A convenient method for the preparation of n-heterocyclic bromophosphines: excellent precursors to the corresponding n-heterocyclic phosphenium salts

Article abstract of DOI:10.1021/om900420g

An in situ redox method is employed to prepare N-heterocyclic bromophosphines in good yield and purity. Such bromophosphines may be treated with a variety of bromide-abstracting reagents to produce the corresponding N-heterocyclic phosphenium salts in exc

Full text of DOI:10.1021/om900420g

Organometallics 2009, 28, 4377–4384 4377
DOI: 10.1021/om900420g
A Convenient Method for the Preparation of N-Heterocyclic
Bromophosphines: Excellent Precursors to the Corresponding
N-Heterocyclic Phosphenium Salts
Jonathan W. Dube, Gregory J. Farrar, Erin L. Norton, Kara L. S. Szekely,
Benjamin F. T. Cooper, and Charles L. B. Macdonald*
Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario N9B 3P4, Canada
Received May 20, 2009
An in situ redox method is employed to prepare N-heterocyclic bromophosphines in good yield
and purity. Such bromophosphines may be treated with a variety of bromide-abstracting reagents to
produce the corresponding N-heterocyclic phosphenium salts in excellent yield.
Salts containing phosphenium cations have played an im-
portant role in the history and development of modern main-
group chemistry. In the most general definition, a phosphe-
nium cation (1) is a cation that contains a dicoordinate
phosphorus center bearing a total of six valence electrons and
is the isovalent analogue of a carbene (2).1,2 While numerous
types of phosphenium cations have been prepared and studied,
the most important class of phosphenium compounds is the
relatively stable species in which the dicoordinate phosphorus
center is supported by two adjacent amido substituents.
Although such compounds are analogous to the now-ubiqui-
tous N-heterocyclic carbenes (NHCs, 3)3,4 and may be labeled
N-heterocyclic phosphenium cations (NHPs, 4), it is worth
noting that well-characterized NHPs were reported more than
35 years ago5,6 and thus predate the first report of a stable NHC
considerably. In fact, the structural characterization of a salt
containing a 1,3,2-diazaphospholenium cation, an unsaturated
NHP directly analogous to the most common type of “Ardu-
engo” NHC,7 was reported as early as 1990.8
and include reversible cycloaddition chemistry,9 azide chemistry
for the preparation of phosphazenes,10 hydride chemistry in-
cluding small-molecule activation,11,12 and the use of NHP frag-
ments as components of highly polarized diphosphines that
exhibit exceptional reactivity.13-16 Perhaps more importantly,
given the importance of NHCs as ligands for transition-metal
complexes,17 interest in NHPs as analogues of NHC ligands has
been great and is growing significantly.18
Computational work demonstrates that, in spite of their
isovalent “carbon copy”21 relationship, NHPs are consider-
ably electrophilic, whereas NHCs are typically considered as
strongly nucleophilic in nature.22 In this light, NHP ligands
may be used to engender altered and perhaps improved
reactivity on the resultant organometallic complexes and
are being used to develop new types of catalysts for organic
transformations.18,23-28
(9) Caputo, C. A.; Price, J. T.; Jennings, M. C.; McDonald, R.; Jones,
N. D. Dalton Trans. 2008, 3461–3469.
(10) Burck, S.; Gudat, D.; Nieger, M.; Schalley, C. A.; Weilandt, T.
Dalton. Trans. 2008, 3478–3485.
(11) Gudat, D.; Haghverdi, A.; Nieger, M. Angew. Chem., Int. Ed.
2000, 39, 3084–3086.
(12) Burck, S.; Gudat, D.; Nieger, M.; Du Mont, W. W. J. Am. Chem.
Soc. 2006, 128, 3946–3955.
(13) Burck, S.; Gudat, D.; Nieger, M. Angew. Chem., Int. Ed. 2004,
43, 4801–4804.
(14) Burck, S.; Forster, D.; Gudat, D. Chem. Commun. 2006, 2810–
2812.
(15) Burck, S.; Gudat, D.; Nieger, M. Angew. Chem., Int. Ed. 2007,
46, 2919–2922.
(16) Burck, S.; Gudat, D.; Nieger, M.; Vindus, D. Eur. J. Inorg.
The rich and diverse chemistry of NHPs has been studied
extensively.1,2 Some recent highlights are depicted in Scheme 1
*To whom correspondence should be addressed. Tel: (519) 253-3000,
ext. 3991. Fax: (519) 973-7098. E-mail: cmacd@uwindsor.ca.
(1) Cowley, A. H.; Kemp, R. A. Chem. Rev. 1985, 85, 367–382.
(2) Sanchez, M.; Mazieres, M.-R.; Lamande, L.; Wolf, R., Phosphe-
nium Cations. In Multiple Bonds and Low Coordination in Phosphorus
Chemistry, Regitz, M., Scherer, O. J., Eds.; Thieme Verlag: Stuttgart,
Germany, 1990; pp 129-148.
(3) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem.
Rev. 2000, 100, 39–91.
(4) Gudat, D.; Haghverdi, A.; Hupfer, H.; Nieger, M. Chem. Eur. J.
Chem. 2008, 704–707.
(17) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290–1309.
(18) Gudat, D. Coord. Chem. Rev. 1997, 163, 71–106.
(19) Burck, S.; Gudat, D.; Nieger, M.; Tirree, J. Dalton Trans. 2007,
1891–1897.
(20) Burford, N.; Losier, P.; Macdonald, C.; Kyrimis, V.; Bakshi, P.
K.; Cameron, T. S. Inorg. Chem. 1994, 33, 1434–1439.
(21) Dillon, K. B.; Mathey, F.; Nixon, J. F., Phosphorus: The Carbon
Copy; Wiley: Chichester, U.K., 1998.
2000, 6, 3414–3425.
(5) Fleming, S.; Lupton, M. K.; Jekot, K. Inorg. Chem. 1972, 11,
2534–2540.
(6) Maryanoff, B. E.; Hutchins, R. O. J. Org. Chem. 1972, 37, 3475–
3480.
(22) Tuononen, H. M.; Roesler, R.; Dutton, J. L.; Ragogna, P. J.
Inorg. Chem. 2007, 46, 10693–10706.
