Synthesis, reactivity, and computational analysis of halophosphines supported by dianionic guanidinate ligands

Article abstract of DOI:10.1021/ja300587z

The reported chemistry and reactivity of guanidinate supported group 15 elements in the +3 oxidation state, particularly phosphorus, is limited when compared to their ubiquity in supporting metallic elements across the periodic table. We have synthesized a series of chlorophosphines utilizing homo- and heteroleptic (dianionic)guanidinates and have completed a comprehensive study of their reactivity. Most notable is the reluctancy of these four-membered rings to form the corresponding N-heterocyclic phosphenium cations, the tendency to chemically and thermally eliminate carbodiimide, and the scarcely observed ring expansion by insertion of a chloro(imino)phosphine into a P-N bond of the P-N-C-N framework. Computational analysis has provided corroborating evidence for the unwillingness of the halide abstraction reaction by demonstrating the exceptional electron acceptor properties of the target phosphenium cations and the underscoring strength of the P-X bond.

Full text of DOI:10.1021/ja300587z

Article
pubs.acs.org/JACS
Synthesis, Reactivity, and Computational Analysis of Halophosphines
Supported by Dianionic Guanidinate Ligands
Allison L. Brazeau,† Mikko M. Hanninen,‡ Heikki M. Tuononen,‡ Nathan D. Jones,†
̈
and Paul J. Ragogna*,†
†Department of Chemistry, The University of Western Ontario, Chemistry Building, 1151 Richmond Street, London, N6A 5B7
Ontario, Canada
‡Department of Chemistry, University of Jyvaskyla, P.O. Box 35, FI-40014 Finland
̈
̈
S
* Supporting Information
ABSTRACT: The reported chemistry and reactivity of
guanidinate supported group 15 elements in the +3 oxidation
state, particularly phosphorus, is limited when compared to their
ubiquity in supporting metallic elements across the periodic table.
We have synthesized a series of chlorophosphines utilizing
homo- and heteroleptic (dianionic)guanidinates and have
completed a comprehensive study of their reactivity. Most
notable is the reluctancy of these four-membered rings to form
the corresponding N-heterocyclic phosphenium cations, the
tendency to chemically and thermally eliminate carbodiimide, and the scarcely observed ring expansion by insertion of a
chloro(imino)phosphine into a P−N bond of the P−N−C−N framework. Computational analysis has provided corroborating
evidence for the unwillingness of the halide abstraction reaction by demonstrating the exceptional electron acceptor properties of
the target phosphenium cations and the underscoring strength of the P−X bond.
CO2,12,13 unsaturated bonds,10 and even catalytic, metal-free
INTRODUCTION
■
hydrogenation.14−18
A major theme in the field of synthetic main group chemistry is
to push the limits of the bonding environments surrounding a
central p-block element and impose a stress that will then allow
for access to unprecedented yet controlled reactivity. There are
many variables associated with forcing such a condition within a
molecule that include modifications to the electronics, sterics,
charge, and ring strain. Over the past decade there has been a
shift in research efforts to design main group molecules that are
capable of reactivity previously reserved for the transition
metals. Highlights include the ability of “frustrated Lewis pairs”,
low-valent group 13 species, and heavy group 14 analogues of
N-heterocyclic carbenes, alkenes, and alkynes to activate small
molecules, including H2, NH3, PH3, and P4.
The silylene LSi (L = CH{(CCH2)(CMe)(2,6-iPr2C6H3N)2})
will oxidatively add ammonia to give four-coordinate LSi(H)-
19
NH2 and is the only reported main group molecule that has
been successfully used in the activation of PH3, yielding
LSi(H)PH2.20
Phosphenium cations [R2P]+ are a class of phosphorus-
containing molecules that have a rich history of reactivity,
including reactions with Lewis bases21−23 and inorganic and
organic unsaturated bonds,24 and acting as ligands on transition
metals.25−32 Phosphenium cations are also known to catenate,
which is demonstrated by the many examples from Burford et
al. in which a [R2P]+ fragment is inserted into cyclic and acyclic
P−P bonds, to expand the ring or chain by an additional P
atom to give catena-polyphosphorus cations.33 Weigand has
recently shown the ability of phosphenium cations to activate
small molecules by novel examples of controlled reactivity with
P4 using a N-heterocyclic phosphenium cation (NHP) sup-
ported within a strained four-membered ring system to form
cationic P5 clusters.34,35 Substitution reactions on acyclic
cations of the type [LPCl2]+ and [L2PCl]2+ to replace the Cl
with CN- or N3-groups have also been accomplished by this
same group.36
Power et al. were the first to demonstrate the bifurcation of
dihydrogen by a main group metal, with the addition of H2
across the GeGe bond in unsaturated digermynes at ambient
temperature and pressure to give a combination of digermenes
and germanes.1 Several years later he also showed the addition
of dihydrogen to distannynes to give tin(II) hydrides.2−8
A
notable surge in the reported reactivity of “frustrated Lewis
pairs” (FLP) was kickstarted by Stephan et al., where they re-
ported metal-free reversible hydrogen activation by the un-
quenched reactivity of a phosphine-borane ((C6H2Me3)2P-
(C6F4)B(C6F5)2), which features a Lewis acidic boron and
Lewis basic phosphorus.9 Since then, many different FLP
systems have been developed,10 and reactivity with a variety of
substrates has been observed, including B−H bonds,11
The phosphenium cation in particular presents a unique en-
vironment where the molecule is Lewis amphoteric given that
Received: January 18, 2012
Published: February 21, 2012
© 2012 American Chemical Society
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Chart 1. (a) NHC versus NHP and (b) Known and Target Four-Membered NHP Species
the cationic phosphorus atom can be an electron donor or elec-
tron acceptor, which opens the floodgates for unique structures,
bonding arrangements, and reactivity. The NHP is isovalent
with the N-heterocyclic carbene (NHC) but has inverse
electronic properties, in that they are poor σ-donors but have
excellent π-accepting capabilities (Chart 1a).
We envisaged designing an NHP within a highly strained
four-membered ring so that the phosphorus atom could be
probed for new and unobserved reactivity. While several four-
membered rings incorporating NHP are known (Chart 1b),
they are all of the type P−N−X−N where X = P,34,37 Si, or
Al,35,38 all electropositive elements; however, the example
where X = C has surprisingly been absent from the literature.
We sought to extend this series and fill the void by using
the nitrogen-based, chelating, guanidinate ligand (Chart 2a,b);
chlorophosphine (2,4,6-trimethylphenyl = Mes) has been
studied extensively. We have explored the tendency of these
compounds to retain the halogen and therefore the reluctancy
to form the “free” NHP. A base-stabilized NHP however was
easily accessible, and computational analysis has provided
insight on this rather counterintuitive result and gave valuable
data on the nature of the electronics about the phosphorus
atom, and why these particular systems are resistant to halide
abstraction. Novel chemically instigated carbodiimide elimi-
nation was observed in conjuction with metal coordination, re-
sulting in the first structurally characterized metal complex with
a cationic iminophosphine ligand. A study between chloro-
phosphine and a gentle one-electron reductant was explored to
give the clean synthesis of the reductively coupled product fea-
turing a dimeric structure with μ-N,N′ bridging guanidinates
and a P−P bond. Upon investigating the thermal stability of the
tris(Mes)guanidinato chlorophosphine, we discovered the
thermally induced ejection of carbodiimide and subsequent
insertion of chloro(imino)phosphine into the P−N bond of the
initial diaminochlorophosphine resulting in a new ring
expansion product with a μ-N,N′ bridging guanidinate and a
μ-bridging N-Mes. This ring expansion chemistry is extremely
rare within P(III)−N chemistry. These observations collectively
give a deeper perception on the nature and reactivity of
diaminochlorophosphines constrained in a four-membered ring.
Chart 2. (a) Monoanionic versus (b) Dianionic Guanidinate
Ligands, and (c) Amidinate Ligands
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were performed in an
inert atmosphere in a nitrogen filled MBraun Labmaster dp glovebox
or by using standard Schlenk techniques unless stated otherwise.
Reagents were obtained from commercial sources. Triethylamine was
distilled from CaH2, and phosphorus(III) chloride and phosphorus-
(III) bromide were distilled prior to use, while all other reagents were
used without further purification. All solvents were dried using an
MBraun controlled atmospheres solvent purification system and stored
in Straus flasks under an N2 atmosphere or over 4 Å molecular sieves
in the glovebox. Chloroform-d was dried over CaH2, distilled prior to
use, and stored in the glovebox over 4 Å molecular sieves. The syn-
thesis of N,N′-bis(2,6-diisopropylphenyl)carbodiimide (2,6-diisopro-
pylphenyl = Dipp),46 N,N′-bis(Mes)carbodiimide,46 N,N′,N″-
tris(Mes)guanidine44 (1), N,N′,N″-tris(Dipp)guanidine47 (2), and
N,N′,N″-tris(cyclohexyl)guanidine48 (5) followed literature procedures.
1H, 13C{1H}, 19F{1H}, and 31P{1H} data were collected on a 400 MHz
despite its widespread use for supporting metallic elements
spanning the periodic table, little is known about the role in
stabilizing group 15 centers in the +3 oxidation state. Jones et
al. have synthesized several (monoanionic)guanidinate
pnictogen(III) dihalides (pnictogen = Pn = P, As, Sb) as
precursors to amido-dipnictenes (Chart 3c).39 Related
(monoanionic)amidinate ligands (Chart 2c) have been used
in stabilizing heavier group 15 atoms (As, Sb, Bi)40−43 as shown
in Chart 3a,b. We have previously reported a (monoanionic)-
guanidinate ligand capable of supporting a dicationic arsenic
center with additional stabilization from a Lewis base (Chart 3c).44
For the synthesis of an NHP it would be ideal to have the “free”
cation, which therefore calls for the use of a (dianionic)-
guanidinate (Chart 2b). There is only one previous report of a
(dianionic) guanidinate being used to support a group 15 atom
involving antimony45 (Chart 3d), and one example of a phos-
phorus center39 being supported by a guanidinate ligand.
