2,5-Bis(1-phenyliminoethyl)pyrazine (bpip): A conjugated metal-metal bridging acceptor ligand and its homodinuclear complexes with low-valent metal centres

Article abstract of DOI:10.1039/a808293f

Complexes of 2,5-bis(1-phenyliminoethyl)pyrazine (bpip), a symmetrical bis(bidentate) ligand which acts through two different chelate donor centres, one imine and one azine nitrogen atom per metal chelate site, have readily been obtained with Cr(CO)₄, Mo(CO)₄, W(CO)₄, Mn(CO)₃Cl, Re(CO)₃Cl, [Ru(bpy)₂]²⁺ and [Cu(PPh₃)₂]⁺ fragments. The 'free' ligand and the dinuclear [{Cu(PPh₃)₂}₂(μ-bpip)][BF₄] ₂ were characterised crystallographically. The low-energy molecular conformation of free bpip is qualitatively different from the planar syn/trans/syn arrangement required in the bpip-bridged dicopper(I) compound (Cu...Cu distance 6.944 A). Structural preferences and the π acceptor properties of the conjugated bridging ligand bpip were studied using DFT calculations of the bpip^0/-/2- chromophore and combined electrochemical and spectroscopic methods; bpip is a stronger n acceptor than the related 2,5-bis(2-pyridyl)pyrazine but a poorer mediator of metal-metal interaction.

Full text of DOI:10.1039/a808293f

View Article Online / Journal Homepage / Table of Contents for this issue
2,5-Bis(1-phenyliminoethyl)pyrazine (bpip): a conjugated metal–
metal bridging acceptor ligand and its homodinuclear complexes
with low-valent metal centres
Axel Klein,a Volker Kasack,a Ralf Reinhardt,a Torsten Sixt,a Thomas Scheiring,a
Stanislav Zalis,b Jan Fiedlerb and Wolfgang Kaim*a
a Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55,
D-70550 Stuttgart, Germany
b J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic,
Dolejsˇkova 3, CZ-18223 Prague, Czech Republic
Received 26th October 1998, Accepted 21st December 1998
Complexes of 2,5-bis(1-phenyliminoethyl)pyrazine (bpip), a symmetrical bis(bidentate) ligand which acts through
two different chelate donor centres, one imine and one azine nitrogen atom per metal chelate site, have readily been
obtained with Cr(CO)4, Mo(CO)4, W(CO)4, Mn(CO)3Cl, Re(CO)3Cl, [Ru(bpy)2]2ϩ and [Cu(PPh3)2]ϩ fragments. The
‘free’ ligand and the dinuclear [{Cu(PPh3)2}2(µ-bpip)][BF4]2 were characterised crystallographically. The low-energy
molecular conformation of free bpip is qualitatively different from the planar syn/trans/syn arrangement required in
the bpip-bridged dicopper() compound (Cu ؒ ؒ ؒ Cu distance 6.944 Å). Structural preferences and the π acceptor
properties of the conjugated bridging ligand bpip were studied using DFT calculations of the bpip0/Ϫ/2Ϫ chromophore
and combined electrochemical and spectroscopic methods; bpip is a stronger π acceptor than the related 2,5-bis(2-
pyridyl)pyrazine but a poorer mediator of metal–metal interaction.
Conjugated bridging ligands with low-lying unoccupied
molecular orbitals are sought after materials in various areas of
supramolecular and co-ordination chemistry. Such molecules
can support the formation of dendrimers and co-ordination
polymers,1 mediate magnetic interactions between metal
centres,2 give rise to mixed-valent dimers,3 show low-lying
charge transfer excited states,4 facilitate energy or electron
transfer between the bridged metal centres,1a,c,3,5 and exhibit a
rich electrochemistry.6 While µ-η1 :η1 co-ordinating ligands such
as pyrazine or 4,4Ј-bipyridine are quite useful in that respect,7
the superior stability of complexes with bis(chelating) bridging
ligands (µ-η2 :η2) is more helpful in the construction of robust
materials. For instance, co-ordinating bis(chelate) ligands with
“S frame”2c conformation such as 2,5-bis(2-pyridyl)pyrazine
(2,5-dpp also abbreviated as bppz or dppz) have been much
used recently in assembling oligonuclear compounds for the
purpose of light-induced energy transfer.1a
While 2,5-dpp can be considered as a replicated 2,2Ј-
bipyridine ligand with potentially conjugating aromatic rings,
and homodinuclear complexes with low-valent metal com-
the previously used8–10 and partially characterised11 bis(chelat-
plex fragments M(CO)4 (M = Cr, Mo or W), MЈ(CO)3Cl
ing) acceptor 2,5-bis(1-phenyliminoethyl)pyrazine (bpip) may
(MЈ = Mn or Re), [Ru(bpy)2]2ϩ and [Cu(PPh3)2]ϩ are presented,
be viewed as a centrosymmetrically replicated 2-pyridine-
including a crystal structure of [{Cu(PPh3)2}2(µ-bpip)][BF4]2.
The electronic situation in the complexes was studied by means
of cyclic voltammetry, absorption spectroscopy (solvato-
carbaldimine (pyca)11–17 system. As imine/azine hybrid chelat-
ing systems the pyca and bpip ligands contain the more flexible
non-aromatic CRЈ᎐NR groups which can bear different kinds
᎐
chromism, spectroelectrochemistry) and EPR of reduced
forms. Homodinuclear co-ordination compounds of bpip with
of substituents; related to bpip is 2,5-bis(4-dimethylamino-
phenyliminomethyl)pyrazine which contains strongly electron-
donating aryl groups as nitrogen substituents.18 Other attempts
to couple 2-pyridinecarbaldimine moieties involved the inser-
tion of potentially conjugating aromatics,5b,6a however, such
systems contain a large number of π centers which usually
diminishes the potential for metal–metal interaction.
