An unusual Ni2Si2P8cluster formed by complexation and thermolysis

Article abstract of DOI:10.1039/d0cc05365a

[LSi(η2-P4)] (L = CH[C(Me)N(Dipp)][C(CH2)N(Dipp)], Dipp = 2,6-diisopropylphenyl) forms well-defined 1?:?1 and 2?:?1 complexes with N-heterocyclic carbene nickel fragments. The cluster compound [(IDipp)Ni2P8(SiL)2] (IDipp = 1,3-bis(2,6-diisopropylphenyl)im

Full text of DOI:10.1039/d0cc05365a

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An unusual Ni2Si2P8 cluster formed by
complexation and thermolysis†
Cite this:DOI: 10.1039/d0cc05365a
a
Christoph G. P. Ziegler,
Clemens Taube,b John A. Kelly,a Gabriele Hierlmeier,a
Maria Uttendorfer,a Jan J. Weigand *b and Robert Wolf
*
a
Received 6th August 2020,
Accepted 8th September 2020
DOI: 10.1039/d0cc05365a
rsc.li/chemcomm
[LSi(g2-P4)] (L = CH[C(Me)N(Dipp)][C(CH2)N(Dipp)], Dipp = 2,6- (Fig. 1, A)10 and Radius and co-workers accessed the P2 complex
diisopropylphenyl) forms well-defined 1 : 1 and 2 : 1 complexes with [{Ni(ImiPr2)2}2(m-Z2:2-P2)] (Fig. 1, B, ImiPr2 = 1,3-bis(iso-propyl)
N-heterocyclic carbene nickel fragments. The cluster compound imidazolin-2-ylidene).16 More recently, our group described the
[(IDipp)Ni2P8(SiL)2] (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin- aggregation of P4 using (NHC)Ni complexes leading to novel
2-ylidene) is selectively formed by thermolysis of the complex [(IDipp)- di- and trinuclear complexes (see e.g. Fig. 1, C).17
Ni(l-g2:2-P4)SiL].
Molecular main-group doped nickel phosphide complexes
were reported by Driess in 2009 (Fig. 1, D).18 This type of
Nickel phosphides are of interest in material science and industry complex is obtained from [LSi(Z2-P4)]19 (L = CH[C(Me)N(Dipp)]
due to their favourable magnetic and catalytic properties1 [C(CH2)N(Dipp)]) and [(L0Ni)2ꢀtoluene] (L0 = CH[CMeN(Dipp)]2)
and their potential in hydrogen evolution reactions (HER),2 and [(L00Ni)2ꢀtoluene] (L00
=
CH[CMeN(2,6-Et2C6H3)]2).18
water splitting,3 hydrodenitrogenation and hydrodesulfuration The molecular structures of these complexes feature an intact
reactions.4,5 Doping nickel phosphides with different transition SiP4 unit coordinating to the NiI centre.
metal elements is a common approach to modify their electro-
We reasoned that reactions with a more strongly reducing
and photocatalytic activity.6 Including main group elements, such Ni0 precursor could lead to P–P bond cleavage and subsequent
as sulphur, has been shown to improve the catalytic activity of
nickel phosphides in HER.7 Although promising, the synthesis of
such metal phosphides remains challenging5,8 and contemporary
methods require sensitive and toxic phosphine precursors
(e.g. P(SiMe3)3), special additives (e.g. tri-n-octylphosphine, oleyl-
amine) and high temperatures.9–11 A possible alternative is the
mild thermolysis of carefully designed transition metal polypho-
sphido complexes,12 which can be accessed by the direct activa-
tion of white phosphorus (P4). The use of nickel(0) complexes
already dates back to 1979, when Sacconi and co-workers reported
the synthesis of a monohapto P4 complex [(k3-P,P,P-NP3)Ni(Z1-P4)]
[(NP3
=
tris(2-diphenylphosphinoethyl)amine)].13 Since then,
several other groups have shown that Ni0 sources are suitable
reagents for the targeted synthesis of nickel polyphosphido
compounds.10,14–17 Le Floch and Mezailles prepared Ni2P nano-
´
particles directly by reacting [Ni(cod)2] (cod = 1,4-cyclooctadiene)
with P4 in the presence of TOPO (tri-n-octylphosphine oxide)
a University of Regensburg, Institute of Inorganic Chemistry, 93040 Regensburg,
Germany. E-mail: robert.wolf@ur.de
b TU Dresden, Faculty of Chemistry and Food Chemistry, 01062 Dresden, Germany.
E-mail: jan.weigand@tu-dresden.de
Fig. 1 Examples of P4 activation by Ni0 precursors (top); heterodinuclear
silicon–nickel polyphosphido complexes (bottom); Dipp = 2,6-iPr2C6H3,
Dep = 2,6-Et2C6H3.
