Cationic nickel(II) complexes with azine diphosphines - Structural and electrochemical study

Article abstract of DOI:10.1016/S0020-1693(02)00982-9

Nickel(II) complexes of azine diphosphine ligands, PR₂CH₂C(Bu^t)NNC(Bu^t)CH ₂PR₂ (R=Ph, Bu^t, Prⁱ, C₆H₁₁), were prepared for the first time by reactions of anhydrous NiX₂ (X=Cl, Br, I) with ligands. Complexes are cationic, the counterion being either simple halide anion or, in two cases, 1/2[NiCl₄]^2-. The azine diphosphines are coordinated terdentately in (E,Z) configuration forming thereby a bicyclic ligand frame (five and six-membered ring) with diphosphine bite angles 160-165°. A balance of stereoelectronic factors governing square planar versus tetrahedral coordination of the ligands is suggested. Structures of four of the complexes with bulky ligands (R=Bu^t, X^-=Cl^-, Br^-, I^-; R=C₆H₁₁, X^-=1/2[NiCl]^2-) were determined by X-ray diffraction and qualitative Hu?ckel MO calculations on model undistorted square planar [trans-NiX(NH₂)(PH₃)₂]⁺ were carried out to elucidate the contributions of steric and electronic factors. Two reduction and two oxidation processes depending on the substituent R and the halide were identified from cyclic voltammetry data on the complexes.

Full text of DOI:10.1016/S0020-1693(02)00982-9

Inorganica Chimica Acta 338 (2002) 201ꢂ209
/
www.elsevier.com/locate/ica
Cationic nickel(II) complexes with azine diphosphines*
/
structural
and electrochemical study
Fernanda M.T. Almeida a, M. Fernanda N.N. Carvalho a, Adelino M. Galvao a,
˜
Jan Cerma´k *, Vratislav Blechta , Armando J.L. Pombeiro , Bernard L. Shaw
b,
b
a
c
ˇ
a Centro de Qu´ımica Estrutural, Complexo I, Instituto Superior Te´cnico, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
b Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Rozvojova 135, CZ-16502 Prague 6, Suchdol, Czech
Republic
c University of Leeds, Leeds LS2 9JT, UK
Received 18 February 2002; accepted 8 April 2002
Abstract
Nickel(II) complexes of azine diphosphine ligands, PR2CH2C(But)NNC(But)CH2PR2 (Rꢀ
for the first time by reactions of anhydrous NiX2 (XꢀCl, Br, I) with ligands. Complexes are cationic, the counterion being either
simple halide anion or, in two cases, 1/2[NiCl4]2ꢁ. The azine diphosphines are coordinated terdentately in (E,Z) configuration
forming thereby a bicyclic ligand frame (five and six-membered ring) with diphosphine bite angles 160ꢂ1658. A balance of
stereoelectronic factors governing square planar versus tetrahedral coordination of the ligands is suggested. Structures of four of the
complexes with bulky ligands (Rꢀ
But, Xꢁ ꢀClꢁ, Brꢁ, Iꢁ; RꢀC6H11, Xꢁ ꢀ1/2[NiCl]2ꢁ) were determined by X-ray diffraction
and qualitative Huckel MO calculations on model undistorted square planar [trans-NiX(NH )(PH ) ]ꢃ were carried out to
/
Ph, But, Pri, C6H11), were prepared
/
/
/
/
/
/
¨
2
3 2
elucidate the contributions of steric and electronic factors. Two reduction and two oxidation processes depending on the substituent
R and the halide were identified from cyclic voltammetry data on the complexes.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Diphosphines; Bite angle; Nickel complexes; PNP ligands; Cyclic voltammetry
1. Introduction
catalyst in cross-coupling of Grignard reagents with
alkyl bromides [2].
Transition metal complexes with rigid terdentate Xꢂ
/
A similar Pꢀ
/
Cꢀ
/
P ligand backbone was studied by
YꢂZ ligands (sometimes called ‘pincer ligand’) are of
/
Milstein et al. in a search for stoichiometric and possibly
catalytic activation of strong bonds, not amenable to
activation by usual means. In this way, solvent-con-
considerable interest in organometallic chemistry and
catalysis by transition metals and were extensively
studied by several groups in recent years. Various
combinations of hard or soft atoms X, Y, Z are possible.
Van Koten and coworkers introduced a combination
of two nitrogen atoms and one carbon atom to form an
trolled selectivity towards Cꢀ
as well as CꢀCF3 bond activation [4] and Cꢀ
activation, were achieved.
/
H or Cꢀ/C activation [3],
/
/Si [5] bond
The pioneering work of the two above mentioned
groups has been extended recently to other donor atom
Nꢀ
/
Cꢀ
/
N ligand frame supplying the transition metal
center with electron density while at the same time
keeping it in a strictly T-shape ligand environment [1].
The ligand frame was used to prepare active nickel
catalysts for the Kharasch addition of halomethanes to
alkenes; dendritic versions of the catalysts were also
developed, manganese(II) complex was used as a
combinations like, e.g. Pꢀ
/
Cꢀ
/
N [6], Oꢀ
/
Cꢀ
/
O [7], Sꢀ
/
Cꢀ
/
S
[8] and PꢀOꢀP [9]. All the so far mentioned ligand
/
/
frames share a common feature, which is the formation
of two annulated, equal-size metallarings (usually five-
membered).
Shaw and coworkers [10ꢂ
PꢀNꢀ ligand, bis(diphenylphosphino)pinacolone
azine, which coordinates to metals with formation of
/
12] developed a terdentate
/
/
P
* Corresponding author. Fax: ꢃ420-2-2092 0661
/
ˇ
E-mail address: cermak@icpf.cas.cz (J. Cerma´k).
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 0 9 8 2 - 9

202
F.M.T. Almeida et al. / Inorganica Chimica Acta 338 (2002) 201ꢂ209
/
two unequal-size condensed metallarings. In such com-
plexes, the two phosphine arms of different lengths are
expected to have different coordination properties. This
is a good basis for increased stoichiometric and possibly
catalytic reactivity of the complexes. In addition, the
ability of the ligand to promote migration of a proton
from ligand backbone to metal [13] or another ligand
[14] results in the even more rigid ene-hydrazone
coordination mode.
Recently, we synthesized four new bis(dialkyl- or
diarylphosphino)pinacolone
azines
and
their
chloro(h3-methallyl)palladium complexes. In this case,
the azine diphosphine ligands bridged two palladium
units, in agreement with their recognized high coordina-
tion versatility. The electronic and steric properties of
the complexes were examined by NMR [15].
