Binuclear complexes of Ni(i) from 4-terphenyldithiophenol

Article abstract of DOI:10.1039/c5dt01178g

Binuclear complexes of Ni(i) have been prepared from a 4-terphenyldithiophenol ligand. Steric effects were found to determine the formation of coordination isomeric structures that differ in the nature of metal-to-ligand bonding. Coordination of spatially

Full text of DOI:10.1039/c5dt01178g

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Binuclear complexes of Ni(I) from
4-terphenyldithiophenol†
Cite this: DOI: 10.1039/c5dt01178g
Felix Koch, Hartmut Schubert, Peter Sirsch and Andreas Berkefeld*
Binuclear complexes of Ni(I) have been prepared from a 4-terphenyldithiophenol ligand. Steric effects
were found to determine the formation of coordination isomeric structures that differ in the nature of
metal-to-ligand bonding. Coordination of spatially demanding phosphine ligands PR3, R = C6H6, C6H11, at
nickel sites results in a butterfly shaped thiolate-bridged Ni2(μ-S)2 motif. For smaller PMe3, the central
π-system of the 4-terphenyl backbone adopts a bis-allyl like μ-syn-η3:η3-C6H4 structure due to significant
d-π* Ni(I)-to-ligand charge transfer. Delocalisation indices δ(Ni–Ni) derived from DFT calculations provide
a metric to assess the strength of electronic coupling of the Ni sites based on solid state structural data,
and indicated less strong electronic coupling for the bis-allyl like structure with δ(Ni–Ni) = 0.225 as com-
pared to 0.548 for the Ni2(μ-S)2 structural motif. A qualitative reactivity study toward CNCH3 as an auxiliary
ligand has provided the first insight into the chemical properties of the bimetallic complexes presented.
Received 25th March 2015,
Accepted 15th June 2015
DOI: 10.1039/c5dt01178g
www.rsc.org/dalton
reactivity displayed by binuclear Ni(I) complexes. An early
example of binuclear reactivity is the tetramerisation of ethyne
Introduction
The class of binuclear complexes of Ni(I) features a remarkable to cyclooctatetraene as put forward by Wilke.26 Despite the
structural diversity, and the ligand design plays a pivotal role exact mechanism of this process remaining unresolved,
in this regard. One can differentiate between structural motifs dinickel(I) complexes have been reported to catalyse reductive
in which electronic coupling of the Ni(I) ions either occurs C–C bond formation in a cooperative fashion,15,21,22 to act as
through bonding to (i) bridging conjugated π-systems in both precursors in bimetallic catalytic group transfer reactions to
syn- and antarafacial fashion,1–8 (ii) bridging amido,9 phos- form carbodiimides and isocyanates,17,27 and to activate sec-
phido,10 thiolato,11,12 sulfido,13 halogenido,14–17 and hydrido18 ondary silanes for the catalytic hydrosilation of unsaturated
ligands in the form of Ni2(μ-X)2 cores, (iii) bridging diphos- substrates.28
phine,19,20 and biphenyldiyl15,21,22 ligands, or in the form of
The capability of thiolate ligands to bridge a pair of metals
unsupported Ni–Ni bonds.23–25 Antiferromagnetic coupling through bis-µ-thiolate coordination aids the formation of
through a ligand and direct Ni–Ni bonding results in diamag- binuclear and higher nuclearity complexes,29–33 including well
netic behaviour in most instances, albeit triplet ground states characterised examples for Ni(I) of type I in Scheme 1.10–12 In
have been proposed for the Ni2(μ-Br)2 core bound to the the context of a reactivity study of thiolate complexes of Ni(I),
neutral form of a redox active 1,8-naphthyridine diimine Tatsumi and co-workers reported the structure of a binuclear
ligand.16
Ni(I) complex with bridging arene and thiolate groups, see II
Based on solution reactivity and VT EPR studies on solid in Scheme 1.11
samples dinickel biradicals coexist in thermal equilibria with
We have set out to develop the coordination chemistry of
diamagnetic ground states. In the biradical state the Ni–Ni binucleating terphenyldithiophenol ligands which combine
interaction is non-bonding and dissociation is a favourable the properties of metal–sulphur34,35 and labile metal–arene36
and facile process.2,5,9 The reversible formation of reactive Ni(I)
species appears to represent one of the two major modes of
Institut für Anorganische Chemie, Eberhard Karls Universität Tübingen, Auf der
Morgenstelle 18, 72076 Tübingen, Germany.
E-mail: andreas.berkefeld@anorg.uni-tuebingen.de; Tel: +49 7071 29 76213
†Electronic supplementary information (ESI) available: Additional compu-
tational, experimental, and NMR spectroscopic details. CCDC 1042645 (1),
1042646 (2), 1042647 (3), 1042648 (4), and 1042649 (5). For ESI and crystallo-
Scheme 1 General structural motifs of dinuclear Ni(I) complexes with
L = neutral ligand, e.g. tert-phosphine, NHC, and R = aryl, tert-alkyl, H.
graphic data in CIF or other electronic format see DOI: 10.1039/c5dt01178g
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bonds. The use of a dianionic ligand framework eliminates the
necessity for incorporating secondary anionic ligands that
might bridge the nickel ions, which may provide structural
flexibility to the bimetallic core. Herein we report on the syn-
thesis and properties of mono- and binuclear complexes of
nickel with such a type of ligand.
Results and discussion
Synthesis and structure of mono- and binuclear complexes 1–5
Fig. 1 ORTEP representation and selected structural parameters of the
molecular structure of 2 (thermal ellipsoids at the 50% probability level).
Given Ni–C, and –S distances are averaged, H- and lattice solvent atoms
are not shown for clarity.