† Electronic supplementary information (ESI) available. CCDC 2009284–2009287.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0cc05365a
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Scheme 1 Synthesis of [(NHC)Ni{(m-Z2:2-P4)SiL}2] [NHC = IDipp (1a),
NHC = IMes (1b)].
aggregation of the resulting intermediates. Herein, we report
the preparation of (NHC)Ni0 complexes with the tetraphospha-
silatricyclopentane [LSi(Z2-P4)]. Mild thermolysis of one of the
complexes affords an unusual Ni2Si2P8 cluster by the nickel-
mediated dimerization of two [LSi(Z2-P4)] units.
Fig. 2 Solid-state molecular structure of 1a. Hydrogen atoms and solvate
molecules are omitted for clarity; thermal ellipsoids are drawn at the
40% probability level; selected bond lengths [Å] for 1a: P1–P3 2.2651(6),
P1–P4 2.2511(7), P2–P3 2.2565(7), P2–P4 2.2724(7), P3–P4 2.2686(6),
Si1–P1 2.2512(8), Si1–P2 2.2422(7), Ni1–P3 2.2923(8), Ni1–P4 2.2674(8),
P5–P7 2.2451(6), P5–P8 2.2603(7), P6–P7 2.2714(9), P6–P8 2.2607(7),
P7–P8 2.4749(7), Si2–P5 2.2475(9), Si2–P6 2.2455(6), Ni1–P7 2.2657(6),
Ni1–P8 2.2847(7), Ni1–C1 1.9761(16); bond distances and angles of
derivatives 1b are presented in the ESI† (see Fig. S26).
Vinyltrimethylsilane complexes [(NHC)Ni(Z2-vtms)2] (NHC =
IDipp, IMes, vtms = Me3SiCHQCH2) were selected as well-
proven ‘‘(NHC)Ni0’’ equivalents and reacted with [LSi(Z2-P4)]
in a 1 : 1 and 1 : 2 stoichiometry in toluene at ꢁ50 1C.19,20 Using
two equivalents of [LSi(Z2-P4)], [(NHC)Ni{(m-Z2:2-P4)SiL}2] 1a
(NHC = IDipp) and 1b (NHC = IMes) are isolated as red powders
in 50% and 26% yield, respectively (Scheme 1). A single crystal
X-ray diffraction (XRD) study revealed that both compounds are
isostructural and therefore only 1a will be discussed here.
The molecular structure shows two [LSi(Z2-P4)] ligands side-
on coordinated to the bridging [(IDipp)Ni] unit. The P–P bond
distances of the coordinated
P atoms (P3–P4 2.2686(6),
P7–P8 2.4749(7) Å) are considerably longer than the corres-
ponding P–P bond length in [LSi(Z2-P4)] (2.159(2) Å), although
similar to complex D (2.351(3) Å).18,19 This might be explained
by a significant back-donation of electron density from the Ni0
centre. The Ni–P distances (av. 2.2744 Å) in 1a lie in a similar
range to those of D (av. 2.266 Å) (Fig. 2).18
Scheme 2 Synthesis of [(IDipp)Ni(m-Z2-P4)SiL] (2) and [(IDipp)NiP8{SiL}2] (3).
The 31P{1H} NMR spectra of 1a (Fig. S5, ESI†) shows two very
broad resonances at d = ꢁ234.3 ppm and 190.6 ppm with an
integral ratio of 1 : 1, which do not resolve at differing tempera-
tures, suggesting a fluxional behaviour in solution (Fig. S7 and and bond analysis).18 When conducted with the less sterically
S8, ESI†). Complex 1b shows comparatively sharper multiplets demanding [(IMes)Ni(Z2-vtms)2] the exclusive formation of 1b
at d = ꢁ236.4 ppm and 194.2 ppm in the 31P{1H} NMR spectrum was observed.
(Fig. S11, S13 and S14, ESI†). Despite the broad resonances
Compound 2 slowly decomposes in solution and over the
observed in the 31P NMR spectra, both 1a and 1b give rise to course of three weeks gives rise to a 31P{1H} NMR spectrum with
well-resolved 1H NMR spectra (Fig. S3 and S9, ESI†), which two new ABCDEMSX spin systems, which are assigned to
indicate a symmetric structure. The 29Si NMR spectrum of 1a [(IDipp)Ni2P8(SiL)2] (3, vide infra). The clean formation of 3 can
reveals a very broad signal at d = ꢁ50.5 ppm (n1/2 = 90 Hz), also be achieved by heating the reaction of [(IDipp)Ni(Z2-vtms)2]
whereas 1b displays a broad pseudo-triplet resonance at and [LSi(Z2-P4)] to 60 1C, circumventing the isolation of 2.
d = ꢁ51.1 ppm which is slightly shifted to higher field com- Significant amounts of by-product IDipp were removed via sub-
pared to the starting material [LSi(Z2-P4)] (d = ꢁ40.4 ppm).19
limation (95 1C and 1 ꢂ 10ꢁ5 mbar). Subsequent recrystallization
The reaction with equimolar amounts of [(IDipp)Ni(Z2-vtms)2] from toluene layered with n-hexane afforded compound 3 as
and [LSi(Z2-P4)] in toluene at room temperature results in the brown crystals in a 33% yield.
selective formation of a new species [(IDipp)Ni(m-Z2:2-P4)SiL] (2).
