^-), we report on the synthesis and characterisation, including the X-ray structures, of HgR₂ and BCl(Ph)R.

Full text of DOI:10.1016/S0277-5387(01)01037-3

Polyhedron 21 (2002) 635–640
www.elsevier.com/locate/poly
Some 2,6-bis(dimethylamino)phenyl–mercury(II) and –boron
complexes
David Cornu, Peter B. Hitchcock, Michael F. Lappert *, Patrick G.H. Uiterweerd
The Chemistry Laboratory, School of Chemistry, Physics and En6ironmental Science, Uni6ersity of Sussex, Falmer, East Sussex,
Brighton BN1 9QJ, UK
Received 28 September 2001; accepted 27 November 2001
Abstract
As a continuation of our studies on the 2,6-bis(dimethylamino)phenyl ligand (R−), we report on the synthesis and
characterisation, including the X-ray structures, of HgR2 and BCl(Ph)R. © 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Arylmetal complexes; Mercury(II); Boron; The 2,6-bis(dimethylamino)phenyl ligand
1. Introduction
In 1997, we drew attention to the potential of the
2,6-bis(dimethylamino)phenyl ligand −C6H3(NMe2)2-
2,6 (ꢀ R−); it was suggested that R− might find a useful
role in organometallic chemistry by virtue of facile
on/off co-ordination of a pendant NMe2 group [1],
especially in catalytic systems. The ligand R− differs
from the more widely studied −C6H3(CH2NMe2)2-2,6
(ꢀ R%) [2], in that in their metal complexes intramolecu-
lar N“M ligation is largely strain-free for (MꢁR%)-, but
not (MꢁR)-, containing derivatives. This is clearly illus-
trated when comparing solid state and solution struc-
tures of Sn(Cl)R% (I) [3], SnR2 (II) [1] and Sn(Cl)R (III)
[1,4]. In the crystalline complexes the angle h at the
ipso-carbon is close to the sp2 value in I [3], but in II it
has an average value of 104° [1], while in III it is
112.1(4)° [4]. Furthermore, whereas in toluene-d8 solu-
1
tion, H NMR VT spectra showed that in II and III
there was a rapid exchange process:
2-Me2N“Sn X 6-Me2N“Sn
at ambient temperature [1] (as also proved to be the
rule for the Group 14 metal(II) complexes listed in
Table 1), this was not the case for I [3].
The conjugate acid RH of the ligand R− was first
prepared in 1970 from 1,3-diaminobenzene and
* Corresponding author.
E-mail address: kafj5@sussex.ac.uk (M.F. Lappert).
trimethyl phosphate [5]. Almost 20 years later, LiR (1)
0277-5387/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(01)01037-3

636
D. Cornu et al. / Polyhedron 21 (2002) 635–640
was obtained by direct lithiation of RH and shown to
have a trinuclear structure in the crystal; this was
retained in benzene or toluene solution, but in THF
there was a monomer = trimer equilibrium [2].
four-coordinate, whereas in SnR2(BH3) the tin atom is
five-coordinate. In both, the MR2 moiety behaves as a
Lewis acid, as is also the case when SnX2 (IV) [7] rather
than BH3 [6] is the electron pair acceptor. By contrast,
with SnR2 or Sn[N(SiMe3)2]R as reagent, treatment
with the thermally stable silylene V [abbreviated as
Si(NN)] led to its product of insertion into the SnꢁR or
SnꢁN(SiMe3)2 bond, respectively [8].
2. Experimental
All manipulations were carried out in flame-dried
glassware under argon, using standard Schlenk tech-
niques. Solvents were distilled from drying agents and
degassed. Elemental analyses were carried out by
Medac Ltd, Uxbridge. Melting points were determined
in sealed capillaries under an argon atmosphere on an
electrothermal apparatus and are uncorrected. The
NMR spectra were recorded in benzene-d6 at 298 K
using a Bruker DPX 300 (1H, 300.1; 13C, 75.4 MHz) or
AMX 500 (11B, 160.4; 199Hg, 89.1 MHz) instrument;
the solvent resonances were used as internal references
1
for H and 13C, while 11BF3(OEt2) and 199HgMe2 were
the external references for the 11B and 199Hg NMR
spectra, respectively. Electron-impact mass spectra (70
eV) were recorded using a Kratos MS 80 RF instru-
ment. The compound [Li(m-R)]3 (1) was prepared as
described in the literature [2]. The other chemicals used
were commercial samples (Aldrich).
