Assembly and stabilization of {E(cyclo-P3)2} (E = Sn, Pb) as a bridging ligand spanning two triaryloxyniobium units

Article abstract of DOI:10.1039/c5dt03383g

Complexes (THF)_0-2E[P₃Nb(ODipp)₃]₂ (E = Sn, Pb; Dipp = 2,6-ⁱPr₂C₆H₃) were isolated (>90%) from the salt metathesis of [Na(THF)₃][P₃Nb(ODipp)

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Assembly and stabilization of {E(cyclo-P3)2}
(E = Sn, Pb) as a bridging ligand spanning two
Cite this: DOI: 10.1039/c5dt03383g
triaryloxyniobium units†
Alexandra Velian, Brandi M. Cossairt and Christopher C. Cummins*
Complexes (THF)0–2E[P3Nb(ODipp)3]2 (E = Sn, Pb; Dipp = 2,6-iPr2C6H3) were isolated (>90%) from the salt
metathesis of [Na(THF)3][P3Nb(ODipp)3] with E2+ salts. The reaction of (THF)Sn[P3Nb(ODipp)3]2 with pyri-
dine-N-oxide was investigated as a method to deposit a new SnP6 phase. Additionally, the neutral
complex P3Nb(ODipp)2(py)2 (py = pyridine) was prepared from [Na(THF)3][P3Nb(ODipp)3] in the presence
of pyridine and salts of coordinating cations (Mg(II), Sn(II), Pb(II), Ge(II), Hg(II) and Ag(I)). P3Nb(ODipp)2(py)2
was found to successfully produce AsP3 upon treatment with AsCl3. The characterization of complexes
(THF)0–1Sn[P3Nb(ODipp)3]2, (THF)2Pb[P3Nb(ODipp)3]2 and P3Nb(ODipp)2(py)2, including their solid state
structures, is discussed.
Received 31st August 2015,
Accepted 2nd October 2015
DOI: 10.1039/c5dt03383g
www.rsc.org/dalton
unit remained coordinated to niobium. In the case of germa-
nium no well-defined new germanium containing species
Introduction
Metallophosphides are attractive materials especially for their could be isolated, but interestingly, in the case of tin a
potential to function as electron reservoirs, but exploring and complex featuring an unusual [SnP6] metallophosphide ligand
exploiting their properties hinges on improving the synthetic stabilized by coordination to niobium was formed quantitati-
methods to control their composition, morphology and vely (along with NaCl); this behavior is also replicated by lead.
structure.1–3 For example, it has been shown recently that Herein we show that this metallophosphide ligand can be
better control over the morphology of tin phosphide particles, liberated from the niobium coordination sphere using an
interesting as anodic4–6 and photocatalytic materials,7 could oxygen-atom transfer reagent, or can facilitate the formation of
be achieved using solvothermal and mechanochemical complex 2-P3 (2 = Nb(ODipp)2(py)2) – a neutral cyclo-P33− trans-
synthetic methods8 than with traditional ones that involve fer reagent (see Scheme 1).
heating the constituent elements together at high tempera-
tures.9 The successful synthesis of molecular arsenic phos-
phide AsP3 from the triple salt metathesis reaction of AsCl3
Assembling the “EP6” (E = Sn, Pb)
3−
with the cyclo-P3
transfer reagent [Na(THF)3][1-P3], (1 =
Nb(ODipp)3, Dipp = 2,6-iPr2C6H3),10,11 made us wonder if the
same molecular strategy could be used to prepare new mole-
cular or metastable phases of bulk main group phosphides, in
particular those of group 14 elements.