other metal centres have been reported, involving RhI,II,III
,
IrI,II,III,9 PtII,III,IV10 and MgII.11 The redox system [{Ru(bpy)2}2-
(bpip)]nϩ was referred to in an earlier EPR study.8
In this work we describe the crystal structure of the bpip
ligand, its spectroelectrochemistry, and DFT calculation results
for neutral, anionic and dianionic states of bpipЈ = 2,5-bis(1-
phenyliminomethyl)pyrazine. Mononuclear [Re(bpip)(CO)3Cl]
J. Chem. Soc., Dalton Trans., 1999, 575–582
575

View Article Online
Table 1 Carbonyl stretching frequencies ν of complexesa
Table 2 Selected distances (Å) and bond angles (Њ) of bpip in the
crystal
Compound
νC O/cmϪ1
᎐
᎐
C(6)–N(1)
C(7)–N(1)
C(7)–C(8)
C(7)–C(71)
1.419(2)
1.279(2)
1.489(2)
1.501(2)
C(8)–N(2)
C(8)–C(9A)
C(9)–N(2)
1.336(2)
1.392(2)
1.331(2)
[{Cr(CO)4}2(µ-bpip)]
[{Mo(CO)4}2(µ-bpip)]
[{W(CO)4}2(µ-bpip)]
[{Mn(CO)3Cl}2(µ-bpip)]
[Re(bpip)(CO)3Cl]
1989vs
2000vs
1986vs
2025s
2027vs
2020vs
1918vs
1920vs
1909vs
1945s
1934s
1940s
1863s
1865s
1860s
1910vs
1905s
1910s
C(5)–C(6)–N(1)
C(1)–C(6)–N(1)
N(1)–C(7)–C(8)
N(1)–C(7)–C(71)
117.54(12)
122.57(13)
116.00(13)
127.10(13)
N(2)–C(8)–C(7)
C(9A)–C(8)–C(7)
N(2)–C(9)–C(8A) 122.69(13)
C(7)–N(1)–C(6)
C(9)–N(2)–C(8)
117.33(13)
122.25(12)
[{Re(CO)3Cl}2(µ-bpip)]
a Measured in THF solution.
121.94(12)
116.90(13)
N(2)–C(8)–C(9A) 120.40(13)
Average C–C distance in the phenyl ring 1.390 Å [1.383(2)–1.396(2) Å].
Average C–C–C angle in the phenyl ring 120.0Њ [119.45(13)–
120.79(14)Њ].
Table 3 Selected distances (Å) and bond angles (Њ) of [{Cu(PPh3)2}2-
(µ-bpip)][BF4]2 in the crystal
Cu(1)–N(2)
Cu(1)–N(1)
Cu(1)–P(1)
Cu(1)–P(2)
N(1)–C(7)
N(1)–C(6)
2.085(4)
2.101(4)
2.2666(15)
2.2741(14)
1.281(6)
1.448(6)
N(2)–C(8)
N(2)–C(9)
C(71)–C(7)
C(7)–C(8)
C(8)–C(9A)
1.348(6)
1.349(6)
1.502(7)
1.506(7)
1.384(7)
Fig. 1 Molecular structure of bpip in the crystal.
Results and discussion
Synthesis and characterisation
N(2)–Cu(1)–N(1)
N(2)–Cu(1)–P(1)
N(1)–Cu(1)–P(1)
N(2)–Cu(1)–P(2)
N(1)–Cu(1)–P(2)
P(1)–Cu(1)–P(2)
C(7)–N(1)–C(6)
C(7)–N(1)–Cu(1)
C(6)–N(1)–Cu(1)
C(8)–N(2)–C(9)
C(8)–N(2)–Cu(1)
78.0(2)
C(9)–N(2)–Cu(1)
C(1)–C(6)–C(5)
C(1)–C(6)–N(1)
C(5)–C(6)–N(1)
N(1)–C(7)–C(71)
N(1)–C(7)–C(8)
C(71)–C(7)–C(8)
N(2)–C(8)–C(9A) 121.6(4)
N(2)–C(8)–C(7)
C(9A)–C(8)–C(7)
128.9(3)
120.2(5)
120.2(5)
119.6(5)
126.3(5)
114.6(4)
119.1(4)
109.39(12)
122.56(12)
121.89(11)
109.95(11)
111.95(5)
117.7(4)
116.9(3)
125.1(3)
116.3(4)
114.0(3)
The compound bpip was obtained from aniline and 2,5-
diacetylpyrazine in hexane, using acidic alumina as catalyst and
molecular sieve for dehydration.11 Primary alkylamines did not
react under such conditions or in the benzene–toluene-p-
sulfonic acid system. Thermal reactions of the photogenerated
solvates [M(THF)(CO)5] (M = Cr, Mo or W) with bpip pro-
duced the bis(tetracarbonylmetal) complexes; in the case of
M = Mo, the use of the tetracarbonyl(norbornadiene)molyb-
denum precursor gave better yields. Typically, mononuclear
complexes could not be isolated even with an excess of bpip
although intermediate colouration was observed e.g. in the
reaction with [W(THF)(CO)5]. In the thermal reaction between
bpip and 2 molar equivalents of [Re(CO)5Cl] in toluene19 the
intermediate red colour before the appearance of the blue
dinuclear complex is attributed to a mononuclear species [Re-
(bpip)(CO)3Cl] which was subsequently synthesized separately
and studied for comparison with the dinuclear compound.
The dinuclear compounds of MnI, CuI and RuII were
prepared according to standard procedures;19–21 the isolated
diruthenium complex was found to contain varying amounts of
the one-electron reduced form.
115.8(4)
122.5(4)
N(2)–C(9)–C(8A) 122.0(5)
The metal carbonyl compounds exhibit typical CO stretching
bands in the infrared region (Table 1). The compound bpip and
[{Cu(PPh3)2}2(µ-bpip)][BF4]2 could be characterised crystallo-
graphically.
Crystal structures of bpip and [{Cu(PPh3)2}2(ì-bpip)][BF4]2 and
DFT calculations
Fig. 2 Molecular structure of the complex dication in the crystal of
[{Cu(PPh3)2}2(µ-bpip)][BF4]2.
Both molecules bpip and [{Cu(PPh3)2}2(µ-bpip)][BF4]2 exhibit
inversion symmetry in the respective crystals; significant inter-
molecular interactions were not observed. The crystal data and
structural results are summarised in Tables 2 and 3 and the
molecular structures are illustrated by Figs. 1 and 2.
A main difference between the conformations of free and
metal–metal bridging bpip lies in the co-ordination-induced
syn/trans/syn arrangement III as observed here for the dicopper
complex (Fig. 2) whereas unco-ordinated bpip adopts a lower-
energy form, an anti/trans/anti conformation I of the virtually
planar bis(iminoethyl)pyrazine π system with significantly
(54.9Њ) tilted phenyl rings (Fig. 1). Owing to the sterically
demanding metal complex fragments the tilt angle increases to
89.3Њ in the dicopper complex in which the imine functions
adopt a ca. 10Њ dihedral angle with respect to the pyrazine ring.
The existence of rotamers I–III (Scheme 1) has been inferred
previously for the one-electron reduced (bpip) Ϫ from EPR and
᝽
ENDOR (electron nuclear double resonance) studies;11 in
agreement with calculations (see below), conformations close to
I have the advantage of non-interference between the methyl
groups and the pyrazine protons.11
The dicopper complex with a metal-metal distance of 6.944
Å exhibits the typical co-ordination of copper() centres in an
N2P2 donor setting,22,23 i.e. distorted tetrahedral symmetry. The
bond lengths to both P atoms are rather similar as are both Cu–
N distances (slightly shorter for the bond to pyrazine N, Table
3). As has been demonstrated before,2c the stronger donation
from one nitrogen centre (here pyrazine N) may be compen-
576
J. Chem. Soc., Dalton Trans., 1999, 575–582

View Article Online
Table 4 Selected experimental and calculated structural dataa for the
bpip ligand
bpip
Exptl. for
dicopper
complexc
Exptl.