(23) Nakazawa, H. J. Organomet. Chem. 2000, 611, 349–363.
(24) Abrams, M. B.; Scott, B. L.; Baker, R. T. Organometallics 2000,
19, 4944–4956.
(7) Arduengo, A. J. Acc. Chem. Res. 1999, 32, 913–921.
(8) Litvinov, I. A.; Naumov, V. A.; Griaznova, T. V.; Pudovik, A. N.;
Kibardin, A. M. Dokl. Akad. Nauk SSSR 1990, 312, 623–625.
(25) Nakazawa, H., Transition metal complexes bearing a phosphe-
nium ligand. In Advances in Organometallic Chemistry; Academic Press:
San Diego, CA, 2004; Vol. 50, pp 107-143.
r
2009 American Chemical Society
Published on Web 07/13/2009
pubs.acs.org/Organometallics

4378 Organometallics, Vol. 28, No. 15, 2009
Dube et al.
Scheme 1. Examples of Some of the Reactivity Reported Recently for N-Heterocyclic Chlorophosphines and the Corresponding
N-Heterocyclic Phosphenium Saltsa
a Reagents: (a) NaCp, -NaCl;19 (b) NaN3, LiCl, -NaCl;10 (c) GaCl3,20 Me3SiOTf, -Me3SiCl or AgOTf, -AgCl;9 (d) LiPR2, -LiCl;13 (e) LiAlH4,
-LiCl.11
Scheme 2. Quantitative Redox Cycloaddition Routes to N-Heterocyclic Phosphenium Salts
Phosphenium salts have typically been prepared from a
unusually low oxidation states,31 we have developed new
methods for the convenient generation of phosphorus(þ1)
centers.32 Recently, the research group of Cowley and our
group showed that the use of PI synthons in the presence of
R-diimines yields phosphenium cations as either chlorostan-
nate or triiodide salts, as illustrated in Scheme 2.33-37
Although these methods are simple and quantitative, the nature
of the counteranions may diminish the utility of the resultant
salts. In this note, we demonstrate that our newest method for
the generation of PI bromide salts38 can be employed for
the clean, convenient, and high-yield one-step synthesis of
N-heterocyclic bromophosphines, which are excellent precur-
sors for a wide variety of NHP salts by bromide anion
abstraction or metathesis. This method thus combines the
simplicity of a one-step redox synthetic approach with the
utility of halide substitution and should provide a very useful
route to phosphenium reagents and ligands.
P-chlorinated precursor, either by chloride anion abstraction
or by metathesis.1,2 Given their long history and potential
utility, it is surprising that convenient and high-yielding
synthetic approaches to the P-chlorinated precursors to
unsaturated NHPs (2-chloro-1,3,2-diazaphospholenes,
NHP-Cl) remained absent until relatively recently. While
logical approaches to such compounds, such as the treatment
of salts of unsaturated R-diamide dianions with PCl3, had
been reported, the yields of the reactions were invariably low
and unwanted byproducts were generally observed. Metath-
esis reactions of 1,3,2-diazasiloles and PCl3 were employed as
an alternative route; however, the yields of the reactions
remained low.4,29 More recently, Gudat and co-workers
reported a multistep, one-pot route to the desired N-hetero-
cyclic chlorophosphines that generates the products in good
to excellent yield.12,30
As part of our continuing investigation of compounds
containing phosphorus and other main-group elements in
Experimental Section
General Procedures. All manipulations were carried out using
standard inert-atmosphere techniques. Phosphorus(III) bro-
mide and phosphorus(III) chloride were purchased from
Strem Chemicals Inc., and all other chemicals and reagents
(26) Ackermann, L.; Spatz, J. H.; Gschrei, C. J.; Born, R.;
Althammer, A. Angew. Chem., Int. Ed. 2006, 45, 7627–7630.
(27) Hardman, N. J.; Abrams, M. B.; Pribisko, M. A.; Gilbert, T. M.;
Martin, R. L.; Kubas, G. J.; Baker, R. T. Angew. Chem., Int. Ed. 2004,
43, 1955–1958.
(28) Burck, S.; Daniels, A.; Gans-Eichler, T.; Gudat, D.; Nattinen,
K.; Nieger, M. Z. Anorg. Allg. Chem. 2005, 631, 1403–1412.
(29) Denk, M. K.; Gupta, S.; Ramachandran, R. Tetrahedron Lett.
1996, 37, 9025–9028.
(33) Reeske, G.; Hoberg, C. R.; Hill, N. J.; Cowley, A. H. J. Am.
Chem. Soc. 2006, 128, 2800–2801.
(34) Reeske, G.; Cowley, A. H. Chem. Commun. 2006, 1784–1786.
(35) Ellis, B. D.; Macdonald, C. L. B. Inorg. Chim. Acta 2007, 360,
329–344.
(30) Burck, S.; Gudat, D.; Naettinen, K.; Nieger, M.; Niemeyer, M.;
Schmid, D. Eur. J. Inorg. Chem. 2007, 5112–5119.
(36) Reeske, G.; Hoberg, C. R.; Cowley, A. H. Inorg. Chem. 2007, 46,
4358–4358.
(37) Powell, A. B.; Brown, J. R.; Vasudevan, K. V.; Cowley, A. H.
(31) Macdonald, C. L. B.; Ellis, B. D., Low Oxidation State Main
Group Chemistry. In Encyclopedia of Inorganic Chemistry, 2nd ed.;
King, R. B., Ed.; Wiley: Chichester, U.K., 2005.
Dalton Trans. 2009, 2521–2527.
(32) Ellis, B. D.; Macdonald, C. L. B. Coord. Chem. Rev. 2007, 251,
936–973.
(38) Norton, E. L.; Szekely, K. L. S.; Dube, J. W.; Bomben, P. G.;
Macdonald, C. L. B. Inorg. Chem. 2008, 47, 1196–1203.

