Selective P4 activation by an organometallic nickel(i) radical: Formation of a dinuclear nickel(ii) tetraphosphide and related di- and trichalcogenides

Article abstract of DOI:10.1039/c4cc02601b

The reaction of the 17e nickel(i) radical [CpNi(IDipp)] (1, IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with P₄ results in a nickel tetraphosphide [{CpNi(IDipp)}₂(μ-η¹: η¹-P₄)] with a butterfly-P₄^2- ligand; related chalcogenides [{CpNi(IDipp)}₂(μ-E₂)] (E = S, Se, Te) and [{CpNi(IDipp)}₂(μ-E₃)] (E = S, Se) are formed with S₈, Se_∞ and Te_∞. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02601b

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Selective P4 activation by an organometallic
nickel(I) radical: formation of a dinuclear nickel(II)
tetraphosphide and related di- and
trichalcogenides†‡
Cite this: Chem. Commun., 2014,
50, 7014
Received 8th April 2014,
Accepted 14th May 2014
Stefan Pelties,a Dirk Herrmann,a Bas de Bruin,b Frantisek Hartlc and Robert Wolf*a
ˇ
DOI: 10.1039/c4cc02601b
www.rsc.org/chemcomm
The reaction of the 17e nickel(I) radical [CpNi(IDipp)] (1, IDipp =
1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with P4 results in a
nickel tetraphosphide [{CpNi(IDipp)}2(l-g1:g1-P4)] with a butterfly-P4
2À
ligand; related chalcogenides [{CpNi(IDipp)}2(l-E2)] (E = S, Se, Te) and
[{CpNi(IDipp)}2(l-E3)] (E = S, Se) are formed with S8, SeN and TeN
.
Scheme 1 Synthesis of nickel(I) complexes 1À3.
The P4 molecule is the most reactive allotrope of phosphorus; its
activation and transformation by transition metal compounds
has attracted substantial interest over the years.1 While many cyclopentadienyl ligand, and an initial reactivity study of
low-valent metal complexes, e.g. transition metal carbonyls or complex 1 with P4 and related small molecules.
anionic metalates, react with P4, it is still challenging to design
highly selective transformations.2,3
Complexes 1–3 are accessible according to Scheme 1 by the
reduction of the appropriate nickel(II) halides with KC8 in THF.¶
White phosphorus is able to efficiently trap organic and main 1H NMR monitoring shows that 1–3 are formed very selectively; they
group element radicals.4 Therefore, one potential solution to the can be isolated as yellow crystalline solids in modest to high yields.
selectivity issue is to use a radical pathway in transition metal- Single X-ray structure analyses (ESI‡) revealed that the nickel centre
mediated P4 transformations. While 2nd and 3rd row metalloradi- is surrounded by the carbene carbon and one Z5-coordinated Cp
cals are well-established,5 nickel(I) radicals have attracted significant or Cp* moiety. No further significant interactions between nickel
attention recently.6,7 Importantly, Driess et al. have shown that and the diisopropylphenyl groups are apparent. Nonetheless, the
reactions of b-diketiminato nickel(I) complexes with P4 yield dinuc- cyclopentadienyl ligand is tilted with respect to the nickel carbene
lear complexes [(LRNi)2(m-Z3:Z3-P4)] (LR = HC[CMeN(2,6-R2C6H3)]2 bond with an angle Ccarbene–Ni–(C5R5)centroid of 154.3(1)1 for 1,
with R = Et, iPr).8 The P–P bond activation in the doubly 151.9(1)1 for 2 and 164.6(1)1 for 3.§
Z3-coordinated ligand is reversible and occurs without the
Cyclic voltammograms show one electrochemically quasi-
2À
reduction of P4 to formally P4
.
reversible wave at E1/2 = À1.02 and À1.06 V vs. Fc/Fc+ for
We have been interested in designing new reactive nickel(I) Cp-substituted 1 and 2, respectively, and a reversible wave at À1.18 V
radicals for element–element bond activations. We now report vs. Fc/Fc+ for the Cp* complex 3 (ESI‡). UV/vis-spectroelectrochemistry
the synthesis of complexes 1–3§ featuring an NHC and a (see Fig. 1 for 1) confirms that these processes correspond to chemically
reversible oxidations of neutral 1–3 to stable cationic nickel(II)
complexes, which probably bind THF in the case of 1 and 2.