Scheme 1. Synthesis of cationic nickel(II) complexes.
We now extend the study of the coordination ability
of bis(dialkyl- or diarylphosphino)pinacolone azines to
nickel. The syntheses and characterization of new
Table 1
Reaction conditions (Scheme 1)
R
X
Reaction time (h)
Yield (%)
cationic chloroꢂ
of the type [NiX(Pꢀ
diphosphines (PꢀNꢀPꢁ
CH2PR2; Rꢀtert-butyl, iso-propyl, cyclohexyl, phenyl)
are reported. In order to fully characterize the com-
pounds the crystal structures of four of them (Rꢀ
But,
XꢀCl, Br or I and Rꢀcyclohexyl, XꢀCl) were
determined by X-ray diffraction.
/
, bromoꢂ
Nꢀ
P)]ꢃXꢁ, with several azine
R2PCH2C(But)ꢁ C(But)-
NꢀNꢁ
/
, and iodoꢂ
/
Ni(II) complexes
Ph
Cl 1a
Br 1b
3
49
72
45
94
97
72
45
77
37
64
95
41
/
/
a
3
1c 0.5
/
/
/
/
/
/
I
Cl 2a
/
But
3
Br 2b 1.5
2c 2.5
/
I
a
a
a
Pri
Cl 3a 24
Br 3b 24
/
/
/
I
3c 24
The effect of the R groups on the redox properties of
the complexes was studied by cyclic voltammetry.
C6H11
Cl 4a
Br 4b
2
2
2
I
4c
a
Ambient temperature.
2. Experimental
measured by using the [Fe(h5-C5H5)2]0/ꢃ redox couple
(Eꢀ0.54 V vs. SCE) as internal standard.
2.1. Materials and methods
/
The magnetic susceptibilities were measured at room
temperature (r.t.) using a Johnson Matthey magnetic
susceptibility balance.
All the manipulations were carried out under argon or
dinitrogen atmosphere using standard Schlenk glass-
ware techniques. The solvents were dried by standard
methods before use. Anhydrous nickel(II) halides [16],
except nickel(II) iodide (Aldrich) and bis(dialkylpho-
sphino)pinacolone azines [15], were prepared by pub-
lished methods.
2.2. Syntheses of the azine diphosphines nickel complexes
As a general procedure, dry acetone (20ꢂ
30 ml gꢁ1 of
/
The NMR spectra were recorded on a Varian UNITY
1
200 spectrometer at 200.1, 50.3, and 81.0 MHz for H,
azine diphosphine) was added to equimolar amounts of
azine diphosphine and anhydrous nickel(II) halide and
the mixture stirred at r.t. or under reflux for a variable
period of time. After cooling to ambient temperature (if
necessary) a small amount of precipitate was filtered off,
the solution was evaporated to approximately half of the
initial volume and an equivalent volume of dry diethyl
ether was added. After several days, a precipitate
appeared that was filtered off and dried under vacuum.
In order to obtain analytically pure samples, the
13C and 31P, respectively, or on a Varian UNITY 500
1
spectrometer at 499.9 and 125.7 MHz for H and 13C,
respectively. The spectra were referenced to internal
hexamethyldisilane (1H, dꢀ
/
0.04 and 13C, dꢀ
/
ꢁ2.49) or
/
external H3PO4 (31P, dꢀ
0.0).
/
Mass spectra were measured with Varian CH5 (EI
mode, 70 eV) and LCQ Finnigan (ESI) instruments.
The electrochemical studies were performed at an
EG&G PAR 173 potentiostat/galvanostat and an
EG&G PARC 175 universal programmer, using a 0.2
products were recrystallized from chloroformꢂ
/diethyl
ether.
The reaction time, temperature and isolated yields of
the products (Scheme 1) are displayed in Table 1.
M [NBu4][BF4]ꢂ/THF or CH2Cl2 solution and a Pt-wire
as the working electrode. The potential values were

F.M.T. Almeida et al. / Inorganica Chimica Acta 338 (2002) 201ꢂ
/209
203
Suitable crystals for X-ray analysis were obtained
directly from the reaction solution for 2c and 4a, while
for 2a and 2b, the crystals were obtained upon recrys-
4.07 (t, J not resolved, 2H); 13C{1H} NMR (CDCl3):
16.61 (s, CH2), 29.20 (s, C(CH3)3), 29.38 (s, C(CH3)3),
30.26 (s, PC(CH3)3), 30.9 (d, 2JPC
ꢀ
/
1.5 Hz, PC(CH3)3),
3.9 Hz, PC(CH3)3),
3.0 Hz, PC(CH3)3),
1
37.56 (dd, JPC
3
11.2 Hz, JPC
tallization from chloroformꢂ
/
diethyl ether using the
ꢀ
ꢀ
/
ꢀ
/
1
37.78 (dd, JPC
3
11.0 Hz, JPC
technique of solvent diffusion from gas phase.
/
ꢀ
/
3
41.23 (d, JPC
3
ꢀ
/
4.0 Hz, C(CH3)3), 41.48 (d, JPC
ꢀ3.9
/
2.2.1. Characterization of products
Hz, C(CH3)3), 41.8 (d, 1JPC
N), 189.68 (s, CꢁN).
ꢀ
/
16.0 Hz, CH2), 177.8 (s, Cꢁ
/
/
2.2.1.1. [NiCl{PPh2CH2C(But)NNC-
(But)CH2PPh2}]2[NiCl4] (1a). Anal. Calc. for 1a×
/
2.2.1.6. [NiI{PBut2CH2C(But)NNC(But)CH2PBut2}]I
(2c). 31P{1H} NMR (CDCl3): 79.1, 87.5 (AB system
1.5CHCl3 (C72H84Cl6N4P4Ni3×
5.1; N, 3.3. Found: C, 52.3; H, 5.4; N, 3.2%.
Mass spectrum (FAB), m/z: 657 (Mꢃ ꢀ
(NiCl3).