Calc.b
C(6)–N(1)
1.419(2)
1.279(2)
1.489(2)
1.501(2)
1.336(2)
1.331(2)
1.392(2)
1.407
1.287
1.497
1.512
1.347
1.334
1.410
1.448(6)
1.281(6)
1.506(7)
1.502(7)
1.348(6)
1.349(6)
1.384(7)
C(7)–N(1)
C(7)–C(8)
C(7)–C(71)
C(8)–N(2)
N(2)–C(9)
C(8)–C(9A)
N(1)–C(7)–C(8)
N(1)–C(7)–C(71)
C(7)–C(8)–N(2)
N(2)–C(9)–C(8A)
C(9)–N(2)–C(8)
116.00(13)
127.10(13)
117.33(13)
122.69(13)
116.90(13)
116.3
127.2
117.8
122.2
117.3
114.6(4)
126.3(5)
115.8(4)
122.0(5)
116.3(4)
Fig. 3 π Molecular orbital representations of DFT calculated virtual
MOs of bpip (syn/trans/syn conformation, methyl and phenyl groups
not depicted).
a Bond lengths (Å) and angles (Њ). b For anti/trans/anti conformation
from DFT calculations on bpip. c For syn/trans/syn conformation.
Table 5 Changes of the DFT calculated bond lengths (Å) of bpipЈ
after electron uptake
Ϫ
bpipЈ
(bpipЈ)
(bpipЈ)2Ϫ
᝽
C(6)–N(1)
C(7)–N(1)
C(7)–C(8)
C(8)–N(2)
N(2)–C(9)
C(8)–C(9A)
1.407
1.285
1.502
1.347
1.335
1.410
1.387
1.318
1.433
1.380
1.318
1.429
1.359
1.347
1.405
1.418
1.307
1.448
energies and compositions of the π acceptor MOs which are
shown in Fig. 3. While the highest lying occupied MOs are
formed by combinations of nitrogen lone pairs and phenyl
orbitals, the lowest-lying unoccupied molecular orbitals
(LUMO, LUMO ϩ 1) are two π MOs of au symmetry (Fig. 3),
one lower-lying MO with strong imine contribution (π*1,
LUMO) and one MO centred at the pyrazine ring (π*2).
Depending on the conformation of the phenyl rings, the bg π
MO with large phenylimino contributions may come to lie
slightly lower than π*2, especially in a coplanar arrangement.
The geometry change as a function of electron addition has
been calculated by example of bpipЈ (Table 5). As illustrated by
resonance structures for the dianion of the bpip chromophore,
the reduction of bpipЈ leads to an increase of bond orders for
C(6)–N(1), C(7)–C(8) and N(2)–C(9) and to bond lengthening
(decreasing bond order) for C(7)–N(1), C(8)–N(2) and C(8)–
C(9). Electron acquisition also diminishes the optimum tor-
Scheme 1
sated by the better π acceptor capacity and thus stronger back
bonding capability of the other centre (here imine N). The
overall π back donation from both copper() centres to the bpip
sional angle C(7)–N(1)–C(6)–C(1) from 34.3 via 28.1Њ for
Ϫ
(bpipЈ) to 8.3Њ for (bpipЈ)2Ϫ, reflecting increasing π conju-
᝽
gation with the phenyl rings. Clearly, the addition of space-
demanding methyl groups on going from bpipЈ to bpip results
in a larger tilt angle for the phenyl rings (34.3 to 55.5Њ, both
calculated; experimental value 54.9Њ).
ligand is weak as illustrated by the regular C᎐N and C–C bond
᎐
lengths (Table 3).24
Ab initio Hartree–Fock (HF) and density functional DFT/
HF hybrid methods25 were used in calculations of bpip and
bpipЈ. Geometry optimisation within constrained Ci symmetry
led to an anti/trans/anti conformation of the bpip central π sys-
tem with tilted (55.5Њ) phenyl rings. Local minima were also
observed for syn/trans/syn and syn/trans/anti rotamer situations
(Scheme 1) where the tilt angle to the phenyl rings is lower than
55Њ. The calculation results and the experimental values from
the crystal structure analysis of bpip show very good agreement
(Table 4). As mentioned before, the bond parameters of bpip in
the dicopper complex are also rather similar despite the differ-
ent, co-ordination-induced conformation in the latter; the main
difference concerns the slightly longer bond C(6)–N(1) due
to the orthogonality between the phenyl ring and the imine
function which precludes π conjugation.
Electrochemistry
Two typical cyclic voltammograms of bpip complexes are
shown in Figs. 4 and 5; the results are listed in Table 6. The
ligand bpip and its complexes are reduced reversibly at poten-
tials which are somewhat less negative (0.1–0.4 V) than those of
corresponding 2,5-dpp systems.4b (Potentials vs. SCE4b are con-
verted into values vs. ferrocene–ferrocenium by subtracting ca.
The DFT calculations of one-electron orbitals yield the
J. Chem. Soc., Dalton Trans., 1999, 575–582
577

View Article Online
Table 6 Electrochemical dataa of bpip and its complexes
E1/2
Compound
bpip
ox II
ox I
red I
red II
Solvent
Ϫ2.05 (80)
Ϫ1.87 (70)
Ϫ1.07 (150)
Ϫ1.06 (83)
Ϫ0.86 (160)
Ϫ0.78 (150)
Ϫ1.17 (117)
Ϫ0.54 (200)
Ϫ0.60 (60)
Ϫ0.75 (63)
Ϫ0.92 (96)
Ϫ0.63 (69)
Ϫ2.39 irr.b
Ϫ2.28 irr.b
Ϫ1.84 (180)
Ϫ1.70 (103)
Ϫ1.58 (190)
Ϫ1.39 irr.b
Ϫ1.89 irr.b
Ϫ1.30 (200)
Ϫ1.27 (130)
Ϫ1.52 (153)
Ϫ1.48 (63)
Ϫ1.26 (72)e
THF
MeCN
THF
THF
THF
THF
THF
THF
MeCN
CH2Cl2
MeCN
MeCN
[{Cr(CO)4}2(µ-bpip)]
[{Mo(CO)4}2(µ-bpip)]
[{W(CO)4}2(µ-bpip)]
[{Mn(CO)3Cl}2(µ-bpip)]
[Re(bpip)(CO)3Cl]
0.13 (230)c
1.12 irr.d
[{Re(CO)3Cl}2(µ-bpip)]
1.00 irr.d
1.02 irr.d
0.92 irr.d
1.01 (69)
[{Cu(PPh3)2}2(µ-bpip)][BF4]2
[{Ru(bpy)2}2(µ-bpip)][PF6]4
<1.09 (69)
a From cyclic voltammetry at 100 mV sϪ1 scan rate. Potentials in V vs. ferrocene–ferrocenium couple. Half-wave potentials E1/2, peak potential
differences ∆Ep = Epa Ϫ Epc in mV in parentheses. b Cathodic peak potential Epc for irreversible reduction step. c Two-electron wave. d Anodic peak
potential Epa for irreversible oxidation step. e Additional bpy-centred reduction at Ϫ1.75 V.