We have used the ligand R− to prepare the 12
crystalline germanium(II), tin(II) and lead(II) com-
plexes listed in Table 1 [1,4,6–8]. For 1 [2] and eight
others X-ray data are available; the principal point of
interest in the present context and highlighted in Table
1 is that each R− ligand has a single N···M intramolec-
ular close contact in every case except in GeR2(BH3)
[6]. The germanium atom in the latter compound is
2.1. Preparation of HgR2 (2)
Solid HgBr2 (0.63 g, 1.75 mmol) was added in small
portions to a solution of LiR (0.60 g, 3.53 mmol) in
Et2O (30 cm3) at room temperature. The reaction mix-
ture was stirred for 20 h; the volatiles were then re-
moved in vacuo. The residue was extracted with
pentane (20 cm3), the filtrate was concentrated in vacuo
and stored at −25 °C, yielding compound 2 (0.55 g,
60%), as colourless plate-shaped crystals. Anal. Found:
C, 45.6; H, 5.71; N, 10.5. Required for C20H30HgN4: C,
45.6; H, 5.74; N, 10.6%. M.p. 63.0–64.0 °C. Mass
spectrum [m/z (%)]: 528 (35, [M+1]+), 163 (100, [R]+),
147 (59, [RꢁMe−1]+), 120 (24, [RꢁNMe2+1]+), 77
Table 1
Previously reported metal (M) crystalline complexes containing the
2,6-bis(dimethylamino)phenyl ligand (R−
)
Complex
Coordination number of M Reference
[Li(m-R)]3 (1)
GeR2
SnR2 (II)
PbR2
GeR2(BH3)
2+2N
2+2N
2+2N
2+2N
3+1N
3+2N
[2]
[1]
[1]
[1]
[6]
[6]
[1,4]
[1]
[1]
[7]
[7]
[8]
[8]
1
(19, [C6H5]+). H NMR: l 2.71 (s, 12H, NMe2), 6.92
(d, 2H, m-H in R), 7.23 (t, 1H, p-H in R). 13C{1H}
NMR: l 45.90 (s, NMe2), 113.78 (s, m-C in R), 129.24
(s, p-C in R), 160.45 (s, o-C in R), 161.32 (s, ipso-C in
R). 199Hg{1H} NMR: l −642.0 (s).
SnR2(BH3)
Sn(Cl)R
2+1N
a
Sn{N(SiMe3)2}R
Sn{CH(SiMe3)2}R
GeR2(SnX2) b
SnR2(SnX2) b
Sn{Si(NN)R}R c
Sn{Si(NN)N(SiMe3)2}R c
a
a
3+2N
2+2N
2.2. Preparation of BCl(Ph)R (3)
a
Dichloro(phenyl)borane (1.25 cm3, 9.63 mmol) was
added dropwise at −78 °C to a stirred solution of LiR
(1.61 g, 9.46 mmol) in Et2O (75 cm3). The resulting
a Not determined.
b SnX2 has the structure IV.
c Si(NN) is an abbreviation for V.

D. Cornu et al. / Polyhedron 21 (2002) 635–640
637
Table 2
R/Ph), 134.61 (s, ipso-C in Ph), 144.21 (s, ipso-C in R),
153.70 (s, o-C in R).
Crystal data and structure refinement for HgR2 (2) and BCl(Ph)R (3)
[R=C6H3(NMe2)2-2,6]
2.3. Crystal data and refinement details for 2 and 3
2
3
Formula
M
T (K)
Crystal system
Space group
C
527.1
173(2)
20H30HgN4
C16H20BClN2
286.6
173(2)
Suitable single crystals of HgR2 (2) and BCl(Ph)R (3)
were obtained from a concentrated diethyl ether (2) or
THF–hexane (3) solution at −25 °C. Data were col-
lected on an Enraf–Nonius CAD4 diffractometer using
orthorhombic
Pbca (no. 61)
13.311(6)
19.253(9)
8.012(3)
90
2053(2)
4
1803
1803
953
1803/0/115
0.033
monoclinic
P21/n (no. 14)
9.213(2)
12.193(4)
14.389(4)
108.60(2)
1531.9(7)
4
3885
3676 [Rint=0.08]
2037
3676/0/181
0.066
,
,
a (A)
monochromated Mo Ka radiation, u 0.71073 A, with
,
b (A)
the crystals under a stream of cold nitrogen gas. Inten-
sities were measured by an ꢀ−2q scan. Corrections
were made for absorption using psi-scan measurements.