The preparation of the nickel complexes followed two different
protocols. The first utilises the comproportionation of Ni(II)
precursors 1 and 2 with Ni(COD)2, COD being 1,5-cyclo-
octadiene. Alternatively, reacting 4-terphenyldithiophenol
with 2 equiv. of (PPh3)2NiN(SiMe3)2 as a source of Ni(I) affords
bimetallic
5 directly, eliminating bis(trimethylsily)amine
and PPh3.11
tance of 2.614(2) Å is significantly longer, as expected. The
molecular structure of 1 is shown in Fig. S9, ESI.† Low-temp-
Ni(II) precursor complexes 1 or 2 have been prepared by salt
metathesis, with a reaction of the dipotassium salt of dithio-
phenol with Ni(II) chloride precursors in toluene solution at
ambient temperature as shown in Scheme 2. The compounds
have been isolated as purple solids in yields of 88 and 60%
with λmax(thf) for 1 at 511 (ε/dm3 mol−1 cm−1 1128) and 2 at
550 (3545) nm. Note that 2 is a kinetic reaction product and
was found to gradually convert into an unknown red precipi-
tate, most likely an oligomeric composition. Substituting PCy3
for PPh3 affords a mixture of ill-defined products.
1
erature 13C{1H} and H NMR data are consistent with η2-arene
bonding to Ni(II) in 2. At 193 K in d2-CH2Cl2 solution, charac-
teristic resonances of a Cs symmetric structure were observed
at 108.4, broadened due to unresolved 2JC–P coupling, and 8.21
as well as 127.4 and 7.44 ppm which account for the Ni-bound
and unbound, yet slowly exchanging C2H2 edges of the central
aryl ring as defined in Scheme 2. At 298 K pseudo C2v sym-
metric spectra result for both complexes with an N-line pattern
|2+4|
at 0.77 ppm,
J
= 7.8 Hz, for trans-(Me3P)2Ni(II) along
H–P
with resonances of the nuclei at the central arene ring at
7.56 and 123.9 ppm for 1, and at 7.76 and 118.6 ppm in the
case of 2.
Binuclear complexes 3–5 have been isolated in the form of
distinct coordination isomers as depicted in Scheme 3 in
yields of 51% (3, brown), 75% (4, yellow), and 73% (5, green)
with λmax (thf)/nm 3, 520 (ε/dm3 mol−1 cm−1 2044), 4, 352
(3005), and 5, 450 (7821). Additional weak absorption bands
Single crystal XRD structural analysis verified square planar
coordination geometries at the trans-dithiolatonickel(II) centres
with average Ni–S bond lengths of 2.249(4) and 2.241(2) Å
for 1 and 2 respectively which compare well with the Ni–S
bond lengths reported for structurally related complexes.10–12
The molecular structure of 2 is depicted in Fig. 1. While two
PMe3 donors complete the ligand shell of Ni(II) in 1, the use of
the more bulky PCy3 (ΔΘTolman = 52°) ligand in 2 allows for
binding just one phosphine ligand, with a Ni–P bond distance
of 2.353(2) Å, and enforces η2-coordination of the central aryl
ring with an averaged Ni–C distance of 2.285(4) Å.
Also in the case of 1 the nickel ion resides above one of the
C2H2 edges of the central arene ring, but the average Ni–C dis-
Scheme 3 Synthesis, structure, and characteristic δC (11.7 T) NMR
chemical shifts of binuclear complexes 3–5 (R’ = 2,4,6-(H3C)3C6H2, R =
tBu).
Scheme 2 Structure and characteristic δC (11.7 T) NMR chemical shifts
of Ni(II) precursor complexes 1 and 2 (R’ = 2,4,6-(H3C)3C6H2, R = tBu).
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Fig. 2 Left: ORTEP representation and selected structural parameters
of 3 (thermal ellipsoids at the 50% probability level; H- and lattice
solvent atoms omitted for clarity). Right: schematic representation of
the overall exchange process of the Ni(I)2 fragment (as a visual aid, aster-
isks highlight identical C atoms).
Fig. 3 ORTEP representation and selected structural parameters of 5
(thermal ellipsoids at the 50% probability level; H- and lattice solvent
atoms omitted for clarity), and schematic representation of a seesaw-
like motion of the Ni2(μ-S)2 fragment.
were detected for each of the dinickel complexes at wave-
lengths of 940–990 nm (ε/dm3 mol−1 cm−1 200–500).
ions to the arene system. Instead, the P(1)–Ni(1)–Ni(2)–P(2)
moiety is tilted upward relative to the aryl ring plane which
allows one nickel ion, Ni(1), to weakly interact with the un-
distorted π-system as indicated in Fig. 3.
Single crystal XRD structure analysis verified the binding
modes of the dinickel cores to the dithiophenolate ligand as
exemplified for 3 and 5 in Fig. 2 and 3. A distinct structural feature
of 3 is the boat shaped conformation of the central π-system
which is present in both the solid state and solution. The vari-
ation of bond lengths and deformation from planarity indicate
the formation of a syn-μ-η3:η3 bonding motif, which likely
results from a significant overlap of the occupied d-orbitals at
both nickel ions with a π*-orbital of the arene π-system.36–38
Short distances of 1.949(4), 2.035(4), and 2.149(4) Å between
Ni and o-, i-, and m-C atoms, and a long Ni–Ni separation of
2.659(1) Å support this description. The observed κ1-S–Ni bond
lengths of 2.205(1) and 2.193(3) Å compare well with those in
the literature.11 Such tight Ni2-arene bonding is likely a conse-
quence of a strong charge donation from the thiolate and
phosphine ligands to nickel. A structurally similar confor-
mation was reported for the central π-system of a 4-terphenyl-
diphosphine ligand that stabilises a bridging μ-η1:η1-o,o′-
biphenyldiyl dinickel fragment.15
The observed average Ni(1)–C distances of 2.569(6) and
2.443(1)39 Å in 4 and 5 respectively are significantly longer
than those found in 2 (2.285(4) Å). The formation of a syn-
endo-Ni2(μ-S)2 structure29 compensates for the lack of metal–
arene interactions at Ni(2). As a result, slightly shorter average
μ-S–Ni(2) and Ni(2)–P bond distances of 2.213(1) (Δd = 0.02)
and 2.148(1) (Δd = 0.009) Å as compared to the respective
bonds to Ni(1) have been found. Acute Ni(1)–S–Ni(2) angles of
65° within the Ni2(μ-S)2 entity are accompanied by a short Ni–
Ni separation of 2.383(1) Å.10,11,29
1
Exchange averaged singlet H NMR resonances of the C2H2
moieties of the central aryl rings in 4 and 5 have been observed
at 8.41 (ν1/2 ∼ 95 Hz) and 7.72 (ν1/2 ∼ 33 Hz) ppm at 168 K.