Article
Organometallics, Vol. 28, No. 15, 2009 4379
were purchased from Aldrich. Phosphorus(III) bromide and
phosphorus(III) chloride were distilled before use, and all other
reagents were used without further purification. All solvents were
dried using a series of Grubbs-type columns39 and were degassed
prior to use. CD2Cl2, and CDCl3 were dried over calcium
hydride. The compounds 1,4-bis(2,4,6-trimethylphenyl)-2,3-di-
methyl-1,4-diazabutadiene (MesDAB), 1,4-bis(2,6-diisopropylphenyl)-2,
3-dimethyl-1,4-diazabutadiene (DippDAB), 1,2-bis(2,4,6-trimethyl-
phenylimino)acenaphthene (MesBIAN), and 1,2-bis(2,6-diisopropyl-
phenylimino)acenaphthene (DippBIAN) were synthesized according
to literature procedures.40
cyclohexene (180 mg, 2.22 mmol); DippDAB (300 mg, 0.74 mmol).
Reaction mixture color changes: initially bright red and gradually
became dark purple. Product: light green solid powder character-
ized as 6 1/2CH2Cl2. Yield: 86% (356 mg, 0.64 mmol). 31P{1H}
3
NMR (CDCl3): δ 191.7. 1H NMR (CDCl3): δ 7.42 (t, 3JHH = 7.8,
p-CDipp), 7.25 (d, 3JHH = 7.8, m-CDipp), 5.29 (s, 0.4 H, CH2Cl2),
2.66 (m, iPr-H, 4H), 2.13 (s, 6H, CH3), 1.15 (m, 24H, iPr-CH3).
13C{1H} NMR (CDCl3): δ 146.8 (N-C), 131.1 (o-CDipp), 125.1
(m-CDipp), 124.2 (p-CDipp), 29.2 (iPr CH), 28.8 (iPr CH3), 26.1
(iPr CH3), 24.0 (CH3), 23.7 (iPr CH). HRMS: calcd for
C28H40N2Pþ 435.2929, found 435.2928 (-0.3 ppm). Anal. Calcd
NMR spectra were recorded at room temperature in CDCl3
solutions on a Bruker Advance 300 MHz spectrometer. Chemi-
cal shifts are reported in ppm, relative to external standards
for C28H40N2PBr 1/2CH2Cl2: C, 61.35; H, 7.41; N, 5.02. Found:
3
C, 61.67; H, 7.69; N, 5.02.
(
MesBIAN)P-Br (7). Reagents: PBr3 (200 mg, 0.74 mmol);
(SiMe4 for 1H and 13C, 85% H3PO4 for 31P, BF3 OEt2 for 11B,
cyclohexene (180 mg, 2.22 mmol); MesBIAN (207 mg, 0.74 mmol).
Reaction mixture color changes: gradually darkens from
orange to dark red. Product: purple solid powder characterized
3
CFCl3 for 19F, AlCl3 in water for 27Al). Coupling constant
magnitudes, |J|, are given in Hz. The high-resolution mass
spectra (HRMS) were obtained using electrospray ionization
of acetonitrile solutions of species either by The McMaster
Regional Centre for Mass Spectrometry, Hamilton, Canada,
or in house; calculated and reported mass to charge ratios are
reported for the most intense signal of the isotopic pattern.
Melting points were obtained on samples sealed in glass
capillaries under dry nitrogen using an Electrothermal melting
point apparatus. Elemental analysis was performed by Atlantic
Microlabs, Norcross, GA.
as 7 1.5CH2Cl2. Yield: 71% (334 mg, 0.53 mmol). 31P{1H} NMR
3
(CDCl3): δ 202.0. 1H NMR (CDCl3): δ 7.67 (d, 3JHH = 8.4, 2H),
7.33 (t, 3JHH = 8.1, 2H), 7.09 (s, 4H), 6.82 (d, 3JHH = 6.9, 2H),
5.30 (s, 1.3 H, CH2Cl2), 2.52 (s, 12H, o-CH3), 2.41 (s, 6H, p-CH3).
13C{1H} NMR (CDCl3): δ 139.6 (N-C), 136.5, 130.7, 130.4,
128.1, 127.9, 127.2, 120.9, 119.9, 21.6 (p-CH3), 19.8 (o-CH3).
HRMS: calcd for C30H28N2Pþ 447.1990, found 447.2003
(þ2.9 ppm). Anal. Calcd for C30H28N2PBr 1.5CH2Cl2: C, 57.78;
3
H, 4.77; N, 4.28. Found: C, 57.84; H, 5.15; N, 4.06.
Specific Procedures. General Synthetic Route to Diamino-
bromophosphines 5-8. A solution of the given R-diimine
(0.74 mmol) in CH2Cl2 (ca. 20 mL) was added to a solution of
PBr3 (0.74 mmol) and cyclohexene (2.22 mmol) in CH2Cl2
(ca. 20 mL). Upon addition, each solution undergoes a color
change specific to the diimine employed and the resulting
reaction mixture was stirred for 36 h. The volatile components
were removed under reduced pressure to afford very dark
colored foamlike solids which were covered with pentane and
sonicated for 1 h. The remaining solids were filtered and washed
with pentane, and then any remaining volatile components were
removed under reduced pressure to afford solid powders of
the diaminobromophosphines 5-8. The reactions are almost
quantitative, as assessed by 31P NMR spectroscopy, but the
purification procedure reduces the isolated yield; the specific
observations and characterization data for the materials are
detailed below. Microanalytical data and 1H NMR spectro-
scopy indicate that varying amounts of CH2Cl2 coprecipitate
with the diaminobromophosphines prepared as described. If
desired, 1H NMR experiments indicate that the dichloro-
methane solvent may be removed completely by dissolving the
solid in toluene (ca. 15 mL) and distilling the solution to
approximately half its original volume.
Crystals of 7 suitable for analysis by single-crystal X-ray
diffraction experiments were obtained by the slow evaporation
of a solution of the compound in acetonitrile.
(
DippBIAN)P-Br (8). Reagents: PBr3 (200 mg, 0.74 mmol);
cyclohexene (180 mg, 2.22 mmol); DippBIAN (373 mg, 0.74 mmol).
