Diorganophosphanylphosphinidenes as complexed ligands: Synthesis via an anionic terminal phosphide of niobium

Article abstract of DOI:10.1002/anie.200352779

Reductive cleavage of a bridging diphosphide complex with sodium amalgam affords the sodium salt of the terminal niobium phosphide anion [PNb(N[Np]Ar)₃]^- (1; Np = neopentyl, Ar = 3,5-Me ₂C₆H₃), which is best formulated as containing an Nb-P triple bond. The phosphorus atom of 1 is nucleophilic. Treatment of 1 with CIP(tBu)₂ or CIP(Ph)₂ provides η²- phosphanylphosphinidene complexes, which are the first examples of such complexed ligands bound to an early-transition-metal fragment.

Full text of DOI:10.1002/anie.200352779

Communications
P Ligands
Diorganophosphanylphosphinidenes as
Complexed Ligands: Synthesis via an Anionic
Terminal Phosphide of Niobium**
Joshua S. Figueroa and Christopher C. Cummins*
Compounds with multiple bonding between adjacent phos-
phorus atoms are of interest both as free molecules and as
complexed ligands.[1] As one proceeds formally from triply
bonded P2,via intermediate states of reduction,all the way to
ꢀ
two equivalents of PH3,several interesting P P bonded
systems (and isomers thereof) may be considered. Phospha-
=
nylphosphinidene,PPH 2,a neutral isomer of HP PH (diphos-
phene) is an intriguing parent species in this regard and its
geometric and electronic structure have been the subject of
several theoretical investigations.[2–5]
Organic derivatives of phosphanylphosphinidene have
generated interest as complex ligands capable of h2-binding to
transition-metal centers.[2] A prototypical example is the
structurally characterized complex [(h2-tBu2PP)Pt(PPh3)2].[6]
That organic phosphanylphosphinidenes,exemplified by
tBu2PP,are novel p-electron ligands in a manner reminiscent
of alkenes or alkynes is recognized and was the subject of a
recent review article.[7] Fritz and co-workers have pioneered
the synthesis of such derivatives by utilization of the
=
“phosphanylphosphinidene transfer” reagent, tBu2PP
PX(tBu2) (X = Me or Br),[6,7] which represented,until now,
the only synthetic entry to complexes of phosphanylphosphi-
nidenes.
Herein we report an alternative synthetic entry to
complexed organic derivatives of phosphanylphosphinidene.
Our method takes advantage of our ability to synthesize an
unprecedented anionic,niobium,terminal phosphide com-
plex,Na +[PNb(N[Np]Ar)3]ꢀ (Np = neopentyl,Ar = 3,5-
Me2C6H3) that permits assembly[8] of R2PP ligands directly
within the metal's protective coordination sphere. Further-
more,the complexes [( h2-R2PP)Nb(N[Np]Ar)3],are the first
examples of h2-phosphanylphosphinidene complexation by an
early-transition-metal fragment.
Our recent discovery that the niobaziridine-hydride com-
2
=
plex [Nb(H)(h -tBu(H)C NAr)(N[Np]Ar)2] reacts with
white phosphorus (P4) to afford the dinuclear,bridging
diphosphide complex [(m2:h2,h2-P2){Nb(N[Np]Ar)3}2] (1)[9]
presented us with a unique entry into low-coordinate
[*] J. S. Figueroa, Prof. Dr. C. C. Cummins
Department of Chemistry, Room 2-227
Massachusetts Institute of Technology
77 Massachusetts Ave., Cambridge, MA 02139-4307 (USA)
Fax: (+1)617-258-5700
E-mail: ccummins@mit.edu
[**] We gratefully acknowledge the U.S. National Science Foundation
(CHE-9988806) for financial support. Diorganophosphanylphos-
phinidenes (1,1-R2PP; R=tBu, Ph).