/
1.5CHCl3): C, 52.0; H,
1
2JPP
(d, 3JPH
2.31 (d, JPH
ꢀ
/
311 Hz); H NMR (CDCl3): 1.37 (s, 18H), 1.65
/
Cl), 163
ꢀ
/
13.2 Hz, 18H), 1.70 (d, 3JPH
ꢀ13.2 Hz, 18H),
/
2
ꢀ10.0 Hz, 2H), 4.08 (t, J not resolved,
/
2H); 13C{1H} NMR (CDCl3): 15.51 (s, CH2), 28.22 (s,
C(CH3)3), 29.22 (s, C(CH3)3), 30.23 (s, PC(CH3)3), 30.88
2.2.1.2. [NiBr{PPh2CH2C(But)NNC-
(But)CH2PPh2}]Br (1b). Anal. Calc. for 1b×
(C36H42Br2N2P2Ni×CHCl3): C, 49.2; H, 4.8; N, 3.1.
Found: C, 49.3; H, 5.1; N, 3.0%.
Mass spectrum (FAB), m/z: 703 (Mꢃ ꢀ
/CHCl3
(s, PC(CH3)3), 37.63 (d, 1JPC
ꢀ16.9 Hz, CH2), 38.0 (dd,
/
3
10.0 Hz, JPC
/
1JPC
1JPC
3JPC
ꢀ
/
ꢀ
/
2.5 Hz, PC(CH3)3), 38.59 (dd,
3
11.7 Hz, JPC
ꢀ
/
ꢀ/3.4 Hz, PC(CH3)3), 41.23 (d,
3
4.1 Hz, C(CH3)3), 41.36 (d, JPC
/Br).
ꢀ
/
ꢀ
ꢀ
/
2.3 Hz,
2
N), 187.59 (dd, JPC
C(CH3)3), 177.99 (s, Cꢁ
/
/5.9 Hz,
2.2.1.3. [NiI{PPh2CH2C(But)NNC(But)CH2PPh2}]I
(1c). Anal. Calc. for 1c×0.75CHCl3 (C36H42I2N2P2Ni×
0.75CHCl3): C, 45.6; H, 4.4; N, 2.9. Found: C, 45.8; H,
4JPC
ꢀ
/
2.3 Hz, CꢁN).
/
/
/
2.2.1.7. [NiCl{PPri2CH2C(But)NNC-
(But)CH2PPr2i }]Cl (3a). 31P{1H} NMR (CDCl3): 62.0;
1H NMR (CDCl3): 1.28 (s, 9H), 1.35 (s, 9H), 1.41ꢂ
1.60
4.6; N, 2.5%.
Mass spectrum (FAB), m/z: 749 (Mꢃ ꢀ
/
I).
31P{1H} NMR (CDCl3): 46.3, 51.0 (AB system,
/
(m, 24H), 2.41 (b, 2H), 2.46 (m, 4H), 3.9 (b, 2H);
13C{1H} NMR (CDCl3): 14.53 (bs, CH2), 17.87 (s,
CH3), 18.17 (s, CH3), 18.64 (s, CH3), 18.69 (s, CH3),
1
2JPP
ꢀ
/
344 Hz); H NMR (CDCl3): 0.65 (s, 9H), 1.02
(s, 9H), 3.43 (b, 2JPH
8.2 (bm, 20H); 13C{1H} NMR (CDCl3): 26.10 (d,
1JPC
12.0 Hz, CH2), 26.72 (s, CH3), 28.06 (s, CH3),
40.18 (s, C(CH3)3), 40.43 (s, C(CH3)3), 46.18 (d, 1JPC
25.2 Hz, CH2), 128.10 (d, Jꢀ8.7 Hz, ꢀCHꢁ), 128.62 (d,
Jꢀ6.9 Hz, ꢀCHꢁ), 128.65 (d, Jꢀca. 45 Hz, ꢂCHꢁ),
129.63 (d, Jꢀca. 45 Hz, ꢂCHꢁ), 131.53 (d, Jꢀ19.2
Hz, ꢀCHꢁ), 134.32 (d, Jꢀca. 6 Hz, ꢀCHꢁ), 135.66 (s,
CHꢁ), 171.77 (s, CꢁN), 186.31 (s, CꢁN).
ꢀ
/
ca. 8 Hz, 2H), 4.41 (b, 4H), 7.3ꢂ
/
1
23.15 (t, j JPCꢃ3
/
JPCjꢀ
/
12 Hz, CH), 23.76 (t,
12 Hz, CH), 27.19 (s, C(CH3)3), 28.15
1
ꢀ
/
j JPCꢃ3
/
JPCjꢀ
/
ꢀ
/
(s, C(CH3)3), 33.14 (m, J not resolved, CH2), 40.74 (s,
/
/
/
C(CH3)3), 41.74 (s, C(CH3)3), 174.81 (s, Cꢁ
/N), 193.49
/
/
/
/
/
(s, CꢁN).
/
/
/
/
/
/
/
/
/
2.2.1.8. [NiBr{PPri2CH2C(But)NNC-
(But)CH2PPr2i }]Br (3b). 31P{1H} NMR (CDCl3): 64.2,
ꢀ
/
/
/
/
2
65.0 (AB system, JPP
1
ꢀ
/
325 Hz); H NMR (CDCl3):
1.73 (m, 24H), 2.46 (b,
2.2.1.4. [NiCl{PBut2CH2C(But)NNC-
1.32 (s, 9H), 1.51 (s, 9H), 1.59ꢂ
/
(But)CH2PBu2t }]Cl (2a). 31P{1H} NMR (CDCl3): 64.4,
2H), 2.54 (m, 4H), 3.84 (b, 2H); 13C{1H} NMR
(CDCl3): 16.88 (bs, CH2), 18.40 (s, CH3), 18.91 (s,
CH3), 19.13 (s, CH3), 19.45 (s, CH3), 24.11 (m, J not
2
70.5 (AB system JPP
ꢀ
325 Hz); 1H NMR (CDCl3):
/
1.38 (s, 18H), 1.66 (b, 36H), 2.32 (b, 2H), 4.00 (b, 2H);
13C{1H} NMR (CDCl3): 14.75 (s, CH2), 28.05 (s,
C(CH3)3), 28.81 (s, C(CH3)3), 29.35 (s, PC(CH3)3),
resolved, 2ꢄ
C(CH3)3), 37.10 (m, J not resolved, CH2), 40.54 (s,
C(CH3)3), 41.64 (s, C(CH3)3), 174.52 (s, CꢁN), 193.13
(s, CꢁN).