Whether or not this exchange involves preliminary dissociation
of one phosphine ligand is unclear albeit no spectral differ-
ences have been observed in the presence of excess (≤5%)
phosphine. As for 3, the apparent exchange of the C2H2 sites
requires an overall seesaw-like motion as schematised in
Fig. 3.
The geometry at each Ni ion may be best described as a dis-
torted square planar defined by the sulphur, phosphorus, and
the peripheral C-atoms of the η3-allyl moiety. The shortest dis-
tance between hydrogen atoms at the PMe3 ligands is 2.5 Å.
The dinickel core of 3 is dynamic. For an 11.7 T magnetic
Computational study
field, the resonance at δH 5.41 ppm of protons 8/9-H at the A comparison of Ni–Ni and Ni–C(arene) bond lengths calcu-
central π-system was observed to coalesce at 203 K. Further lated from single crystal X-ray diffraction data suggests a
cooling to 178 K gave rise to broad 1H and 13C NMR reso- higher contribution of direct metal–metal (M–M) bonding to
nances at 4.97 (ν1/2 ∼ 26 Hz) and 5.59 (ν1/2 ∼ 23 Hz) as well as the coupling of the nickel ions in the case of the syn-endo-
109 (i-C, ν1/2 ∼ 42 Hz), 74.2 (o-C, ν1/2 ∼ 60 Hz), and 98.5 (m-C, Ni2(μ-S)2 structural motif in 4 and 5 than in 3. To gain deeper
ν1/2 ∼ 55 Hz) ppm. Exchange cross peaks between protons 8/9- insight into the nature and actual strength of Ni–Ni bonding
1
H are well established in H–1H NOESY data at 178 K. Appar- in these structurally different complexes, we carried out a
ent exchange of 8-H for 9-H requires the Ni(I) ions to formally topological analysis of their electron density distribution,
change sites across the syn-μ-η3:η3-C6H4 system as schematised ρ(r), as derived from the DFT calculations of 3 and 5, using
in Fig. 2. Whether this process evolves through an intermedi- the “quantum theory of atoms in molecules” (QTAIM)
ate structure analogous to that of 4 and 5 is unknown.
approach.40,41 DFT calculations were performed using
Steric interactions between the mesityl and phosphine sub- Gaussian42 at the B3LYP/6-311G(d,p) level43–49 with dispersion
stituents in 4 and 5 impair a close approach of both nickel corrections, including Becke–Johnson damping,50,51 and
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substituting the tBu groups on the ligand for CH3 in the Solution properties of 3–5
theoretical model systems. The optimised geometries were in
excellent agreement with the experimental counterparts, and
the Ni–Ni distances differed by 2 and 6.1 pm for 3 and 5,
respectively. For computational details and the full geometries,
see the ESI.†
1
VT H NMR data of 3 and 5 revealed a marked dependence of
chemical shifts and line-widths of specific proton resonances
with temperature and solvent polarity. In the case of 3, substi-
tuting the solvent d8-thf with d6-C6H6 at 299 K resulted in a
ΔδH of +1 ppm for protons 8/9-H at the central π-system. As
shown in Fig. 4, variation of the temperature of a d8-thf solu-
tion of 3 results in a reversible, non-linear variation of δH
between 5.39 at 198 K and 11 ppm at 405 K, ΔδH(8/9-H) =
5.61 ppm.
Although numerous experimental and theoretical electron
density studies on M–M bonding in dinuclear complexes have
been reported, a fundamental understanding of this inter-
action remains elusive.52,53 In both 3 and 5, a stable bond
path54 between the two nickel ions could be identified which
implies direct M–M interactions in both systems. This obser-
vation is noteworthy, as the presence of bridging ligands
usually results in the loss of M–M bond paths, in particular for
metal atoms linked by formal single bonds, as concluded by
Farrugia and Macchi in a recent review.53 The topological para-
meters of ρ(r) at the Ni–Ni bond critical points (BCPs)55 in 3
and 5 are listed in Table 1, and the extended topological data
and the molecular graphs of 3 and 5 are provided in the ESI.†
In both systems, the value for ρ(rb) is relatively small, a feature
commonly observed for M–M bonds which has been attributed
to the diffuse nature of this interaction.53,56 The slightly nega-
tive values for the total energy density, H(rb), imply that the
potential energy density at these BCPs is somewhat larger than
the kinetic energy density which is indicative of covalent
bonding, despite the positive value for the Laplacian of ρ(rb),
∇2ρ(rb).56,57 A better metric than ρ(rb) to assess the actual
strength of metal–metal bonding is provided by the delocaliza-
tion index, δ(A–B), which measures the number of electron
pairs shared between two atoms A and B, irrespective of the
existing bond paths.53,56,58 For 3 and 5, the values of δ(Ni–Ni)
are equal to 0.225 and 0.548, respectively. These smaller, frac-
tional bond orders indicate what has been termed as a partial
covalent character for formally M–M single bonds, in the sense
that not a whole pair of electrons is involved in direct inter-
actions between the atoms.53 To summarise, the bonding
between the Ni atoms in both 3 and 5 can be described as a
combination of through-ligand and direct M–M bonding. The
latter contribution was clearly identified through the existence
of Ni–Ni bond paths. The large increase in the Ni–Ni separ-
ation by 27.6 pm [DFT: 31.7 pm] in 3 compared to 5 also
reflects a significant reduction of Ni–Ni bond strength: the
respective value of δ(Ni–Ni) in 5 is more than 2.4 times higher
than the one in 3, which can therefore be classified as a bi-
metallic complex exhibiting only very weak Ni–Ni interactions.
In comparison, the observed chemical shift changes for the
protons at the flanking phenyl rings, ΔδH(3-H) = 0.42, and 0.88
(5-H) ppm, and for the PMe3 ligands (0.58 ppm) are smaller by
about one order of magnitude but vary in the same non-linear
fashion. The largest shift difference in the latter series
accounts for the protons most distant from the nickel ions
(5-H; for numbering cf. to Scheme 3) whereas resonances of
the mesityl (26-H) and tBu substituents are unaffected.