instances, however, this metal-based oxidation occurs as a
virtual two-electron process (Fig. 4), with only indirectly
detectable formation of a RuIII–RuII mixed-valent intermediate
form (Fig. 5). The comproportionation constant Kc for the
mixed-valent form is 103.2 for the [{Ru(bpy)2}2(µ-2,5-dpp)]5ϩ
reference27 but less than 101.4 for [{Ru(bpy)2}2(µ-bpip)]5ϩ, see
eqns. (1) and (2).
Kc = 10∆E/59 mV = [X(n Ϫ 1)]2/[X][X(n Ϫ 2)
X ϩ X(n Ϫ 2) 2 X(n Ϫ 1)
]
(1)
(2)
Fig. 4 Cyclic voltammogram of [{Cr(CO)4}2(µ-bpip)] in THF–0.1 M
Bu4NClO4 at 100 mV sϪ1 scan rate.
Usually, the better π acceptor bridging ligand (here bpip)
should induce a stronger metal–metal coupling in terms of
stability of mixed-valent states27 than a weaker π acceptor
(here 2,5-dpp), especially if the metal–metal distances are
comparable. The unexpected deviation from this rule27 as
observed here is attributed to the predominantly imine-centred
π*1 MO for metal–metal communication (Fig. 3); the metals
thus mainly interact with the peripheral acceptor part of the
bridging ligand. In contrast, the mediating π*1 MO of 2,5-dpp
has the main acceptor functionality in the central pyrazine
ring,4b and similar situations are found for all related bridging
ligands.4b Nevertheless, the bpip bridging ligand has been
unique in allowing us to observe the uncommon d7/d8 mixed-
valent state of complexes [(C5Me5)M(µ-bpip)M(C5Me5)]ϩ,
M = Rh or Ir.9
UV/VIS spectroscopy, solvatochromism and spectroelectro-
chemistry
Fig. 5 Experimental and simulated cyclic voltammograms of [{Ru-
(bpy)2}2(µ-bpip)][PF6]4 in MeCN–0.1 M Bu4NPF6.
Complexes between π acceptor ligands and metal centres in low
oxidation states are usually distinguished by intense, low-energy
and often highly solvatochromic28–30 metal-to-ligand charge
transfer (MLCT) transitions. Table 7 summarises the solvent
dependence of the long-wavelength MLCT absorption for the
carbonylmetal compounds and Table 8 contains additional
information for some reduced systems from spectroelectro-
chemistry (Fig. 6).
0.45 V.26) Substitution of the 2-pyridyl by two N-phenylimine
functions clearly causes an energetic stabilisation of the LUMO
and thus of reduced forms. Under the conditions employed, the
second reduction is irreversible for the ‘free’ ligand and the
halide containing species; otherwise, a difference of about 0.7 V
is observed between first and second reversible reductions.
Facilitated reduction of π acceptor ligands after single and
especially twofold metal co-ordination is a well known phen-
omenon,4b,19,21 the increase in half-wave reduction potentials
E1/2redI from the ‘free’ ligand via the complexes of Cr0, Mo0, W0,
MnI, CuI, RuII to the rhenium() dinuclear complex follows
corresponding sequences for related complexes with bis(chelat-
ing) bridging ligands.19,20b
In comparison with corresponding 2,5-dpp compounds,4b,19,21
all dinuclear bpip complexes exhibit first MLCT absorption
bands (MLCT1, d → π*1) at distinctly longer wavelengths.
The reason for this bathochromic shift, e.g. from 584 [650 (sh)]
to 721 [800 (sh)] nm for the dinuclear bis(2,2Ј-bipyridine)-
ruthenium() complexes, lies in the lower lying LUMO of bpip
compounds relative to 2,5-dpp systems which is evident from
facilitated reduction (Table 6). The occupied metal d orbitals,
on the other hand, have about the same energy in bpip and 2,5-
dpp complexes as illustrated by comparative absolute potentials
As expected for compounds with metal centred HOMOs,
the oxidation of the complexes proceeds irreversibly except for
the dichromium(0) and diruthenium() systems. In both
578
J. Chem. Soc., Dalton Trans., 1999, 575–582

View Article Online
Table 7 Solvent dependent MLCTa absorption maxima λmax/nm of dinuclear carbonyl metal complexes of L = µ-bpip and solvatochromism
parametersb
c
Solvent
Hexane
Toluene
Benzene
THF
E*MLCT
0.00
0.30
0.34
0.59
0.64
0.82
0.90
0.95
[{Cr(CO)4}2L]
[{Mo(CO)4}2L]
[{W(CO)4}2L]
[{Mn(CO)3Cl}2L]
[{Re(CO)3Cl}2L]
Insoluble
Insoluble
Insoluble
382
642
376
635
377
636
364 (sh)
640
376
639
362 (sh)
635
362
634
360 (sh)
628
15610
220
26130
1770
Insoluble
513
862
519
858
490
817
510
836
475
785
463
771
450
482
817
488
808
460
765
475
790
452 (sh)
734
395
651
395
649
376 (sh)
603
378
636
366
820
823
440
787
DCEd
800
427 (sh)
759
Acetone
MeCN
DMF
584
368
578
450
715
749
44 (sh)
715
11470
2640
19600
3330
766
741
11510
2050
 Insufficient
data
567
14350
3420
24220
3500
b
A1
10950
2150
18060
4030
B1
A2
B2


a Metal-to-ligand charge transfer to π1* (LUMO; MLCT1) and π2* (SLUMO; MLCT2). b Solvatochromism analysed via linear regressions νMLCTn
/
cmϪ1 = An ϩ BnE*MLCT c Solvent parameter from ref. 28. d 1,2-Dichloroethane.
.