Reaction mixture color changes: gradually changes from orange
to dark red. Product: light brown solid 8. Yield: 83% (375 mg,
0.61 mmol). 31P{1H} NMR (CDCl3): δ 202.1. 1H NMR (CDCl3):
3
δ 7.88 (d, JHH =6.0, 2H), 7.56 (m, 2H), 7.3-7.1 (m, 6H), 6.64
(d, 3JHH = 6.3, 2H), 3.03 (m, 4H), 1.24 (d, 3JHH=6.3, 12H), 0.97
3
(d, JHH =6.7, 12H). 13C NMR (CDCl3): δ 161.0, 147.9, 135.6,
129.0, 128.0, 127.3, 125.1, 123.5, 120.8, 29.0, 28.7, 25.6, 25.0, 23.4,
23.2. HRMS: calcd for C36H40N2Pþ 531.2929, found 531.2946
(þ3.2 ppm). Anal. Calcd for C36H40N2PBr: C, 70.70; H, 6.59; N,
4.58. Found: C, 70.95; H, 7.32; N, 4.85.
Synthesis of [(MesDAB)P][PF6] (9[PF6]). To a flask containing
KPF6 (115 mg, 0.50 mmol) was added a solution of MesDABP-
Br (215 mg, 0.50 mmol) in CH3CN (40 mL). The reaction
mixture was stirred overnight before the resulting KBr was
removed by filtration. The volatile components were removed
from the filtrate under reduced pressure to give a crude product,
which was washed in pentane and sonicated for 1 h. The product
was extracted with acetonitrile, and removal of the volatile
components provided the dark red solid [(MesDAB)P][PF6].
Yield: 85% (210 mg, 0.423 mmol). 31P{1H} NMR (CDCl3):
δ -143.9 (sept, 1JP-F = 712), 199.8. 1H NMR (CDCl3): δ 7.14
(s, 4 H, m-HMes), 2.40 (s, 6H, CH3), 2.24 (s, 6H, p-CH3), 2.11
(s, 12H, o-CH3). 13C NMR (CDCl3): δ 144.93 (s, N-C), 142.14
(s, i-CMes), 134.96 (s, o-CMes), 130.84 (s, m-CMes), 129.78 (s, p-
2-Bromo-1,3-dimesityl-1,3,2-diazaphospholene ((MesDAB)P-Br,
5). Reagents: PBr3 (200 mg, 0.74 mmol); cyclohexene (180 mg,
2.22 mmol); MesDAB (236 mg, 0.74 mmol). Reaction mixture
color changes: initially bright red and gradually became
dark green. Product: brown solid powder characterized as
5
3/4CH2Cl2. Yield: 85% (310 mg, 0.63 mmol). 31P{1H} NMR
3
1
C
mes), 21.64 (s, CH3), 18.10 (s, o-CH3), 13.01 (s, p-CH3). 19F{1H}
(CDCl3): δ 188.8. H NMR (CDCl3): δ 7.03 (s, 4 H, m-HMes),
5.30 (s, 0.8 H, CH2Cl2), 2.27 (s, 6 H, CH3), 2.11 (s, 12 H, o-CH3),
2.06 (s, 6 H, p-CH3). 13C{1H} NMR (CDCl3): δ 142.4 (N-C),
141.0 (i-CMes), 136.2 (o-CMes), 131.1 (m-CMes), 129.7 (p-CMes),
21.1 (CH3) 19.2 (o-CH3), 12.8 (p-CH3). HRMS: calcd for
C22H28N2Pþ 351.1990, found 351.2005 (þ4.2 ppm). Anal. Calcd
1
NMR (CDCl3): δ -72.26 (d, JP-F = 712). HRMS: calcd for
C22H28N2Pþ 351.1990, found 351.1984 (-1.7 ppm); calcd for
PF6- 144.9642, found 144.9780 (þ95 ppm).
Synthesis of [(MesDAB)P][B(C6F5)4] (9[B(C6F5)4]). To a flask
containing LiB(C6F5)4 OEt2 (380 mg, 0.50 mmol) was added a
for C22H28N2PBr 3/4CH2Cl2: C, 55.19; H, 6.01; N, 5.66. Found:
3
3
solution of [(MesDAB)P][Br] (215 mg, 0.50 mmol) in CH3CN
(40 mL). The reaction mixture was stirred overnight before
filtration to remove LiBr. The filtrate was concentrated under
reduced pressure to give a crude product, which was washed
in pentane and sonicated for 1 h. Any remaining volatile
components were removed under reduced pressure to afford
the light brown solid [(MesDAB)P][B(C6F5)4]. Yield: 92%
(476 mg, 0.463 mmol). 31P{1H} NMR (CDCl3): δ 200.8.
1H NMR (CDCl3): δ 7.18 (s, 4 H, m-HMes), 2.42 (s, 6 H, CH3),
C, 55.08; H, 6.63; N, 5.51.
2-Bromo-1,3-bis(2,6-diisopropylphenyl)-1,3,2-diazaphospho-
lene ((DippDAB)P-Br, 6). Reagents: PBr3 (200 mg, 0.74 mmol);
(39) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518–1520.
(40) Dieck, H. T.; Svoboda, M.; Greiser, T. Z. Naturforsch., B: Chem.
Sci. 1981, 36, 823–832.

4380 Organometallics, Vol. 28, No. 15, 2009
Table 1. Summary of X-ray Crystallographic Data for (MesBIAN)P-Br (7) and [(MesDAB)P][I3] (9[I3])
MesBIAN)P-Br [(MesDAB)P][I3]
Dube et al.
(
empirical formula
formula wt
temp (K)
wavelength (A)
habit, color
cryst syst
C
527.42
174(2)
30H28BrN2P
C22H28I3N2P
732.13
174(2)
˚
0.710 73
prism, red
monoclinic
P21/c
plate, orange
orthorhombic
P212121
space group
unit cell dimens
˚
a (A)
11.4785(12)
17.6796(18)
37.784(4)
90
93.030(2)
90
7657.1(14)
12
1.373
1.694
3264
14.0085(14)
15.5780(16)
24.014(2)
90
90
90
5240.5(9)
8
1.856
3.650
2784
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
calcd density (g cm-3
)
abs coeff (mm-1
F(000)
)
θ range for data collection (deg)
limiting indices
1.08-27.50
-14 e h e 14, -22 e k e 22, -49 e l e 47
1.56-27.50
-18 e h e 18, -19 e k e 20, -30 e l e 31
no. of rflns collected
no. of indep rflns
Rint
84 740
17 321
0.3695
59 223
11 915
0.0550
abs cor
SADABS
full-matrix least squares on F2
refinement method
no. of data/restraints/params
goodness of fit on F2
final R indicesa (I > 2σ(I))
R indices (all data)
17 321/0/919
0.975
R1 = 0.1038, wR2 = 0.1603
R1 = 0.3088, wR2 = 0.2386
1.544 and -0.511
11 915/0/506
1.029
R1 = 0.0464, wR2 = 0.0900
R1 = 0.0772, wR2 = 0.1068
1.950 and -0.534
-3
˚
largest diff map peak and hole (e A
P
)
P
P
P
a R1(F) = (|Fo| - |Fc|)/ |Fo|} for reflections with Fo > 4(σ(Fo)). wR2(F2) = { w(|Fo|2 - |Fc|2)2/ w(|Fo|2)2}1/2, where w is the weight given each
reflection.
