Carbon-phosphorus triple bond formation through multiple bond metathesis of an anionic niobium phosphide with carbon dioxide

Article abstract of DOI:10.1016/j.poly.2011.07.016

A novel borane-capped niobium phosphide anion ([B(C₆F ₅)₃-1]) has been prepared in 87% yield by reaction of [PNb(N[Np]Ar)₃]^- (1, Ar = 3,5-C₆H ₃Me₂ and Np = neopentyl)

Full text of DOI:10.1016/j.poly.2011.07.016

Polyhedron 32 (2012) 10–13
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Carbon–phosphorus triple bond formation through multiple bond metathesis
of an anionic niobium phosphide with carbon dioxide
⇑
Ivo Krummenacher, Christopher C. Cummins
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 27 July 2011
A novel borane-capped niobium phosphide anion ([B(C6F5)3-1]) has been prepared in 87% yield by reac-
tion of [P„Nb(N[Np]Ar)3]À (1, Ar = 3,5-C6H3Me2 and Np = neopentyl) with the Lewis acidic borane
B(C6F5)3. Room-temperature reaction of this adduct with carbon dioxide readily yields the OCPÀ ion 3,
isolated as its sodium salt, [Na(dme)2][OCP] (dme = dimethoxyethane), in 70% yield, with concomitant
formation of a borane-substituted niobium oxo complex [B(C6F5)3-2] (2 = ONb(N[Np]Ar)3).
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Anions
Boranes
Multiple bonds
Niobium
Phosphaalkynes
P ligands
Phosphorus
Low coordination number
Carbon dioxide
Terminal phosphide
1. Introduction
diethyl ether solution of previously reported terminal phosphide
anion [Na(OEt2)][1] [5]. After recrystallization from a mixture of
Terminal phosphide complexes are a unique class of compounds
involvinga metal-phosphorus triplebond [1]. One particularlyinter-
esting aspect of this rare functionality is its potential for P-atom
transfer reactions to organic substrates, thus enabling the construc-
tion of diverse organophosphorus compounds. Such reactivity has,
however, only recently materialized for an anionic niobium phos-
phide by metathetical conversion of aliphatic acyl chlorides into
phosphaalkynes, a process largely driven by the oxophilicity of the
early transition metal niobium [2]. The involved formal [2+2] cyclo-
addition and cycloreversion steps bear analogy to the reaction path-
way of alkyne metathesis by Schrock-type alkylidyne complexes [3].
The possibility for formal metathetical exchange of M–P for C–P tri-
ple bonds prompted us to explore the reactivity of the d0 metal–
phosphido complex toward other carbonyl-containing electrophiles
[4]. Herein we report a transition-metal based method for the gener-
ation of phosphaethynolate ions (OCPÀ) fromthe reaction ofa Lewis-
acid adduct of [P„Nb(N[Np]Ar)3]À (1, Ar = 3,5-C6H3Me2 and
Np = neopentyl) with carbon dioxide.
diethyl ether and n-pentane the product was obtained as red crys-
talline solid in 87% yield. The increased coordination number at the
phosphorus and boron atoms upon Lewis acid–base adduct forma-
tion is clearly reflected in the low frequency shift of the 31P and 11
B
NMR resonances (dP 514 ppm and dB À26 ppm),1 the former being
upfield-shifted as compared to the W(CO)5-capped derivative (dP
588 ppm) [6]. X-ray structural analysis confirmed the expected
phosphidoniobium-borane adduct formation with a bent Nb–P–B
linkage (154.4(1)°, Fig. 1). In accord with the angular structure is a
small increase in the Nb–P bond length on Lewis acid complexation
(from 2.203(1) to 2.239(1) Å), which is more compatible with a
description as a niobium–phosphorus double bond than triple bond
[7]. The P–B distance of 2.042(4) Å is similar to values observed in
related borane adducts involving phosphorus lone-pair coordination
to B(C6F5)3.2 Furthermore, the molecular structure discloses
[Na(OEt2)][B(C6F5)3-1] as a tight ion pair with an interaction of the
phosphorus atom to the sodium cation, which shows further con-
tacts to an aryl unit of one of the anilide ligands, a diethyl ether mol-
ecule, and two ortho CF bonds of the borane unit. The observed
CFÁ Á ÁNa+ interactions of 2.404(3) and 2.599(3) fall within a typical
range for fluorine–sodium distances [8].