Table 8 Spectroelectrochemical data:a UV/VIS absorption of bpip
and its complexes
Compound
λ/nm (10Ϫ3 ε/MϪ1 cmϪ1
)
bpip
291 (23.7), 365 (sh) (5.5)
240 (13.7), 280 (16.9), 460 (13.2), 700
(5.1)
245 (19.5), 272 (11.3), 469 (27.1)
298 (23.0), 386 (6.5), 450 (2.5), 715
(13.0)
368 (13.0), 577 (14.0), 625 (sh), 715 (sh)
343 (sh), 554 (20.0), 583 (23.0)
267 (48.6), 392 (3.9)
274 (48.0), 440 (12.6), 500 (10.9), 622
(10.1), 879 (0.15)
bpipϪ
bpip2Ϫ
[{Mo(CO)4}2(µ-bpip)]b
Ϫ
b
[{Mo(CO)4}2(µ-bpip)]
᝽
[{Mo(CO)4}2(µ-bpip)]2Ϫ b
[{Cu(PPh3)2}2(µ-bpip)]2ϩ
[{Cu(PPh3)2}2(µ-bpip)]ϩ
[{Cu(PPh3)2}2(µ-bpip)]
254 (44.5), 538 (sh) (35.1), 565 (40.2),
716 (1.64)
a From measurements in MeCN unless stated otherwise. For assign-
ments of transitions see text. Reduced forms generated by in situ
electrolysis in solvent–0.1 M Bu4NPF6 at ambient temperatures.
b Measured in DMF.
Fig. 6 Absorption spectra of [{Mo(CO)4}2(µ-bpip)]n from spectro-
electrochemistry in DMF–0.1 M Bu4NPF6. Top: n = 0 to Ϫ1. Bottom:
n = Ϫ1 to Ϫ2.
A comparison between [{Mo(CO)4}2(µ-bpip)] and [{Mo-
(CO)4}2(µ-2,5-dpp)]30a reveals that the former exhibits a lower
A1 value (11470 vs. 13360 cmϪ1) and a diminished sensitivity B1
(2640 vs. 3110 cmϪ1), confirming once again the better π
acceptor capacity of bpip relative to 2,5-dpp.
for the RuII → RuIII oxidation (Table 6, ref. 21). The stronger
acceptor effect of bpip relative to 2,5-dpp is also evident21 from
the hypsochromically shifted d → π*(bpy) absorption of
[{Ru(bpy)2}2(µ-bpip)]4ϩ (422 nm) relative to the 432 nm of
[{Ru(bpy)2}2(µ-2,5-dpp)]4ϩ.21
The ‘free’ ligand and the complexes of the Mo0 and the CuI
were studied in their singly and doubly reduced forms by UV/
Ϫ
VIS spectroelectrochemistry (Fig. 6, Table 8). The ions bpip
᝽
and bpip2Ϫ show absorptions in the visible due to transitions
from the singly or doubly occupied π*1 MO to higher lying
unoccupied orbitals.31 New bands at longer wavelengths are
also observed for the dicopper() complex, where intraligand
(IL) and MLCT transitions are expected. The dimolybdenum(0)
complex, on the other hand, exhibits a long-wavelength
MLCT1 band in the neutral form; on reduction this band
is shifted hypsochromically (anion radical) and then dis-
appears (dianion) whereas other, intraligand transitions
dominate.
The typically29,30 negative solvatochromism of MLCT
absorptions of d6 metal carbonyl complexes has been quantified
using the well established30 E*MLCT solvent parameters as
defined by Manuta and Lees.28 Linear correlations were
observed in all cases, and the values of the regressions
νMLCT = A ϩ BE*MLCT are listed in Table 7 for MLCT1 and
MLCT2 bands. The A1 values reflect the established4b,19
sequence of MLCT1 energies, increasing from compounds of
Cr0 via Mo0 and W0 to ReI and MnI. The sensitivity as
measured by B1 shows large differences; the manganese()
dimer shows almost no solvent dependence whereas the
rhenium() analogue exhibits rather large hypsochromic shifts
with increasing solvent polarity. This contrast suggests30b that
σ donation and π acceptance through the ligand are more
balanced in the dimanganese() compound.
EPR spectroscopy of reduced forms
The magnetic resonance behaviour of the bpip radical anion
was previously reported.11 Temperature-dependent EPR and
ENDOR studies revealed a dynamic situation resulting from
J. Chem. Soc., Dalton Trans., 1999, 575–582
579

View Article Online
Table 9 The EPR dataa of paramagnetic species
c
Compound
giso
aM
aN
Ϫ
b
bpip
2.0033
2.0021
2.0029
2.0036
2.0005
2.0030
1.9980
0.159
0.339
0.335
0.330
n.o.
0.34
n.o.
᝽
Ϫ
d
[{Cr(CO)4}2(µ-bpip)]
[{Mo(CO)4}2(µ-bpip)]
[{W(CO)4}2(µ-bpip)]
[{Re(CO)3Cl}2(µ-bpip)]
[{Cu(PPh3)2}2(µ-bpip)]
n.o.
᝽
Ϫ
d
0.247 (95,97Mo)
n.o.
᝽
Ϫ
d
᝽
Ϫ
1.74 (185,187Re)
1.32 (63,65Cu; 31P)
n.o.
d
᝽
ϩ
e
᝽
3ϩ d, f
ؒ
[{Ru(bpy)2}2(µ-bpip)]
a Coupling constants a in mT (1 T = 104 G). b Generated by in situ elec-
trolysis at ambient temperatures in THF–0.1 M Bu4NClO4. c Within the
linewidth the 14N hyperfine splitting was found identical for imine
and azine nitrogen centres. d Generated by reduction with Bu4NBH4
in acetone. e Generated by reduction with Bu4NBH4 in CH2Cl2. f g
Components at 1.979, 1.996, 2.019 in acetone at 110 K.
Ϫ
dinuclear complexes such as the stabilisation of d7/d8 and the
poor support of d5/d6 mixed-valent states; the structural and
electronic potential of this kind of species thus deserves to be
explored further.
Fig. 7 The EPR spectrum of [{Re(CO)3Cl}2(µ-bpip)]
solution with computer simulation (linewidth 1.93 mT).
in acetone
᝽
Experimental
Instrumentation
The EPR spectra were recorded in the X band on a Bruker
System ESP 300 equipped with a Bruker ER035M gaussmeter
1
and a HP 5350B microwave counter, H NMR spectra on a
Bruker AC 250 spectrometer, infrared spectra using Perkin-
Elmer 684 and 283 instruments and UV/VIS/NIR absorption
spectra on Shimadzu UV160 and Bruins Instruments Omega 10
spectrophotometers. Cyclic voltammetry was carried out using
a
three-electrode configuration (glassy carbon electrode,
platinum counter electrode, Ag–AgCl or SCE reference) and a
PAR 273 potentiostat and function generator. The ferrocene–
ferrocenium couple served as reference. Cyclic voltammograms
were simulated with the help of the program DigiSim 2.1
(Bioanalytical Systems Inc., V 2.1, 1996). Spectroelectro-
chemical measurements were performed using an optically
transparent thin-layer electrode (OTTLE) cell33 for UV/VIS
spectra and a two-electrode capillary for EPR studies.8
Preparations
Fig. 8 The EPR spectrum of [{Cu(PPh3)2}2(µ-bpip)] ϩ in acetone solu-
᝽
The compound bpip was synthesized as described.11
tion with computer simulation (linewidth 0.43 mT).