/CH), 27.31 (s, C(CH3)3), 28.78 (s,
2
1
29.90 (d, JPC
16.1 Hz, CH2), 36.85 (dd, JPC
ꢀ
/
1.5 Hz, PC(CH3)3), 35.82 (d, JPC
1
ꢀ
3.5
/
/
3
10.6 Hz, JPC
ꢀ
/
ꢀ
/
/
1
Hz, PC(CH3)3), 37.1 (dd, JPC
PC(CH3)3), 41.51 (d, 3JPC
3
9.3 Hz, JPC
3.9 Hz, C(CH3)3), 41.64 (d,
ꢀ
/
ꢀ2.9 Hz,
/
ꢀ
/
2.2.1.9. [NiI{PPri2CH2C(But)NNC(But)CH2PPri2}]I
(3c). 31P{1H} NMR (CDCl3): 70.9, 73.0 (AB system,
3JPC
N).
ꢀ
/
2.0 Hz, C(CH3)3), 177.48 (s, CꢁN), 191.22 (s, Cꢁ
/
/
1
2JPP
(dd, JHH
ꢀ
/
311 Hz); H NMR (CDCl3): 1.29 (s, 9H), 1.39
3
7.1 Hz, JPH
3
ꢀ
/
ꢀ14.6 Hz, 6H), 1.42 (s, 9H),
/
2.2.1.5. [NiBr{PBut2CH2C(But)NNC-
(But)CH2PBu2t }]Br (2b). 31P{1H} NMR (CDCl3): 68.7,
ꢀ
/
ꢀ20.6 Hz, 6H), 1.57 (dd,
/
3
1.42 (dd, JHH
3
7.3 Hz, JPH
3JHH
6.9 Hz, JPH
2.52 (m, 4H), 3.91(dd, JPH
ꢀ
/
7.2 Hz, JPH
ꢀ
/
15.7 Hz, 6H), 1.60 (dd, JHH
ꢀ
/
3
3
75.0 (AB system 2JPP
ꢀ
/
322 Hz); 1H NMR (CDCl3): 1.43
ꢀ
/
15.6 Hz, 6H), 2.49 (2JPH
ꢀ10.1 Hz, 2H),
/
3
(s, 9H), 1.54 (s, 9H), 1.74 (d, 3JPH
ꢀ/17.9 Hz, 18H), 1.81
ꢀ
/
ꢀ3.5 Hz,
/
2
2
7.3 Hz, JHH
3
(d, JPH
2
17.8 Hz, 18H), 2.43 (d, JPH
ꢀ
/
ꢀ/9.4 Hz, 2H),
2H); 13C{1H} NMR (CDCl3): 15.72 (bs, CH2), 18.57 (s,

204
F.M.T. Almeida et al. / Inorganica Chimica Acta 338 (2002) 201ꢂ209
/
Table 2
Crystallographic data for complexes of general formula [NiX(PR2CH2C(But)NNC(But)CH2PR2)]Y (2a RꢀBut, XꢀYꢀCl; 2b RꢀBut, XꢀBr: 2c
RꢀBut, XꢀI; 4a RꢀCy, XꢀCl, Xꢁ ꢀ1/2[NiCl4]2ꢁ
)
2c
4a
2b
2a
Formula
M
C28H58I2N2NiP2×C3H6O C36H66Cl3N2Ni1.50P2 C28H58Br2N2NiP2×CHCl3 C28H58Cl2N2NiP2×2CHCl3
855.42
monoclinic
C2/c
783.39
monoclinic
C2/c
822.71
monoclinic
C2/c
853.15
monoclinic
P21/n
Crystal system
Space group
˚
a (A)
27.212(2)
11.981(1)
24.712(2)
102.09(1)
7878
19.862(5)
14.291(3)
30.20(1)
100.19(3)
8437
28.196(5)
11.8507(7)
23.943(2)
103.67(1)
7774
11.054(4)
25.61(2)
15.43(1)
99.78(4)
4305
˚
b (A)
˚
c (A)
b (8)
3
U (A )
˚
Z
8
1.442
8
1.233
8
1.404
4
1.313
Dcalc (g cmꢁ3
)
m (mmꢁ1) (Mo Ka)
F(000)
2.024
3488
7075
0.967
3352
7627
2.773
3400
6948
0.975
1784
7853
Number of reflections measured
Number of unique reflections (Rint
R1 (I ꢀ2s(I))
)
6874 (0.055)
0.04
0.11
7399 (0.03)
0.08
0.17
6801 (0.08)
0.09
0.13
7562 (0.06)
0.07
0.11
wR2
1
CH3), 19.38 (s, CH3), 19.38 (s, CH3), 19.80 (s, CH3),
1
25.41 (dd, JPC
64.2; H NMR (CDCl3): 1.25ꢂ
/
1.40 (m, 20H), 1.29 (s,
3
22.0 Hz, JPC
ꢀ
/
ꢀ
/
4.6 Hz, CH), 25.65
3.7 Hz, CH), 27.46 (s,
9H), 1.38 (s, 9H), 1.75ꢂ2.00 (m, 20H), 2.28 (b, 4H), 2.36
/
1
(dd, JPC
3
21.1 Hz, JPC
ꢀ
/
ꢀ
/
(b, 2H), 3.85 (b, 2H); 13C{1H} NMR (CDCl3): 16.15 (bt,
J not resolved, CH2), 25.47 (s, CH2), 26.42 (s, CH2),
26.53 (s, CH2), 26.66 (s, CH2), 27.53 (s, C(CH3)3), 28.46
(s, C(CH3)3), 28.59 (s, CH2), 29.24 (s, CH2), 34.23 (t,
1
C(CH3)3), 28.80 (s, C(CH3)3), 36.22 (d, JPC
ꢀ
/
16.5
3.7 Hz,
3
Hz, CH2), 40.81 (s, C(CH3)3), 41.76 (d, JPC
C(CH3)3), 174.52 (s, CꢁN), 192.20 (s, CꢁN).