Supporting information for this article (experimental details,
spectroscopic data, computational details) is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200352779
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phosphorus chemistry. It was recognized that the P2
unit in complex 1 could be further reduced by two
electrons to afford a species featuring a terminal,
uninegative phosphorus atom bound to a single
niobium center. Accordingly,treatment of a green
THF solution of 1 with an excess of 1.0% sodium
amalgam gradually produced a dark orange mix-
ture over 2.5 h. Analysis of the crude reaction
mixture by 31P{1H} NMR spectroscopy indicated
the presence of a new signal centered near d =
1010 ppm,characteristic of early-transition-metal
phosphido species containing one-coordinate phos-
phorus.[10]. Solvent removal followed by diethyl
ether extraction and crystallization produced the
hydrocarbon-soluble,sodium etherate dimer
[{[Na(Et2O)]2}2] (Scheme 1),as yellow orange
blocks in 65% yield.[11]
The 31P{1H} NMR spectrum of pure
[{[Na(Et2O)]2}2] in C6D6 solution gives rise to a
single signal located at d = 949.2 ppm (n1/2
=
495 Hz),which shifts to 1019.8 ppm ( n1/2 = 166 Hz)
when the spectrum is obtained in THF solution.
Whereas the broad nature of the signals is caused
by relaxation effects owing to the quadrupolar
niobium nucleus,[12] the chemical shift difference
presumably arises from formation of a monomeric
[Na(thf)x]2 species in THF solution. Such down-
field movement in 31P chemical shift in polar
solvents may be attributed to diminished Na–P
contact ion pairing,with concomitant strengthening
of P(pp)!Nb(dp) bonding interactions as the electro-
positive niobium center compensates for the build-
up of negative charge.[13] Complete sequestration of
the sodium counterion from the phosphorus nuclei
can be achieved by treatment of a THF solution of
[{[Na(Et2O)]2}2] (i.e. [Na(thf)x]2) with 2.0 equiva-
Scheme 1.
lents of the crown ether [12]crown-4 ([12]c-4). The resulting
hydrocarbon insoluble salt,[Na([12]c-4) 2]2,gives rise to a
single 31P NMR signal at 1110.2 ppm (n1/2 = 170 Hz,
[D8]THF),further emphasizing the downfield progression of
chemical shift as the [PNb(N[Np]Ar)3]ꢀ unit becomes discrete
and separated from its counterion.
Crystallization from a THF/n-pentane mixture produced
red-orange plates of [Na([12]c-4)2]2 in 80% yield. The single-
crystal X-ray structure of [Na([12]c-4)2]2 (Figure 1) clearly
shows the anion–cation separation imposed by 2.0 equivalents
of [12]c-4 and confirms the terminal nature of the phosphido
ꢀ
substituent. The Nb P bond length of 2.186(2) Š in
[Na([12]c-4)2]2 is short when compared to the related four-
=
coordinate,tantalum phosphinidene complex,[PhP Ta(OSi-
Figure 1. ORTEP diagram of [Na([12]c-4)2]2 (ellipsoids set at 35%
probability). Selected bond lengths [Š] and angles [8]: Nb1-P1 2.186(2),
Nb-N 2.073 av; P-Nb-N 103.43 av.
ꢀ
tBu3)3] (Ta P 2.317(4) Š),which can be viewed as containing
a short Ta P double bond. However,the Nb P bond length
in [Na([12]c-4)2]2 agrees well with the Mo P triple bonds in
[P Mo(N[tBu]Ar)3] (2.119(4) Š) and [P Mo(N[iPr]Ar)3]
(2.116(3) Š),[16] when the 0.07 Š difference in covalent radii
between Nb and Mo is considered.[17] Thus,we suggest
[14]
ꢀ
ꢀ
ꢀ
[15]
ꢁ
ꢁ
It is also noteworthy that [Na([12]c-4)2]2 is the first definitive
example of a terminal phosphide complex of a transition
metal outside of group 6.[19]
The terminal phosphorus atom in [Na(thf)x]2 has proven
to be nucleophilic and readily provides terminal phosphini-
ꢀ
[Na([12]c-4)2]2 is accurately described as containing a Nb P
triple bond,the notion of which has also been substantiated
by DFT calculations on the model complex [PNb(NH2)3]ꢀ.[18]
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denes upon treatment with electrophiles. For example,treat-
phanylphosphinidene complex,[( h2-Ph2PP)Nb(N[Np]Ar)3]
(6) in 60% isolated yield.