2. Results and discussion
The borane adduct [Na(OEt2)][B(C6F5)3-1] was synthesized by
addition of one equivalent of tris(pentafluorophenyl)borane to a
1
The phosphorus–boron coupling in the 11B NMR spectrum was not resolved.
A Cambridge Structural Database (CSD) search identified more than 20 structures
2
⇑
Corresponding author.
E-mail address: ccummins@mit.edu (C.C. Cummins).
with P–B distances ranging from 2.0 to 2.2 Å.
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.07.016

I. Krummenacher, C.C. Cummins / Polyhedron 32 (2012) 10–13
11
F2
F1
F3
F10
B1
F4
O1
O1
B1
F15
N2
F5
Na1
P1
Nb1
N3
N2
N1
Nb1
N3
N1
Fig. 1. Molecular structure of [Na(OEt2)][B(C6F5)3-1]. Displacement ellipsoids are
drawn at the 50% probability level. Selected bond distances (Å) and angles (°): P1–
Nb1 2.239(1), P1–B1 2.042(4), Na1–P1 2.933(2), Na1–O1 2.299(4), Nb–N (avg)
2.022, Na1Á Á ÁF10 2.404(3), Na1Á Á ÁF15 2.599(3); Nb1–P1–B1 154.4(1).
Fig. 2. Molecular structure of [B(C6F5)3-2]. Displacement ellipsoids are drawn at the
50% probability level. Selected bond distances (Å) and angles (°): Nb1–O1 1.800(1),
O1–B1 1.535(2), Nb–N (avg) 1.975; Nb1–O1–B1 175.9(1).
of this product was further confirmed by X-ray structural analysis
(Fig. 2) along with an independent synthesis (vide infra).
Reaction
of
the
borane-capped
phosphide
anion
[Na(OEt2)][B(C6F5)3-1] with a stoichiometric amount of CO2 in
diethyl ether led to a color change from dark red to bright yellow
over the course of several minutes. Reaction monitoring by
31P{1H} NMR spectroscopy indicated that the starting phosphide
anion (d 514 ppm) was completely consumed and that a single
new resonance appeared at extremely low frequency (d
À393 ppm), reminiscent of the lithium phosphaethynolate
[Li][O–C„P] (dP À384 ppm) [9] (Scheme 1). Removal of the solvent
in vacuo and washing with portions of diethyl ether and pentane
left a grayish residue, from which the new phosphorus species
[Na(dme)2][OCP] (3) was isolated in 70% yield as long colorless
needles after layering a dimethoxyethane (DME) solution with n-
pentane at À30 °C. Multinuclear NMR spectroscopic analysis of
the diethyl ether/pentane washings revealed a new terminal oxo-
borane adduct of niobium [B(C6F5)3-2] (2 = ONb(N[Np]Ar)3), indic-
ative of oxo-transfer from CO2 to the metal center. The constitution
Single crystals of the salt 3 could be obtained, however they
proved to be of insufficient quality to obtain a properly refined
structure, although the connectivity between the atoms could be
verified (Scheme 2). Inspection of the data revealed a dimeric
structure with two linear OCP anions bridging two sodium cations
that are each solvated by two chelating DME ligands, thereby con-
firming the chemical composition as [Na(dme)2][OCP]. Besides the
distinctive 31P NMR signal, the observed 13C NMR signal of the
cyaphido group carbon at 169 ppm with a 1J(31P, 13C) coupling con-
stant of 52 Hz and a PC stretching frequency of 1773 cmÀ1 compare
well with available experimental (dP 167 ppm, 1J(31P, 13C) = 42 Hz,
and 1763 cmÀ1 for [Li][OCP]) [9] and theoretically calculated data
[10]. It is worth mentioning that after the original synthesis re-
ported by Becker et al. almost two decades ago from lithium
bis(trimethylsilyl)phosphanide and dimethyl carbonate [9] only
one other route to access salts of the phosphaethynolate anion
P
C
O
Na
3
C6F5
C6F5
Na
C6F5
B
P
C6F5
C6F5
C6F5
1 equiv CO2
Et2O, 25 ºC
B
Nb
N[Np]Ar
N[Np]Ar
N
O
Nb
N[Np]Ar
N[Np]Ar
N
[B(C6F5)3-2]
[Na(OEt2)][B(C6F5)3-1]
Scheme 1. Metathetical exchange between CO2 and niobium phosphide [Na(OEt2)][B(C6F5)3-1].