[{Cr(CO)4}2(ì-bpip)]. The [Cr(THF)(CO)5] complex as gen-
erated by 3 h irradiation of [Cr(CO)6] (70 mg, 0.318 mmol) in 70
cm3 THF was treated with 50 mg (0.159 mmol) bpip to yield a
green solution. After stirring for 20 h the volume was reduced
by half and 40 cm3 n-hexane were added to precipitate the dark
green product. Yield: 41 mg (40%) (Found: C, 51.31; H, 3.27;
N, 8.13. Calc. for C28H18Cr2N4O8ؒH2O: C, 50.87; H, 3.03; N,
8.48%).
restricted rotation around the C (imine)–C (pyrazine) bonds
(Scheme 1).
The one-electron reduced forms of the dinuclear complexes
also exhibit EPR spectra at ambient temperature (Figs. 7 and
8), and the g values and hyperfine constants are summarised in
Table 9. The dinuclear manganese() complex showed only a
broad, unresolved signal on reduction.
The EPR data are compatible with results from related anion
radical complexes (LnM)(BL Ϫ)(MLn).8,19,20b,32 This concerns
᝽
the decrease of g values from the tungsten(0) complex via CuI,
Mo0, Cr0 and ReI to RuII, the detection of sizeable metal hyper-
fine splitting for 63,65Cu, 95,97Mo and 185,187Re, and the strong
increase, in fact more than doubling, of the 14N coupling con-
stants after metal binding. Remarkably, the hyperfine coupling
from the chemically different imine and pyrazine nitrogen
donor atoms is identical for the ligand and complex radical
species within the respective EPR linewidths. This observation
agrees with results from Hückel MO calculations of π spin
populations;11 the DFT calculations suggest slightly higher
MO coefficients at the imine nitrogen centres in the LUMO
(Fig. 3).
[{Mo(CO)4}2(ì-bpip)]. A solution of the norbornadiene
complex [Mo(nbd)(CO)4] (60 mg, 0.20 mmol) in 30 cm3 THF
was treated with 30 mg (0.096 mmol) bpip to yield a deep blue
solution. After stirring for 20 h 40 cm3 n-hexane were added
to precipitate the dark blue product which was washed with
n-hexane and dried under vacuum. Yield: 57 mg (74%) (Found:
C, 45.74; H, 2.68; N, 7.50. Calc. for C14H9MoN2O4: C, 46.05; H,
2.48; N, 7.67%). 1H NMR (THF-d8): δ 2.46 (s, 6 H, CH3), 7.13
(dd, 4 H, o-H), 7.34 (tt, 2 H, p-H), 7.54 (m, 4 H, m-H) and 9.66
(s, 2 H, pyrazine H).
[{W(CO)4}2(ì-bpip)]. The [W(THF)(CO)5] complex as gener-
ated by 3 h irradiation of [W(CO)6] (112 mg, 0.318 mmol) in 70
cm3 THF was treated with 50 mg (0.159 mmol) bpip to yield a
Summarising, the conjugated π system underlying the
bpip bridging ligand is causing several unexpected effects in
580
J. Chem. Soc., Dalton Trans., 1999, 575–582

View Article Online
Table 10 Crystallographic data for bpip and its dicopper complexa
deep blue solution. After heating to reflux for 1 h the volume
was reduced by half and 40 cm3 n-hexane were added to precipi-
tate the dark blue product. Yield: 48 mg (34%) (Found: C,
34.73; H, 3.24; N, 5.50. Calc. for C14H9N2O4Wؒ2H2O: C, 34.36;
H, 2.66; N, 5.78%).
[{Cu(PPh3)2}2(µ-bpip)]
[BF4]2
bpip
Formula
C20H18N4
314.38
Orthorhombic
Pbca (no. 61)
6.3013(4)
7.8308(7)
33.242(3)
C92H78B2Cu2F8N4P4
1664.16
Monoclinic
P21/n (no. 14)
13.844(3)
11.111(3)
26.604(6)
105.03(2)
3952.20(17)
2
0.689
7271
6974
4882
Formula weight (M)
Crystal system
Space group
a/Å
b/Å
c/Å
[{Mn(CO)3Cl}2(ì-bpip)]. A solution of [Mn(CO)5Cl] (73.3
mg, 0.318 mmol) in 30 cm3 THF was treated with 50 mg (0.159
mmol) bpip to yield a dark red solution. After heating to reflux
for 2 h the solution had turned blue; 30 cm3 n-hexane were
added to precipitate the dark blue product which was washed
with n-hexane and dried under vacuum. Yield: 56 mg (53%)
(Found: C, 45.67; H, 2.87; N, 8.00. Calc. for C26H18Cl2Mn2-
N4O6ؒH2O: C, 45.81; H, 2.94; N, 8.22%).
β/Њ
V/Å3
1640.3(2)
4
0.078
2370
2370
1680
Z
µ(Mo-Kα)/mmϪ1
Measured reflections
Unique data
Data with I > 2σ(I)
R1 [I > 2σ(I)]
wR2 (all data)
[{Re(CO)3Cl}2(ì-bpip)]. A solution of [Re(CO)5Cl] (72 mg,
0.20 mmol) in 50 cm3 toluene was treated with 31 mg (0.10
mmol) bpip to yield a dark red solution which turned purplish
blue after heating to reflux for 1 h. To complete the precipi-
tation 50 cm3 n-hexane were added; the dark blue product was
washed with toluene and n-hexane and dried under vacuum.
Yield: 80 mg (86%) (Found: C, 34.32; H, 1.98; N, 5.58. Calc.
0.0480
0.1449
0.0622
0.1844
Crystallography
Yellowish crystals of bpip were obtained by cooling a saturated
solution in toluene. Wyckoff scans and 2255 reflections were
used for refinement. Crystals of [{Cu(PPh3)2}(µ-bpip)][BF4]2
were obtained as purplish black needles by slow evaporation of
a saturated solution in acetone. Wyckoff scans and 6568 reflec-
tions were used for refinement.