A similarly marked dependence emerged for 5 in d8-thf
with ΔδH(8,9-H) = 4.94 ppm and δH(8,9-H, 299 K) = 10.12 ppm
(12.01 ppm in d6-C6H6 and d8-toluene) whereas it was found to
be small in the case of 4, with ΔδH(8,9-H) = 0.51 ppm and
δH(8,9-H, 299 K) = 8.69 ppm. Plots of δH vs. temperature for
selected types of protons for complexes 4 and 5 are provided in
Fig. S7 and S8, ESI.† The respective values of δH and the vari-
ation in temperature have been observed for both raw products
and single as well as double recrystallized samples of 3–5 that
were obtained from different batches. This indicates that puta-
tive Ni(I) impurities most likely are not the reason for this solu-
tion behaviour.
Whereas the H and 13C NMR chemical shifts determined
1
at low temperatures account for the respective structural fea-
tures observed in the solid state structures of 3–5, the continu-
ous shift of δH to lower fields at higher temperatures,
especially δH(8/9-H) being progressively greater than 11 ppm,
suggests thermal population of a paramagnetic biradical (S = 1)
Table 1 Selected topological properties at the Ni–Ni bond critical
points in the model systems of 3 and 5
da
ρ(rb)b
∇2ρ(rb)c
ε
G(rb)/ρ(rb)
H(rb)d
3
5
2.639
2.322
0.194
0.407
+1.856
+1.686
0.02
0.36
0.879
0.674
−0.041
−0.156
a In units of Å. b In units of e Å−3
hartree Å−3
.
c In units of e Å−5
.
d In units of
Fig. 4 Temperature dependence of δH observed for 3 (500 MHz, 21 mM
in d8-thf).
.
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state2,9 which would add contact and dipolar shift contri- 2 equiv. of PPh3 and L′ in an NMR tube reaction under other-
butions to the observed chemical shift.28,59,60 An independent wise identical conditions.
parameter to probe for variable net electron spin moments is
IR spectra obtained with a dip probe at 193 K showed the
the trend of the temperature dependence of the 1H spin–lattice appearance of an intense band at 2051 cm−1, along with a
relaxation time, T1, which is expected to be positive for weak band at 2152 cm−1, for 5*2L′ whose intensity reached a
protons in diamagnetic environments. Values of T1 for 8/9-H maximum after the addition of 2 equiv. of L′. As expected,
of 3–5 were found to decrease markedly with increasing temp- both bands disappeared upon warming the solution to room
eratures, e.g. T1(8/9-H) for 3 in d8-thf decreased from 1300 ms temperature due to the disproportionation of 5*2L′, and were
at 223 K to 110 ms at 323 K; cf. to Fig. S3–S6, ESI.† Surpris- replaced by broad bands at 2109 and 2060 cm−1. These bands
ingly, invariable effective magnetic moments of 0.9 B.M. for 3 compare well with the literature reported values for complexes
and 1.0 B.M. for 5 in both toluene and thf solutions were of the type (Ph3P)n(RNC)4−nNi(0), with n = 1–3, and R = tBu, Cy,
determined by Evan’s method61,62 over the same temperature and PhCH2.63
range at a field of 11.7 T. This finding challenges the hypo-
In clear contrast, addition of 1 equiv. of L′ to 3 at 213 K
thesis of the thermal equilibrium population of a paramagnetic afforded an asymmetric compound which we assign as 6
1
spin state. Heating a solution of 5 in d8-toluene to 373 K for shown in Scheme 5. The complete assignment of H and 31P
3 h was found to slightly increase the effective magnetic NMR data of putative 6 is given in the ESI.† The PMe3 ligands
moment by ∼0.1 B.M. but did not have any effect on the temp- in 6 are non-equivalent but appear to coordinate to the same
erature dependence of δH.
nickel site, with 2JP–P = |54| Hz.64 The magnitude of 3JP–P of the
putative isomeric structure 6iso may be expected to be signifi-
cantly smaller if detectable at all.20 Selective 31P decoupling of
1H NMR data aided in the assignment of the two PMe3
Chemical properties of dinickel cores
1
ligands. Both phosphine ligands showed H–1H NOE contacts
Compounds 3–5 displayed appreciable thermal stability in VT
NMR spectroscopic studies of their toluene and thf solutions.
While heating a solution of 3 in d6-benzene to 343 K for 2 h
resulted in low conversion, ∼3%, back into 1 no changes were
observed in d8-thf under identical conditions. When dissolved
in d2-CH2Cl2 under ambient conditions gradual deterioration
to yet unknown products occurs in all cases. To gain a more
detailed insight into the chemical properties of these bi-
metallic systems the reactivities of 3–5 were studied qualitat-
ively toward methyl isocyanide (CNCH3 = L′) by VT NMR
spectroscopy.
to L′ (δH 3.32 ppm), which would not have been possible if
PMe3 and L′ were coordinating in a trans-fashion to the same
nickel ion as in 6iso. Ni-arene d-π* back bonding appears to be
insignificant, judging from the chemical shifts of the four dis-
tinct resonances of the central ring protons, δH being 7.74,
6.53, 6.47, and 6.18 ppm at 193 K. The central π-system of the
4-terphenyl backbone may act as a σ-donor to complete the
ligand shell at the Ni–L′ site as indicated in Scheme 5. As
1
observed for complex 2, the shift of one of the above H reso-
nances to a lower field may reflect dative bonding of the
π-system to nickel which binds another strong σ-donor ligand
such as PCy3 and L′ in the trans-position.
The structure of 6 is dynamic in solution. Exchange spectra
show that the PMe3 ligands slowly exchange Ni sites with L′ at
1H and 31P NMR spectroscopic monitoring showed that 5
(δP 68 ppm) reacts with L′ in a 1 : 2 stoichiometry at T ≤ 213 K
to produce a single product 5*2L′ with characteristic reso-
nances δH(Ni–L′) 2.10 and δP 22.51 ppm as described in
Scheme 4. Coordination of both PPh3 and L′ to both Ni sites is
supported by 1H–1H COSY and 1H–31P HSQC data which estab-
lished the presence of scalar JH and JP coupling of the methyl
protons of L′ with the o- and m-1H and 31P nuclei of coordinate
PPh3. Albeit we cannot assign a molecular structure based on
the NMR data available, 5*2L′ must be a bimetallic complex.