ꢀ
/
/
/
j JPCꢃ3
/
JPCjꢀ
/
12.6 Hz, CH), 34.98 (t, j JPCꢃ3
/
JPCjꢀ
/
1
1
2.2.1.10. [NiCl{P(C6H11)2CH2C(But)NNC-
11.7 Hz, CH), 36.43 (t, j JPCꢃ3
/
JPCjꢀ
/
9.0 Hz, CH2),
N),
1
(But)CH2P(C6H11)2}]2[NiCl4] (4a). 31P{1H} NMR
40.80 (s, C(CH3)3), 41.46 (s, C(CH3)3), 175.1 (s, Cꢁ
/
1
2
191.38 (d, JPC
(CDCl3): 53.7; H NMR (CDCl3): 1.20ꢂ
/1.45 (m, 20H),
ꢀ
/
5.0 Hz, Cꢁ/N).
1.37 (s, 9H), 1.7ꢂ
/
2.0 (m, 20H), 2.15ꢂ
/
2.30 (m, 4H), 2.39
(b, 2H), 3.89 (b, 2H); 13C{1H} NMR (CDCl3): 15.61 (bs,
CH2), 25.59 (s, CH2), 25.63 (s, CH2), 26.58 (s, CH2),
26.91 (s, CH2), 27.49 (s C(CH3)3), 27.98 (s, CH2), 28.03
2.3. X-ray crystallographic analysis
Dark red (2a, 2b) or black (2c, 4a) crystals were
mounted in thin-walled glass capillaries and X-ray data
collected at r.t. on an Enraf-Nonius MACH3 diffract-
ometer with graphite-monochromatized Mo Ka radia-
(s, CH2), 28.13 (s, CH2), 28.42 (s, C(CH3)3), 29.08 (s,
1
CH2), 32.54 (t, j JPCꢃ3
/
JPCjꢀ
/
10 Hz, CH), 33.67 (t,
1
j JPCꢃ3
/
JPCjꢀ
/
10 Hz, CH), 33.95 (bt, j JPCꢃ3
/
JPCjꢀ
/
10
Hz, CH2), 40.93 (s, C(CH3)3), 41.77 (s, C(CH3)3),
1
tion, using an v ꢂ2u scan mode. Unit cell dimensions
/
175.37 (s, Cꢁ
/
N), 193.19 (s, Cꢁ
/
N).
were obtained by least-squares refinement of the setting
angles of 25 reflections.
2.2.1.11. [NiBr{P(C6H11)2CH2C(But)NNC-
(But)CH2P(C6H11)2}]Br
The data were corrected [17] for Lorentz and polar-
ization effects (Lp), for linear decay and empirically for
absorption. The heavy atom positions were located by
Patterson methods using SHELXS-86 [18]. The remaining
atoms were located in successive Fourier-difference
maps and refined by least-squares on F2 using
SHELXL-93 [19]. The crystal data are summarized in
Table 2.
(4b).
31P{1H}
NMR
1.40 (m, 20H),
1
(CDCl3): 64.2; H NMR (CDCl3): 1.25ꢂ
/
1.28 (s, 9H), 1.40 (s, 9H) 1.75ꢂ
/
2.00 (m, 20H), 2.20ꢂ2.35
/
(m, 4H), 2.42 (b, 2H), 3.91 (b, 2H); 13C{1H} NMR
(CDCl3): 16.39 (bs, CH2), 25.62 (s, CH2), 26.60 (s, CH2),
26.74 (s, CH2), 26.86 (s, CH2), 27.63 (s C(CH3)3), 28.76
(s, CH2), 28.76 (s, C(CH3)3), 28.84 (s, CH2), 29.44 (s,
1
CH2), 34.55 (t, j JPCꢃ3
/
JPCjꢀ
/
12.7 Hz, CH), 35.18 (t,
1
Solvent molecules were located in the Fourier-differ-
ence maps for complexes 2; chloroform in the cases of 2a
or 2b or acetone in the case of 2c.
1
j JPCꢃ3
/
JPCjꢀ
/
11.7 Hz, CH), 36.78 (t, j JPCꢃ3
/
JPCjꢀ
/
8.7
Hz, CH2), 40.87 (s, C(CH3)3), 41.62 (s, C(CH3)3),
174.93 (s, Cꢁ
/
N), 191.75 (s, Cꢁ
/
N).
All the non-hydrogen atoms were refined with aniso-
tropic thermal motion parameters. The hydrogen atoms
were included in calculated positions, constrained to
ride at fixed distances of the parent carbon atom.
2.2.1.12. [NiI{P(C6H11)2CH2C(But)NNC-
(But)CH2P(C6H11)2}]I (4c). 31P{1H} NMR (CDCl3):

F.M.T. Almeida et al. / Inorganica Chimica Acta 338 (2002) 201ꢂ
/209
205
Fig. 2. ORTEP view of molecular structure of [NiCl{P(C6H11)2-
CH2C(But)NNC(But)CH2P(C6H11)2}][NiCl4] in 4a. Selected atoms
are labeled and hydrogen atoms are omitted for clarity.
Rꢀ
Br, I (2a, 2b, 2c); Rꢀ
RꢀC6H11, XꢀCl, Yꢀ
/
Ph, Xꢀ
/
Yꢀ
/
Br, I (1b or 1c); Rꢀ
Pri, Xꢀ
YꢀCl, Br, I (3a, 3b, 3c);
1/2[NiCl4]2ꢁ (4a); Xꢀ
YꢀBr,
/
But, Xꢀ
/
Yꢀ
/
Cl,
/
/
/
/
/
/
/
/
I (4b, 4c), were prepared from the corresponding
anhydrous nickel(II) halides and azine diphosphines.
The best yields were obtained for the nickel bromide
derivatives and they tended to increase with the bulki-
ness of the R group on phosphorus, i.e. Ph$
/
PriB
c-
/
C6H11B
/
But.
The complexes were characterized by conventional
techniques (see Section 2). The counterion of the
compounds is the halide in all complexes, but for 4a
(verified by X-rays, see Fig. 2) and 1a (on the basis of
elementary analysis and negative FAB) it is [NiCl4]2ꢁ
.
Apart from complexes [NiX{PPh2CH2C(But)-
NNC(But)CH2PPh2}]ꢃ (1) all the species display well
defined NMR spectra consistent with their formulation.
In the case of complexes 1a and 1b the 1H and 31P NMR
spectra are so broad that no attribution of the signals
could be made.
The 1H and 31P NMR spectra of complexes 2, 3 and 4
suggest that no appreciable structural differences exist
between them. In addition, the spectra of 1c and that of
Pd(II) complex [11] with the analogous bis(diphenylpho-
sphino)pinacolone azine ligand are quite similar in what
concerns the coordination of the phosphine, i.e. the
trans coordination of the two phosphorus atoms is
Fig. 1. ORTEP view of molecular structure of (a) [NiCl{PBut2CH2C-
(But)NNC(But)CH2PBut2}]ꢃ in 2a×2CHCl3, (b) [NiBr{PBu2t CH2-
C(But)NNC(But)CH2PBu2t }]ꢃ in 2b×CHCl3, (c) [NiI{PBu2t CH2C(But)-
NNC(But)CH2PBut2}]ꢃ in 2c×
(CH3)2CO. Hydrogen atoms are omitted
for clarity.