1
for C13H9ClN2O3Re: C, 33.73; H, 1.96; N, 6.05%). H NMR
(acetone-d6): δ 2.58 (s, 6 H, CH3), 7.24 (dd, 4 H, o-H), 7.45 (tt,
2 H, p-H), 7.63 (m, 4 H, m-H) and 9.60 (s, 2 H, pyrazine H).
[Re(bpip)(CO)3Cl]. A solution of [Re(CO)5Cl] (92 mg, 0.255
mmol) in 30 cm3 toluene–dichloromethane (3:1) was treated
with 80 mg (0.255 mmol) bpip under reflux for 1 h to yield a
dark red solution. The solution was cooled to 4 ЊC for 20 h to
precipitate the purplish blue dinuclear compound. After fil-
tration the solvent was removed in vacuo and the red residue
recrystallised from 20 cm3 toluene–dichloromethane (3:1) to
yield 100 mg (63%) (Found: C, 45.50; H, 3.48; N, 8.51. Calc. for
C23H18ClN4O3Re: C, 44.55; H, 2.93; N, 9.04%). 1H NMR
(acetone-d6): δ 2.45 (s, 3 H, CH3), 2.71 (s, 3 H, CH3), 6.99 (dd,
4 H, o-H), 7.19 (m, 2 H, m-H), 7.42 (tt, 1 H, p-H), 7.44 (m, 2 H,
m-H), 7.60 (tt, 1 H, p-H), 9.72 (s, 1 H, pyrazine H) and 9.79 (s,
1 H, pyrazine H). UV/VIS (THF): λmax = 461 and 295 nm.
Data were collected at 183 K on a Siemens P4 diffractometer
with graphite monochromator and Mo-Kα radiation (λ 0.71073
Å). The structures were solved by direct methods (Siemens
SHELXTL PC) and refined (SHELXL 93)34 by full-matrix
least squares on F2. Hydrogen atoms were introduced at calcu-
lated positions and refined freely. Other details are given in
Table 10.
CCDC reference number 186/1291.
See http://www.rsc.org/suppdata/dt/1999/575/ for crystallo-
graphic files in .cif format.
Calculations
Ab initio Hartree–Fock (HF) and density-functional (DFT)/HF
hybrid25 methods were used for electronic structure calculations
of the bpip ligand. Within the DFT/HF procedure, the three-
parameter Becke–Lee–Yang–Parr (B3LYP) functional poten-
tial was employed. Calculations were performed using the
GAUSSIAN 94 program package.35 Dunning’s36 valence
double-ζ basis with polarisation functions was used for H, C
and N atoms. The geometry variation as a function of reduc-
tion was examined for bpipЈ where the methyl groups of bpip
were replaced by H atoms. The calculations were done within Ci
symmetry constraint.
[{Cu(PPh3)2}2(ì-bpip)][BF4]2. A solution of [Cu(PPh3)4]BF4
(240 mg, 0.20 mmol) in 30 cm3 CH2Cl2 was treated with 31 mg
(0.10 mmol) bpip to yield a purplish brown solution. After stir-
ring for 2 h 60 cm3 n-hexane were added to precipitate the pur-
ple product which was redissolved in acetone and reprecipitated
with n-hexane. The solid formed after keeping at 0 ЊC for 12 h
was recrystallised from methanol and dried under vacuum.
Yield: 90 mg (54%) (Found: C, 65.82; H, 4.65; N, 3.16. Calc.
for C92H78B2Cu2F8N4P4ؒCH3OH: C, 65.80; H, 4.83; N, 3.30%).
1H NMR (acetone-d6): δ 2.37 (s, 6 H, CH3), 6.52 (dd, 4 H,
o-H), 7.12 (tt, 2 H, p-H) and 7.3–9.2 (m, 66 H, m-H, PPh3,
pyrazine H).
Acknowledgements
Support of this work and German-Czech co-operation from
Deutsche Forschungsgemeinschaft and Volkswagenstiftung is
gratefully acknowledged.
[{Ru(bpy)2}2(ì-bpip)][PF6]4 ؊ x. In a typical experiment a
solution of cis-[Ru(bpy)2Cl2]ؒ2H2O (100 mg, 0.19 mmol) in 50
cm3 ethanol was heated together with 30 mg (0.096 mmol) bpip
for 3 h to yield a red-brown solution. Following volume reduc-
tion to about 15 cm3 a solution of 5 mg NH4PF6 in 50 cm3 water
was added to produce a red-brown precipitate. After washing
with water the product was redissolved in acetone, filtered and
reprecipitated with diethyl ether. Column chromatography
(neutral alumina) of the acetone-dissolved compound with
water/acetone (1:1) yielded a main brown fraction which was
repeatedly dissolved in acetone, precipitated with diethyl ether
and dried under vacuum. Yield: 80 mg (53%). Elemental
analysis and spectroscopy revealed that the complex contained
varying amounts of the one-electron reduced species [{Ru-
(bpy)2}2(µ-bpip)][PF6]3, depending on reaction conditions.
References
1 (a) V. Balzani, S. Campagna, G. Denti, A. Juris, S. Serroni and
M. Venturi, Acc. Chem. Res., 1998, 31, 26; (b) S. Bodige, A. S.
Torres, D. J. Maloney, D. Tate, G. R. Kinsel, A. K. Walker and
F. M. MacDonnell, J. Am. Chem. Soc., 1997, 119, 10364; (c)
E. Constable, Chem. Ind. (London), 1994, 56; (d) A. Neels, B. M.
Neels, H. Stoeckli-Evans, A. Clearfield and D. M. Poojary, Inorg.
Chem., 1997, 36, 3402.
2 (a) G. De Munno, G. Viau, M. Julve, F. Lloret and J. Faus, Inorg.
Chim. Acta, 1997, 257, 121; (b) G. De Munno, T. Poerio, M. Julve,
F. Lloret, J. Faus and A. Caneschi, J. Chem. Soc., Dalton Trans.,
1998, 1679; (c) W. Kaim, S. Kohlmann, J. Jordanov and D. Fenske,
J. Chem. Soc., Dalton Trans., 1999, 575–582
581

View Article Online
Z. Anorg. Allg. Chem., 1991, 598/599, 217; (d) G. Brewer and
E. Sinn, Inorg. Chem., 1985, 24, 1580.
3 J. Poppe, M. Moscherosch and W. Kaim, Inorg. Chem., 1993,
32, 2640; V. Kasack, W. Kaim, H. Binder, J. Jordanov and E. Roth,
Inorg. Chem., 1995, 34, 1924; M. Ketterle, J. Fiedler and W. Kaim,
Chem. Commun., 1998, 1701.
4 (a) S. Kohlmann, S. Ernst and W. Kaim, Angew. Chem., 1985, 97,
698; Angew. Chem., Int. Ed. Engl., 1985, 24, 684; (b) W. Kaim and
S. Kohlmann, Inorg. Chem., 1987, 26, 68; (c) W. Kaim, S. Kohlmann,
A. J. Lees, T. L. Snoeck, D. J. Stufkens and M. M. Zulu, Inorg. Chim.
Acta, 1993, 210, 159.