Warming the sample to 263 K in the NMR probe resulted in
the progressive formation of (Ph3P)2(L′)2Ni(0) at the expense of
the resonances of 5*2L′, with characteristic 1H and 31P NMR
resonances at 2.85 ppm (N-line pattern, L′) and δP 33.2 ppm.
The identity of this Ni(0) product has been established un-
equivocally through independent synthesis from Ni(COD)2,
Scheme 5 Reactivity of 3 toward CNCH3 (L’) monitored by in situ VT
t
Scheme 4 Reactivity of binuclear
5
toward CNCH3 (L’) monitored
NMR spectroscopy (L = PMe3, R’ = 2,4,6-(H3C)3C6H2, R = Bu). Dashed
in situ by VT NMR spectroscopy in d8-thf (δNucleus in ppm at 11.7 T).
line accounts for the capability of L’ to μ-bonding.
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213 K, see Fig. S1 and S2 in the ESI.† The known capability of also reflected by the calculated delocalisation index from solid
L′ to coordinate in a μ-fashion19,65 likely triggers this exchange state structural data.
process as indicated in Scheme 5.
The magnitude of metal–metal electronic coupling may be
Monitoring the addition of L′ to 3 at 193 K in thf by IR taken as a parameter which determines the electronic flexi-
spectroscopy showed the gradual appearance of a broad band bility and thus the reactivity of bimetallic structures.66 The bi-
at 2064 cm−1, along with a significantly weaker broad band metallic complexes of nickel described herein provide a
centred at 2124 cm−1. Bands at wavenumbers greater than complementary set of model compounds suitable for studies
2000 cm−1 have been reported as characteristic for terminal on the structural effects on the reactivity of binuclear systems
rather than bridging coordination of isocyanide ligands toward electrophilic reactants.
in binuclear complexes of low-valent nickel.19,65 Compared
to a reference sample with no added 3, L′ coordination
results in a shift of the CuN stretching mode by 100 cm−1 to
Experimental
General considerations
lower energy, indicating a significant d-π* charge transfer
from Ni to L′. At 234 K, 1H NMR resonances of the central
arene ring protons are subject to exchange broadening and All reactions were carried out under a dry argon atmosphere
resonances of 1 began to gradually grow into the spectra. The using standard Schlenk or glove box techniques (MBraun, MB
addition of two equivalents of L′ to 3 resulted in complete 150-GI). All solvents were purified and dried prior to use.
conversion of 6 into 1. IR spectra obtained at the same temp- Dichloromethane and hexane were dried over Grubbs columns
eratures showed a broad band centred at 1855 cm−1 that ir- of a MBraun solvent purification system. Benzene, diethyl
reversibly formed at the expense of the bands at 2124 and ether, pentane, tetrahydrofuran, and toluene were pre-dried
2064 cm−1 that had originally been observed upon L′ addition over activated 3 Å molecular sieves and distilled from sodium
at 193 K.
benzophenone ketyl or potassium metal under argon. Metha-
nol was dried over activated neutral alumina. d6-Benzene, d8-
toluene, and d8-thf were dried over and distilled from the NaK
alloy whereas d-CHCl3 and d2-CH2Cl2 were dried over and
vacuum transferred from 3 Å molecular sieves. All solvents
Conclusions
Mono- and binuclear complexes of Ni(I) have been prepared were stored over 3 Å molecular sieves under argon. Molecular
from a 4-terphenyldithiophenol ligand. Steric interactions sieves and neutral alumina were activated by heating under
between phosphine and substituents at the 4-terphenyl back- dynamic vacuum (10−3 mbar) at 250 °C for 24–48 hours.
bone result in the formation of coordination isomeric struc- UV-Vis spectra were collected on a PerkinElmer Lambda 35
tures with thiolate bonding either in a bridging or terminal spectrophotometer. The range from 200 to 1100 nm was
fashion. In the latter case, significant d-π* charge transfer scanned at a speed of 480 nm per minute, using 1 cm quartz
causes the 4-terphenyl ligand backbone to coordinate to the Ni cuvettes sealed with Teflon stoppers. Combustion analyses
ions in a syn-μ-η3:η3 fashion. Electronic coupling of metal were performed on an Elementar Vario MICRO instrument.
centres within the Ni2(μ-S)2 core displayed in 4 and 5 involves NMR data were recorded on Bruker Avance II 400 and DRX 250
bonding interactions via the bridging thiolates and also direct instruments. VT NMR spectra were collected on a Bruker
M–M bonding with calculated δ5(Ni–Ni) = 0.548, whereas a AVII+500 spectrometer. δ Values are given in ppm, J values in
1
delocalisation index of 0.225 indicates significantly weaker Hz. H and 13C{1H} NMR chemical shifts are referenced to the
coupling of the Ni(I) ions in 3. The origin of the chemical shift residual proton and naturally abundant carbon resonances of
dependence on temperature observed for 3–5 is currently not the solvents: 7.16/128.06 (d6-C6H6), 3.58/67.21 (d8-thf), 5.32/
understood but most likely reflects changes of the electronic 53.84 (d2-CH2Cl2), and 7.26/77.16 (d-CHCl3) ppm. 31P NMR
properties of the bimetallic core. Whereas metal–thiolate chemical shifts are referenced to an external standard sample
bonding couples protons 3/5-H to the Ni ions, 8/9-H at the of 85% H3PO4 set to 0 ppm. VT solution IR spectra were
central π-system bind directly to the latter. Albeit plausible obtained with a Mettler Toledo ReactIR 15 system equipped
this hypothesis necessitates further studies.