The structure of 3 shows an unusual asymmetrical Ni2Si2P8
After filtration and recrystallisation from n-hexane, 2 was isolated cluster with strongly varying P–P distances (range: 2.1702(8)–
as red crystalline blocks (Scheme 2). The solid-state structure of 2 3.4219(8) Å). The phosphorus atoms P1, P3, P8, and P6 are aligned
reveals a heterodinuclear [Si(m-Z2:2-P4)Ni] core highly reminiscent in a plane (torsion angle - 1.81) and coordinated by the central
of complex D (see the ESI† for full crystallographic data, NMR Ni1 atom. This plane is fused to a P3 ring (P2, P4 and P5) with one
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The Ni1–Ni2 distance (2.4126(4) Å) also deserves to be
commented on. This value is consistent with an estimated
value of 2.48 Å derived from the covalent radius of a single
nickel atom (1.24 Å).21 While covalent metal–metal bonds are
common for nickel(I) complexes,22 only a few related dinickel(0)
complexes have been described with Ni0–Ni0 distances ranging
from 2.437 to 2.572 Å.23 A notable example is the isonitrile
complex [Ni2(m-CNMe)(CNMe)2(m-PPh2CH2PPh2)2] reported by
Kubiak and co-workers with a Ni–Ni separation of 2.572(1) Å.23a
In order to investigate the bonding situation in more detail,
the electronic structure was analysed by calculating intrinsic
bond orbitals (IBOs)24 at the PBE/def2-TZVP level of theory.
A truncated model of 30 (with iPr of 3 replaced by Me groups)
was used for the calculations. The composition of the IBOs
suggests a 3d10 configuration for the two Ni atoms. Seven two-
centre-two-electron P–P bonds, four doubly occupied IBOs
involving the Si–P bonds and, additionally, three three-centre-
two-electron bonds were calculated. One of the multicentre
bonds (see Fig. S35, ESI†) suggests a Ni1ꢀ ꢀ ꢀNi2 interaction.
The presence of a weak Ni–Ni bond is also supported by the
calculated Mayer bond order of 0.3. The short Ni1ꢀ ꢀ ꢀNi2
distance might thus mainly be explained by the constrained
alignment of the core of the cluster and additional significant
contributions of the multicentre bonds.
Multinuclear NMR spectra of 3 suggest the presence of two
isomers in solution in an approximate ratio of 3 : 1 due to the
asymmetric L ligand. It is worth noting a similar ratio was
found in the solid state, in the disorder of the diketiminate
ligand (L) backbone (see Fig. S29, ESI†). As a result, the 1H,
13C NMR spectra are complex, but confirm the presence of all
molecular components. The 29Si NMR spectrum shows two
broad resonances at d = ꢁ3.9 ppm and 60.8 ppm. However,
the 31P{1H} NMR spectrum is particularly informative, showing
two ABCDEMSX spin systems independent of temperature
(see Fig. 4, Fig. S23 and S24, ESI†). To aid in assigning the
signals, 31P and 29Si NMR chemical shieldings were calculated
for the main isomer (see Tables S3 and S4, ESI†) of the slightly
Fig. 3 Solid-state molecular structure of 3. Hydrogen atoms and disorder
are omitted for clarity; thermal ellipsoids are drawn at the 40% probability
level; selected bond distances [Å] and angles [1] for 3: P1–P3 2.1702(8),
P1ꢀ ꢀ ꢀP4 3.4219(8), P1ꢀ ꢀ ꢀP6 3.0176(7), P2–P4 2.2160(9), P2–P5 2.1704(10),
P4ꢀ ꢀ ꢀP5 2.5499(9), P3–P8 2.4018(7), P6ꢀ ꢀ ꢀP7 2.1946(7), P7–P8 2.3083(7),
Si1–P1 2.2539(8), Si1–P2 2.2832(11), Si2–P6 2.2265(7), Si2–P7 2.2709(7),
Ni1ꢀ ꢀ ꢀNi2 2.4126(4), Ni1–P1 2.2717(6), Ni1–P3 2.4280(6), Ni1–P6 2.3132(6),
Ni1–P8 2.4143(6), Ni2–P4 2.1983(8), Ni2–P5 2.2448(6), Ni2–P7 2.1811(7);
P1–P3–P8 98.26(3), P3–P8–P6 97.79(3), P3–P1–P6 82.91(2), P3–P1–P4
84.04(3), P1–P4–P5 68.31(2), P4–P5–P3 104.92(4), P2–P4–P5 53.63(3),
P4–P5–P2 55.29(3), P5–P2–P4 71.08(3), P5–P3–P8 95.97 (3), P3–P8–P7
107.97(3), P6–P8–P7 79.88(3), P8–P7–P6 48.896(18), P7–P6–P8 51.23(2),
P4–P1–P6 85.977(19), P1–P6–P7 96.32(2), P1–Ni1–P4 98.25(3), P3–Ni1–
P5 56.10(2), P6–Ni1–P7 79.0(2), P8–Ni–P7 59.00(2).