In this context, we report the first synthesis and comprehensive
characterization of a series of chlorophosphines supported by
homo- and heteroleptic (dianionic)guanidinates. The reactivity
of the model compound tris(2,4,6-trimethylphenyl)guanidinato
1
Varian Inova spectrometer (399.762 MHz for H, 100.52 MHz for
13C, 376.15 for 19F, and 161.825 MHz for 31P). Spectra were recorded
at room temperature (rt), unless otherwise indicated, in CDCl3 using
the residual protons of the deuterated solvent for reference and are
listed in ppm, with coupling constants listed in Hz. Phosphorus and
fluorine NMR spectra were recorded unlocked relative to an external
standard (85% H3PO4, δP = 0.00; CF3C6H5, δF = −63.9). Single crystal
X-ray diffraction data were collected on a Nonius Kappa-CCD area
detector or a Bruker Apex II-CCD detector using Mo Kα radiation
(λ = 0.710 73 Å) and at a temperature of 150(2) K. Crystals were
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Chart 3. Literature Examples of (a) Monomeric (Monoanionic)amidinate, (b) Dimeric (Monoanionic)amidinate, (c)
(Monoanionic)guanidinate, and (d) (Dianionic)guanidinate Ligands
3
selected under Paratone-N oil, mounted on MiTeGen micromounts or
nylon loops, and then immediately placed in a cold stream of N2.
Structures were solved and refined using SHELXTL. FT-IR spectra
were collected on samples as KBr pellets using a Bruker Tensor 27
spectrometer, with a resolution of 4 cm−1. Samples for FT-Raman
spectroscopy were packed in capillary tubes and flame-sealed. Data was
collected using a Bruker RFS 100/S spectrometer, with a resolution of
4 cm−1. Melting points (Mp) and decomposition points were recorded
in flame-sealed capillary tubes using a Gallenkamp variable heater.
High resolution mass spectrometry (HRMS) was collected using a
Finnigan MAT 8200 instrument.49 Elemental analyses (C, H, N) were
(m, 2H, CH(CH3)2), 2.41 (d, 3H, CH(CH3)2, JH−H = 4.8), 2.37 (d,
3
3H, CH(CH3)2, JH−H = 6.0), 2.30 (s, 3H, CH3), 2.25 (s, 3H, CH3),
1.39 (d, 3H, CH(CH3)2, 3JH−H = 6.8), 1.35 (d, 3H, CH(CH3)2, 3JH−H
=
7.2), 1.11 (m, 3H, CH3). 13C{1H} NMR: δ 148.3, 147.2, 146.9, 146.4,
145.5, 144.4, 144.2, 144.0, 141.2, 137.8, 137.6, 136.6, 136.5, 136.3,
133.5, 133.3, 133.2, 132.3, 132.2, 130.9, 129.7, 129.6, 129.2, 129.1,
128.9, 127.9, 123.9, 123.4, 123.0, 122.4, 28.5, 28.1, 25.1, 24.0, 23.5,
22.5, 21.1, 20.9, 18.8, 18.6. FT-IR (cm−1 (ranked intensity)): 551(10),
693(11), 757(6), 799(8), 863(4), 1034(12), 1104(13), 1151(14),
1222(9), 1292(7), 1378(15), 1473(5), 1650(1), 2963(2), 3399(3).
FT-Raman (cm−1 (ranked intensity)): 142(5), 236(14), 266(13),
572(3), 689(12), 883(15), 1159(10), 1255(8), 1306(6), 1381(4),
1444(11), 1608(2), 1650(7), 2867(9), 2919(1). HRMS: C31H41N3
calcd (found) 455.3300 (455.3298). Anal. Calcd for C31H41N3: C
81.71, H 9.07, N 9.22. Found: C 81.23, H 9.28, N 9.28.
́
performed by Laboratoire d’Analyze Elem
́ ́
entaire de l’Universite de
Montreal, Montreal, QC, Canada.
́
́
Synthesis. General Synthesis for Guanidine. After cooling a
THF solution (15 mL) of aniline to 0 °C, n-butyllithium (nBuLi)
was added to the cooled solution and stirred for 20 min. The
yellow solution was allowed to warm to room temperature.
A THF solution (6 mL) of carbodiimide was added to the
solution via cannula transfer causing the solution to turn
orange. The reaction mixture was refluxed for 2 h. After cooling
to rt, H2O (1.5 mL) was added dropwise, resulting in a white
precipitate from the dark orange solution. The reaction mixture
was dried over MgSO4 and filtered. The filtrate was dried by
rotary evaporation, resulting in an orange wax. Recrystallization
from hexanes at −30 °C yielded a white powder.
Compound 4 was prepared from 2,4,6-trimethylaniline (1.4 mL,
9.96 mmol), BuLi (2.0 M in cyclohexane, 6.0 mL, 12.0 mmol), and
n
N,N′-bis(Dipp)carbodiimide (3.58 g, 9.87 mmol). Yield: 33%. Mp:
156−160 °C. X-ray quality colorless crystals were obtained from a
1
concentrated CH3CN solution after 5 days at −20 °C. H NMR: δ
7.33−7.11 (5H, aryl), 7.01−6.86 (3H, aryl), 5.00 (s, 1H, NH), 4.74
(br, 1H, NH), 3.64−3.36 (m, 4H, CH(CH3)2), 2.42 (s, 2H, CH3),
2.36 (s, 2H, CH3), 2.29 (s, 3H, CH3), 2.25 (d, 2H, CH(CH3)2, 3JH−H
=
9.2), 1.38−1.04 (24H, CH3). 13C{1H} NMR: δ 148.4, 148.3, 147.4,
147.3, 147.2, 146.3, 145.6, 144.2, 143.9, 141.5, 141.1, 137.8, 136.8,
136.4, 133.8, 133.5, 132.8, 132.5, 132.2, 131.7, 130.9, 129.7, 129.0,
128.9, 127.9, 127.7, 124.1, 123.8, 123.5, 123.1, 123.0, 122.5, 28.9, 28.8,
28.6, 28.5, 28.4, 28.0, 25.8, 25.3, 24.8, 24.1, 23.6, 23.1, 23.0, 22.2, 21.0,
20.8, 18.5, 18.3. FT-IR (cm−1 (ranked intensity)): 764(5), 800(8),
849(7), 935(10), 1059(15), 1109(9), 1255(13), 1295(6), 1361(11),
1383(14), 1493(4), 1586(12), 1647(1), 2962(2), 3409(3). FT-Raman
(cm−1 (ranked intensity)): 147(6), 275(8), 445(14), 574(3), 676(9),
Specific Procedures for Guanidines. Compound 3 was prepared
n
from 2,6-diisopropylaniline (90%, 2.3 mL, 11.0 mmol), BuLi (2.0 M
in cyclohexane, 6.6 mL, 13.2 mol), and N,N′-bis(Mes)carbodiimide
(3.06 g, 11.0 mmol). Yield: 58%. Mp: 146−149 °C. X-ray quality
colorless crystals were obtained from a concentrated CH3CN solution
1
after 8 days at −20 °C. H NMR: δ 7.33−7.10 (3H, aryl), 7.00−6.86
(4H, aryl), 5.00 (br, 1H, NH), 4.78−4.71 (3s, 1H, NH), 3.63−3.37
5400
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884(15), 1046(13), 1108(11), 1261(4), 1308(10), 1382(12), 1444(5),
1588(2), 2866(7), 2917(1). HRMS: C34H47N3 calcd (found) 497.3770
(497.3771). Anal. Calcd for C34H47N3: C 82.04, H 9.52, N 8.44.
Found: C 81.93, H 9.47, N 8.46.
35%. Mp: 142−146 °C. X-ray quality colorless crystals were obtained
1
after three days from a concentrated hexanes solution at −20 °C. H
NMR (−30 °C):50 δ 7.38 (t, 1H, aryl, 3JH−H = 7.6), 7.30 (d, 1H, aryl,
3JH−H = 7.6), 7.26 (d, 1H, aryl, 3JH−H = 7.6), 6.76 (s, 1H, aryl), 6.58 (s,
1H, aryl), 6.54 (s, 1H, aryl), 6.25 (s, 1H, aryl), 3.81 (sept, 1H,
1PCl. To a toluene solution (30 mL) of 1 (2.64 g, 6.39 mmol) were
added PCl3 (0.72 mL, 8.31 mmol) and NEt3 (2.30 mL, 16.5 mmol)
sequentially. The cloudy reaction mixture was stirred at rt for 2.5 h.
The reaction mixture was cannula transferred to a Schlenk filter frit,
and the yellow filtrate was dried in vacuo to give a yellow powder. The
powder was dissolved in CH2Cl2 (2 mL), and CH3CN (5 mL) was
added with vigorous stirring to precipitate a white powder. The
suspension was centrifuged and the yellow solution decanted. This
process was repeated, and the decanted solutions were combined and
placed in the freezer (−30 °C) for 30 min, during which more white
powder precipitated. The solution was decanted, and the powder was
dissolved in CH2Cl2 (2 mL) and added to those collected by cen-
trifugation. The volatiles were removed in vacuo to give 1PCl as a
white powder (2.11 g, 4.42 mmol). Yield: 69%. Mp: 143−146 °C.