with a Sicomp dip probe at a spectral range from 2600 to
Coordination of a π-acceptor ligand such as methyl iso- 650 cm−1. Ligand preparation is described in the ESI.† The
cyanide to 3 and 5 subjects the bimetallic fragments to dispropor- compounds (Me3P)2NiCl2,67 Ni(COD)2,68 (Ph3P)2NiN(SiMe3)2,69
tionation. Interestingly, the nature of the phosphine ligand (pyridine)4NiCl2,70 and methyl isocyanide71 were prepared fol-
determines the character of intermediately formed species and lowing the procedures adapted from the literature. Caution:
the temperature at which Ni(0) species are extruded. The Methyl isocyanide is toxic and has a very unpleasant odour. All
binuclear Ni2(μ-S)2 structure in 5 persists upon binding of 2 manipulations with this reagent should be carried out in a
equiv. of isocyanide at low temperatures. In the case of 3, fume hood. X-ray data were collected on a Bruker Smart APEX
apparent PMe3 migration to the same nickel site is triggered II diffractometer with graphite-monochromated Mo Kα radi-
by isocyanide bonding. This structural reorganisation reaction ation. The programs used were Bruker’s APEX2 v2011.8-0,
is remarkably facile even at low temperature which may be a including SADABS for absorption correction and SAINT for
consequence of the weak electronic coupling of the Ni ions, as structure solution,72 the WinGX suite of programs version
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2013.3,73 SHELXS for structure solution, SHELXL for structure evacuation of solid 3 must be avoided since concomitant
refinement,74,75 and PLATON.76 Crystals were, unless otherwise removal of volatile PMe3 leads to gradual sample degradation.
noted, coated in a perfluorinated polyether oil and mounted on
δH (500 MHz, d8-thf, 178 K) 7.29 (2 H, d, J3−5 1.7, 4, 3-,
a 100 µm MiTeGen MicroMounts™ loop that was placed on the 18-H), 6.87 (2 H, d, J5−3 1.7, 4, 5-, 16-H), 6.83 (4 H, s, 21-, 23-,
goniometer head under a stream of dry nitrogen at 100 K.
32-, 34-H), 5.59 (2 H, br s), 4.97 (2 H, br s), 2.28 (6 H, s, 26-,
37-H), 1.95 (12 H, s, 25-, 27-, 36-, 38-H), 1.33 (18 H, s, 4-, and
17-tBu), 1.29 and 1.28 (s, P(CH3)3).
Preparation and characterization of 1
Ligand (500 mg, 0.776 mmol), benzyl potassium (210 mg,
δC (126 MHz, d8-thf, 178 K) 152.42 (C-1, -14), 144.05, 140.80,
1.550 mmol), and toluene (40 mL) were combined in a Schlenk 139.03 (C-19, -30), 136.00, 135.34, 134.81, 127.53 (C-21, -23,
flask. The resulting mixture was stirred for 0.5 hour (h) at -32, -34), 125.75 (C-5, -16), 122.21 (C-3, -18), 109.43, 98.51,
room temperature (r.t.) to form a clear yellow solution. After 74.23, 34.12 (C-28, -40), 31.11 (C-29, -39), 20.5 (C-25, -27, -36,
this time, (Me3P)2NiCl2 (220 mg, 0.780 mmol) was added and and -38), 20.5 (C-26, -37), 15.72 (JC−7−P 26.5, P(CH3)3).
stirring was continued for an additional 2 h at r.t. The solution
gradually turned dark purple and a white solid separated. The
δP (202 MHz, d8-thf, 178 K) −13.66.
Elemental analysis found: C, 66.08; H, 7.45; S, 6.83. Calc.
solid was filtered off, the solvent removed under vacuum, and for C50H66S2: C, 65.96; H, 7.31; S, 7.04%.
the residual dark purple solid repeatedly (3×) washed with
UV-Vis: λmax (thf)/nm 266 (ε/dm3 mol−1 cm−1 25 089), 328
10 mL portions of methanol, and dried under dynamic (20 496), 373 (15 074), 520 (2044) and 943 (310).
vacuum to produce a purple solid 1 (576 mg, 88%). Bulk crys-
Crystal data: C50H66Ni2P2S2·C5H12, M = 982.65, monoclinic,
tallisation by slow diffusion of methanol layered on top of a a = 15.8803(8), b = 20.1160(10), c = 17.6539(9) Å, U = 5371.9(5) Å3,
concentrated solution of 1 in benzene produced single crystals T = 100(2) K, space group P121/n, Z = 4, 75839 reflections
also suitable for XRD analysis.
measured, 15 591 unique (Rint = 0.0321) which were used in all
δH (400 MHz; d6-C6H6; 298 K) 7.76 (2 H, d, J3−5 2.4, 4, calculations. The final wR(F2) was 0.1389 (all data).
3-, 18-H), 7.56 (4 H, s, 8-, 9-, 11-, 12-H), 7.05 (2 H, d, J5−3 2.6, 4,
5-, 16-H), 6.95 (4 H, s, 21-, 23-, 32-, 34-H), 2.29 (12 H, s, 25-, 27-,
Preparation and characterization of 2
36-, 38-H), 2.17 (6 H, s, 26-, 37-H), 1.33 (18 H, s, 4-, and
17-tBu), 0.77 (18 H, N-line pattern |2+4|JH–P 7.83, P(CH3)3).
Ligand (500 mg, 0.776 mmol), benzyl potassium (210 mg,
δC (100 MHz; d6-C6H6; 298 K) 144.22, 143.41, 141.83 1.550 mmol), and toluene (40 mL) were combined in a
ˉ
(t, JC–P 2.1, C-1, -14), 141.63 (C-19, -30), 140.75, 140.12, 136.10 Schlenk flask. The resulting mixture was stirred for 30 minutes
(C-20, -24, -31, -35), 135.65 (C-22, -33), 128.36 (C-21, -23, -32, at r.t. to form a clear yellow solution. After (pyridine)4NiCl2
-34), 126.34 (C-5, -16), 123.85 (C-8, -9, -11, -12), 122.77 (C-3, (360 mg, 0.776 mmol) and tricyclohexylphosphine (210 mg,
-18), 34.19 (C-28, -40), 31.63 (C-29, -39), 21.19 (C-26, -37), 0.776 mmol) were added, stirring was continued for an
|1+3|
20.97 (C-25, -27, -36, -38), 12.47 (N-line pattern
P(CH3)3).
J
C–P
27.7, additional 2 h at r.t. The solution turned brown and a white
solid separated. After filtration, the solvent was removed under
vacuum, the residual brown solid repeatedly (3×) washed with
δP (162 MHz, d6-C6H6, 298 K) −27.98 (Ni-P(CH3)3).