To start, we explored the possibility of preparing the
unknown anions “GeP3−” and “SnP3−”, isoelectronic and
expected to be isostructural with the reported AsP3 and SbP3
tetrahedra.10 We quickly learned that when treating the
[Na(THF)3][1-P3] complex with germanium or tin(II) salts, only
a partial salt metathesis reaction occurred and the cyclo-P3
metallophosphide ligand
Treating a bright orange solution of [Na(THF)3][1-P3] (2 equiv.)
in tetrahydrofuran (THF) with tin or lead dichloride (1 equiv.)
resulted in the quantitative formation of the complexes (THF)
Sn[1-P3]2 and (THF)2Pb[1-P3]2, respectively (see Scheme 1), as
well as sodium chloride. n-Pentane extraction of the residue
obtained upon removing the volatile materials from the crude
reaction mixture allowed the selective dissolution of the metal
phosphide complex, and facilitated its isolation as a spectro-
scopically pure material, dark red in the case of tin (94% yield)
and maroon in the case of lead (96% yield). H and 31P NMR
1
spectroscopic analysis of the two complexes at 21 °C indicated
that in solution they are three-fold symmetric, suggestive of
fluxional coordination of the tin and lead atoms to the cyclo-P3
units. The 31P NMR spectra of the complexes in benzene
display broad singlet resonances at −219 ppm (Δν1/2 = 185 Hz)
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts
Avenue, Cambridge, MA 02139, USA. E-mail: ccummins@mit.edu
†Electronic supplementary information (ESI) available. CCDC 1421298–1421303.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/C5DT03383G
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Scheme 1 Synthesis of complexes (THF)xE[1-P3]2 (E = Sn, Pb; x = 0–2) and 2-P3 from the reported salt [Na(THF)3][1-P3]. Both [Na(THF)3][1-P3] and
2-P3 can transfer the cyclo-P3 unit to arsenic and form AsP3.
for the tin phosphide and at −212 ppm (Δν1/2 = 180 Hz) for the distances of 2.683(1) and 2.672(1) Å respectively are close to
lead phosphide, shifting to −202 ppm for both species in THF. 2.62 Å, the mean value for a P–Sn single bond reported in the
These values are upfield of −200 and −158 ppm, the chemical CCDC database,12 but longer than 2.51 Å, the calculated value
shifts of the [Na(THF)3][1-P3] salt in benzene and THF, for a single P–Sn bond based on the sum of the covalent radii
respectively, but downfield of −235 ppm, the chemical shift of the elements.13 The almost orthogonal P12–Sn2–P7 angle of
observed for the SnIV complex Ph3SnP3Nb(ODipp)3 in 92.23(4)° is suggestive of p-orbital involvement in the bonding
benzene.11 Empirically, we note that the upfield shift for the interactions necessary for Sn(II) to complete its octet. With two
31P NMR chemical shift of the [1-P3] unit correlates with an THF molecules satisfying its coordination sphere, the lead
increase of the interaction of the cyclo-P3 unit with the co- atom in (THF)2Pb[1-P3]2 interacts with each [NbP3] unit
ordinated cation, in the order Na+ < Pb2+ < Sn2+ < Ph3Sn+.
through mainly one Pb–P interaction of 2.843(1) Å, slightly
The solid-state structures of (THF)Sn[1-P3]2 and (THF)2Pb- longer than the mean Pb–P interaction reported in the CCDC
[1-P3]2 indicate the loss of the local C3v symmetry of the 1-P3 of 2.75 Å.12 The perturbation of the [NbP3] tetrahedra by Sn
moieties observed in solution (see Fig. 1 and 2). With one THF and Pb coordination to the cyclo-P3 units is best reflected in
molecule coordinated, the tin atom in (THF)Sn[1-P3]2 bridges the elongation of the Nb–P interatomic distances from an
almost symmetrically two 1-P3 units by interacting with one average of 2.51 Å in [Na(THF)3][1-P3] to 2.55 Å in (THF)Sn-
P vertex of each of the [NbP3] tetrahedra. The P–Sn interatomic [1-P3]2 and 2.56 Å in (THF)2Pb[1-P3]2.
Examples of molecular species in which tin or lead have an
all-phosphorus coordination environment are rare and include
cage compounds such as [Sn10(tBuP)4] and [Pb7(tBuP)7],14,15
Fig. 1 Solid-state molecular structure of (THF)Sn[1-P3]2 with ellipsoids
at the 50% probability level and rendered using PLATON.18 Hydrogen
atoms and the disordered solvents (Et2O) were omitted for clarity.