Single crystals suitable for X-ray diffraction experiments were grown by
diffusion of CH3CN into a concentrated CH2Cl2 solution of the bulk
3
3
CH(CH3)2, JH−H = 6.4), 3.42 (sept, 1H, CH(CH3)2, JH−H = 6.8),
2.49 (s, 3H, CH3), 2.27 (s, 3H, CH3), 2.21 (s, 3H, CH3), 2.15 (s, 3H,
CH3), 2.02 (s, 3H, CH3), 1.90 (s, 3H, CH3), 1.38 (m, 9H, CH(CH3)2,
50
1.32 (d, 3H, CH(CH3)2, JH−H = 6.8). 13C{1H} NMR (−30 °C):
δ
3
150.2, 148.1, 145.5 (d, 2J13C−P = 5.9), 138.4, 137.6, 137.1, 134.6, 131.4,
2
130.2, 129.2, 129.1 (d, J13C−P = 2.8), 128.95, 128.9, 128.6, 128.1,
127.8, 127.3, 124.4, 123.9, 29.5 (d, J13C−P = 4.2), 28.8, 25.8, 25.5 (d,
J13C−P = 4.8), 24.6, 24.3, 21.0, 20.7, 20.1 (d, J13C−P = 9.9), 19.8, 19.1,
18.3. 31P{1H} NMR: δ 180.5 (s). FT-IR (cm−1 (ranked intensity)):
462(4), 561(13), 681(10), 714(15), 767(9), 803(5), 846(12), 985(6),
1227(11), 1266(2), 1317(8), 1466(7), 1608(14), 1698(1), 2963(3). FT-
Raman (cm−1 (ranked intensity)): 97(15), 143(7), 226(6), 461(8),
575(2), 1009(14), 1316(10), 1358(5), 1455(9), 1590(12), 1609(4),
1701(3), 2863(11), 2917(1), 2964(13).
4PCl. This compound was synthesized from 4 (0.75 g, 1.51 mmol),
PCl3 (0.17 mL, 1.96 mmol), and NEt3 (0.55 mL, 3.92 mmol). Yield:
61%. Mp: 138−141 °C. X-ray quality colorless crystals were obtained
after two weeks from a concentrated CH2Cl2/Et2O solution
50
1
material at −30 °C for 4 weeks. H NMR (−30 °C): δ 6.87 (s, 2H,
aryl), 6.77 (s, 2H, aryl), 6.57 (s, 1H, aryl), 6.24 (s, 1H, aryl), 2.49 (s,
6H, CH3), 2.41 (s, 6H, CH3), 2.26 (s, 3H, CH3), 2.22 (s, 6H, CH3),
2.02 (s, 3H, CH3), 1.88 (s, 3H, CH3). 13C{1H} NMR (−30 °C):50
δ
50
1
3
at −20 °C. H NMR (−30 °C): δ 7.43 (t, 1H, aryl, JH−H = 7.6), 7.34
(d, 1H, aryl, 3JH−H = 7.6), 7.30 (d, 1H, aryl, 3JH−H = 7.6), 6.92 (d, 1H,
2
144.8 (d, J13C−P = 6.0), 138.5, 138.3, 137.8, 135.9 (br), 131.3, 130.0,
2
3
3
3
129.2, 128.9 (d, J13C−P = 5.3), 128.0, 127.7 (d, J13C−P = 3.7), 127.3,
127.2, 21.1, 20.7, 20.3, 19.9 (br), 19.1, 18.4. 31P{1H} NMR: δ 181.1
(s). FT-IR (cm−1 (ranked intensity)): 448(6), 562(8), 682(10), 714(15),
756(11), 849(4), 948(14), 986(3), 1173(13), 1263(2), 1312(9), 1480(5),
1608(12), 1720(1), 2915(7). FT-Raman (cm−1 (ranked intensity)):
120(9), 193(11), 225(14), 253(7), 391(15), 448(8), 574(1), 1008(12),
1347(5), 1381(6), 1485(10), 1610(3), 1716(4), 2916(2), 3018(13). Anal.
Calcd for C28H33N3PCl: C 70.35, H 6.96, N 8.79. Found: C 69.32, H
7.18, N 8.65.
aryl, JH−H = 7.2), 6.78 (s, 1H, aryl), 6.75 (t, 1H, aryl, JH−H = 7.6),
6.53 (d, 1H, aryl), 6.51 (s, 1H, aryl), 3.80 (sept, 1H, CH(CH3)2, 3JH−H
=
=
3
6.8), 3.39 (m, 2H, CH(CH3)2), 2.94 (sept, 1H, CH(CH3)2, JH−H
6.4), 2.47 (s, 3H, CH3), 2.15 (s, 3H, CH3), 2.13 (s, 3H, CH3), 1.41
3
(m, 9H, CH(CH3)2), 1.35 (d, 3H, CH(CH3)2, JH−H = 6.4), 1.32 (d,
3
3H, CH(CH3)2, JH−H = 6.8), 0.95 (m, 6H, CH(CH3)2), 0.83 (d, 3H,
50
3
CH(CH3)2, JH−H = 6.4). 13C{1H} NMR (−30 °C): δ 150.7, 148.4,
142.7 (d, 2J13C−P = 5.6), 139.7, 138.6, 138.3, 137.1, 137.0, 134.4, 130.3,
129.5 (d, J13C−P = 4.0), 129.0, 128.95, 128.9, 124.2, 123.6, 122.7,
2
General Synthesis for Halophosphine. To a toluene solution (20 mL)
of guanidine (1−5) were added 1.3 and 2.6 equiv of PX3 (X = Cl, Br) and
NEt3, respectively, in a sequential fashion. The reaction mixture was
left to stir at rt overnight, during which time the solution became
cloudy. Volatiles were removed in vacuo giving an off-white solid, which
was resuspended in THF. The white solid was removed by centrifugation,
and the liquid was concentrated in vacuo to give an off-white solid. The
product was washed with CH3CN (2 × 4 mL), decanting the colored
solution each time. The product was dried in vacuo to give a white solid.
Specific Procedures for Halophosphines. 2PCl. This compound
was prepared from 2 (0.75 g, 1.39 mmol), PCl3 (0.16 mL,
1.83 mmol), and NEt3 (0.50 mL, 3.59 mmol). Yield: 72%. Mp:
127−130 °C. X-ray quality colorless crystals were obtained
from a concentrated CH2Cl2 solution after four weeks at −20 °C.
121.6, 120.6, 29.5 (d, J13C−P = 2.9), 29.0, 28.9, 28.4, 26.5 (d, J13C−P
=
8.0), 26.4, 25.2, 24.9, 24.1, 23.9, 21.2, 21.0, 20.5, 19.8 (d, J13C−P = 10.9),
19.2. 31P{1H} NMR: δ 178.6 (s). FT-IR (cm−1 (ranked intensity)):
432(11), 461(9), 558(15), 675(12), 754(5), 782(7), 883(14), 986(4),
1268(3), 1317(8), 1362(13), 1465(6), 1587(10), 1705(1), 2965(2). FT-
Raman (cm−1 (ranked intensity)): 109(9), 200(14), 462(8), 578(5),
625(11), 887(7), 1011(13), 1259(10), 1319(12), 1360(4), 1448(6),
1590(2), 1708(3), 2925(1), 3065(15). Anal. Calcd for C34H45N3PCl: C
72.64, H 8.07, N 7.47. Found: C 71.97, H 8.06, N 7.37.
5PCl. This compound was synthesized from 5 (0.55 g, 1.80 mmol),
PCl3 (0.20 mL, 2.34 mmol), and NEt3 (0.65 mL, 4.68 mmol). Yield:
50
1
31%. Mp: 75−78 °C. H NMR (−30 °C): δ 3.53−3.35 (m, 3H,
CH), 2.10−2.01 (m, 4H, CH2), 1.82−1.14 (m, 26H, CH2). 13C{1H}
NMR (−30 °C):50 δ 145.6, 55.9, 54.5, 51.4, 35.5, 35.3, 33.6, 33.5, 33.4,
32.1, 31.8, 25.61, 25.56, 25.4, 25.2, 25.1, 25.0. 31P{1H} NMR: δ 181.5
(s). FT-IR (cm−1 (ranked intensity)): 433(7), 655(3), 699(8),
725(14), 839(10), 890(9), 988(4), 1109(13), 1148(12), 1215(5),
1298(11), 1361(15), 1449(6), 1704(2), 2930(1). FT-Raman (cm−1
(ranked intensity)): 85(8), 143(14), 216(6), 430(10), 659(13),
728(12), 811(7), 1027(9), 1253(11), 1347(15), 1444(4), 1702(3),
2854(2), 2887(5), 2938(1). Anal. Calcd for C19H33N3PCl: C 61.69, H
8.99, N 11.36. Found: C 61.35, H 9.26, N 11.17.
50
3
1H NMR (−30 °C): δ 7.30 (t, 2H, aryl, JH−H = 7.6), 7.22
(d, 2H, aryl, 3JH−H = 7.6), 7.13 (m, 2H, aryl), 7.00 (d, 1H, aryl,
3
3JH−H = 7.6), 6.81 (t, 1H, aryl, JH−H = 7.6), 6.61 (d, 1H, aryl,
3JH−H = 7.6), 3.42 (m, 4H, CH(CH3)2), 3.31 (br sept, 1H,
CH(CH3)2), 2.47 (br sept, 1H, CH(CH3)2), 1.41 (d, 6H,
CH(CH3)2, 3JH−H = 6.4), 1.32 (d, 6H, CH(CH3)2, 3JH−H = 6.8),
3
1.26 (d, 6H, CH(CH3)2, JH−H = 6.8), 1.18 (m, 9H,
3
CH(CH3)2), 0.86 (d, 6H, CH(CH3)2, JH−H = 6.4), 0.56 (d,
δ
50
3
6H, CH(CH3)2, JH−H = 6.0). 13C{1H} NMR (−30 °C):
1PBr. This compound was synthesized from 1 (0.93 g, 2.24 mmol),
PBr3 (0.27 mL, 2.92 mmol), and NEt3 (0.81 mL, 5.81 mmol). Yield:
70%. Mp: 117−123 °C. Single crystals suitable for X-ray diffraction
experiments were grown by diffusion of CH3CN into a concentrated
149.5, 147.5, 141.3 (d, 2J13C−P = 5.6), 140.0, 138.5, 137.4, 130.6
(d, 2J13C−P = 4.8), 128.8, 124.1, 124.0, 123.4, 123.0, 121.2, 30.1,
30.0, 28.9, 28.2, 27.4, 27.3, 23.6, 23.3, 22.9, 22.6, 22.4. P{1H}
31
1
NMR: δ 179.9 (s). FT-IR (cm−1 (ranked intensity)): 527(15),
674(11), 752(8), 786(4), 984(3), 1121(10), 1219(7), 1260(5),
1323(9), 1363(14), 1386(12), 1438(6), 1588(13), 1717(1),
2965(2). FT-Raman (cm−1 (ranked intensity)): 137(6), 279(9),
451(13), 887(5), 1048(15), 1104(12), 1252(14), 1299(10),
1353(8), 1443(4), 1589(1), 1718(3), 2867(7), 2909(2), 3062(11).