large P4ꢀꢀꢀP5 distance of 2.5499(9) Å. In addition, this unit is
connected to a second P3 ring (P6, P7 and P8) via a P–P single
bond (P3–P7 2.1702(8) Å). The P8 framework is stabilised by two
LSi moieties. Each silicon atom is connected to two phosphorus
atoms. The whole framework is capped by a [(IDipp)Ni] fragment
connected to three P atoms (P4, P5 and P7) (Fig. 3).
Fig. 4 31P{1H} NMR spectrum of cluster 3 in C6D6 at room temperature with nuclei assigned to the ABCDEMSX spin system; two isomers are present in
solution (signals colour-coded in black and red, assigned based on integration); chemical shifts for isomer A: d/ppm = ꢁ237.1 (PA), ꢁ190.3 (PB), ꢁ162.2
(PC), ꢁ149.2 (PD), ꢁ123.5 (PE), ꢁ60.5 (PM), 51.3 (PS), 191.1(PX); isomer B: d/ppm ꢁ232.2 (PA), ꢁ199.3 (PB), ꢁ164.8 (PC), ꢁ145.0 (PD), ꢁ113.7 (PE), ꢁ46.5 (PM),
46.4 (PS), 208.8 (PX); DFT-calculated and simulated coupling constants are presented in the ESI† (see Table S1); insets: representation of the core of the
cluster; thermal ellipsoids are drawn at the 40% probability level.
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truncated model cluster 30 at the TPSS/pcSseg-2 level of theory.
Complex 2 was chosen as a reference system and the reso-
nances of the 31P{1H} NMR spectrum (Fig. 4) were assigned
according to the calculation, allowing for the iterative fitting of
the 31P{1H} NMR spectrum of 3 (see the ESI†). DFT-calculated
J(31P,31P) coupling constants were used as an initial starting
point for the fitting procedure (see Table S1, ESI†). Five large
1J(31P,31P) coupling constants (ꢁ204.2 to ꢁ409.7 Hz) and one
unexpectedly small 1J(31P,31P) coupling constant of ꢁ7.5 Hz
between nuclei PA and PB were derived. The small 1J(PA,PB)
(ꢁ7.5 Hz) coupling constant might be explained by the rela-
tively long distance of the nuclei PA and PB observed by X-ray
crystallography (P3–P8 2.4018(7) Å). Notably, a rather large
coupling constant of 218.1 Hz between nuclei PD and PM is
observed despite the long P–P distance (P1ꢀ ꢀ ꢀP6 3.0176(7) Å)
deduced from the solid-state structure. We reason this finding
as a through space coupling as observed also in other polypho-
sphorus compounds.25
In summary, we have shown that the (NHC)Ni synthon
[(NHC)Ni(Z2-vtms)2] (NHC = IDipp, IMes, vtms = Me3SiCHQCH2)
effects a unusual dimerisation of [LSi(Z2-P4)] to form the Ni2Si2P8
cluster 3. Additionally, the classical (NHC)Ni complexes 1a, b and
2 have been isolated alongside. Such Ni complexes show great
potential as starting materials for the synthesis of ternary phos-
phorus cluster such as compound 3 as they are well-defined and
conveniently prepared. Derivatisation reactions of the cluster core
3 through the substitution of the diketiminate ligands L may
further enhance the diversity of this class of cluster molecules.
An extension of the synthetic methodology reported here and the
use of 3 and related clusters as single source precursors for
phosphorus-based materials will be of significant future interest.
We thank Dr Peter Coburger for assistance with the DFT
calculations and Julia Leitl for help with preparing the manu-
script. Generous financial support by the Deutsche Forschungs-
gemeinschaft (WE4621/3-1 and WO1496/7-1), the European
Research Council (CoG 772299) and the Fonds der Chemischen
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