Elemental analysis found: C, 68.26; H, 8.35; S, 6.73. Calc. 10 mL portions of methanol, and dried under dynamic
for C50H66P2S2·2CH3OH: C, 68.18; H, 8.16; S, 7.00%. vacuum to produce a dark purple solid 2 (456 mg, 60%). Crys-
UV-Vis: λmax (thf) nm 257 (ε/dm3 mol−1 cm−1 67 671), 280 tallisation by slow diffusion of pentane vapour into a concen-
(52 736), 351 (13 220) and 511 (1128). trated solution of 2 in thf produced single crystals suitable for
Crystal data: C50H66P2S2·2CH3OH, M = 915.88, monoclinic, XRD analysis.
a = 13.5105(2), b = 12.1898(2), c = 29.9385(5) Å, U = 4928.25(14)
δH (500 MHz; d2-CH2Cl2; 273 K) 7.76 (4 H, s, 8-, 9-, 11-,
Å3, T = 100(2) K, space group P121/c, Z = 4, 27715 reflections 12-H), 7.27 (2 H, d, J3−5 2.3, 4, 3-, 18-H), 6.78 (4 H, s, 21-, 23-,
measured, 10 728 unique (Rint = 0.0223) which were used in all 32-, 34-H), 6.73 (2 H, d, J5−3 2.3, 4, 5-, 16-H), 2.19 (6 H, s, 26-,
calculations. The final wR(F2) was 0.1114 (all data).
37-H), 1.91 (12 H, s, 25-, 27-, 36-, 38-H), 1.27 (18 H, s, 4-, and
17-tBu), 2.04–0.75 (m, P(C6H11)3).
Preparation and characterization of 3
δC (126 MHz; d2-CH2Cl2; 273 K) 146.25 (C-4, -17), 141.72
1 (200 mg, 0.234 mmol), Ni(COD)2 (65 mg, 0.234 mmol), and (C-6, -15), 141.02 (C-19, -30), 140.71 (C-2, -13), 139.60 (C-7, -10),
thf (25 mL) were combined in a Schlenk flask. The resulting 137.93 (C-1, -14), 136.07 (C-20, -24, -31, -35), 135.88 (C-22, -33),
mixture was stirred for 2 h at r.t. to form a brown solution. The 128.08 (C-21, -23, -32, -34), 127.59 (C-5, -16), 121.49 (C-3, -18),
solvent was removed under vacuum, and the residual brown 118.62 (C-8, -9, -11, -12), 34.56 (C-28, -40), 33.53 (d, P(C6H11)3),
solid repeatedly washed with 5 ml portions of pentane (3×), 31.42 (C-29, -39), 30.11 (d, P(C6H11)3), 27.19 (d, P(C6H11)3),
and dried under dynamic vacuum to produce a brown solid 3 26.82 (d, P(C6H11)3), 21.15 (C-26, -37), 20.88 (C-25, -27, -36, -38).
(110 mg, 51%). Slow evaporation of a concentrated solution of
δP (162 MHz, d2-CH2Cl2, 273 K) 15.42.
3 in pentane produced single crystals suitable for XRD ana-
Elemental analysis found: C, 74.32; H, 8.44; S, 6.27. Calc.
lysis. Further purification was carried out by cooling a saturated for C62H81S2: C, 75.98; H, 8.33; S, 6.54%.
solution of 3 (200 mg) in diethyl ether/pentane (1 : 1) to
UV-Vis: λmax (thf)/nm 253 (ε/dm3 mol−1 cm−1 28 630), 269
238 K to yield brown crystals of 3 (110 mg, 55%). Prolonged (27 354), 283 (24 012), 298 (21 892), 453 (4545) and 550 (3545).
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Crystal data: C62H81NiPS2·C4H8O, M = 1052.17, monoclinic, hydrocarbon, the material contains varying amounts of
a = 9.7947(5), b = 31.5469(16), c = 19.7427(11) Å, U = 6084.0(6) trapped thf, rendering elemental analysis difficult.
Å3, T = 100(2) K, space group P121/n, Z = 4, 81 481 reflections
δH (500 MHz, d6-C6H6, 299 K) 12.01 (4 H, br s, 8-, 9-, 11-, 12-
measured, 12 065 unique (Rint = 0.0491) which were used in all H), 7.99 (2 H, d, J3−5 1.6, 4, 3-, 18-H), 7.86 (2 H, d, J5−3 1.8, 4,
calculations. The final wR(F2) was 0.2111 (all data).
5-, 16-H), 7.48 (m, PPh3), 7.02 (m, PPh3), 6.84 (m, PPh3), 6.78
(4 H, s, 21-, 23-, 32-, 34-H), 2.28 (6 H, s, 26-, 37-H), 1.68 (12 H,
s, 25-, 27-, 36-, 38-H), 1.22 (18 H, s, 4-, and 17-tBu).
Preparation and characterization of 4
Compound 2 (100 mg, 0.102 mmol), Ni(COD)2 (28 mg,
δC (126 MHz, d6-C6H6, 299 K) 164.71, 152.34, 142.79,
0.102 mmol), tricyclohexylphosphine (29 mg, 0.103 mmol), 136.41, 136.37, 135.04, 133.70, 129.49, 128.02, 126.01, 115.33,
and thf (25 mL) were combined in a Schlenk flask. The result- 33.20, 32.25, 21.27, 20.73.
ing mixture was stirred for 2 h at r.t. to form a yellow solution.
δH (500 MHz, d8-thf, 298 K) 10.12 (4 H, br s, 8-, 9-, 11-, 12-
After removing the solvent under dynamic vacuum, the H), 7.53 (2 H, d, J3−5 2.2, 4, 3-, 18-H), 7.36 (2 H, d, J5−3 2.3, 4,
residual yellow solid was repeatedly (3×) washed with 5 mL 5-, 16-H), 7.29 (m, PPh3), 7.17 (m, PPh3), 6.55 (4 H, s, 21-, 23-,
portions of pentane, and dried under dynamic vacuum to 32-, 34-H), 2.26 (6 H, s, 26-, 37-H), 1.36 (18 H, s, 4-, and
produce solid yellow 4 (80 mg, 75%). Bulk crystallisation by 17-tBu), 1.26 (12 H, s, 25-, 27-, 36-, 38-H).
slow diffusion of pentane layered on top of a concentrated thf
δC (126 MHz, d8-thf, 298 K) 159.11, 150.80 (C-4, -17), 147.85,
solution produced single crystals suitable for XRD analysis.