12
I. Krummenacher, C.C. Cummins / Polyhedron 32 (2012) 10–13
obtained from the reaction of niobium(V) oxo complex 2 with
B(C6F5)3, supported the formulation of [B(C6F5)3-2].
P
C
O
3. Conclusion
In summary, we have shown that an unprecedented borane ad-
duct of a terminal niobium phosphide can readily transform carbon
dioxide, under mild conditions, into the O–C„PÀ ion, the phospho-
rus analogue of cyanate. Borane-capped niobium oxo was identi-
fied as the byproduct in this multiple bond metathesis reaction.
The reported activation and functionalization of carbon dioxide
further establishes the synthetic potential of the niobium phos-
phide functionality in P-atom transfer chemistry.
(dme) Na
Na(dme)
2
2
O
C
P
Scheme 2. Molecular structure of 3 in the solid state.
4. Experimental
has been reported which consist in the hydrophosphination of CO;
NaPH2 + CO ? NaOCP + H2 [11]. Presumably due to the inherent
instability or high reactivity of these anions, only a handful of ex-
tant studies are devoted to its coordination chemistry. The prepa-
4.1. General
All manipulations were performed under an atmosphere of
purified dinitrogen using a Vacuum Atmospheres glove box (model
MO-40 M) or Schlenk techniques at room temperature unless
otherwise stated. [Na(Et2O)][PNb(N[Np]Ar)3] ([Na(Et2O)-1],
Ar = 3,5-C6H3Me2, Np = neopentyl) [5], ONb(N[Np]Ar)3 (2) [16],
and B(C6F5)3 [18] were synthesized according to literature meth-
ods. Anhydrous-grade, oxygen-free dimethoxyethane (DME) was
purchased from Aldrich and further dried by passing through a col-
umn of molecular sieves and stirring with Na for at least 12 h prior
to filtration through Celite to remove Na. All other solvents were
obtained anhydrous and oxygen-free by bubble degassing (N2)
and purification through columns of alumina. Deuterated solvents
were purchased from Cambridge Isotope Laboratories. Benzene-d6
and tetrahydrofuran-d8 were degassed and stored over molecular
sieves for at least 2 d prior to use. Celite 435 (EM Science) was
dried by heating above 200 °C under a dynamic vacuum for at least
24 h prior to use. NMR spectra were obtained on Varian Inova 300
or 500, or Bruker Avance 400 instruments equipped with Oxford
Instruments superconducting magnets and referenced to residual
C6H5D (1H = 7.16 ppm, 13C = 128.06 ppm). 31P NMR spectra were
referenced externally to 85% H3PO4 in D2O. Abbreviations are as
follows: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet;
br, broad. Infrared spectra (thin films on KBr plates) were recorded
on a Perkin–Elmer Model 2000 FTIR spectrometer under an N2
purge at 25 °C. Abbreviations for infrared intensity: s, strong; m,
medium; w, weak; vw, very weak. Elemental analyses were per-
formed by Midwest Microlab LLC, Indianapolis, IN.
ration of
a series of alkaline earth metal complexes by
Westerhausen et al. [12] and 1,3-diferrio-1,3-diphosphetane-2,4-
diones by Weber et al. [13] through salt metathesis reactions with
[Li][OCP] are highlights in the development of phosphaethynolate
chemistry.
Phosphorus–carbon triple bond formation likely results from
initial attack of the nucleophilic phosphido group at the carbon
atom of CO2 to afford a P–CO2À adduct [14]3 or rather a niobacyclic
Nb–P–C(O)–O intermediate upon coordination of one oxygen atom
to niobium. It is possible that the borane migrates away from phos-
phorus in this initial step to stabilize the negative charge on oxygen,
which in that case would represent the product of formal CO2 inser-
tion into the P–B bond. The corresponding metallacycle then can un-
dergo
a [2+2]-cycloreversion to form the terminal metal-oxo
complex and the C„P bond found in the anion of salt 3. The migra-
tion of the Lewis acid B(C6F5)3 in this process to form a strong
Nb„O–B(C6F5)3 interaction is quite remarkable and suggests that
the terminal niobium oxo group is more nucleophilic than the oxy-
gen atom in the [OCP] anion. Mechanistic insight into the observed
multiple bond metathesis reaction comes from our studies on the
transformation of acyl chlorides into phosphaalkynes mediated by
complex [Na(OEt2)][1] [2]. In this case, it was possible to isolate
and to characterize structurally 4-membered Nb–P–C–O niobacyclic
intermediates, which readily underwent retro [2+2]-fragmentation
to furnish a carbon–phosphorus triple bond. The ease of carbon diox-
ide activation and functionalization under these mild conditions is
likely a consequence of the concomitant formation of strong nio-
bium– and boron–oxygen bonds in the final metal oxo–borane com-
plex [B(C6F5)3-2]. The role of borane is however not critical in this
metathetical process as we have found that the parent complex
[Na(OEt2)][1] reacts similarly with carbon dioxide under mild condi-
tions (1 atm, 25 °C) to produce metal-oxo O„Nb(N[Np]Ar)3 (2) and
phosphaethynolate 3 [15].