Indeed, the preparative oxidation of 1 with [Cp2Fe]PF6 affords
the THF adduct [(C5H5)Ni(IDipp)(THF)]PF6 (1-THF) (ESI‡).§
a University of Regensburg, Institute of Inorganic Chemistry, 93040 Regensburg,
Germany. E-mail: robert.wolf@ur.de
Complexes 1–3 show identical magnetic moments of 2.3(1),
2.3(1), and 2.2(1) mB in [D8]THF, which indicate the presence of
one unpaired electron per molecule. The EPR spectrum of 1
is characteristic for an S = 1/2 system and reveals a rhombic g-tensor
with significant deviations from ge pointing to metalloradical
character. DFT calculated g11 and g22 values are somewhat
smaller than the experimental ones, but show a similar rhombicity
(Fig. 1).
b University of Amsterdam, Van’t Hoff Institute for Molecular Sciences,
Science Park 904, 1098 XH Amsterdam, The Netherlands
c University of Reading, Department of Chemistry, Whiteknights, Reading,
RG6 6AD, UK
† Dedicated to the memory of Prof. Michael F. Lappert.
‡ Electronic supplementary information (ESI) available. Full experimental
details, electrochemical, EPR and crystallographic data. CCDC 995931–995941
and 999501. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c4cc02601b
7014 | Chem. Commun., 2014, 50, 7014--7016
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and TEMPO with 1 in THF afforded the known thiolate
[(C5H5)Ni(SPh)(IDipp)] (4)9 and the new TEMPO adduct 5 in
quantitative yield (Fig. 2). The molecular structure of 5 shows a
side-on Z2-coordinated TEMPO ligand and an Z1-coordinated
Cp ligand at the distorted square planar nickel(II) atom. The
structural parameters agree with presence of a formally anionic
TEMPOÀ ligand.10 A sharp 1H NMR singlet at 5.93 ppm is observed
for the Cp moiety even at À90 1C presumably due to rapid
haptotropic migration.
Fig. 1 Left: UV/Vis monitoring of the oxidation of 1 performed at À0.83 V
vs. Fc/Fc+ within an OTTLE cell equipped with a Pt minigrid working
electrode, THF/TBAH under Ar, 293 K. Right: experimental and simulated
X-band EPR spectrum of 1 in frozen THF. Freq. 9.3646 GHz, 0.063 mW,
20 K, mod. 4 Gauss; g-tensor parameters obtained from simulations
We next investigated the reactivity of 1 with the heavier
chalcogens. The reaction with S8 (1/8 equivalents) gave the blue
disulfide 6-S and the purple trisulfide 7-S (Fig. 3) in a 7 : 3 ratio
1
according to H NMR analysis. 6-S is soluble in n-hexane and
and DFT calculations (b3-lyp, def2-TZVP) are: g11 = 2.377 (2.220), g22
=
diethyl ether and can thus be separated from 7-S by extraction
and subsequent crystallisation (ESI‡). Disulfide-bridged dinuc-
lear complexes with an M–S–S–M motif are well-known,11 while
2.306 (2.187), g33 = 2.050 (2.078) (DFT-calculated values in parentheses).