2.18 (s, 6 H, p-CH3), 2.06 (s, 12 H, o-CH3). 13C NMR (CDCl3):
(s, 6H, p-CH3), 2.12 (s, 12H, o-CH3). 13C NMR (CDCl3): δ 143.8
(N-C), 142.0 (i-CMes), 134.2 (s, o-CMes), 130.6 (s, m-CMes),
129.1 (s, p-CMes), 21.2 (s, CH3), 18.1 (s, o-CH3), 13.3 (s, p-
CH3). 27Al{1H} NMR (CDCl3): δ 80.4. HRMS: calcd for
C22H28N2Pþ 351.1990, found 351.2005 (þ4.2 ppm); calcd for
AlBr-4 342.6549, found 342.6568 (þ5.5 ppm).
δ 148.2 (d(m), JC-F = 234, C6F5 m-C), 143.4 (N-C), 142.6
(i-CMes), 133.7 (o-CMes), 130.7 (m-CMes), 21.1 (CH3), 17.3
(o-CH3), 12.2 (p-CH3). 11B{1H} NMR (CDCl3): δ -12.27.
3
19F{1H} NMR (CDCl3): δ -166.22 (t, 8F, JF-F = 17.8, m-F),
-162.42 (t, 4F, 3JF-F = 20.9, o-F), -132 (d, 8F, 3JF-F = 24.0, o-
F). HRMS: calcd for C22H28N2Pþ 351.1990, found 351.1987
(-0.9 ppm); calcd for BC24F-20 679.0358, found 678.9781
(-1.1 ppm).
Synthesis of [(MesDAB)P]“[SnCl5]” (9“[SnCl5]”). A solution
of PCl3 (100 mg, 0.74 mmol) and MesDAB (237 mg, 0.74 mmol)
in dichloromethane (40 mL) was added to a flask containing
SnCl2 (139 mg, 0.74 mmol). Upon addition the solution
turned from green to dark red. The resulting reaction mixture
was stirred overnight before the volatiles were removed
under reduced pressure to produce the dark red solid
[(MesDAB)P][SnCl5]. Crude yield: 78% (375 mg, 0.58 mmol).
Anal. Calcd for C22H28N2PSnCl5: C, 40.81; H, 4.36; N, 4.33.
Found: C, 41.60; H, 4.57; N, 4.21. 31P{1H} NMR (CDCl3): δ
199.9, 198.1 (please see the Results and Discussion); HRMS:
calcd for C22H28N2Pþ 351.1990, found 351.1989 (-0.3 ppm);
calcd for SnCl-3 224.8088, found 224.7971 (-52.0 ppm); calcd
for SnCl-5 294.7465, found 294.7616 (þ51.2 ppm).
Synthesis of [(MesDAB)P][OTf] (9[OTf]). To a flask contain-
ing trimethylsilyltrifluoromethanesulfonate (TMS-OTf; 111 mg,
0.50 mmol) was added a solution of [(MesDAB)P][Br] (215 mg,
0.50 mmol) in CH2Cl2 (40 mL). The reaction mixture was
stirred overnight. The volatile components were removed under
reduced pressure to give a crude product, which was washed in
pentane and sonicated for 1 h. The product was extracted with
acetonitrile, and removal of the volatile components provided
the dark brown solid [(MesDAB)P][OTf]. Yield: 90% (224 mg,
0.448 mmol). 31P{1H} NMR (CDCl3): δ 199.6. 1H NMR
(CDCl3): δ 7.14 (s, 4 H, m-HMes), 2.40 (s, 6 H, CH3), 2.28 (s,
12 H, o-CH3), 2.12 (s, 6 H, p-CH3). 13C NMR (CDCl3): δ 144.90
(s, N-C), 142.15 (s, i-CMes), 134.89 (s, o-CMes), 130.84 (s, m-
Synthesis of [(MesDAB)P][I3] (9[I3]). To a flask containing PI3
(200 mg, 0.485 mmol) was added a solution of MesDAB (155 mg,
0.485 mmol) in toluene (40 mL). Upon addition the solu-
tion turned from red to dark red. The resulting reaction mixture
was stirred overnight before the volatiles were removed under
reduced pressure to produce the dark red solid [(MesDAB)P][I3].
Recrystallization by slow evaporation in acetonitrile yielded red
crystals suitable for analysis by single-crystal X-ray diffraction.
Crude yield: 76% (270 mg, 0.369 mmol). 31P{1H} NMR
(CDCl3): δ 201.78. 1H NMR (CDCl3): δ 7.18 (s, 4 H, m-HMes),
C
Mes), 129.78 (s, p-CMes), 21.62 (s, CH3), 18.17 (s, o-CH3), 13.23
(s, p-CH3). 19F{1H} NMR (CDCl3): δ -78.62. HRMS: calcd for
C22H28N2Pþ 351.1990, found 351.1996 (þ1.7 ppm); calcd for
SO3CF-3 148.9520, found 148.9662 (þ96 ppm).
Synthesis of [(MesDAB)P][AlBr4] (9[AlBr4]). To a flask con-
taining AlBr3 (95 mg, 0.357 mmol) was added a solution of
[(MesDAB)P][Br] (154 mg, 0.357 mmol) in toluene (40 mL).