Anal. Calcd for C37H51N3PCl: C 73.55, H 8.51, N 6.95. Found: C
72.69, H 8.66, N 6.90.
CH2Cl2 solution of the bulk material at −30 °C for 3 days. H NMR
(−30 °C):50 δ 6.88 (s, 2H, aryl), 6.79 (br s, 2H, aryl), 6.61 (s, 1H,
aryl), 6.26 (s, 1H, aryl), 2.49 (s, 6H, CH3), 2.47 (br s, 6H, CH3), 2.29
(s, 3H, CH3), 2.24 (s, 6H, CH3), 2.04 (s, 3H, CH3), 1.89 (s, 3H, CH3).
50
2
13C{1H} NMR (−30 °C): δ 144.1 (d, J13C−P = 5.13 Hz), 138.2,
137.8, 135.6, 131.4, 129.8, 129.4, 129.0, 127.9, 127.8, 127.3, 21.1, 20.7,
20.3, 19.9, 19.8, 19.3, 18.3. 31P{1H} NMR: δ 200.2 (s). FT-IR (cm−1
(ranked intensity)): 414(15), 559(7), 674(11), 756(9), 803(14), 849(6),
951(12), 983(3), 1268(2), 1315(8), 1374(13), 1478(4), 1609(10),
1698(1), 2916(5). FT-Raman (cm−1 (ranked intensity)): 97(8),
3PCl. This compound was synthesized from 3 (1.10 g, 2.42 mmol),
PCl3 (0.27 mL, 3.12 mmol), and NEt3 (0.87 mL, 6.25 mmol). Yield:
5401
dx.doi.org/10.1021/ja300587z | J. Am. Chem. Soc. 2012, 134, 5398−5414

Journal of the American Chemical Society
Article
147(4), 177(15), 207(12), 240(9), 324(7), 413(13), 570(2), 1010(14),
1358(6), 1379(11), 1489(10), 1610(3), 1700(1), 2918(5). Anal. Calcd
for C28H33N3PBr: C 64.37, H 6.37, N 8.04. Found: C 64.06, H 6.42, N
7.93.
General Synthesis for 6M, M = Al, Ga. To a CH2Cl2 solution
(5 mL) of 1PCl, a CH2Cl2 solution (3 mL) of MCl3 was added slowly
with stirring. In all cases the reaction mixtures turned yellow. The
reactions were stirred at rt for 3 h, during which time the yellow color
became less intense. The volatiles were removed in vacuo to give
slightly yellow powders.
313(13), 415(11), 618(15), 1001(2), 1029(10), 1075(12), 1094(9),
1302(7), 1382(14), 1492(1), 1585(6), 1605(3), 3059(5). Anal. Calcd for
C64H56F3NO3P4PtS: C 59.35, H 4.36, N 1.08. Found: C 59.04, H 4.42, N
1.07.
8. To a CH2Cl2 solution (5 mL) of 1PCl (0.40 g, 0.83 mmol) was
added a CH2Cl2 solution (2 mL) of 2,2′-bipyridine (0.13 g, 0.83 mmol)
and Me3SiOTf (150 μL, 0.83 mmol). The previously colorless solu-
tion turned yellow after the addition of Me3SiOTf and was allowed
to stir at rt for 3.5 h before removing the volatiles in vacuo to give an
orange waxy product. Washing with hexanes (2 × 5 mL) produced an
orange powder. The powder was redissolved in CH2Cl2 (3 mL), and
hexanes (6 mL) were added to produce an oil. The supernatant was
decanted and the oil dried to give an orange powder. This process was
repeated three times. Yield: 72%. Decomposition point: 128−133 °C.
1H NMR: δ 9.22 (d, 2H, bpy, 3JH−H = 8.0), 8.59 (br d, 2H, bpy), 8.50
6Al. Compound 6Al was prepared from 1PCl (0.20 g, 0.41 mmol)
and AlCl3 (0.055 g, 0.41 mmol). The crude product was purified by
liquid diffusion of hexane into a concentrated CH2Cl2 solution of the
bulk powder at −30 °C for 48 h. This gave a microcrystalline white
powder. Yield: 53%. Decomposition point: 160−164 °C. X-ray quality
crystals were obtained from slow evaporation of a concentrated
3
(t, 2H, bpy, JH−H = 8.0), 7.64 (br t, 2H, bpy), 2.76 (br s, 6H, CH3),
1
CH2Cl2 solution of the bulk powder at rt for a two week period. H
2.21 (s, 6H, CH3), 2.06 (s, 3H, CH3), 1.99 (s, 3H, CH3), 1.82 (br s,
2
6H, CH3), 1.77 (s, 3H, CH3). 13C{1H} NMR: δ 146.6 (d, J13C−P
=
NMR: δ 7.02 (s, 1H, aryl), 7.00 (s, 1H, aryl), 6.76 (s, 1H, aryl), 6.68
(s, 1H, aryl), 6.52 (s, 1H, aryl), 6.19 (s, 1H, aryl), 2.72 (s, 3H, CH3),
2.64 (s, 3H, CH3), 2.55 (s, 3H, CH3), 2.51 (d, 3H, CH3 JH−P = 2.0),
2.35 (s, 3H, CH3), 2.20 (s, 3H, CH3), 2.11 (s, 3H, CH3), 2.07 (s, 6H,
CH3). 13C{1H} NMR: δ 160.8 (d, 2J13C−P = 9.95), 140.8, 140.0, 139.0,
138.8, 137.7, 137.3, 136.6, 135.3, 133.6, 133.3, 130.5, 130.2, 130.0,
129.6, 129.0, 128.5, 127.3, 21.4, 21.3 (d, J13C−P = 7.34), 21.0, 20.9,
20.8, 20.7, 20.6, 20.3, 19.2. 31P{1H} NMR: δ 171.4 (s). FT-IR (cm−1
(ranked intensity)): 412(14), 429(9), 471(11), 505(2), 661(6),
711(10), 748(4), 855(3), 885(15), 981(8), 1195(12), 1282(5),
1352(13), 1555(1), 2922(7). FT-Raman (cm−1 (ranked intensity)):
186(7), 221(9), 283(10), 430(8), 495(5), 574(2), 661(13), 1018(14),
1321(12), 1352(4), 1386(6), 1482(11), 1609(3), 2925(1), 3024(15).
6Ga. Compound 6Ga was synthesized from 1PCl (0.25 g, 0.52 mmol)
and GaCl3 (0.093 g, 0.53 mmol). The crude product was purified
by redissolving in CH2Cl2 and precipitating white solids by the
addition of hexanes with rapid stirring. The solution was decanted, and
the white product was dried in vacuo. Yield: 66%. Decomposition
5.0), 145.1, 144.6, 139.8, 138.1, 136.6, 136.1, 132.0, 131.3, 130.0,
2
2
128.8, 128.1, 127.8, 127.6 (d, J13C−P = 4.0), 125.8 (d, J13C−P = 5.0),
21.4, 21.0, 20.7, 19.8, 18.7, 18.5. 31P{1H} NMR: δ 106.0 (s). 19F{1H}
NMR: δ −78.6 (s). FT-IR (cm−1 (ranked intensity)): 476(14),
517(8), 564(12), 637(3), 724(13), 770(7), 854(10), 988(15),
1031(2), 1155(5), 1260(1), 1478(6), 1613(11), 1695(4),
2921(9). FT-Raman (cm−1 (ranked intensity)): 85(2), 394(14),
570(4), 633(13), 770(9), 1016(7), 1251(15), 1304(11), 1331(3),
1384(12), 1497(8), 1563(10), 1608(1), 1694(6), 2922(5).
9. A THF (3 mL) solution of Cp2Co (0.43 g, 2.27 mmol) was
added to a colorless THF (3 mL) solution of 1PCl (1.08 g, 2.27
mmol). The reaction mixture turned a dark brown color, and after 48
h of stirring at rt, copious amounts of green precipitate were visible.
The green precipitate was separated by centrifugation, and the dark
solution was decanted and dried in vacuo to give an off-white solid.
The product was washed with CH3CN (2 × 6 mL) to remove
remaining Cp2Co, and the suspension was centrifuged. The solid white
product was dissolved in CH2Cl2 (3 mL) and precipitated with the
addition of CH3CN, the solution was decanted and discarded, and this
process was repeated once more. After the second precipitation the
vial was placed in the freezer (−30 °C) for 45 min, the slightly colored
solution was discarded, and the white precipitate was dried in vacuo. X-
ray quality crystals were grown at −30 °C from the liquid diffusion of
CH3CN into a concentrated CH2Cl2 solution of the bulk material over
a one-week period. Yield: 61%. Decomposition point: 310−320 °C.