ment of thawing THF solutions of [Na(thf)x]2 with 0.95 equiv-
alents[20] of trimethylsilyl or trimethylstannyl chloride pro-
The solid-state structures of 5 and 6 (Figure 2 and
duced the trimethysilyl,[Me 3SiP Nb(N[Np]Ar)3] (3),and
Figure 3,respectively) show the h2-interaction of the R2PP
=
=
trimethylstannyl,[Me 3SnP Nb(N[Np]Ar)3] (4),phosphini-
dene complexes in 75% and 85% yield,respectively (see
Scheme 2). The 31P{1H} NMR spectral resonances for 3 (d =
528.6 ppm) and 4 (d = 607.0 ppm) indicate bent[14,21] phosphi-
nidene moieties typical for four-coordinate early-transition-
metal phosphinidene complexes supported by hard, p-donat-
ing,ancillary ligands. [22] A solid-state structural determination
of 4[23] confirmed this spectroscopic assignment and revealed
ꢀ
an elongated Nb P bond (2.2731(8) Š) relative to that in
[Na([12]c-4)2]2,consistent with formulation of this product as
=
a doubly bonded Nb P phosphinidene. Furthermore,the
ꢀ
P Sn bond length of 2.4778(8) Š matches the sum of covalent
radii for these atoms,[17] attesting to a single bond between tin
and phosphorus in 4.[24]
Our attention then turned to the construction of h2-bound
phosphanylphosphinidenes,species known to contain multi-
ple bonding between adjacent phosphorus atoms.[7] Accord-
ingly,treatment of a thawing THF solution of [Na(thf) x]2 with
0.95 equivalents[20] of di-tert-butylchlorophosphane (tBu2PCl)
readily provided the orange phosphanylphosphinidene com-
plex,[( h2-tBu2PP)Nb(N[Np]Ar)3] (5) in 65% yield
(Scheme 2). Formation of phosphanylphosphinidene 5 from
2 and tBu2PCl represents a synthetic method complementary
to Fritz and co-workers' use of a transfer reagent[6,7] in which
Figure 2. ORTEP diagram of 5 (ellipsoids set at 35% probability).
Selected bond lengths [Š] and angles [8]: Nb1-P1 2.4149(12), Nb1-P2
2.6126(11), P1-P2 2.0888(15), Nb1-N 2.027 av; Nb1-P1-P2 70.49(4),
P1-P2-Nb1 60.61(4), P1-Nb1-P2 48.90(4).
ꢀ
the P P bond is already intact. However,our method is
applicable to the synthesis of novel,Nb-complexed R 2PP
derivatives. Thus,treatment of [Na(thf) x]2 with diphenyl-
chlorophosphane (Ph2PCl) afforded the red diphenylphos-
Figure 3. ORTEP diagram of 6 (ellipsoids set at 35% probability).
Selected bond lengths [Š] and angles [8]: Nb1-P1 2.429(3), Nb1-P2
2.589(3), P1-P2 2.073(4), Nb1-N 2.016 av; Nb1-P1-P2 69.71(11), P1-
P2-Nb1 61.62(10), P1-Nb1-P2 48.66(9).
ꢀ
unit with the niobium center. The Nb1 P1
bond for both 5 and 6 (2.4149(12) and
2.429(3) Š,respectively) are much longer
than typically found in early-transition-
metal phosphinidene complexes[22] and more
ꢀ
closely approximate an Nb P single bond.