The volatile components were removed from the filtrate under
reduced pressure to give a crude product, which was washed in
pentane and sonicated for 1 h. The product was extracted with
acetonitrile, and removal of the volatile components provided
the dark purple solid [(MesDAB)P][AlBr4]. Crude yield: 72%
2.43 (s, 6H, CH3), 2.31 (s, 6H, p-CH3), 2.18 (s, 12H, o-CH3). 13
C
NMR (CDCl3): δ 142.80 (s, N-C), 141.99 (s, i-CMes), 134.33
(s, o-CMes), 130.81 (s, m-CMes), 129.40 (s, p-CMes), 21.49 (s,
CH3), 18.71 (s, o-CH3), 13.62 (s, p-CH3). HRMS: calcd for
C22H28N2Pþ 351.1990, found 351.1973 (-4.9 ppm); calcd for I3-
380.7134, found 380.7133 (-0.3 ppm). Anal. Calcd for
(180 mg, 0.258 mmol). 31P{1H} NMR (CDCl3): δ 197.89. H
1
NMR (CDCl3): δ 7.16 (s, 4 H, m-HMes), 2.41 (s, 6 H, CH3), 2.28

Article
Organometallics, Vol. 28, No. 15, 2009 4381
C22H28N2PI3: C, 36.09; H, 3.85; N, 3.83. Found: C, 38.32; H,
4.12; N, 3.91.
Scheme 3. Quantitative Synthetic Approach to a Chelated PI
Bromide Salt38
X-ray Crystallography. Each crystal was covered in Nujol and
placed rapidly into a cold N2 stream of the Kryo-Flex low-
temperature device. The data were collected using the
SMART41 software package on a Bruker APEX CCD diffract-
ometer employing a graphite-monochromated Mo KR radia-
˚
tion (λ=0.710 73 A) source. Hemispheres of data were collected
using counting times of 10-30 s per frame at -100 ꢀC. The
details of crystal data, data collection, and structure refinement
are given in Table 1. Data reductions were performed using the
SAINT42 software package, and the data were corrected for
absorption using SADABS.43 The structures were solved by
direct methods using SIR9744 and refined by full-matrix least-
squares on F2 with anisotropic displacement parameters for the
non-H atoms using SHELXL-9745 and the WinGX46 software
package, and thermal ellipsoid plots were produced using
SHELXTL.45
Scheme 4. Redox Cycloaddition Preparation of N-Heterocyclic
Bromophosphines using Aryl-DAB ligands: 5 (Ar = Mes) and 6
(Ar = Dipp)
formal transfer of two electrons from the putative PI center
to the diimine and thus produces the observed PIII center
ligated by the corresponding unsaturated diamido dianion.
It should be emphasized that mechanistic investigations on
related phosphorus(þ1) systems with nonreducible ligands
such as diphosphines indicate that such reactions most likely
occur in a stepwise manner and the redox process occurs
after the phosphorus atom is chelated by the ligand;47 it is
probable that the reactions described above occur in a
similar manner.
As Cowley and co-workers had demonstrated that related
redox cycloaddition reactions occur with the extended aro-
matic R-diimines of the BIAN family,33,48 we also investi-
gated the reactivity of the “PI-Br” synthon with two aryl-
BIAN ligands; they both produce the desired bromopho-
sphines 7 and 8 in quantitative yield according to 31P NMR
spectroscopy. All of the other spectroscopic and spectro-
metric methods were consistent with the proposed formula-
tions and warrant no additional comment. In the case of the
mesitylene-substituted BIAN ligand, we were able to obtain
a solid from the recrystallization of 7 in acetonitrile that was
suitable for examination by single-crystal X-ray diffraction
studies. Although the data from even the best of the crystals
that we examined was of low quality, the solution and
refinement of the structure confirms the identity and con-
nectivity of the compound.
Results and Discussion
Recently, we reported that the reaction of PBr3 with
chelating diphosphines such as 1,2-bis(diphenylphosphino)-
ethane (dppe) produces the corresponding PI bromide salt in
quantitative yield with the concomitant elimination of 1,2-
dibromocyclohexane, as illustrated in Scheme 3.38 The cy-
clohexene behaves as a sequestering agent for the 1 equiv of
Br2 that is formally eliminated in the redox process, and the
volatility of both cyclohexene and 1,2-dibromocyclohexane
greatly simplifies the purification and isolation of the salts
prepared using this approach.
In light of the successful redox cycloaddition syntheses of
NHP salts using the PI synthons described above, we rea-
soned that a similar approach might be suitable for the
preparation of N-heterocyclic bromophosphines. Thus, the
treatment of a solution containing an R-diimine of the
general type 1,4-bis(aryl)-2,3-dimethyl-1,4-diazabutadiene
with a solution containing 1 equiv of PBr3 in the presence
of excess cyclohexene results in the formation of the corre-
sponding N-heterocyclic bromophosphines, as illustrated
in Scheme 4. Test reactions confirmed that the starting
diimines do not undergo cycloaddition reactions, or exhi-
bit any other observable reactivity, with cyclohexene. The
resultant materials were completely characterized by
multinuclear NMR spectroscopy, mass spectrometry, and
elemental analysis and confirm the formation of the anti-
cipated cyclic diaminobromo-phosphines (MesDAB)P-Br 5
and (DipDAB)P-Br 6. It should be noted that, in the absence
of cyclohexene, the reaction of PBr3 and the R-diimines is
considerably slower and generates several P-containing pro-
ducts, including modest amounts of the diaminobromopho-
sphines.
The formation of the cyclic diaminobromophosphines is
consistent with the formal generation of the synthon “PI-
Br”, with the concomitant elimination of 1,2-dibromocyclo-
hexane (identified by multinuclear NMR spectroscopy),
which undergoes a formal 4 þ 2 electron cycloaddition with
the R-diimine. This cycloaddition process results in the
The bromophosphine 7 crystallizes in the space group P21/c
with three crystallographically independent molecules pre-
sent in the asymmetric unit, as illustrated in Figure 1. The
three independent molecules adopt very similar structures
and, in each case, exhibit metrical parameters (Table 2) that
are consistent with the BIAN ligand having been reduced
during the progress of the reaction. For example, the en-
(41) SMART; Bruker AXS, Madison, WI, 2001.
(42) SAINTPlus; Bruker AXS, Madison, WI, 2001.
(43) SADABS; Bruker AXS, Madison, WI, 2001.
(44) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.;
Spagna, R. J. Appl. Crystallogr. 1999, 32, 115–119.
(45) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112–122.
(46) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
˚
docyclic C-N bonds range from 1.380(10) to 1.402(10) A, in
comparison to the distance of 1.2662(16) A reported for the
˚
(47) Dillon, K. B.; Monks, P. K. Dalton Trans. 2007, 1420–1424.