Crystals of [B(C6F5)3-2] suitable for X-ray diffraction studies
were grown from a saturated diethyl ether solution. The Nb1–O1
distance of 1.800(1) is significantly elongated with respect to the
parent compound 2 (1.696(3)) [16], reflecting a reduction in bond
order on complexation of the borane to the oxo moiety. The Nb–O–
B fragment shows a linear disposition (175.9(1)°) with a O–B dis-
tance (1.535(2) Å) that lies well in the range of other terminal me-
tal-oxo adducts of B(C6F5)3 [17]. Spectroscopic and combustion
data as well as analysis of an independently synthesized sample,
4.2. [Na(Et2O)][(C6F5)3BPNb(N[Np]Ar)3] ([Na(OEt2)][B(C6F5)3-1])
To
a diethyl ether solution of [Na(Et2O)][PNb(N[Np]Ar)3]
(700 mg, 0.88 mmol, 20 mL) at room temperature was added a
diethyl ether solution of B(C6F5)3 (452 mg, 0.88 mmol, 5 mL) drop-
wise. The mixture became immediately dark red and was allowed
to stir for 30 min. The solvent was removed in vacuo and the result-
ing orange powder washed with several portions of n-pentane
(10 mL). Recrystallization from a mixture of diethyl ether and n-
pentane at room temperature afforded large red crystals (1.00 g,
0.77 mmol, 87%). M. p.: 130 °C (decomp.). 1H NMR (400 MHz,
C6D6, 20 °C): d 6.84 (s, 6H, o-Ar), 6.42 (s, 3H, p-Ar), 3.62 (s, 6H,
N-CH2), 3.17 (q, 4H, Et2O), 2.14 (s, 18H, Ar-CH3), 1.03 (t, 6H,
Et2O), 0.73 (s, 27H, tert-Bu). 11B NMR (128.4 MHz, C6D6, 20 °C): d
À26 (br,
m
1/2 = 160 Hz). 13C NMR (100.6 MHz, C6D6, 20 °C): d
1
156.2 (s, ipso-Ar), 148.4 (dm, JCF = 247 Hz, B(C6F5)3), 139.6 (dm,
1JCF = 247 Hz, B(C6F5)3), 138.8 (s, m-Ar), 137.6 (dm, JCF = 242 Hz,
1
3
B(C6F5)3), 125.2 (s, p-Ar), 121.9 (s, o-Ar), 71.8 (s, N-CH2), 65.9 (s,
Et2O), 35.0 (s, C(CH3)3), 29.1 (s, C(CH3)3), 21.3 (s, Ar-CH3), 15.1 (s,
Alternatively, a [2+2] cycloaddition reaction of carbon dioxide with the terminal
metal–phosphido complex can be considered.

I. Krummenacher, C.C. Cummins / Polyhedron 32 (2012) 10–13
13
Et2O); not observed B(C6F5)3 (ipso-C). 19F NMR (376.5 MHz, C6D6,
20 °C): d À164.4 (m, m-C6F5), À160.4 (t, J = 21 Hz, p-C6F5), À132.4
(d, J = 21 Hz, o-C6F5). 31P{1H} NMR (162 MHz, C6D6, 20 °C): d 514
thanks the Swiss National Science Foundation for a post-doctoral
research fellowship.
(br, m
1/2 = 570 Hz). IR (Et2O solution, KBr plates, cmÀ1): 2955 sh,
Appendix A. Supplementary data
2866 w, 1517 m, 1465 s, 1281 w, 1095 m, 979 m. Anal. Calc.: C,
56.19; H, 5.41; N, 3.22. Found: C, 55.89; H, 5.34; N, 3.27%.
CCDC 821010 and 821011 contain the supplementary crystallo-
graphic data for [Na(OEt2)][B(C6F5)3-1] and [B(C6F5)3-2]. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/con-
ts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223
336 033; or e-mail: deposit@ccdc.cam.ac.uk.