5 (a) L. De Cola, V. Balzani, F. Barigelletti, L. Flamigni, P. Belser,
A. von Zelewsky, M. Frank and F. Vögtle, Inorg. Chem., 1993, 32,
5228; (b) I. S. Calonaci, Ph.D. Thesis, University of Amsterdam,
1995.
6 (a) H. Haga and K. Koizumi, Inorg. Chim. Acta, 1985, 104, 47;
S. C. F. Maloney, M. R. McDevitt and F. L. Urbach, Inorg. Chim.
Acta, 1989, 164, 123; (b) M. Krejcik, S. Zalis, J. Klima, D. Sykora,
W. Matheis, A. Klein and W. Kaim, Inorg. Chem., 1993, 32, 3362;
(c) F. Baumann, W. Kaim, M. Garcia Posse and N. E. Katz, Inorg.
Chem., 1998, 37, 658; (d) F. Baumann, A. Stange and W. Kaim,
Inorg. Chem. Commun., 1998, 1, 305.
7 C. Creutz, Prog. Inorg. Chem., 1983, 30, 1; D. E. Richardson and
H. Taube, Coord. Chem. Rev., 1984, 60, 107; R. J. Crutchley, Adv.
Inorg. Chem., 1994, 41, 273; W. Bruns, W. Kaim, E. Waldhör and
M. Krejcik, Inorg. Chem., 1995, 34, 663.
8 W. Kaim, S. Ernst and V. Kasack, J. Am. Chem. Soc., 1990, 112, 173.
9 W. Kaim, R. Reinhardt and J. Fiedler, Angew. Chem., 1997, 109,
2600; Angew. Chem., Int. Ed. Engl., 1997, 36, 2493; W. Kaim,
S. Berger, S. Greulich, R. Reinhardt and J. Fiedler, J. Organomet.
Chem., in the press.
10 A. Klein, S. Hasenzahl, W. Kaim and J. Fiedler, Organometallics,
1998, 17, 3532.
20 (a) W. Kaim and S. Kohlmann, Inorg. Chem., 1987, 26, 1469;
(b) C. Vogler and W. Kaim, Z. Naturforsch., Teil B, 1992, 47, 1057.
21 S. D. Ernst and W. Kaim, Inorg. Chem., 1989, 28, 1520.
22 J. R. Kirchhoff, D. R. McMillin, W. R. Robinson, D. R. Powell,
A. T. McKenzie and S. Chen, Inorg. Chem., 1985, 24, 3928.
23 C. Vogler, H.-D. Hausen, W. Kaim, S. Kohlmann, H. E. A. Kramer
and J. Rieker, Angew. Chem., 1989, 101, 1734; Angew. Chem., Int.
Ed. Engl., 1989, 28, 1659; C. Vogler, W. Kaim and H.-D. Hausen,
Z. Naturforsch., Teil B, 1993, 48, 1470.
24 S. Greulich, W. Kaim, A. Stange, H. Stoll, J. Fiedler and S. Zalis,
Inorg. Chem., 1996, 35, 3998.
25 A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
26 N. G. Connelly and W. E. Geiger, Chem. Rev., 1996, 96, 877.
27 S. Ernst, V. Kasack and W. Kaim, Inorg. Chem., 1988, 27, 1146.
28 C. M. Manuta and A. J. Lees, Inorg. Chem., 1983, 22, 3825.
29 E. S. Dodsworth and A. B. P. Lever, Coord. Chem. Rev., 1990, 97,
271; Inorg. Chem., 1990, 29, 499.
30 (a) W. Kaim and S. Kohlmann, Inorg. Chem., 1986, 25, 3306;
(b) W. Kaim, S. Kohlmann, S. Ernst, B. Olbrich-Deussner,
C. Bessenbacher and A. Schulz, J. Organomet. Chem., 1987, 321,
215.
31 P. S. Braterman, J.-I. Song, S. Kohlmann, C. Vogler and W. Kaim,
J. Organomet. Chem., 1991, 411, 207; M. Krejcik, S. Zalis, M.
Ladwig, W. Matheis and W. Kaim, J. Chem. Soc., Perkin Trans. 2,
1992, 2007.
32 W. Kaim and S. Kohlmann, Inorg. Chem., 1986, 25, 3442.
33 M. Krejcik, M. Danek and F. Hartl, J. Electroanal. Chem.
Interfacial Electrochem., 1991, 317, 179.
34 G. M. Sheldrick, SHELXTL PLUS: An Integrated System for
Solving, Refining and Displaying Crystal Structures from
Diffraction Data, Siemens Analytical X-Ray Instruments Inc.,
Madison, WI, 1989; SHELXL-93, Program for Crystal Structure
Determination, Universität Göttingen, 1993.
11 T. Stahl, V. Kasack and W. Kaim, J. Chem. Soc., Perkin Trans. 2,
1995, 2127.
12 G. van Koten and K. Vrieze, Recl. Trav. Chim. Pays-Bas, 1981, 100,
129; Adv. Organomet. Chem., 1982, 21, 152; K. Vrieze and G. van
Koten, Inorg. Chim. Acta, 1985, 100, 79.
13 G. van Koten, J. T. B. H. Jastrzebski and K. Vrieze, J. Organomet.
Chem., 1983, 250, 49.
14 H. Brunner and W. A. Herrmann, Chem. Ber., 1972, 105, 770;
J. Organomet. Chem., 1973, 57, 183.
15 D. J. Stufkens, Coord. Chem. Rev., 1990, 104, 39.
16 D. P. Drolet, L. Chan and A. J. Lees, Organometallics, 1988, 7, 2502.
17 M. Maruyama, H. Matsuzawa and Y. Kaizu, Inorg. Chem., 1995, 34,
3232.
35 M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G.
Johnson, M. A. Robb, J. R. Cheeseman, F. G. A. Keith, A. Peterson,
J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G.
Zakrzewki, J. V. Ortziz, J. V. Foresman, J. Cioslowski, B. B.
Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y.
Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle,
R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees,
J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez and J. A.
Pople, GAUSSIAN 94, Revision B.3, Gaussian, Inc., Pittsburgh,
PA, 1995.
36 T. H. Dunning, Jr. and P. J. Hay, in Modern Theoretical Chemistry,
ed. Electronic Structure Theory, H. F. Schäfer III, Plenum, New
York, 1977, vol. 3.
18 M. B. Lawson and E. B. Fleischer, J. Coord. Chem., 1972, 2, 79.
19 W. Kaim and S. Kohlmann, Inorg. Chem., 1990, 29, 2909.
Paper 8/08293F
582
J. Chem. Soc., Dalton Trans., 1999, 575–582