(48) Hill, N. J.; Vargas-Baca, I.; Cowley, A. H. Dalton Trans. 2009,
240–253.

4382 Organometallics, Vol. 28, No. 15, 2009
Dube et al.
Figure 1. Asymmetric unit for 7 showing (a) the three crystallographically independent molecules and (b) a thermal ellipsoid plot (30%
probability surface) illustrating the numbering scheme employed for each molecule. In each instance, hydrogen atoms are omitted for
clarity.
Table 2. Selected Metrical Parameters for Bromophosphine 7 and NHP Salt 9[I3]a
7
9[I3]
param
molecule 1
molecule 2
molecule 3
molecule 1
molecule 2
Distances
P(x)-Br(x)
2.432(3)
1.673(7)
1.675(7)
1.399(10)
1.402(10)
1.350(11)
2.452(3)
1.685(7)
1.692(7)
1.380(10)
1.389(10)
1.362(10)
2.506(3)
1.680(7)
1.696(7)
1.387(9)
1.398(10)
1.348(11)
P(x)-N(x1)
P(x)-N(x2)
C(x1)-N(x1)
C(x2)-N(x2)
C(x1)-C(x2)
I(x1)-I(x2)
I(x2)-I(x3)
1.665(5)
1.667(6)
1.376(8)
1.371(8)
1.356(8)
2.9814(9)
2.8632(9)
1.658(5)
1.680(6)
1.380(8)
1.370(8)
1.359(9)
2.8821(8)
2.9762(8)
Angles
N(x1)-P(x)-N(x2)
91.2(3)
298.4(5)
91.2(3)
295.7(5)
91.7(3)
296.8(5)
88.5(3)
89.0(3)
P
—@P
I(x1)-I(x2)-I(x3)
a Distances are reported in angstroms and angles in degrees.
177.22(2)
179.10(2)
free ligand,49 and the endocyclic C-C distances for the five-
membered ring range from 1.348(11) to 1.362(10) A, in
Scheme 5. “Bond-No-Bond” Resonance Postulated for Some
Substituted NHP Derivatives (X = H, Cl, Br, PR2, etc.)
˚
˚
comparison to the distance of 1.528(2) A in the uncomplexed
ligand. Furthermore, the C-N and C-C bond distances in 7
are also only marginally different from those observed in the
related cations in the salts [(DippBIAN)P][A] (A = SnCl5
3
THF, I3),33 which have C-N distances ranging from
˚
1.351(5) to 1.366(5) A and C-C distances of 1.395(5) and
2.282(1)-2.324(1) A).51,52 The long bonds in the bromopho-
˚
˚
˚
1.380(8) A; the P-N distances of 1.673(7)-1.696(7) A in 7
are also consistent with the P-N distances in the cations of
sphine are consistent with the long phosphorus-element
bonds observed for NHP-H, NHP-Cl, and other compounds
containing the unsaturated NHP core. The unusual length of
these bonds has been explained as arising from the hyper-
conjugation of the π system on the NHP core with the
antibonding σ* orbital of the phosphorus-element bond,
which increases the relative weight of the “no-bond” cano-
nical structure in the putative “bond-no-bond” resonance
˚
1.694(4)-1.700(4) A.
There are surprisingly few compounds that contain the
N2PIII-Br moiety reported in the Cambridge Structural
Database (CSD)50 for comparison, and the P-Br distances
˚
in 7 (2.432(3)-2.506(3) A) are very long in relation to the
P-Br distances in those diamidobromophosphines (range:
(49) El-Ayaan, U.; Murata, F.; El-Derby, S.; Fukuda, Y. J. Mol.
Struct. 2004, 692, 209–216.
(50) Allen, F. H. Acta Crystallogr., Sect. B: Struct. Sci. 2002, 58, 380–
(51) Chivers, T.; Fedorchuk, C.; Schatte, G.; Brask, J. K. Can. J.
Chem. 2002, 80, 821–831.
(52) Burford, N.; Conroy, K. D.; Landry, J. C.; Ragogna, P. J.;
388.
Ferguson, M. J.; McDonald, R. Inorg. Chem. 2004, 43, 8245–8251.

Article
Organometallics, Vol. 28, No. 15, 2009 4383
Scheme 6. Bromide Abstraction Reactions of 5 That Produce NHP Salts of 9
scheme illustrated in Scheme 5.4,53 In spite of the poor quality
of the data, it is perhaps also worth noting that the specific
P-Br distances in the three independent molecules of 7 in the
the proposed formula for 9[SnCl5], we were surprised to
observe two distinct singlet signals in the solution 31P NMR
spectrum at 200 and 198 ppm. Each of these signals is
consistent with the cation 9, and we hypothesized that the
close interaction of two different anions with the cation may
explain the observation of two signals.20 In fact, negative ion
mode ESI mass spectra of solutions of “9[SnCl5]” contain
signal manifolds that confirm the presence of both [SnCl-3 ]
and [SnCl-5 ] anions and are consistent with our supposition.
The reason for the unexpected, but reproducible, behavior
remains unclear; however, it is tangential to the work pre-
sented herein and was not pursued further. Thus, in order to
obtain crystalline material containing 9 suitable for single-
crystal X-ray diffraction experiments, we employed the other
redox approach involving the disproportionation of phos-
phorus triiodide in the presence of the diimine. Gratifyingly,
the corresponding phosphenium triiodide salt 9[I3] is ob-
tained quantitatively by the treatment of MesDAB with PI3 in
dichloromethane and concentration of a solution of the salt
in acetonitrile did indeed produce crystals suitable for X-ray
crystallography.
crystal structure appear to correlate qualitatively to the
P
number of close contacts (within rvdw) to the phosphorus
and bromine atoms in the given molecule: there are three
close contacts to the molecule with the longest bond, two for
the molecule with the intermediate distance, and one for the
molecule with the shortest bond.