The programs used for structure solutions and refine-
ment (full-matrix on all F2) were SHELXS-97 [9] and
SHELXL-97 [10], respectively. Further details are in
Table 2.
,
c (A)
i (°)
3
,
U (A )
Z
Total reflections
Independent reflections
Reflections with I\2|(I)
Data/restraints/parameters
R1 [I\2|(I)]
wR2 (all data)
0.094
0.167
3. Results and discussion
The objectives of the present study were to prepare
2,6-bis(dimethylamino)phenyl–mercury(II) and –boron
compounds.
The mercury(II) aryl HgR2 (2) was sought as a
potentially useful R− or R transfer agent. A particular
’
’
target was SnR3; crystalline, mononuclear tin(III) com-
pounds are unknown, although earlier researches had
’
revealed that in toluene solution Sn[CH(SiMe3)2]3 [11]
’
and nitrogen analogues such as Sn[N(SiMe3)2]3 [11,12]
were indefinitely stable at ambient temperature. In the
event, there was no reaction between SnR2 [1] and
HgR2. [Alternative strategies, based on Sn(Br)R3 as a
precursor, failed since this tin(IV) bromide proved to be
inaccessible]. An added interest in HgR2 was as a
possible source of RHgꢁHgR {such mercury(I) organic
compounds are unknown, an exception being
XHgꢁHgX, X=Si[Si(Me)2SiMe3]3 [13]}; but this also
proved to be disappointing, as attempted reduction of 2
invariably yielded elemental mercury.
Scheme 1.
The compound B(Cl)R2 appeared to be an attractive
source of the crystalline BR2 radical or cation [BR2] ,
viscous white suspension was allowed to warm to room
temperature, stirred for 4 h and filtered. The filtrate was
concentrated in vacuo and stored at −25 °C, then
filtered; the residue was washed with pentane (2×10
cm3) and dried in vacuo, yielding compound 3 (2.18 g,
80%) as a cream solid. Anal. Found: C, 66.8; H, 6.97;
N, 9.63. Required for C16H20BClN2: C, 67.1; H, 7.03;
N, 9.77%. M.p. 104–106 °C. Mass spectrum [m/z (%)]:
286 (100, [M−1]+), 251 (32, [MꢁCl]+), 209 (73,
[MꢁPh−1]+), 163 (49, [R]+), 147 (13, [RꢁMe−1]+).
1H NMR: l 2.42 (s, 12H, NMe2), 6.23 (d, 2H, m-H in
R), 7.12–7.28 (m, 4H, R/Ph), 7.83 (d, 2H, m-H in Ph).
11B{1H} NMR: l 14.7 (br s, w1/2=200 Hz). 13C{1H}
NMR: l 43.85 (s, NMe2), 106.33 (s, m-C in R), 126.76
(s, R/Ph), 127.77 (s, R/Ph), 128.26 (s, R/Ph), 130.86 (s,
+
’
both of which would be highly novel. However, we
were unable unambiguously to displace a second chlo-
ride ion from BCl3 using an excess of LiR, perhaps for
steric reasons. An alternative strategy led us to the less
hindered compound BCl(Ph)R (3).
Treatment of HgBr2 with 2 equiv. of LiR in diethyl
ether at ambient temperature yielded (step i of Scheme
1) HgR2 (2) in 60% yield after crystallisation from
pentane. The reaction between B(Cl)2Ph and an equiva-
lent portion of LiR in diethyl ether at −78 °C af-
forded (step ii of Scheme 1) crystalline BCl(Ph)R (3) in
high yield.