Selected interatomic distances (Å) and angles (°): Sn2–P12 2.683(1),
Sn2–P10 2.929(1), Sn2–P11 3.386(2), Sn2–P7 2.672(1), Sn2–P8 3.216(1),
Sn2–P9 3.270(1), P11–Nb4 2.549(1), P10–Nb4 2.518(1), P12–Nb4
2.580(1), P9–Nb3 2.542(2), P8–Nb3 2.560(1), P7–Nb3 2.533(1), P11–P12
2.192(2), P12–P10 2.234(2), P10–P11 2.176(2), P9–P8 2.174(2), P8–P7
2.206(2), P7–P9 2.201(2), P12–Sn2–P7 92.23(4), P10–Sn2–P8 171.09(4).
Fig. 2 Solid-state molecular structure of (THF)2Pb[1-P3]2 with ellipsoids
at the 50% probability level and rendered using PLATON.18 Hydrogen
atoms and the disordered solvent (C6H6) were omitted for clarity.
Selected interatomic distances (Å) and angles (°): Pb1–P1 2.8434(6),
Pb1–P3 3.1375(6), Pb1–P2 3.6390(7), Pb1–O1S 2.592(2), Nb1–P2
2.5402(6), Nb1–P3 2.4999(7), Nb1–P1 2.5843(6), P1–P2 2.185(1), P2–P3
2.177(1), P3–P1 2.2101(8), P3–Pb1–P1 43.02(2).
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which in addition to E–P bonds (averaging 2.57 Å for Sn), also layers of SnP3, a tin atom bridges three puckered P6 rings with
contain metal–metal interactions. Other relevant examples an interatomic distance Sn–P of 2.662(3) Å, and interacts with
3−
are the isomorphic metallopolyphosphides [SnP15
]
and three additional phosphorus atoms from a neighboring layer
[PbP15]3−, which could be isolated in low yields by the salt with long Sn–P interatomic distances of 2.925(3) Å.19 Ana-
metathesis of the Zintl salt K3P7 and the element diiodide, logous to what was observed in the case of the (THF)Sn[1-P3]2
in a 2 : 1 ratio.16 X-ray analysis of these anions established complex, the Nb–P interactions of the [NbP3] tetrahedra are
their overall geometry, best described as two nortricyclic P7 slightly weaker than in the parent [Na(THF)3][1-P3] salt, with
units connected by an μ2, η2, η2-[PE] bridge, but crystallo- Nb–P interatomic distances averaging 2.56 Å. Interestingly,
graphic disorder of the P and E atoms precluded any discus- addition of THF to a benzene solution of Sn[1-P3]2 does not
sion of bond metrical parameters. Another noteworthy lead to the formation of (THF)Sn[1-P3]2, but to a mixture of un-
instance of tetrel incorporation in all-phosphorus ligands is identified species with no detectable 31P NMR signal and with
the formation of the triatomic, aromatic cyclo-EP2 (E = Ge, Sn, broad features observed in the 1H NMR spectrum.