1H NMR (−50 °C):50 δ 6.95 (s, 2H, aryl), 6.80 (s, 2H, aryl), 6.63 (s,
2H, aryl), 6.42 (s, 2H, aryl), 6.10 (s, 2H, aryl), 5.96 (s, 2H, aryl), 2.82
(s, 6H, CH3), 2.44 (s, 6H, CH3), 2.39 (s, 6H, CH3), 2.30 (s, 6H, CH3),
2.22 (s, 6H, CH3), 2.06 (s, 6H, CH3), 1.98 (s, 6H, CH3), 1.52 (s, 6H,
CH3), 1.48 (s, 6H, CH3). 13C{1H} NMR (−50 °C):50 δ 146.2 (dd,
2J13C−P = 6.0), 142.2, 138.4, 138.2, 137.3 (br), 136.8, 135.8 (br), 134.5,
130.2 (br), 129.6 (br), 129.2, 128.3 (br), 128.1, 126.8 (dd, 2J13C−P = 7.2),
125.6, 21.8, 21.3 - 20.8 (br), 20.6, 20.2 (br), 19.0 (d, J13C−P = 2.6), 18.7
(br). 31P{1H} NMR: δ 59.2 (s). FT-IR (cm−1 (ranked intensity)):
532(10), 562(7), 604(14), 719(5), 747(9), 848(6), 949(11), 1005(3),
1188(12), 1232(2), 1308(15), 1375(13), 1477(4), 1642(1), 2915(8). FT-
Raman (cm−1 (ranked intensity)): 108(9), 243(14), 403(15), 430(10),
474(6), 531(11), 576(5), 968(12), 1019(13), 1220(8), 1307(4), 1381(7),
1609(1), 1660(3), 2915(2). Anal. Calcd for C56H66N6P2: C 75.99, H 7.52,
N 9.49. Found: C 75.67, H 7.44, N 9.47.
1
point: 171−179 °C. H NMR: δ 7.02 (s, 1H, aryl), 7.01 (s, 1H, aryl),
6.75 (s, 1H, aryl), 6.68 (s, 1H, aryl), 6.51 (s, 1H, aryl), 6.20 (s, 1H,
aryl), 2.73 (s, 3H, CH3), 2.65 (s, 3H, CH3), 2.53 (s, 3H, CH3), 2.51 (d,
3H, CH3 JH−P = 2.4), 2.35 (s, 3H, CH3), 2.20 (s, 3H, CH3), 2.07
2
(virtual d, 9H, CH3). 13C{1H} NMR: δ 159.7 (d, J13C−P = 10.3),
140.8, 139.9, 139.0, 138.8, 137.9, 137.4, 136.5, 135.3, 133.4, 133.3,
3
130.7, 130.1 (d, J13C−P = 3.32), 130.06, 129.6, 129.4, 129.0, 128.4,
127.0 (d, 3J13C−P = 1.81), 21.4, 21.2 (d, J13C−P = 3.22), 21.0, 20.9, 20.8,
20.7, 20.6, 20.1, 19.2. 31P{1H} NMR: δ 172.7 (s). FT-IR (cm−1
(ranked intensity)): 463(15), 493(2), 552(5), 657(9), 705(14),
>750(7), 855(3), 980(8), 1280(4), 1321(10), 1352(12), 1477(6),
1560(1), 1611(13), 2921(11). FT-Raman (cm−1 (ranked intensity)):
199(6), 241(13), 360(4), 388(11), 493(7), 573(2), 657(15),
1017(12), 1321(10), 1352(5), 1385(8), 1482(9), 1609(3), 2924(1),
3023(14). Anal. Calcd for C28H33N3PCl4Ga: C 51.41, H 5.09, N 6.42.
Found: C 50.95, H 5.31, N 6.32.
7. A CH2Cl2 solution (8 mL) of 1PCl (0.35 g, 0.73 mmol) was
added to solid Pt(PPh3)4 (0.91 g, 0.73 mmol) to give a clear orange
solution. After 45 min of stirring at rt, Me3SiOTf was added, and the
color changed instantly to dark red. The volatiles were removed after
stirring at rt for 30 min resulting in a waxy brown product. Hexanes
washes (3 × 5 mL) yielded a brown powder. The bulk material was
dissolved in CH2Cl2 (3 mL) and stirred vigorously as hexanes (8 mL)
were added leading to the precipitation of brown powder. The
supernatant was decanted and discarded and the bulk material once
again dissolved in CH2Cl2 (3 mL) and set up for recrystallization by
10. Compound 1PCl (0.54 g, 1.1 mmol) was dissolved in toluene
(6 mL) and heated to 90 °C for 48 h. The solvent was removed in
vacuo giving a colorless waxy material, which became a powder after
stirring in pentane for 5 min. The solvent was decanted, and the white
powder was dried in vacuo. The product was further purified by
recrystallization from slow diffusion of CH3CN into a concentrated
CH2Cl2 solution of the bulk material at −30 °C. Single crystals suitable
for X-ray diffraction studies were grown in a similar fashion after 11
layering hexanes (6 mL) and keeping at −30 °C for 4 days. Yield: 56%.
1
Decomposition point: 87−92 °C. H NMR: δ 7.34 (t, 9H, aryl, 3JH−H
=
8.0), 7.07 (t, 18H, aryl, 3JH−H = 8.0), 6.94 (d, 18H, aryl, 3JH−H = 8.0), 6.68
(s, 2H, aryl), 2.15 (s, 3H, CH3), 1.67 (s, 6H, CH3). 13C{1H} NMR: δ
133.5, 131.0, 129.0, 128.9, 21.6, 19.2. 31P{1H} NMR: δ 147.0 (br t), 23.3
(2JP−P = 65, JP‑195Pt = 3990). 19F{1H} NMR: δ −78.3 (s). FT-IR (cm−1
1
1
days in the freezer. Yield: 34%. Mp: 178−182 °C. H NMR: δ 6.94
(ranked intensity)): 418(12), 516(2), 637(7), 694(1), 744(5), 853(13),
998(14), 1030(6), 1092(9), 1147(10), 1222(15), 1272(3), 1435(4),
1480(8), 3054(11). FT-Raman (cm−1 (ranked intensity)): 97(8), 244(4),
(s, 4H, aryl), 2.64 (s, 12H, CH3), 2.29 (s, 6H, CH3). 13C{1H} NMR:
δ 138.3, 138.1, 132.0, 129.8, 21.1, 19.6. 31P{1H} NMR: δ 211.0 (s).
FT-IR (cm−1 (ranked intensity)): 408(2), 485(5), 511(7), 558(3),
5402
dx.doi.org/10.1021/ja300587z | J. Am. Chem. Soc. 2012, 134, 5398−5414

Journal of the American Chemical Society
Article
600(10), 716(14), 896(1), 1035(13), 1158(9), 1218(6), 1263(11),
1308(15), 1375(12), 1476(4), 2916(8). FT-Raman (cm−1 (ranked
intensity)): 181(14), 221(2), 379(3), 405(11), 486(10), 543(12),
577(4), 632(8), 984(15), 1274(6), 1317(1), 1386(9), 1482(13),
Scheme 1. Synthetic Route for N,N′,N″-Tris(aryl)guanidines
(1−4)
+
1612(5), 2920(7). HRMS for C18H22N2Cl2P2 calcd(found)
398.0635(398.0622). Anal. Calcd for C18H22Cl2N2P2: C 54.15, H
5.55, N 7.02. Found: C 53.97, H 5.51, N 6.92.
11. In a pressure tube 1PCl (0.54 g, 1.1 mmol) was dissolved in
CDCl3 (3 mL) and heated in an oil bath at 90 °C overnight. The
solvent was removed in vacuo, producing a wax-like colorless product.
This was washed with pentane (6 mL), which produced a white
powder. The suspension was centrifuged, and the decanted solution
was transferred to a vial and kept in the freezer (−35 °C) for 1 h. The
centrifuged white solid was dissolved in CH2Cl2, transferred to a vial,
and dried in vacuo to give a sticky white solid. Resuspension in pentane
and drying in vacuo produced a fine white powder. The cooled
solution had a white precipitate, which was isolated by decanting the
pentane and drying in vacuo to give a white powder, which was
combined with the previous isolated product. Single crystals suitable
for X-ray diffraction studies were grown from a concentrated CH3CN
solution of the bulk material at −30 °C for two weeks. Yield: 33%. Mp:
and will not be discussed further (see Supporting Information, SI,
for sample spectra). Single crystals of 3 and 4 were grown from
concentrated CH3CN solutions of the bulk powder at −20 °C,
and subsequent X-ray diffraction studies confirmed the synthesis
of the heteroleptic guanidines in moderate to low yields (58% and
33%, respectively; see SI for solid-state structures). Synthesis of
N,N′,N″-tris(cyclohexyl)guanidine (5) followed the reported facile
procedure of Richeson et al. where cyclohexylamine and N,N′-
dicyclohexylcarbodiimide are combined and heated at 90 °C in
hexane in a pressure tube for two days.48
1
204−209 °C. H NMR: δ 6.94 (br s, 2H, aryl), 6.88 (br s, 2H, aryl),
6.61 (br s, 2H, aryl), 6.30 (br s, 2H, aryl), 2.86 (s, 6H, CH3), 2.56 (br
s, 6H, CH3), 2.35 (br s, 6H, CH3), 2.27 (br s, 3H, CH3), 2.22 (s, 3H,
CH3), 2.21 (s, 3H, CH3), 2.04 (s, 3H, CH3), 1.87 (s, 6H, CH3).