Each of the crystalline colourless 2 or cream 3 com-
pounds was characterised by satisfactory elemental

638
D. Cornu et al. / Polyhedron 21 (2002) 635–640
dimer. Although in each experiment the colour of the
reaction mixture changed from colourless to fluorescent
green, the 1H NMR spectra showed only signals at-
tributable to 3. Likewise, efforts to generate the salts
analysis, melting point (63–64 °C, 2; 104–106 °C, 3),
multinuclear NMR and mass (molecular ions91 mass
units) spectra, and single crystal X-ray diffraction (see-
below). The mercury compound 2 was light-sensitive
both in the solid state and in solution, a feature well
documented for compounds containing Hg(II)ꢁC bonds
[14]. Substantial decomposition was significant in ben-
zene solution only after approximately 3 weeks at ambi-
ent temperature.
[B(Ph)R][BX4], by treating
3
with NaBPh4 or
[NEt3H][B(C6F5)4], were unsuccessful; no evidence was
found for a reaction having taken place.
Treatment of BCl3 with 2 equiv. of LiR in diethyl ether
or toluene gave a white, sparingly soluble powder and
unreacted LiR. This suggests that it was not possible to
introduce two R− ligands onto the boron atom, which
may have a steric explanation. When an excess of boron
trichloride was added to a solution of LiR, a compli-
cated reaction mixture was obtained, from which a
compound having somewhat greater solubility was even-
tually isolated. The 11B{1H} NMR spectrum showed two
signals of approximately equal intensity at l 11.4 and
17.5. A plausible formulation would be that shown in 4
(Scheme 1), by analogy with the crystallographically
authenticated compound IX, for which 11B spectral data
were not recorded [20].
The 199Hg{1H} NMR spectrum of 2 in benzene-d6
showed a singlet at l −642. This value is at higher
frequency than the recorded values for HgPh2, which
range from l −808 (acetone-d6) to −742 (CD2Cl2) [15];
but is close to the l −689 (in CDCl3) reported for VI,
for which intramolecular N···Hg interaction was inferred
[16]. A further comparison is available with compound
VII, having l −778 in CDCl3, in which such a close
N···Hg contact was excluded [17].
The 11B{1H} NMR spectrum of BCl(Ph)R (3) in
benzene-d6 showed a singlet at l 14.7, consistent with the
solution of 3 containing a single tetrahedrally co-ordi-
nated boron compound [18] and having a strong N“B
bond [19]. This chemical shift is close to the l 14.4 for
VIII [19].
3.1. The molecular structure of HgR2 (2)
The molecular structure of crystalline HgR2 (2) is
illustrated in Fig. 1; selected bond distances and bond
angles are in Table 3.
Crystalline 2 is a centrosymmetric monomer, with a
linear C(1)ꢁHgꢁC(1)% vector. This structure is thus unex-
ceptional. The Hg···N(1) and Hg···N(2) contacts of
Attempts were made to reduce compound 3 with
potassium or potassium–graphite in order to obtain
’
either the radical B(Ph)R or its boronꢁboron bonded
,
3.077(8) and 3.247(8) A are somewhat longer than those
,
found in Hg(C6H4CH2NMe2-2)2 (X) [2.89(1) A] [21] or
,
the 2-pyridylphenyl complex VI [2.798(7) A] [16], but
still fall well within the sum of the van der Waals radii
,
for nitrogen (1.55 A) [22] and a reasonable value of 1.73
,
A for mercury [23]. Hence we suggest that there is a
single weak Hg···N bonding interaction in 2 involving
each of the R− ligands. This is consistent with (i) the
slight deviation from the sp2 value in the angles
C(1)ꢁC(2)ꢁN(1) and C(1)ꢁC(6)ꢁN(2) and (ii) magnitudes
of the HgꢁC(1)ꢁC(2/6) angles which demonstrate that
the fragment incorporating N(1) is slightly bent towards
the mercury atom. The HgꢁC(1) bond length of 2.081(8)
,
,
A in 2 is close to the 2.085(7) A in HgPh2 [24], the 2.10(2)
,
,
A in X [20] and the 2.098(8) A in VI [16]. The mercury
Fig. 1. The X-ray crystal structure of complex 2.
,
atom in 2 is 0.41 A out of the C6 aromatic mean plane,
a situation similar to that in HgPh2 in which, however,
Table 3
,
Selected bond distances (A) and angles (°) for 2
,
the deviation is only 0.1 A [24]. The HgꢁC(1)···C(4) angle
in 2 is 168.7(6)°, the molecule being slightly bent at the
C(1) atom. The phenyl planes are parallel to one an-
other.