/
/
/
2
confirmed by a large JPP coupling constant (ca. 330
Atomic scattering factors and anomalous dispersion
terms were as in SHELXL-93 [19]. The ORTEP drawings
were made with ORTEX [20].
Hz). The asymmetry of the two moieties of the azine
diphosphine ligand caused by the formation of two non-
equivalent five- and a six-membered rings is clearly
identified in the proton and carbon NMR spectra by
distinct signals of all relevant (But, CH2, Cꢁ
in the two rings.
/N) groups
3. Results and discussion
The broadness of the NMR spectra of 1a and 1b is
consistent with paramagnetic character and suggests
these complexes have an electronic and geometric
structure considerably different from the other com-
pounds under study. A distortion from the essentially
3.1. Synthesis and characterization of complexes
The nickel complexes [NiX{PR2CH2C(But)NNC-
(But)CH2PR2}]Y (Rꢀ
/
Ph, Xꢀ
/
Cl, Yꢀ1/2NiCl4 (1a);
/

206
F.M.T. Almeida et al. / Inorganica Chimica Acta 338 (2002) 201ꢂ209
/
Table 3
groups in complexes 2ꢂ4, compared with phenyl. The
results herein suggest a significant dependence of the
geometry on electronic effects.
/
˚
Selected bond lengths (A) and angles (8) with e.s.d.s in parentheses for
2aꢂc and 4a
/
The geometry/magnetic properties herein are in full
agreement with those of other nickel(II) phosphine
4a
2a
2b
2c
Bond lengths
NiꢀP(1)
NiꢀP(2)
NiꢀN(4)
NiꢀX
complexes [22] that, in the case of Rꢀ
bulky iodide ligand, display a tendency to diamagnetic
planar forms, whereas in the case of Rꢀaryl and the
/alkyl and the
2.192(3)
2.199(2)
1.903(6)
2.157(3)
2.196(2)
2.246(2)
1.916(4)
2.148(3)
2.202(4)
2.260(4)
1.897(10)
2.300(2)
2.213(2)
2.268(2)
1.926(5)
2.4882(9)
/
less steric demanding chloride ligand a paramagnetic
tetrahedral form is preferred.
Bond angles
In the azine diphosphine nickel complexes 1 there is a
considerably lower electron density at the metal site (see
P(1)ꢀNiꢀN(4)
P(1)ꢀNiꢀX
P(2)ꢀNiꢀN(4)
P(2)ꢀNiꢀX
P(1)ꢀNiꢀP(2)
N(4)ꢀNiꢀX
83.0(2)
95.7(1)
91.2(2)
90.9(1)
165.4(1)
176.2(2)
84.7(2)
91.74(8)
92.6(2)
84.3(3)
92.5(1)
90.7(3)
95.9(1)
160.1(2)
168.4(3)
84.2(2)
92.29(5)
90.6(2)
electrochemical studies) than at the complexes 2ꢂ
These electronic properties conceivably are related to
the geometry of the species (tetrahedral for XꢀCl or
/4.
93.28(7)
163.9(1)
170.4(1)
96.77(5)
159.8(1)
167.1(1)
/
Br). In the case of 1c, the electronic properties do not
contribute much to the square-planar geometry, since
iodide behaves essentially as a non p-electron donor.
square-planar structure (diamagnetic), confirmed by X-
rays on complexes 2a, 2b, 2c, Fig. 1, and 4a, Fig. 2,
towards a tetrahedral (paramagnetic) geometry, in the
case of the phenyl substituent (R) on the phosphine arm
would explain the paramagnetic behavior of 1a and 1b.
The absence of an appreciable paramagnetic effect, in
the case of complex 1c, for which reasonable NMR
spectra could be obtained, points to the magnitude of
the square-planar to tetrahedral distortion being con-
trolled by the bulkiness of the X group.
3.2. Structural studies
The molecular structures of the bis(di-t-butylpho-
sphino)pinacolone azine complexes, 2aꢂc, Fig. 1, as well
/
as that of the cyclohexyl analogue 4a, Fig. 2, were
obtained by single crystal X-ray diffraction.
All the complexes show a square planar geometry
with some degree of distortion.
Selected bond lengths and angles are listed in Table 3.
From the available structural data it is evident that
We were unable to detect EPR signals in complexes 1,
either at r.t. or at low (ꢁ196 8C) temperatures. How-
/
the Niꢀ
/
P1 bond lengths in the six-membered rings are
P2 bond lengths in
ever, the effective magnetic moment values measured at
r.t. on complexes 1a (2.57 mB) and 1b (2.2 mB), although
lower than the expected spin only value for two
unpaired electrons (2.83 mB) [21], are still acceptable as
an evidence of paramagnetic character. The measure-
ment of the magnetic susceptibility of 1c gave incon-
sistent results. The meff values measured for the phenyl
substituted diphosphine azine nickel complexes 1 point
to an increasing tendency to distort from square planar
longer than the corresponding Niꢀ
/
the five-membered rings, although in the case of
complex 4a the effect is very small. This small effect is
responsible for the small difference in the value of the
31P NMR chemical shifts observed for the two phos-
phorus atoms in complexes 4 that becomes comparable
to the coupling constant and consequently the usual 31P
AB system (of the other Ni diphosphine azine com-
plexes) collapses into a single line.
to tetrahedral geometry in the order IB
/
BrBCl.
/
In order to rationalize the structural parameters
summarized in Table 3 some extended low level Huckel
This is exactly the inverse trend to that observed in the
azine diphosphine nickel complexes with non-aromatic
R groups, Table 2, structurally characterized within this
study that display square-planar geometries, with a
degree of tetrahedral distortion that increases with the
bulkiness of the coordinated halide, Table 3.
Unfortunately, no X-ray diffraction data could be
obtained for complexes 1, due to crystal disorder,
although apparently good crystals of 1a were obtained.
From the above mentioned data obtained by NMR,
magnetic susceptibility and X-rays it seems there is a fine
balance between square-planar and tetrahedral geome-
try in [NiX{PR2CH2C(But)NNC(But)CH2PR2}]ꢃ ions.
The square-planar versus tetrahedral geometry is un-
expected for the bulkier iodide compared with chloride
in complexes 1, such as it is in the case of the bulkier R
¨
MO calculations were performed using as crude model
the undistorted square-planar [trans-NiX(NH2)-
(PH3)2]ꢃ. We would like to emphasize that these
calculations are used just to qualitatively analyze the
structural trends and do not aim at any quantitative
estimate of structural parameters or energies.
The results of the calculations are depicted in Fig. 3.