13C{1H} NMR: δ 141.9, 138.8, 138.7, 138.0, 137.9, 137.8, 137.7, 136.7,
136.6, 136.1, 135.9, 131.2, 130.2, 129.9, 129.0, 127.8, 126.7, 21.5, 21.4,
21.0, 20.9, 20.6, 20.5, 20.4, 19.7. 31P{1H} NMR: δ 140.2 (s). FT-IR
(cm−1 (ranked intensity)): 452(3), 563(9), 600(11), 722(15), 853(5),
950(7), 981(10), 1056(4), 1140(14), 1183(8), 1222(1), 1475(6),
1608(12), 1649(2), 2919(13). FT-Raman (cm−1 (ranked intensity)):
86(6), 204(7), 396(14), 421(8), 443(10), 511(15), 580(4), 1058(11),
1307(5), 1381(9), 1535(13), 1610(2), 1652(3), 2921(1), 3007(12).
The sequential addition of PCl3 and NEt3 to a toluene
solution of N,N′,N″-tris(aryl)guanidine (1−4) or N,N′,N″-
tris(cyclohexyl)guanidine (5) (Scheme 2) in a 1.3:2.6:1
Scheme 2. General Method for Synthesizing Halophosphines
with (Dianionic)guanidinate Ligands
+
HRMS for C37H44N4Cl2P2 calcd(found) 676.2418(676.2413). Anal.
Calcd for C37H44N4Cl2P2: C 65.58, H 6.54, N 8.27. Found: C 65.81, H
6.94, N 8.15.
Computational Details. All calculations were done with the
program packages Turbomole 6.351 and ADF 2010.2.52 Geometries of
the studied systems were optimized using the PBE1PBE density
functional53−56 in combination with the def2-TZVP basis sets.57,58 The
nature of stationary points found was assessed by calculating full
Hessian matrices at the respective level of theory. Partial atomic
charges were calculated with the natural population analysis (NPA) as
implemented in Turbomole 6.3.59 The nature of phoshporus−chlorine
interaction in the studied chlorophosphines was inspected with the
energy decomposition analysis (EDA) procedure60 as implemented in
ADF 2010.2.61−63 The analyses were performed at the PBE1PBE/
def2-TZVP optimized geometries using the PBE1PBE density
functional in together with the all electron Slater-type TZP basis
sets.64 The program gOpenMol was used for all visualizations of
molecular structures and Kohn−Sham orbitals.65,66
stochiometry at room temperature generated copious amounts
of white precipitate. Aliquots of the reaction mixture were
sampled every hour and analyzed by 31P{1H} NMR spectros-
copy, which in all cases showed the emergence of signals shifted
upfield from PCl3, consistent with the chemical shifts reported
for diaminochlorophosphines (δP = 178−181, Table 1; cf. δP =
102 − 210).25,29,67−70 A distinct difference in reaction com-
pletion times emerged on the basis of the steric bulk of the
ligands; when the steric demand was increased, reaction times
were correspondingly longer from 1 (1, 3−5) to 12 (2) h.
Upon no further change in the 31P{1H} NMR spectra, the
reaction mixtures were filtered or centrifuged. Removal of the
volatiles of the supernatant in vacuo gave light yellow powders
in all cases. Further washing of the powders with CH3CN
yielded white powders, which were sampled for multinuclear
NMR spectroscopic analysis, where the 31P{1H} NMR spectra
showed only a single peak identical to that observed for their
respective reaction mixtures, with the exception of 3PCl. The
initial reaction mixture of 3PCl taken after 1 h showed the
emergence of two peaks (δP = 178 and 181) with equal in-
tensities; no change was observed in the 31P{1H} NMR
spectrum after continuing to monitor the reaction for 24 h. The
purified product had a single resonance (δP = 181). The proton
RESULTS AND DISCUSSION
■
Synthesis. Synthesis of N,N′,N″-tris(aryl)guanidines (1−4,
aryl = Mes and Dipp) followed the literature procedure of
Boere
́
et al.47 Preparations were accomplished by refluxing
N,N′-bis(R)carbodiimide and lithium (R′)anilide in THF for
2 h; the addition of water gave either the homoleptic
guanidines 1 and 2 (R = R′ = Mes for 1; R = R′ = Dipp for
2), or the heteroleptic guanidines 3 and 4 (R = Mes; R′ = Dipp
for 3; R = Dipp; R′ = Mes for 4; Scheme 1). Upon removal of
THF, compounds 3 and 4 deposited as waxlike materials on the
sides of the flask. Further recrystallization from hexanes
produced white powders, which were sampled for analysis by
proton NMR spectroscopy. Spectra of the isolated materials
demonstrated dynamic behavior indicative of sterically bulky
guanidines.47 These phenomena have been documented
elsewhere (for 2)47 using variable temperature NMR spectroscopy
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NMR spectrum of crude 3PCl supported the conclusion that
two products were present on the basis of the extra resonances
observed when compared to a spectrum of pure 3PCl at δP =
181. Presumably, the products at δP = 178 and 181 were the
symmetric and asymmetric chelating guanidinates, respectively
(Chart 4). Single crystals of the product with a phosphorus
Chart 4. (a) Symmetric and (b) Asymmetric Chelating
Guanidinates for 3PCl
chemical shift of 181 ppm were grown, which confirmed an
asymmetric chelating guanidinate (Chart 4b). In an attempt to
isomerize to one product, an aliquot of the reaction mixture
was transferred to an NMR tube and heated at 90 °C overnight.
The 31P{1H} NMR spectrum revealed that two peaks were still
present in approximately the same ratio, but that the reso-
nances had now shifted downfield approximately 30 ppm to
δP = 210.9 and 211.1. There were no observable N−H peaks in
1
the H NMR spectra of 1PCl−5PCl, and the IR spectra of the
solids lacked N−H vibrations in the range ν = 3100−3500 cm−1.44,71
Given these data, the compounds were assigned as the
chlorophosphines 1PCl, 2PCl, 3PCl, 4PCl, and 5PCl isolated
in low to good yields (Table 1). X-ray quality crystals were
grown from samples of the bulk powders for 1PCl−4PCl,
and subsequent X-ray diffraction experiments confirmed the
synthesis of the strained, four-membered, cyclic diaminochloro-
phosphines.
Reactivity. Given the strained nature of these ring systems,
we surmised that conversion to the NHP would generate highly
reactive cationic phosphorus centers. This transformation was
attempted using several metathetical reagents, including a
variety of triflate salts (EOTf; E = Me3Si, Ag, Na, Li; OTf =
[F3CSO3]¯). A stoichiometric amount of Me3SiOTf was added
to CH2Cl2 solutions of 1PCl−5PCl, and reaction mixtures
were monitored by 31P{1H} NMR spectroscopy. At room
temperature for 1PCl−4PCl no change was detected over the
course of 7 days, even after the addition of excess amounts of
Me3SiOTf. Identical reaction mixtures were heated to 100 °C
in toluene and monitored by 31P{1H} NMR spectroscopy,
which showed the slow evolution of multiple products, with
one peak downfield of the chlorophosphines at δP = 211,
indicative of the generation of the phosphetidine Cl2P2N2Mes2
(vide infra).68 Analogous results were observed using the
sources of metal triflates with 1PCl over 7 days at 100 °C. Only
compound 1PCl was employed as a model system in sub-
sequent reactivity studies given the simplicity of its solution 1H
NMR spectrum. In the case of 5PCl the addition of Me3SiOTf
results in several products as evidenced by the many signals in
the 31P{1H} NMR spectrum.
Given the difficulty associated with removing chloride from
the chlorophosphines, we looked to the bromo- derivatives, as
heavier halides generally undergo more facile metathesis.72 The
bromophosphine 1PBr was synthesized in a similar manner to
1PCl (Scheme 2, Table 1) and subjected to identical metathesis
conditions. Again, no reaction or multiple products were
observed, none of which corresponded to a downfield chemical
shift expected of an NHP in the 31P{1H} NMR spectra.
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Given the puzzling failure of the salt metathesis routes, the
Lewis acids AlCl3 and GaCl3 were employed as halide ab-
stracting reagents for 1PCl. These reactions were also
monitored by 31P{1H} NMR spectroscopy and revealed the
formation of single products with upfield chemical shifts
(ΔδP ≈ 10, Table 1). The volatiles were removed in vacuo, and
Given that the isolation of the free NHP was proving to be a
difficult feat, an attempt was made to trap the desired phos-
phenium cation by reaction with a Pt(0) metal center (Scheme 4),
Scheme 4. Using Pt(0) as a Trapping Agent
1
the bulk powders were redissolved in CDCl3 to obtain the H
NMR spectra, which in both cases showed sharp peaks
indicating that there was no longer a slow exchange process
on the NMR time scale, and furthermore that all symmetry in
the molecule was lost. Given the upfield chemical shift in the
31P{1H} NMR spectra and the lack of symmetry noted in the
1H NMR spectra, it was hypothesized that halide abstraction
was not effected: rather, a Lewis acid (AlCl3, GaCl3)/Lewis
base (N or P) adduct was formed (Scheme 3). Single crystals
Scheme 5. Synthesis of Base-Stabilized Cationic 8
Scheme 3. Lewis Acid/Lewis Base Adduct Formation of 6Al
and 6Ga
Scheme 6. Redox Reaction for the Synthesis of 9
suitable for X-ray diffraction studies were obtained for the
product containing AlCl3, and the solid-state structure con-
firmed an N→Al adduct of AlCl3 from the exocyclic nitrogen of
the guanidinate ligand (6Al). Although suitable single crystals
for X-ray diffraction studies could not be grown for the GaCl3
adduct (6Ga), an identical structure was assigned on the basis
of identical 1H NMR spectra and a similar chemical shift in the
31P{1H} NMR spectrum.