Bond distances
Bond angles
HgꢁC(1)
Hg···N(1)
Hg···N(2)
2.081(8)
3.077(8)
3.247(8)
C(1)ꢁHgꢁC(1%)
C(1)ꢁC(2)ꢁN(1)
C(1)ꢁC(6)ꢁN(2)
HgꢁC(1)ꢁC(2)
HgꢁC(1)ꢁC(6)
HgꢁC(1)···C(4)
180
116.7(7)
117.6(7)
117.0(5)
123.3(6)
168.7(6)
3.2. The molecular structure of BCl(Ph)R (3)
The molecular structure of crystalline BCl(Ph)R (3) is
illustrated in Fig. 2; selected bond distances and bond
angles are in Table 4.
Symmetry element % is −x, −y, −z.

D. Cornu et al. / Polyhedron 21 (2002) 635–640
639
2
,
NMe2-2){k -O,O%ꢁOCH2C(Ph)2O} of 1.754(4) A [30] is
similar to that in 3.
4. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Centre, CCDC Nos. 170829 for 2 and 3. Copies of this
information may be obtained free of charge from The
Director, CCDC 12 Union Road, Cambridge CB2 1EZ,
UK (fax: +44-1223-366-033; e-mail: deposit@ccdc.
cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgements
Fig. 2. The X-ray crystal structure of complex 3.
We thank the European Commission for providing a
TMR Category 20 studentship to P.G.H.U. and the
University of Lyon II for granting a period of study
leave to D.C.
Table 4
Selected bond distances (A) and angles (°) for 3
,
Bond distances
Bond angles
BꢁC(1)
BꢁC(11)
BꢁCl
1.599(5)
1.591(5)
1.865(4)
1.740(4)
C(1)ꢁC(2)ꢁN(1)
BꢁC(1)ꢁC(2)
C(1)ꢁBꢁN(1)
BꢁN(1)ꢁC(2)
101.1(3)
92.4(3)
82.3(2)
83.5(2)
References
B···N(1)
[1] C. Drost, P.B. Hitchcock, M.F. Lappert, L.J.-M. Pierssens,
Chem. Commun. (1997) 1141.
[2] S. Harder, J. Boersma, L. Brandsma, J.A. Kanters, W. Bauer,
P.v.R. Schleyer, Organometallics 8 (1989) 1696.
[3] J.T.B.H. Jastrzebski, P.A. van der Schaaf, J. Boersma, G. van
Koten, M.C. Zoutberg, D. Heijdenrijk, Organometallics 8 (1989)
1373.
Crystalline 3 is a monomer, with the boron atom in
a four-coordinate, distorted tetrahedral environment.
This distortion is evidently caused by the chelating R−
ligand, as reflected in the significantly narrower than
the sp2 value of the endocyclic angles at C(1) and C(2).
The latter is also much narrower than in, for example,
SnR2 (II), 113.6(4)° [1], which is attributed to the
relative sizes of the boron and tin atoms, BꢀSn. The
BꢁN(1)ꢁC(2)ꢁC(1) ring is planar, the sum of the endo-
cyclic bond angles being 359.3°.
[4] (a) P.B. Hitchcock, M.F. Lappert, P.G.H. Uiterweerd, in
preparation;
(b) P.G.H. Uiterweerd, D.Phil. Thesis, University of Sussex,
2001.
[5] G. Friedmann, M. Brini, P. Ederle, J. Gasser, P.-J. Holderith,
M. Vernois, J.-M. Widmaier, Bull. Soc. Chim. Fr. (1970) 706.
[6] C. Drost, P.B. Hitchcock, M.F. Lappert, Organometallics 17
(1998) 3838.
[7] C. Drost, P.B. Hitchcock, M.F. Lappert, Angew. Chem., Int.
Ed. Engl. 38 (1999) 1113.
,
The BꢁCl bond in 3, 1.865(4) A, is significantly
[8] C. Drost, B. Gehrhus, P.B. Hitchcock, M.F. Lappert, Chem.
Commun. (1997) 1845.
[9] G.M. Sheldrick, SHELXS-97, Program for the Solution of Crystal
Structures, University of Go¨ttingen, 1997.