The analysis of these results shows that the contribution
of the halide to the p-system drops in the series Clꢀ
BrꢀI and almost vanishes in the case of iodide. This is
also the trend observed for the differences of the Niꢀ
/
/
/P
bond lengths in the two metallacycles. Moreover, the
geometry of the two metallacycles imposes a 58 decrease
in the P1ꢀ
/
Niꢀ
/
N angle (Â
/
858) that improves the overlap
population of p-orbitals of the halide with the xz metal

F.M.T. Almeida et al. / Inorganica Chimica Acta 338 (2002) 201ꢂ
/209
207
Fig. 3. CACAO MO drawings depicting the halogen p-bonding for an ideal square planar Niꢀ
/
X (XꢀCl, Br or I) geometry.
/
orbital, further decreasing the Niꢀ
/
P1 distance when
As a general trend, the complexes display one
reversible reduction wave (I) followed by an irreversible
one (II) at a lower potential. In addition, two anodic
processes are observed for the complexes with bromide
or iodide as the counterion, which were attributed to the
halide oxidation, Table 4. Controlled potential electro-
lysis performed at the reduction potential of wave (I) on
complexes 1b and 1c is consistent with one electron per
molecule in the processes.
The free azine diphosphines, PR2CH2C(But)NNC-
(But)CH2PR2, display by cyclic voltammetry one anodic
irreversible process at potentials considerably dependent
on the R group, Table 4.
compared with a corresponding NiꢀP2 bond length.
/
From these qualitative results, it comes out that the p
interactions may be responsible for the observed trends,
since the analysis of the in plane p interactions and the
contribution of the Niꢀ
model complex, Fig. 3, shows that there is a tendency to
shorter NiꢀP1 compared with NiꢀP2 bond lengths as
P bond length on the
/
P s electron density for the
/
/
well as a dependence of the Niꢀ
/
characteristics of the coordinated halide.
In order to compare the distortion from the square-
planar geometry, induced by the R groups in the
phosphine ligand, the angle defined by the plane formed
by the metal and the two ligand sites in the five-
membered ring and the plane that contains the metal,
the halogen and the phosphorus atoms, in the six-
membered ring, was calculated. The obtained values for
4a (13.88), 2a (17.98), 2b (21.58) and 2c (22.38) show that
the distortions from the square-planar geometry in-
crease with the bulkiness of the R and the X groups. The
higher distortion imposed by the bulkier tert-butyl
compared with the cyclohexyl group is consistent with
3.3.1. Cathodic behavior
The reversible cathodic wave (I) observed with com-
plexes 1, 2 and 3 (in THF) and 4 (in CH2Cl2) is
attributed to the Ni(II)0
/
Ni(I) reduction process in
agreement with the results obtained by controlled
potential electrolysis.
In THF the electrochemical behavior of the cyclo-
hexyl diphosphine azine complexes 4 differs consider-
ably from that of the other complexes, due to slow
decomposition and the instability of the reduced species
that leads to the irreversibility of wave (I). However, in
CH2Cl2, a solvent with lower coordination ability, the
characteristics of wave (I) follow that of the other
complexes in THF.
their relative bite angles (Pꢀ
/
Niꢀ/P), Table 3.
A tentative rationalization of the degree of distortion
imposed by the halide, in complexes 2, was also made on
the basis of the calculation of the difference between the
sum of the P1ꢀ
/
Niꢀ
/
N plus P2ꢀ
/
Niꢀ
/
N angles and the P1ꢀ
/
NiꢀP bite angle. The values vary from 13.48 (Cl) to 14.98
/
(Br) and 15.08 (I), in agreement with a bigger distortion
in the case of the bulkier halide. The JPP values
The complexes 1 with the aromatic R group are
reduced at a less cathodic potential than those with the
2
measured by NMR in 2a (325 Hz), 2b (322 Hz) and 2c
(311 Hz) provide a spectroscopic evidence for this
distortion.
alkyl R group (2ꢂ4), in agreement with the expected
/
weaker electron-donor character of the aromatic azine
diphosphine ligand. However, the difference may be
accounted for by a different geometry. It is also
observed that the aryl or alkyl nature of the R group
on the diphosphine azine ligands plays an important
role on the potentials of the cathodic processes. In
addition, the halogen ligands have a marked influence
3.3. Electrochemical studies
The electrochemical behaviors of the complexes 1ꢂ
and that of the free azine diphosphine ligands were
studied by cyclic voltammetry in 0.2 M [NBu4][BF4]ꢂ
/4,
/
on the redox potentials of the Rꢀ
but not in the Rꢀaryl complexes 1.
The reduction potential values of wave (I) in complexes
2ꢂ4, follow the order ClBBrBI, which is the inverse
/
alkyl complexes 2ꢂ4,
/
THF or dichloromethane solutions using a Pt wire as
the working electrode, and the obtained experimental
data are displayed in Table 4.
/
/
/
/

208
F.M.T. Almeida et al. / Inorganica Chimica Acta 338 (2002) 201ꢂ209
/
Table 4
a
Cyclic voltammetry data for complexes [NiX(PR2CH2C(But)NNC(But)CH2PR2)]ꢃ and diphosphine azines PR2CH2C(But)NNC(But)CH2PR2
R
X
Cathodic
Complex
Anodic
Complex
IEopx
Free diphosphine
I
E
red
1/2
II ox
E
IIErped
Eopx
1/2
b
Ph
Cl
Br
I
1a
1b
1c
ꢁ0.45
ꢁ0.44
ꢁ0.43
ꢁ1.63
ꢁ1.67
ꢁ1.62
1.22
0.74
0.33
1.06
0.63
But
Pri
Cl
Br
I
2a
2b
2c
ꢁ0.71
ꢁ0.61
ꢁ0.52
ꢁ1.66
1.18
1.46
1.42
c
0.79
0.34
1.01
0.64
ꢁ1.44
Cl
Br
I
3a
3b
3c
ꢁ0.75
ꢁ0.68
ꢁ0.62
ꢁ1.32
ꢁ1.60
ꢁ1.96
c
d
0.74
0.31
1.09
0.62
b
d
d
d
C6H11
Cl
Br
I
4a
4b
4c
ꢁ0.71
ꢁ0.66
ꢁ0.63
ꢁ1.26
ꢁ1.53
ꢁ1.65
0.87
0.35
1.09
0.65
a
Potential values in volts (920 mV) vs. SCE, measured in 0.2 M [NBu4][BF4]ꢂ
THF, unless stated otherwise, at 200 mV sꢁ1, using [Fe(h5-
/
C5H5)2]0/ꢃ (E1o/x2ꢀ0.54 V, vs. SCE).
b
The counterion is [NiCl4]2ꢁ
.
c
red
p/2
E
.
d
Values measured in CH2Cl2.
order of the net p-electron donor ability of the halide
ligand (see Fig. 3). Moreover, within the compounds with
3.3.2. Anodic behavior
Two anodic processes are detected in all the com-
plexes except those with chloride or [NiCl4]2ꢁ as
counterions. Typically a lower potential irreversible
process followed by a higher potential reversible process
was detected, Table 4, which were attributed to the
oxidation of the halide counterion.