142.56 (C-19, -30), 137.45, 137.09 (PPh3), 136.05, 135.47 (PPh3),
δH (500 MHz, d8-thf, 253 K) 8.59 (4 H, br s, 8-, 9-, 11-, 12-H), 130.58 (C-8, -9, -11, -12), 130.09 (PPh3), 129.24, 128.80 (PPh3),
7.35 (2 H, d, J3−5 2.3, 4, 3-, 18-H), 6.81 (2 H, d, J5−3 2.2, 4, 128.23 (C-21, -23, -32, -34), 127.73 (C-5, -16), 125.60 (C-1, -14),
5-, 16-H), 6.74 (4 H, s, 21-, 23-, 32-, 34-H), 2.23 (6 H, s, 26-, 37-H), 124.93 (C-3, -18), 34.57 (C-28, -40), 32.39 (C-29, -39), 21.60
1.94 (12 H, s, 25-, 27-, 36-, 38-H), 1.30 (18 H, s, 4-, and 17-tBu), (C-25, -27, -36, -38), 21.10 (C-26, -37).
2.05–0.72 (P(C6H11)3).
δP (202 MHz, d8-thf, 213 K) 68 (ν1/2 ∼ 800 Hz).
δC (126 MHz, d8-thf, 253 K) 164.86 (C-2, -13), 150.45 (C-6,
Elemental analysis found: C, 73.84; H, 6.22; S, 4.87. Calc.
-15), 146.71 (C-4, -17), 143.19 (C-7, -10), 141.81 (C-19, -30), for C80H78S2: C, 74.90; H, 6.13; S, 5.00%.
139.52 (C-1, -14), 136.32 (C-20, -24, -31, -35), 135.47 (C-22, -33),
UV-Vis: λmax (thf)/nm 315 (ε/dm3 mol−1 cm−1 13 205),
127.75 (C-21, -23, -32, -34), 126.30 (C-5, -16), 124.95 (C-8, -9, 380 (17 718), 450 (7821), 725 (576) and 990 (527).
-11, -12), 121.56 (C-3, -18), 35.92 (PCy3), 34.88 (C-28, -40), 31.47
Crystal data: C80H78Ni2P2S2, M = 1282.9, triclinic, a =
(C-29, -39), 30.63 (PCy3), 28.10 (PCy3), 27.20 (PCy3), 23.08 11.9127(3), b = 21.1860(5), c = 28.2328(7) Å, T = 100(2) K, space
ˉ
(C-25, -27, -36, -38), 21.12 (C-26, -37).
group P1, Z = 4, 275458 reflections measured, 30 564 unique
(Rint = 0.0235) which were used in all calculations. The final
δP (202 MHz, d8-thf, 253 K): 35 (ν1/2 ∼ 5000 Hz).
Elemental analysis found: C, 72.28; H, 8.74; S, 4.60. Calc. wR(F2) was 0.0936 (all data).
for C80H114S2: C, 72.84; H, 8.71; S, 4.86%.
Computational details
UV-Vis: λmax (thf)/nm 220 (ε/dm3 mol−1 cm−1 15 789), 248
DFT calculations of 3 and 5 were performed using the
(25 725), 311 (6257), 352 (3005), 789 (233) and 972 (227).
Crystal data: C80H114Ni2P2S2, M = 1319.19, triclinic, a =
11.1421(4), b = 12.9165(5), c = 26.8959(10) Å, T = 130(2) K,
Gaussian 09 program suite,42 using the B3LYP density
functional,43–45 along with the implemented 6-311G(d,p) basis
set46–49 and dispersion corrections, including Becke–Johnson
ˉ
space group P1, Z = 2, 51 911 reflections measured, 17 549
t
damping.50,51 For the theoretical model systems, the Bu sub-
unique (Rint = 0.0224) which were used in all calculations. The
final wR(F2) was 0.1013 (all data).
stituents on the bridging 4-terphenyldithiophenolate ligand
were replaced by methyl groups. All geometry optimizations
were initially carried out without imposing any symmetry con-
straints. The geometry of 3, however, converged close to C2
symmetry and was subsequently optimised within this sym-
metry. All geometrical parameters were in excellent agreement
with their experimental counterparts. The optimized structures
were confirmed as true minima on the respective potential
energy surface by calculating analytical frequencies. Modes
with imaginary frequencies were absent for both 3 and 5. The
topology of the electron density was analysed using the soft-
ware package Aimall.41 Plots were generated using Aimall and
Chemcraft.77
Preparation and characterization of 5
To
a yellow solution of (Ph3P)2NiN(SiMe3)2 (463 mg,
0.622 mmol) in thf (50 mL) was added a solution of the ligand
(200 mg, 0.311 mmol) in thf (5 mL) dropwise, and the result-
ing mixture stirred for 2 h at r.t. The solution gradually turned
dark green. After removing the solvent under dynamic
vacuum, the residual green solid was repeatedly (3×) washed
with 10 mL portions of pentane, and dried under dynamic
vacuum to produce solid 5 (215 mg). The raw product contains
additional PPh3, typically ≤5%, as the only impurity. NMR
spectroscopic properties of raw products are identical to
samples recovered after one and two recrystallization steps.
Recrystallization was performed by slow vapour diffusion of
pentane, or hexane, into a concentrated solution of 5 in thf
Acknowledgements
and produced green single crystals which were also suitable This work was funded by the Fonds der Chemischen Industrie,
for XRD analysis (155 mg 39%). Other than the co-crystallized FCI, in the form of a Liebig scholarship to A.B. which includes
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a
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second crystallographically independent formula unit
results from packing effects between the C6H5 rings of the
respective PPh3 ligands. See the ESI† for details.
40 R. F. Bader, Atoms in molecules : a quantum theory, Claren-
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