Pb) between two niobium centers by salt metathesis involving
the terminal niobium phosphide [PNb(N[CH2 Bu]Ar)3]− (tBu =
t
tert-C4H9, Ar
stoichiometry.17
= a 2 : 1
3,5-Me2C6H3) with E2+ salts in
Releasing “SnP6” from a molecular
precursor
Interested in the interaction of naked group 14 cations with
the 1-P3 moiety, we wondered if THF incorporation could be
circumvented by use of a weakly coordinating solvent. Heating
a bright orange slurry of [Na(THF)3][1-P3] (2 equiv.) and SnCl2
(1 equiv.) in benzene to 70 °C for 3 h resulted in the formation
of a new species, which was isolated in 94% yield as a maroon
powder. C3v symmetric in solution, the product displayed a
single, relatively sharp signal in its 31P NMR spectrum at
−226 ppm (Δν1/2 = 9 Hz) in benzene, upfield of that of the
(THF)Sn[1-P3]2 complex. X-ray analysis confirmed the identity
of the new tin phosphide as a C1 symmetric Sn[1-P3]2 complex
(see Fig. 3). In the absence of a coordinating solvent, the Sn
atom in Sn[1-P3]2 satisfies its octet by interacting with a total
of three phosphorus atoms through three long interactions of
2.8014(7), 2.7541(9) and 2.7009(7) Å. In Sn[1-P3]2 the tin atom
is effectively sandwiched between two staggered cyclo-P3 rings,
an environment reminiscent of that experienced by tin in
extended phosphides. In particular, within the corrugated
Interested in forming new molecular or bulk main group phos-
phides, we explored releasing the metallophosphide [SnP6]
ligand from the coordination sphere of niobium. The reaction
of (THF)Sn[1-P3]2 with an oxygen atom transfer agent may lib-
erate the bridging [SnP6] cluster and effect the formation of
the previously reported oxo dimer {ONb(ODipp)3}2.20
This strategy was previously used to release the [Ph3SnP3]
unit from the Ph3SnP3Nb(ODipp)3 complex with transfer to
1,3-cyclohexadiene.11
Treatment of a solution of (THF)Sn[1-P3]2 (1 equiv.) in Et2O
with pyridine-N-oxide (py–O, 2 equiv.) in THF effected the
immediate formation of a pale yellow supernatant and a fine
black suspension, which was precipitated with Et2O and col-
lected by suction filtration on a sintered frit. Isolated in 69%
yield, the species contained in the yellow filtrate was identified
and structurally characterized as the pyridine adduct of the
niobium oxo monomer, O[1(Py)]. Interestingly, when (THF)Sn-
[1-P3]2 was treated with py–O in the presence of excess 1,3-cyclo-
hexadiene (20–100 equiv.) no soluble phosphorus containing
species was observed to form, in contrast to the observation
reported in the case of the Ph3SnP3Nb(ODipp)3 complex.11
22 °C
ðTHFÞSn½1‐P3ꢀ2 þ 2Py ꢁ O ꢁꢁꢁ! 2O½1ðPyÞꢀ þ black material
THF
ð1Þ
The black particles collected from the reaction mixture
appeared crystalline and displayed a metallic shine. Insoluble
in common laboratory solvents (e.g. toluene, THF, pyridine, di-
methylformamide) they slowly turned yellow upon exposure to
air. Powder diffraction analysis revealed the presence of a few,
broad features21 and indicated that no metallic Sn or any
known tin phosphides were present.9,19,22–24 Energy-dispersive
X-ray spectroscopy analysis of this material indicated an even
Fig. 3 Solid-state molecular structure of Sn[1-P3]2 with ellipsoids at the
50% probability level and rendered using PLATON.18 Hydrogen atoms
and the disordered solvent (C6H6) were omitted for clarity. Selected distribution of tin and phosphorous throughout the sample,
interatomic distances (Å) and angles (°): P1–Sn1 2.8014(7), P2–Sn1
2.7541(9), P6–Sn1 2.7009(7), P5–Sn1 3.0184(7), P4–Sn1 2.9867(7), P1–
P2 2.2073(9), P1–P3 2.1920(9), P3–P2 2.190(1), P6–P5 2.2118(9), P5–P4
2.1849(9), P4–P6 2.2070(9), Nb1–P3 2.5374(7), Nb1–P1 2.5679(7), Nb1–
with an average ratio of Sn : P of 1 : 5.4. In addition to tin and
phosphorus, combustion elemental analysis of this black solid
indicated variable carbon content, from 16 to 27%, possibly
stemming from coordinated THF or intercalated ODipp-
containing species. Solid state 31P NMR analysis using Cross
P2 2.5846(8), Nb2–P6 2.5905(7), Nb2–P5 2.5409(7), Nb2–P4 2.5442(7),
P3–P1–Sn1 77.81(3), P2–Sn1–P6 101.07(2), P1–Sn1–P5 120.23(2).