4.3. [(C6F5)3BONb(N[Np]Ar)3] ([B(C6F5)3-2])
A diethyl ether solution of B(C6F5)3 (75.4 mg, 0.147 mmol, 2 mL)
was added to a solution of ONb(N[Np]Ar)3 (100 mg, 0.147 mmol) in
diethyl ether (10 mL). The solution became dark yellow in color
and was allowed to stir for 30 min, at which point the solvent
was removed in vacuo. The residue was washed thoroughly with
several portions of n-pentane and dried under vacuum to yield
the product as bright yellow powder in 75% yield (132 mg,
0.11 mmol). If desired, the product can be recrystallized from
diethyl ether to give yellow plates. M. p.: ꢀ170 °C. 1H NMR
(400 MHz, C6D6, 20 °C): d 6.54 (s, 3H, p-Ar), 6.26 (s, 6H, o-Ar),
3.87 (s, 6H, N-CH2), 1.99 (s, 18H, Ar-CH3), 0.51 (s, 27H, tert-Bu).
References
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11B NMR (128.4 MHz, C6D6, 20 °C): d À18 (br,
m
1/2 = 220 Hz). 13C
NMR (100.6 MHz, C6D6, 20 °C): d 150.0 (s, ipso-Ar), 149.0 (dm,
1
1JCF = 240 Hz, B(C6F5)3), 140.5 (dm, JCF = 242 Hz, B(C6F5)3), 138.6
1
(s, m-Ar), 137.8 (dm, JCF = 252 Hz, B(C6F5)3), 128.6 (s, p-Ar),
123.1 (s, o-Ar), 68.8 (s, N-CH2), 34.5 (s, C(CH3)3), 28.3 (s, C(CH3)3),
21.2 (s, Ar-CH3); not observed B(C6F5)3 (ipso-C). 19F NMR
(376.5 MHz, C6D6, 20 °C): d À164.0 (dd, J = 8.2, 24 Hz, m-C6F5),
À158.2 (m, p-C6F5), À133.0 (dd, J = 8.2, 24 Hz, o-C6F5). IR (Et2O
solution, KBr plates, cmÀ1): 2956 sh, 2867 w, 1644 w, 1603 m,
1514 m, 1463 s, 1280 w, 1188 w, 1096 m, 973 m. Anal. Calc.: C,
57.44; H, 5.07; N, 3.53. Found: C, 57.23; H, 4.97; N, 3.37.
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4.4. [Na(dme)2][OCP] (3)
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A
dark red solution of [Na(Et2O)][(C6F5)3BPNb(N[Np]Ar)3]
(400 mg, 0.31 mmol) in diethyl ether (30 mL) and DME (1 mL) was
treated via syringe with CO2 (7.0 ml, 0.31 mmol) at 0 °C. The solution
rapidly turned from red to bright yellow and was stirred for 1 h. The
solvent was evaporated and the residue washed with diethyl ether
and pentane in order to remove the niobium oxo compound. The
grayish residue was then dissolved in DME, the turbid solution fil-
tered, the filtrate concentrated, layered with n-pentane and cooled
to À30 °C. Subsequent recrystallization cycles yielded long colorless
needles. Yield: 57 mg (0.21 mmol, 70%). 1H NMR (400 MHz, thf-d8,
20 °C): d 3.43 (s, CH2), 3.27 (s, CH3). 13C NMR (100.6 MHz, thf-d8,
(b) W.E. Buhro, M.H. Chisholm, J.D. Martin, J.C. Huffman, K. Folting, W.E.
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[17] A search of the Cambridge crystal structure database (CSD) revealed an
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early transition metal-oxo compounds. For a recent example, see: J. Sánchez-
Nieves, L.M. Frutos, P. Royo, O. Castaño, E. Herdtweck, M.E.G. Mosquera, Inorg.
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1
20 °C): d 168.6 (d, JPC = 52 Hz, OCP), 72.9 (s, CH2), 59.1 (s, CH3).
31P{1H} NMR (162 MHz, C6D6, 20 °C): d À393 (s, OCP). IR (THF solu-
tion, KBr plates, cmÀ1): 1822 s, 1798 s, 1788 s, 1773 s (
w, 1296 sh, 1103 m, 976 m, 807 w, 740 sh, 518 m.
m(CP)), 1608
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Acknowledgments
For support of this work we are grateful to the US National Sci-
ence Foundation (CHE-719157) and Thermphos International. I.K.