[10] G.M. Sheldrick, SHELXL-97, Program for Crystal Structure
Refinement, University of Go¨ttingen, 1997.
[11] A. Hudson, M.F. Lappert, P.W. Lednor, J. Chem. Soc., Dalton
Trans. (1976) 2369.
longer than in the three-coordinate boron chloride
,
B(C6F5)2Cl, 1.746(5) A [25], but is similar to those in
the four-coordinated chloroboranes B(Cl)Ph2(THF),
1.893(2) A [26], IX (av. 1.86 A) [27], and the pyridine
adduct of 1-chloro-1-borafluorene XI, 1.898(2) A [28].
The mean BꢁC distance in 3 of 1.59 A is slightly
longer than in B(C6F5)2Cl, 1.56 A [25], but similar to
that in the four-coordinated boron chlorides IX, 1.60 A
,
,
,
,
[12] M.J.S. Gynane, D.H. Harris, M.F. Lappert, P.P. Power, P.
Rivie`re, M. Rivie`re-Baudet, J. Chem. Soc., Dalton Trans. (1977)
2004.
,
,
,
[27], or XI, 1.61 A [28]. The B···N(1) distance in 3 is
[13] D. Bravo-Zhivotovskii, M. Yuzefovich,
M Bendikov, K.
significantly longer than in B(Br)2(k2-C,NꢁC6H4CH2-
Klinkhammer, Y. Apeloig, Angew. Chem., Int. Ed. Engl. 38
(1999) 1100.
,
,
NMe2-2), 1.625(1) A [29] or IX, 1.649(5) A [27]. Evi-
[14] Cf. A.G. Davies, J.L. Wardell, Comprehensive Organometallic
Chemistry, In: E.W. Abel, F.G.A. Stone, G. Wilkinson (Eds.),
2nd ed., vol. 3 (J.L. Wardell, ed.), Pergamon, Oxford, 1995.
[15] Cf. B. Wrackmeyer, R. Contreras, Ann. Rep. NMR Spectrosc.
24 (1992) 267.
dently the methylene group in the latter two complexes
facilitates the R%N“B coordination (R%=Me or Et),
whereas this effect in 3 is subject to greater strain. The
B···N separation in the spiro compound B(C6H4CH2-
2

640
D. Cornu et al. / Polyhedron 21 (2002) 635–640
[16] D.St.C. Black, G.B. Deacon, G.L. Edwards, B.M. Gatehouse,
Aust. J. Chem. 46 (1993) 1323.
[24] D. Grdenic´, B. Kamenar, A. Nagl, Acta Crystallogr., Sect. B 33
(1977) 587.
[17] A.J. Canty, P. Barron, P.C. Healey, J. Organomet. Chem. 179
[25] W.E. Piers, R.E.v.H. Spence, L.R. MacGillivray, M.J. Za-
worotko, Acta Crystallogr., Sect. C 51 (1995) 1688.
[26] W.I. Cross, M.P. Lightfoot, F.S. Mair, R.G. Pritchard, Inorg.
Chem. 39 (2000) 2690.
(1979) 447.
(
[18] S. Toyota, T. Futakawa, H. Ikeda, M. Oki, J. Chem. Soc.,
Chem. Commun. (1995) 2499.
[19] M. Lauer, G. Wulff, J. Organomet. Chem. 256 (1983) 1.
[20] R. Schlengermann, J. Sieler, S. Jelonek, E. Hey-Hawkins, Chem.
Commun. (1997) 197.
[21] J.L. Atwood, D.E. Berry, S.R. Stobart, M.J. Zaworotko, Inorg.
Chem. 22 (1983) 3480.
[27] R. Schlengermann, J. Sieler, E. Hey-Hawkins, Main Group
Chem. 2 (1997) 141.
[28] C.K. Narula, H. No¨th, Inorg. Chem. 24 (1985) 2532.
[29] D.S. Brown, C.J. Carmalt, A.H. Cowley, A. Decken, H.S. Isom,
Heteroatom Chem. 9 (1998) 79.
(
[22] A. Bondi, J. Phys. Chem. 68 (1964) 441.
[23] A.J. Canty, G.B. Deacon, Inorg. Chim. Acta 45 (1980) L225.
[30] S. Toyota, M. Oki, Bull. Chem. Soc. Jpn. 65 (1992)
1832.