ꢀ
Contrastingly,the P1 P2 separations of
2.0888(15) and 2.073(4) Š in 5 and 6,respec-
tively are approximately 0.13 Š shorter than
[25]
ꢀ
expected for a P P single bond, lending
credence to the notion of multiple-bond
character in R2PP unit,itself complexed
according to the standard Dewar–Chatt–
Duncanson model. Resonance structures
including those shown in Scheme 2 may
Scheme 2.
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Chemie
procedure was repeated two more times until complete addition of
reasonably approximate the bonding attendant with R2PP
complexation in this system.[26]
the electrophile was achieved. The reaction mixture was then allowed
to warm to room temperature and stirred for an additional 30 min
before being evaporated to dryness in vacuo. The residue was
extracted with n-pentane (3 mL),filtered through Celite,and the
filtrate evaporated to dryness again in vacuo. Crystallizations were
effected by storing Et2O (3, 4, 6) or n-pentane (5) solutions at ꢀ358C
for 1–2 days. Color,yield: 3: Orange,75%; 4: Red,85%; 5: Orange,
65%; 6: Red,60%.
Crystallographic Data: Crystal data can be found in the
Supporting Information. CCDC-218230 ([{[Na(Et2O)]2}2]),CCDC-
218231 ([Na([12]c-4)2]2·(THF)),CCDC-218229 ( 4),CCDC-218232
(5),CCDC-218228 ( 6) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre,12,Union Road,Cambridge CB21EZ,
UK; fax: (+ 44)1223-336-033; or deposit@ccdc.cam.ac.uk).
The 31P{1H} NMR spectrum of 5 (C6D6) exhibits two sets
of doublets located at d = 405.5 and 80.6 ppm for the
phosphinidene (Pa) and phosphanyl (Pb) atoms,respectively,
with a large 1JP,P coupling constant of 425 Hz also reflective of
multiple-bonding interactions[27] (Pa = 401.3 ppm,P
=
b
19.0 ppm, 1JP,P = 439 Hz for 6). NMR calculations carried
out on the hypothetical construct,[( h2-Me2PP)Nb(NH2)3],
support the assignment of a downfield shift for Pa relative to
Pb in this system.[26] However,the highly downfield signals for
Pa in 5 and 6 are in stark contrast to the mononuclear [(h2-
tBu2PP)PtL2] (L = phosphane) complexes of Fritz and co-
workers,[6] in which Pa resonates upfield of Pb,which indicates
a small contribution from Pa in the frontier-orbital region[28]
for the latter and nicely highlighting a major spectroscopic
differentiation between these early- and late-transition-metal
systems.
Received: September 3,2003 [Z52779]
Keywords: anions · multiple bonds · niobium · P ligands ·
In conclusion,nucleophilic,terminal,triply-bonded func-
tional groups are of interest for building novel complexed
moieties “from the ground up”,as demonstrated previously
for niobium nitride[8,29] and molybdenum carbide[13,30] anions.
Now to this category we add the niobium phosphide anion,
the versatility of which is demonstrated by the present
synthesis of novel stannyl and phosphanyl phosphinidene
complexes. Furthermore,incorporation of phosphorus atoms
derived directly from the element into complexed ligands
represents a potentially powerful method for the generation
of synthetically useful phosphorus-containing molecules.
.
phosphorus
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General: Unless otherwise stated,all manipulations were preformed
at room temperature in a glovebox (N2) using dried and deoxygen-
ated solvents and reagents. Full spectroscopic details for all new
compounds can be found in the Electronic Supporting Information.
[{[Na(Et2O)]2}2]: 1.0% sodium amalgam (Na: 0.115 g,3.3 equiv/
Nb) was added to a green THF solution of 1[9] (1.00 g,1.50 mmol,
10 mL). The mixture was stirred vigorously for 2.5 h,it gradually
changed in color to dark orange. The supernatant solution was
decanted from the amalgam,filtered through Celite,and evaporated
to dryness. The resulting dark orange residue was then dissolved in
Et2O (5 mL),filtered through Celite again,and cooled at ꢀ358C for
3 days whereupon large yellow-orange crystals were obtained. Yield:
0.759 g,65% in two crops.