Scheme 7. Carbodiimide Elimination and Ring Expansion
Producing 10 and 11
Given the production of 6Al and 6Ga, two stoichiometric
equivalents of AlCl3 were added to a CH2Cl2 solution of 1PCl
at rt, where it was envisaged that the exocyclic nitrogen could
be blocked and the second Lewis acid would be free to abstract
the chloride ion. After 24 h 6Al was completely consumed, and
two phosphorus resonances at δP = 165 and 184 were observed.
The volatiles were removed in vacuo, and an orange powder
was isolated. A sample of the powder was redissolved in CDCl3,
of which several examples with NHP as ancillary ligands are
known.26,27 The 1:1:1 stoichiometric reaction of 1PCl,
Pt(PPh3)4, and Me3SiOTf was monitored by 31P{1H} NMR
spectroscopy. Combining 1PCl and Pt(PPh3)4 resulted in an
orange solution which had a change in the 31P{1H} NMR
spectrum from a singlet to a downfield triplet with Pt satellites
for the phosphorus atom of 1PCl (P1PCl) and an upfield
doublet with Pt satellites for coordinated PPh3 (PPPh3); also,
free PPh3 was observed. The splitting pattern was indicative of
an AX2 spin system. The addition of Me3SiOTf caused a color
change to red, and an aliquot sampled for 31P{1H} NMR spec-
troscopy showed an upfield shift for the triplet of P1PCl, which
had broadened out significantly. The solvent was removed in
vacuo yielding a red waxy material that became a powder after
agitation in hexanes. A sample of the bulk material was used for
1H NMR spectroscopy (see SI for spectrum), and analysis of
the spectrum showed the presence of two products, one of
1
and a H NMR spectrum of the crude material was obtained,
which confirmed the presence of multiple products none of
which could be isolated separately.
It should be noted that while halide abstraction with these
four-membered diaminochlorophosphines appears to be non-
trivial, the analogous reaction of those with unsaturated five-
membered rings is generally facile.28,73−77 One major difference
between these two species is the additional stabilization gained
from the delocalization of a 6π-electron system in the case of
the five-membered ring, contrary to the 4π-electrons present in
the four-membered ring. While aromaticity is not a necessity
for the isolation of NHPs25,34,35,37,38,67 and has been found to
be a weak factor in their stabilization,75 the lack thereof in our
four-membered diaminochlorophosphines may be a contribut-
ing factor to the difficulty in halide abstraction.
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a
Scheme 8. Proposed Routes to 11 by (a) Insertion of MesNPCl or (b) Insertion of Carbodiimide
a
(a) Reaction pathway for the insertion of MesNPCl into a P−N bond of 1PCl. The following describes the individual steps: (eq 1a) the thermal
ejection of carbodiimide from 1PCl to give phosphetidine 10, (eq 1b) monomer/dimer equilibrium of 10 and MesNPCl, and (eq 1c) the insertion
of MesNPCl into a P−N bond of 1PCl to give 11. Overall, the reaction starts with 2 equiv of 1PCl and ends with equal equivalents of carbodiimide
and 11. (b) Reaction pathway for the insertion of carbodiimide into a P−N bond of 10. The following describes the individual steps: (eq 2a)
the thermal ejection of carbodiimide from 1PCl to give MesNPCl, (eq 2b) dimerization of MesNPCl to 10, and (eq 2c) the insertion of
carbodiimide into a P−N bond of 10 to give 11. Overall, the reaction starts with 2 equiv of 1PCl and ends with equal equivalents of
carbodiimide and 11.
which was identified as free N,N′-bis(Mes)carbodiimide. The
other product could be isolated in pure form from repeated
recrystallizations (×2) from DCM/hexanes at −30 °C. X-ray
diffraction studies revealed that the phosphorus containing pro-
duct was actually the iminophosphine complex of Pt(PPh3)3,
7.78 Although there has been a previous report of cationic
iminophosphines acting as ancillary ligands for Ni(0) and
Pt(0), no solid-state structures have been obtained.79
Similar to previous work reported by our group,44 we sought
to employ a Lewis base to help support the coordinatively
unsaturated phosphenium center. In this regard, 1 equiv of 2,2′-
bipyridine (bpy) was added to 1PCl followed by the addition of
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Me3SiOTf (Scheme 5). The color of the solution changed from
colorless to yellow. The 31P{1H} NMR spectrum showed an
upfield shift by ΔδP = 75 from 1PCl, as well as a shift cor-
responding to the starting material, showing no change in in-
tensity between 3 and 24 h. A proton NMR spectrum was
collected after removing the solvent in vacuo and showed four
resonances corresponding to coordinated bpy, and four re-
sonances for free bpy. The product was purified by redissolving
in CH2Cl2 and adding hexanes to give an oil, which became a
powder after drying in vacuo. Multinuclear NMR spectroscopy
of the purified material showed a single resonance in the
31P{1H} NMR spectrum at δP = 106 and the loss of peaks
1
corresponding to free bpy in the H NMR spectrum. Single
crystals were grown, and a solid-state structure was obtained,
which confirmed that bpy and a dianionic ligand were co-
ordinated to the phosphorus atom to give the base-stabilized
cation, 8.78
Jones et al. attempted to reduce a dichlorophosphine
supported by a bulky (monoanionic)guanidinate with KC8,
anticipating a diphosphene; however, the result was many
nonisolable phosphorus-containing products.39 In this context,
we investigated the reactivity of 1PCl with the one-electron
reducing agent cobaltocene (Cp2Co). The 1:1 stoichiometric
reaction of 1PCl and Cp2Co in THF at room temperature
(Scheme 6) produced copious amounts of green precipitate
(cobaltocenium chloride; δH = 5.9), and monitoring the
reaction by 31P{1H} NMR spectroscopy revealed the pro-
duction of a single product at δP = 59 (ΔδP = 123). The
reaction was complete after 48 h, at which time the green
precipitate was removed by centrifugation, the volatiles were
removed from the supernatant in vacuo, and the product was
1
washed with CH3CN yielding a white powder. The H NMR
spectrum of the white powder in CDCl3 at room temperature
had four signals in the aryl region and five methyl resonances;
however, if the sample was cooled to −50 °C, six aryl and nine
methyl signals were observed. X-ray diffraction studies revealed
that reductive coupling had occurred, resulting in a dimeric
structure with μ-N,N′ bridging guanidinates for 9.
Figure 1. (a) Plot of the integration of δP 1PCl, 10, and 11 relative to
normalized PPh3 and the sum of the integrations as a function of time.
(b) Stacked plot of 31P{1H} NMR spectra for monitoring the
production of 11 from 1PCl.
The thermal stability of the cyclic four-membered diamino-
chlorophosphines was investigated given the multiple instances
of a peak emerging at δP ≈ 211 upon heating of these species.
This was accomplished by heating a toluene solution of 1PCl at
90 °C overnight to deduce what had occurred. Resembling the
attempted isomerization of 3PCl and halide abstraction of
1PCl−4PCl, a 30 ppm downfield chemical shift was noted in
the 31P{1H} NMR spectrum after 2 days at 90 °C, giving a
singlet at δP = 211. The product was waxy after the solvent was
removed in vacuo, but became a white powder after stirring in
pentane for 5 min. Decanting the supernatant and drying the
powder under reduced pressure yielded pure product as
evidenced by 1H NMR spectroscopy, which showed a
drastically simplified spectrum (see SI). X-ray diffraction
studies revealed that the product was indeed the dichlor-
odiazadiphosphetidine, 10 (see SI for solid-state structure). The
proposed route from 1PCl to 10 is by elimination of N,N′-
bis(Mes)carbodiimide resulting in a chloro(2,4,6-
trimethylphenylimino)phosphine (MesNPCl), which dimerizes
to give a dichlorodiazadiphosphetidine.80 An analogous di-
chlorodiazadiarsetidine, [{2-(6-Me)C5H3N}NAsCl]2, has been
observed by the thermal elimination of Me3SiCl from [{2-(6-
Me)C5H3N}NSiMe3]AsCl2.42 The thermally induced removal
of a carbodiimide fragment from guanidinato molecular species
has been comprehensively studied by Barry et al. and is a
1
Figure 2. Stack plot of H NMR spectra for (a) pure 11, (b) crude
completed ring expansion, and (c) pure N,N′-bis(Mes)carbodiimide.
known pathway in their thermal decomposition.81,82 While 10
was the major product from heating 1PCl, there was a minor
product at δP = 140. This species could be isolated from
collecting the CH3CN washes of 10. Single crystals suitable for
X-ray diffraction studies revealed that the minor product was
actually a dinuclear compound featuring a μ-N,N′ bridging
guanidinate and a bridging N−Mes (Scheme 7), 11. Heating an
NMR tube charged with 1PCl in a CDCl3 solution at 90 °C
overnight in an oil bath also produces 11 in nearly quantitative
yields (92%) according to the 31P{1H} NMR spectrum.
Possible routes for the ring expansion are proposed as either
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Figure 3. Solid-state structures of (a) 1PCl, (b) 2PCl, (c) 3PCl, (d) 4PCl, (e) 1PBr, and (f) 6Al. Ellipsoids are drawn to 50% probability. The
i
methyl groups on the Pr substituents, and the hydrogen atoms have been omitted for clarity.