2À
complexes with an unsupported m-S3 bridge are still rather
scarce.11a,b,12 The structure of 7-S shows a similar S1–S2–S3 angle
and S–S bond lengths as the structure of [{(C5H5)Fe(CO)2}2(m-S3)].11a
Diselenide 6-Se (31% isolated) is the major reaction product of 1
with one equivalent of elemental selenium. A 1H NMR spectrum of
the reaction mixture (THF, room temperature) shows that 6-Se is
formed in more than 80% yield whereas the triselenide 7-Se is a
minor by-product. Ditelluride 6-Te was the only product to be
detected after stirring 1 with one equivalent of grey tellurium for
seven days. It was isolated as a dark brown crystalline solid in 31%
yield. The molecular structures of 6-Se, 6-Te and 7-Se are analogous
to the corresponding sulfides 6-S and 7-S (ESI‡).
Considering that a mixture of at least two products is formed
with sulfur and selenium, it was gratifying to discover that
complex 1 reacts with P4 in a highly selective fashion in THF at
room temperature, giving tetraphosphide 8 as the sole product.
The reaction is instantaneous, and compound 8 can be isolated
Fig. 2 Reaction of 1 with TEMPO and solid-state molecular structure of
[(C5H5)Ni(TEMPO)(IDipp)] (5). The hydrogen atoms are omitted for clarity.
Thermal ellipsoids are drawn at 40% level. Selected bond lengths [Å]
and angles [1]: Ni1–O1 1.8408(14), Ni1–N1 1.9581(16), N1–O1 1.3989(20),
Ni1–C1 1.8824(19), Ni1–C4 2.034(2), C1–Ni1–O1 104.50(7), O1–Ni1–N1
43.07(6), C1–Ni1–C4 97.104(4), N1–Ni1–C4 115.325(2).
Initial reactivity studies of 1 established its behavior as a as an analytically pure, dark purple powder in quantitative yield
typical metal-centered radical. The reactions of phenyl disulfide simply by removing the solvent. Its molecular structure (Fig. 3)
Fig. 3 Left: reactions of 1 with P4, S8, SeN and TeN. Right: solid-state molecular structures of the products 6-S, 7-S and 8. The hydrogen atoms
are omitted for clarity. Thermal ellipsoids are drawn at 40% level. Selected bond lengths [Å] and angles [1]: 6-S: Ni1–S1/Ni2–S2 2.1800(1)/2.1797(1),
S1–S2 2.0476(1), Ni1–S1–S2–Ni2 78.601(5), 7-S: Ni1–S1/Ni2–S3 2.1936(6)/2.1748(5), S1–S2/S2–S3 2.0561(7)/2.0522(7), S1–S2–S3 111.58(3), 8: Ni1–P1/
Ni2–P2 2.2107(6)/2.2103(6), P1–P3/P4 2.2334(7)/2.2111(7), P3–P4 2.1649(7), P1–P2 2.8897(8).
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1
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À190.5 Hz. These values are similar to those of [{CpRFe(CO)2}2-
(m-Z1:Z1-P4)] (CpR = C5H3-1,3-tBu2, C5H2-1,2,4-tBu3, C5H-iPr4,
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In conclusion, we have prepared rare mononuclear cyclo-
pentadienyl nickel(I) complexes 1–3 with significant metallo-
radical character.6,7 This feature was successfully utilized for
the high-yield synthesis of the novel tetraphosphido complex
[{(C5H5)Ni(IDipp)}2(m-Z1:Z1P4)] (8), which features an uncommon
m-Z1:Z1-bridging P42À ligand.14 Further reactivity studies of 1–3 and
8 are in progress; the results will be reported in due course.
We thank Christian Hoidn, Christian Preischl and Philipp
Bu¨schelberger for preparing 1–3 as part of their BSc projects.
Financial support by the DFG and NWO (NWO-VICI 016.122.613)
is gratefully acknowledged.
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of 1 was attributed to the asymmetric spin density distribution.
¶ The hydride complex [(C5H5)NiH(IDipp)] (1-H) was identified as a
minor by-product (o5%) of the synthesis of 1. Compound 1-H was
prepared independently and features a distinct molecular structure
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7016 | Chem. Commun., 2014, 50, 7014--7016
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