The free azine diphosphines PR2CH2C(But)NNC-
(But)CH2R2 display in THFꢂ
[NBu4][BF4], ill-defined
alkyl R groups the potentials of the Ni(II)0
process, for the same halide ligand, are little dependent
on the R group. In fact, the potentials of complexes 2a (ꢁ
0.71 V), 3a (ꢁ0.75 V), 4a (ꢁ0.71 V), for which the
/
Ni(I)
/
/
/
coordinated halogen is chloride, do not differ much. The
same trend is verified for bromide and, as expected (see
above), in a lesser extent for iodide, Table 4. Hence, for
/
anodic processes. In CH2Cl2 at least one reasonably
well-defined anodic wave is observed at potential values
highly dependent on the R group, Table 4. The
oxidation potentials of the diphosphine azines do not
follow directly the electronic or steric properties of the R
substituents. Moreover, there is no evident relationship
between the oxidation potentials of the free phos-
complexes 2ꢂ4 (with the alkyl R group) the electronic
/
characteristics of the halide present a dominant effect on
their redox properties. However, this behavior is no
longer found in complexes 1 for which the potentials of
wave (I) are independent of the coordinated halide and
higher than those of complexes 2ꢂ4.
/
The results herein indicate that for the square-planar
geometry (which, as shown by the above theoretical
studies, enables the participation of the halide in the
LUMO of the complexes) the redox properties of the
[NiX{PR2CH2C(But)NNC(But)CH2PR2}]ꢃ complexes
become highly dependent on X and almost independent
of R. A balance between steric and electronic effects
phines and the potential of the Ni(II)0
/
Ni(I) reduction
process.
Acknowledgements
operate in these Niꢂdiphosphine azine complexes which
/
corroborates that mentioned for other Ni(II) systems
[22].The cathodic waves (II) display potential values that
tend to follow the same trend as wave (I), although the
process is irreversible.
In order to fully confirm conclusions suggested
herein, the study needs to be extended to other R
groups.
The authors thank Mr. Jan Storch for skillful experi-
mental assistance. This project was supported by the
Grant Agency of the Czech Republic (grant No. 203/01/
0554), the Fundacao para a Cieˆncia e Tecnologia (FCT,
¸˜
POCTI/36125/QUI/99) and the ICCTI/ASCR Protocol
(ICCTI/ACCR-432/AC.C.Rep.Checa, by
grant).
a
travel

F.M.T. Almeida et al. / Inorganica Chimica Acta 338 (2002) 201ꢂ
/209
209
[11] S.D. Perera, B.L. Shaw, M. Thornton-Pett, J. Chem. Soc., Dalton
Trans. (1993) 3653.
References
[12] S.D. Perera, B.L. Shaw, M. Thornton-Pett, J. Chem. Soc., Dalton
Trans. (1994) 3311.
[1] R.A. Gossage, L.A. Vandekuil, G. van Koten, Acc. Chem. Res.
31 (1998) 423.
[13] S.D. Perera, B.L. Shaw, M. Thornton-Pett, J. Chem. Soc., Dalton
Trans. (1996) 3111.
[14] M.F.N.N. Carvalho, M.T. Duarte, A.M. Galvao, A.J.L. Pom-
[2] J.G. Donkervoort, J.L. Vicario, J.T.B.H. Jastrzebski, R.A.
Gossage, G. Cahiez, G. van Koten, J. Organomet. Chem. 558
(1998) 61.
˜
ˇ
beiro, J. .Cerma´k, S. Sabata, P. Vojt´ısˇek, J. Podlaha, J.
ˇ
[3] B. Rybtchinski, D. Milstein, Angew. Chem., Int. Ed. 38 (1999)
871.
Organomet. Chem. 598 (2000) 318.
ˇ
ˇ
[15] J. .Cerma´k, M. Kv´ıcˇalova´, S. Sabata, V. Blechta, P. Vojt´ısˇek, J.
Podlaha, B.L. Shaw, Inorg. Chim. Acta 313 (2001) 77.
[16] G.W. Watt, P.S. Gentile, E.P. Helvenston, J. Am. Chem. Soc. 77
(1955) 2752.
[4] M.E. van der Boom, Y. Ben David, D. Milstein, J. Am. Chem.
Soc. 121 (1999) 6652.
[5] M.E. van der Boom, J. Ott, D. Milstein, Organometallics 17
(1998) 4263.
[17] C.K. Fair, MO
[18] G.M. Sheldrick, SHELXS 86, Program for the Solution of Crystal
Structure, University of Gottingen, Gottingen, Germany, 1986.
lEN. Enraf-Nonius, Delft, 1990.
[6] M. Gandelman, A. Vigalok, L.J.W. Shimon, D. Milstein,
Organometallics 16 (1997) 3981.
¨
¨
[7] J. Vicente, A. Arcas, M.A. Blasco, J. Lozano, M.C.R. Dearellano,
Organometallics 17 (1998) 5374.
[19] G.M. Sheldrick, SHELXL 93, Program for the Refinement of
Crystal Structure, University of Gottingen, Gottingen, Germany,
1993.
¨
¨
[8] D.E. Bergbreiter, P.L. Osburn, Y.S. Liu, J. Am. Chem. Soc. 121
(1999) 9531.
[20] P. McArdle, J. Appl. Crystallogr. 28 (1995) 65.
[21] A.G. Sharp, Inorganic Chemistry, Longman, London, 1981, p.
487.
[9] P.C.J. Kamer, J.N.H. Reek, P.W.N.M. van Leeuwen, Chemtech
28 (1998) 27.
[10] S.D. Perera, B.L. Shaw, M. Thornton-Pett, J. Chem. Soc., Dalton
Trans. (1992) 1469.
[22] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements,
Pergamon Press, Oxford, 1990, pp. 1346ꢂ1347.
/