Anorg. Allg. Chem. 1997, 623,1917; E. Matern,J. Pikies,G. Fritz,
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[10] G. Wu,D. Rovnyak,M. J. A. Johnson,N. C. Zanetti,D. G.
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[11] Full structural details for [{[Na(Et2O)]2}2] can be found in
CCDC-218230,see Experimental Section.
[Na([12]c-4)2]2:
A
THF solution of [12]crown-4 (0.040 g,
[12] NMR and the Periodic Table (Eds.: R. K. Harris,B. E. Mann),
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Soc. 1994, 116,11159 – 11160.
0.231 mmol,2 mL) was added to a solution of [{[Na(Et 2O)]2}2]
(0.100 g,0.115 mmol) dissolved in THF (5 mL) over the course of
5 min. The reaction mixture was stirred for 30 min,after which it was
concentrated to a volume of 2 mL and filtered through a small plug of
Celite. n-Pentane (2 mL) was added and the resulting solution was
stored at ꢀ358C overnight whereupon small red-orange crystals were
obtained. Yield: 0.099 g,80%.
[15] C. E. Lalplaza,W. M. Davis,C. C. Cummins,
Angew. Chem.
1995, 107,2181 – 2183; Angew. Chem. Int. Ed. Engl. 1995, 34,
2042 – 2044.
3–6: Separately,a solution of [{[Na(Et 2O)]2}2] (0.250 g,
0.316 mmol) dissolved THF (5 mL) and a THF solution (2 mL)
containing 0.95 equiv of the corresponding electrophile (ClSiMe3 for
3,ClSnMe for 4,ClP tBu2 for 5,and ClPPh for 6) were frozen in a
[16] J. P. F. Cherry,F. H. Stephens,M. J. A. Johnson,P. L. Diaco-
nescu,C. C. Cummins, Inorg. Chem. 2001, 40,6820 – 6862.
[17] L. Pauling, The Nature of the Chemical Bond,3rd ed.,Cornell
University Press,Cornell, 1960,chap. 11,p. 405.
[18] Input and results for this calculation can be found in the
Supporting Information,ADF Program Package (release
2000.02).
3
2
glove box cold well (liquid N2). Upon removal from the cold well,
approximately 0.6 mL of the thawing solution containing the electro-
phile was added dropwise over 1 min to the thawing solution of
[{[Na(Et2O)]2}2],eliciting a color change from dark yellow to red-
orange. The reaction mixture stirred for an additional 3 min where-
upon both solutions were placed back into the cold well. This
[19] J. S. Freundlich,R. R. Schrock,W. M. Davis, J. Am. Chem. Soc.
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Communications
[20] Sub-stoichiometric quantities of halo-electrophiles,cold temper-
atures,and slow addition rates are necessary to prevent over
halogenation of the {PNbL3}ꢀ fragment.
[21] A. H. Cowley,B. Pellerin,J. L. Atwood,G. S. Bott, J. Am. Chem.
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[22] A. H. Cowley, Acc. Chem. Res. 1997, 30,445 – 451.
[23] Full structural details for complex 4 can be found in CCDC-
218229,see Experimental Section.
1
[24] J(Sn,P) coupling is not fully resolved in the 31P{1H} NMR spectrum
of 4,but rather observed as shoulders to the main peak.
ꢀ
[25] The experimentally determined P P interatomic distance in
P4(g) is 2.21 Š,N. N. Greenwood,A. Earnshaw, Chemistry of the
Elements,2nd ed.,Butterworth–Heinemann,Oxford,
1997,
chap. 12,p. 479.
[26] Two of many possible resonance structures are presented here.
Detailed DFT calculations to elucidate the bonding in this
system are currently in progress and will be presented in due
course.
[27] S. Shah,J. D. Protasiewicz, Coord. Chem. Rev. 2000, 210,181 –
201.
[28] See for example: G. Schreckenbach,T. Ziegler, J. Phys. Chem.
1995, 99,606 – 611.
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2002, 124,2412 – 2413.
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