To confirm the applicability of the N-heterocyclic bromo-
phosphines obtained using the method outlined above for
the preparation of NHP salts, we examined the reaction of 5
with a selection of common anion metathesis or halide
abstraction agents. As illustrated in Scheme 6, the treatment
of a solution of 5 in either methylene chloride or acetonitrile
with a solution containing an equimolar quantity of AlBr3
results in the rapid and quantitative formation of the NHP
salt [(MesDAB)P][AlBr4] (9[AlBr4]), as indicated by multi-
nuclear NMR experiments. Similarly, anion metathesis re-
actions between 5 and K[PF6] or Li[B(C6F5)4 result in the
complete loss of the 31P NMR resonance at δ 189 ppm for 5
and the observation of a new signal at δ 200 ppm for the
uncoordinated phosphenium cation 9. Finally, the treatment
of 5 with Me3Si-OTf in dichloromethane produced 9[OTf],
as indicated by the 31P resonance at δ 200 ppm.
We have been unable as of yet to obtain crystalline samples
of the NHP salts of 9 derived from the brominated precursor
5 and, although the spectroscopic investigations confirm the
formation of the desired products, we desired structural
authentication of the cation. In an attempt to obtain such
information, we employed the direct NHP redox syntheses
reported previously. Thus, the diimine MesDAB was treated
with PCl3 and SnCl2 in CH2Cl2 in an attempt to produce
9[SnCl5]. While the microanalytical data are consistent with
The salt 9[I3] crystallizes in the space group P212121 with
two pairs of crystallographically independent cations and
anion in theasymmetric unit, as illustrated in Figure2. As one
would anticipate, the metrical parameters (also in Table 2) in
the cation are completely consistent with those reported for
related unsaturated NHP cations and do not mandate ex-
tensive commentary. For example, the P-N distances, which
˚
range from 1.655(5) to 1.680(6) A, are indistinguishable from
˚
the P-N distance of 1.665(3) A reported for the related
triiodide salt in which the carbon atoms in the five-membered
ring are substituted with hydrogen rather than with methyl
groups.54 The metrical parameters of the triiodide anions are
likewise completely typical and are consistent with those
anticipated for salts in which there is not extensive interaction
between the cations and the anions.
(53) Kibardin, A. M.; Litvinov, I. A.; Naumov, V. A.; Struchkov, I.
T.; Griaznova, T. V.; Mikhailov, I. B.; Pudovik, A. N. Dokl. Akad. Nauk
SSSR 1988, 298, 369–373.
(54) Reeske, G.; Cowley, A. H. Inorg. Chem. 2007, 46, 1426–1430.

4384 Organometallics, Vol. 28, No. 15, 2009
Dube et al.
Figure 2. Depiction of the asymmetric unit for [MesNHP][I3] showing (a) the two crystallographically independent ion pairs and (b) a
thermal ellipsoid plot (30% probability surface) illustrating the numbering scheme employed for each molecule. In each instance,
hydrogen atoms are omitted for clarity.
While the details of the metrical parameters of the salt 9[I3]
are relatively unimportant, the structure clearly contains the
cation postulated to exist in the products of the halide-
abstraction reactions described above. Importantly, all of
the spectroscopic data for the cation in 9[I3] are identical with
those observed for the salts generated from the bromopho-
sphine reagent 5 obtained using our new PBr3/cyclohexene
synthetic approach. Thus, the crystallographic results con-
firm the spectroscopic identification and illustrate the viabi-
lity of our new synthetic approach.
The investigations outlined above indicate clearly that the
redox-cycloaddion methodology may be employed for the
convenient synthesis of unsaturated diaminobromopho-
sphine precursors to N-heterocyclic phosphenium cations.
Given that salt metathesis, which is often the most important
and sometimes only synthetic approach to many inorganic or
organometallic compounds, does not work well for the
preparation of unsaturated NHP precursors, the mild and
simple redox approach described above appears to provide
an effective alternative route to this increasingly important
class of reagent.
generally, this reaction adds to the growing number of
syntheses involving the cycloaddition reactions of halogenated
low-oxidation-state precursors and synthons from throughout
the p block55-60 with suitable chelating ligands; such reactions
provide synthetic alternatives to metathesis chemistry for the
preparation of inorganic heterocyclic ligands.
Acknowledgment. We thank the Natural Science and
Engineering Research Council (NSERC) of Canada for
funding and a scholarship (E.L.N.). We thank the
Canada Foundation for Innovation, the Ontario Innova-
tion Trust, and the Ontario Research and Development
Challenge Fund for support of the University of
Windsor’s Centre for Catalysis and Materials Research.
C.L.B.M. thanks the Province of Ontario’s Ministry of
Research and Innovation for an Early Researcher Award
(ERA). We thank Kirk M. Green (McMaster) and
Shangquan Zhang (Windsor) for the collection of mass
spectra.
Supporting Information Available: CIF files giving crystal-
lographic data. This material is available free of charge via the
Internet at http://pubs.acs.org.
Conclusions
(55) West, R.; Moser, D. F.; Guzei, I. A.; Lee, G. H.; Naka, A.; Li,
W. J.; Zabula, A.; Bukalov, S.; Leites, L. Organometallics 2006, 25,
2709–2711.
(56) Dutton, J. L.; Tindale, J. J.; Jennings, M. C.; Ragogna, P. J.
Chem. Commun. 2006, 2474–2476.
(57) Dutton, J. L.; Tuononen, H. M.; Jennings, M. C.; Ragogna, P. J.
J. Am. Chem. Soc. 2006, 128, 12624–12625.
(58) Dutton, J. L.; Sutrisno, A.; Schurko, R. W.; Ragogna, P. J.
The treatment of R-diimines with PBr3 in the presence
of excess cyclohexene produces the corresponding cyclic
diaminobromophosphines in good yield. In each instance,
the results are consistent with product being derived
from the formal 4 þ 2 cycloaddition and electron transfer
between the R-diimine with a putative “PI-Br” fragment.
For one of the R-diimines, the resultant diaminobromopho-
sphine was shown to be a useful reagent for the generation
of the corresponding N-heterocyclic phosphenium salts
by several common methods of anion abstraction. More
Dalton Trans. 2008, 3470–3477.
(59) Martin, C. D.; Jennings, M. C.; Ferguson, M. J.; Ragogna, P. J.
Angew. Chem., Int. Ed. 2009, 48, 2210–2213.
(60) Dutton, J. L.; Martin, C. D.; Sgro, M. J.; Jones, N. D.; Ragogna,
P. J. Inorg. Chem. 2009, 48, 3239–3247.