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Polarization Magic-Angle spinning revealed the presence of a into a tetramer,26,27 a hypothesis supported also by a Diffusion
broad signal around 13 ppm, this being located in a 31P NMR Ordered NMR spectroscopy experiment (see ESI†).28
region characteristic to Hittorf-type bonding in violet phos-
C2v symmetric in solution, 2-P3 is almost Cs symmetric in
phorus (see section S.2).25 Interestingly, heating this material the solid state, with the two pyridine molecules coordinated
to temperatures of 250–290 °C led to the formation of P4 and trans to each other. With an average Nb–P interatomic distance
either Sn4P3 or SnP, well characterized tin phosphides.9,19,22–24 of 2.52 Å, and P–P bond of 2.19 Å, the [NbP3] tetrahedron in
These preliminary characterization data seem to indicate 2-P3 is only slightly perturbed by the replacement of an ODipp
that the black material produced by releasing the [SnP6] ligand ligand by pyridine. Treatment of 2-P3 with a source of
from (THF)Sn[1-P3]2 contains a new phosphorus-rich tin phos- [ODipp]− anion readily regenerates the [1-P3]− core (see
phide, but an improved procedure for its preparation and Scheme 1 and ESI†).
further characterization data will be needed to establish the
details of its structure and chemical formula.
The neutral cyclo-P3 niobium complex 2-P3 is uniquely
poised to test if sodium chloride formation is a necessary
driving force in the synthesis of AsP3 from the anionic [1-P3]−
complex.29 We found that when treating a solution of the
neutral complex 2-P3 with AsCl3, AsP3 forms in 45% spectro-
scopic yield along with complex Cl3Nb(ODipp)2, suggesting
that NaCl formation is in fact not essential for this process.
A neutral cyclo-P3 niobium transfer
reagent
Exposing red solutions of (THF)Sn[1-P3]2 and (THF)2Pb[1-P3]2
complexes to pyridine immediately effected a color change to
bright green, and the formation of a single new species with a
31P NMR chemical shift of −78 ppm, containing ODipp and
Conclusions
Devising synthetic methods to assemble main group phos-
phides from molecular precursors may allow the formation of
binary materials with new stoichiometries and interesting
emergent properties. Herein we introduced the synthesis of
complexes (THF)0–1Sn[P3Nb(ODipp)3]2 and (THF)2Pb[P3Nb-
(ODipp)3]2 in which the phosphorus rich metallophophide
ligand {E(cyclo-P3)2} (E = Sn, Pb) is supported by two niobium
aryloxide platforms. Preliminary studies revealed that releasing
the {Sn(cyclo-P3)2} unit from the coordination sphere of
niobium using an oxygen atom transfer reagent led to the for-
mation of a new tin phosphide with an unusually high phos-
phorus content (P to Sn ratio of ca. 5.4). Further, the neutral
complex 2-P3, prepared in the pyridine-assisted reaction of
[Na(THF)3][1-P3] with salts of coordinating cations, was intro-
duced as a new phosphorus transfer reagent and was found to
successfully transfer the cyclo-P3 unit to arsenic with formation
of the molecular arsenic phosphide AsP3.10
1
pyridine ligands in a ratio of 1 : 1, as determined by H NMR
spectroscopy. Its identity as the neutral cyclo-P3 niobium bis-
ODipp complex 2-P3, was elucidated using X-ray diffraction
(see Fig. 4). Effectively a salt elimination process with for-
mation of Sn(ODipp)2 and the neutral 2-P3 complex, similar
transformations also took place when pyridine and salts of
Ge(II), Hg(II), Ag(I) or Mg(II) were added to solutions of
[Na(THF)3][1-P3]. A convenient synthesis of 2-P3, isolated in
95% yield as a bright green solid, was devised using MgCl2 as
a stoichiometric reagent (see Scheme 1). Interestingly, complex
[Ag][1-P3], formed in the salt metathesis of [Na(THF)3][1-P3]
(1 equiv.) with AgOTf (1 equiv.) displays an intriguing quintet
centered around −243 ppm in the 31P NMR spectrum. This
NMR feature suggests that in solution [Ag][1-P3] aggregates
Acknowledgements
This material is based upon work supported by the National
Science Foundation under CHE-1362118. The authors declare
no competing financial interests.
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