(a) ring-opening of 1PCl to dimerize with MesNPCl, or (b)
insertion of N,N′-bis(Mes)carbodiimide into a P−N bond of 10
(Scheme 8). To elucidate the likely reaction pathway this
reaction was monitored by 31P{1H} NMR spectroscopy where
a spectrum was collected every hour for 43 h (Figure 1). It was
determined that 10 is a short-lived intermediate in the reaction
when CDCl3 is used as the solvent (Figure 1b), as the reaction
appears to be solvent dependent.83 Regardless of the route
following proposed mechanism a or b, a 1:1 stoichiometric ratio
within the 31P{1H} NMR spectra, it was deduced that the
mechanism most likely follows route a. This is similar to the
Lewis acid induced oligomerization observed by Burford et al.,
where an iminophosphine, in equilibrium with the correspond-
ing phosphetidine, is formally inserted into a phosphazane.84
X-ray Crystallographic Studies. Single crystals suitable
for X-ray diffraction studies were grown for compounds 1PCl−
4PCl and 1PBr by various techniques and solvent combina-
tions (see Experimental Section for details). The solid-state
structures of 1PCl−4PCl and 1PBr (Figure 3a−e) are
analogous and were all crystallized in the monoclinic P21/c
space group (Table 2, see Table 1 for summary of important
metrical parameters). In all cases, the coordination of the
guanidinate ligand to phosphorus is κ2-N,N′ chelating, which
generates a strained four-membered ring. The N−P−N bond
angles range from 75.73(7)° to 78.61(7)°, which are on the
1
of 11 and carbodiimide should be observed in the H NMR
spectrum of the crude material upon the completion of the
reaction, which is indeed the case (Figure 2). Given that the
reaction between isolated 10 and N,N′-bis(Mes)carbodiimide
(Scheme 8b, eq 2c) in various stoichiometries (1:1, 1:2, 1:5),
solvents, and temperatures (toluene (90 °C), xylene (140 °C),
and CDCl3 (90 °C)) gave no change in the signals observed
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Table 2. X-ray Details for 1PCl−4PCl, 1PBr, 6Al, 9, 11
1PCl
2PCl
3PCl
4PCl
1PBr
6Al
9
11
empirical
formula
C28H33ClN3P
C37H51ClN3P
C31H39ClN3P
C34H45ClN3P
C28H33BrN3P
C28H33AlCl4N3P
C56H66N6P
C29H94Cl6N10P4
fw (g/mol)
cryst syst
space group
a (Å)
477.99
monoclinic
P21/c
604.23
monoclinic
P21/c
520.07
monoclinic
P21/c
562.15
monoclinic
P21/c
522.45
monoclinic
P21/c
611.32
monoclinic
P21/c
885.09
monoclinic
P21/c
1520.22
triclinic
P1
̅
8.6540(5)
26.8867(14)
12.6400(6)
90
15.829(3)
11.199(2)
24.175(8)
90
20.468(2)
9.2821(10)
15.5195(18)
90
10.744(2)
15.331(3)
22.699(6)
90
8.6103(17)
14.299(3)
21.572(4)
90
14.544(3)
13.657(3)
17.327(7)
90
15.278(3)
21.301(4)
21.126(7)
90
11.562(2)
13.418(3)
14.712(3)
65.35(3)
78.17(3)
88.20(3)
2026.6(7)
1
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
115.843(3)
90
125.42(2)
90
92.061(4)
90
118.03(2)
90
91.13(3)
90
119.39(2)
90
133.195(17)
90
2646.9(2)
4
3492.3(15)
4
2946.6(6)
4
3300.3(12)
4
2655.4(9)
4
2998.7(15)
4
5012(2)
4
Dc (mg m−3
)
1.199
1.149
1.172
1.131
1.307
1.354
1.173
1.246
Rint
0.0787
0.0422
0.1035
1.016
0.0578
0.0550
0.1687
1.069
0.0827
0.0894
0.2347
1.126
0.0756
0.0371
0.1026
1.029
0.0305
0.0533
0.1424
1.056
0.0534
0.0557
0.1887
1.083
0.0609
0.0788
0.2300
1.105
0.1067
a
R1 [I > 2σ(I)]
wR2 (F2)
0.0606
a
0.1461
a
GOF (S)
1.010
a
R1(F[I > 2σ(I)]) = ∑||Fo| − |Fc ||/∑ |Fo|; wR2(F2 [all data]) = [w(Fo2 − Fc2)2]1/2; S(all data) = [w(Fo2 − Fc2)2/(n − p)]1/2 (n = no. of data; p = no.
of parameters varied; w = 1/[σ2(Fo ) + (aP)2 + bP] where P = (Fo + 2Fc2)/3 and a and b are constants suggested by the refinement program.
2
2
smaller end of the scale compared to other similar four-
membered rings derived from chelating nitrogen-based ligands
on phosphorus (cf. 70.66−120.72°, mean = 87.21°).85,86 The
metrical parameters of the guanidinate ligands further con-
firmed its dianionic nature in 1PCl−4PCl and 1PBr. The
C−Nendo bond lengths within the ring are consistent with
carbon−nitrogen single bonds C(1)−N(1) 1.403(2)−1.437(2)
Å and C(1)−N(2) 1.410(3)−1.477(2) Å (cf. 1.42 (av) Å)48
while the C−Nexo bond lengths are representative of a carbon−
nitrogen double bond C(1)−N(3) 1.249(3)−1.261(2) Å (cf.
1.28 Å).48 The exocyclic nitrogen N(3) has a stereochemically
active lone pair, and the aryl substituent on N(3) in 3PCl and
4PCl resides on the side of the endocyclic nitrogen bearing the
substituent with the least bulk to minimize steric interactions in
the case of the heteroleptic guanidinate. The expected trigonal
planar geometry is observed about C(1) (Σang = 360°) and a
pyramidal geometry about P(1) (Σang = 280.8−287.2°). Short
P(1)−Cl(1) bonds are noted for 1PCl−4PCl with bond
lengths ranging from 2.0476(7) to 2.1141(7) Å. This is con-
trary to the characteristically long P−Cl bonds commonly
observed for 6π aromatic cyclic diaminochlorophosphines in
the larger five-membered ring, attributed to the hypercon-
jugation of π(C2N2)−σ*(P−Cl) (e.g., 2.24−2.70 Å).69,75,87 The
P−Cl bond lengths are even shorter than acyclic (NMe2)2PCl
(2.180(4)).88 A possible reason for the failed metathesis
reactions is the strong phosphorus−halide bond, reflected in
the short P−X (X = Cl, Br (2.2936(10) Å, cf. 2.43−
2.95 Å)28,72,74) bond distance for 1PCl−4PCl and 1PBr.
The solid-state structure of 6Al (Figure 3f, Table 2)
confirmed the atom connectivity of N(3) to Al(1) with a
bond length of 1.936(3) Å, which is in the typical range found
for other AlCl3 adducts.89,90 Compared to 1PCl, 6Al has
shortened C(1)−N(1), C(1)−N(2), and P(1)−Cl(1) bond
lengths and an elongated C(1)−N(3) bond (Table 1). The Al−
Cl bond lengths have also elongated from 2.068(4) Å in AlCl3
to 2.1132(16)−2.1304(13) Å in 6Al.91 Exocyclic N(3) and
endocyclic N(1) have a trigonal planar geometry with the
because of the Mes substituent on N(3) residing above, thus
causing a slight pyramidalization to decrease steric interactions.
The most notable structural change of 1PCl upon reaction
with Cp2Co to form 9 (Figure 4a, Table 2) is the change in the
coordination mode of the guanidinate ligand from κ2-N,N′
chelating to μ-N,N′ bridging. The guanidinate ligand 1 retains
its dianionic nature in the bridging mode, as indicated by the
carbon−nitrogen bond lengths. The P(1)−P(2) bond length is
2.2251(14) Å, which is similar to an analogous structure with μ-
N,N′ bridging ureas (P−P 2.222(2) Å),92 and consistent with
typical P−P single bond lengths (cf. 2.21 Å).86,93 Unlike the
planar As2N4C bicyclic fragment in a (monoanionic)guanidinate-
bridged diarsene,39 9 has a puckered arrangement of the two
five-membered P2N2C rings, where the two planes are 100.4°
to each other (Figure 4b). The molecular geometry about
the phosphorus atoms is trigonal pyramidal with a stereo-
chemically active lone pair of electrons indicative of an AX3E
electron pair geometry.
The solid-state structure of 11 revealed that a ring expansion
of 1PCl had occurred (Figure 4c). This was most likely the
result of monomeric chloro(imino)phosphine inserting into the
P−N bond of 1PCl.84 Upon ring expansion the coordination of
the guanidinate ligand changes from κ2-N,N′ chelating to μ-
N,N′ bridging and retains its dianionic charge as indicated by
the C(1)−N bond lengths (C(1)−N(1) 1.411(3) Å, C(1)−
N(2) 1.416(3) Å, C(1)−N(3) 1.270(3) Å). The P(1)−Cl(1)
and P(2)−Cl(2) bond lengths are 2.1228(11) and 2.1290(12) Å,
respectively, and significantly longer than 1PCl. Unlike the
phosphetidine 10 the chlorine atoms are in a trans confor-
mation, and the ring is puckered with a mean deviation from
the plane of 0.1425 Å, compared to nearly planar 10 with a
mean deviation from the plane of 0.0571 Å. The P−N bonds
range from 1.688(2) to 1.704(2) Å, which are shorter than
1PCl (1.7081(14)−1.7235(15) Å) and longer than that of a
monomeric chloro(imino)phosphine (P−N 1.495(4) Å for
Cl−PN−Mes*, Mes*=2,4,6-tri-tert-butylphenyl).94
Computational Studies. To shed light on the resistance
of compounds 1PCl−5PCl and 1PBr to halide abstraction,
the electronic structures of chlorophosphines 5PCl and
Σang = 359.9°, while the sum of the angles about N(2) is 349.8°
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Figure 4. Solid-state structures of (a) 9, (b) view of 9 along P−P bond, and (c) 11. Ellipsoids are drawn to 50% probability. Hydrogen atoms and
solvates have been omitted for clarity. In part b, all but the ipso carbons are removed from the Mes substituents for simplicity. Selected bond lengths
(Å) and angles (deg): (a) P(1)−N(1) 1.725(3), P(1)−N(4) 1.725(3), P(2)−N(2) 1.729(3), P(2)−N(5) 1.721(3), C(1)−N(1) 1.419(5), C(1)−
N(2) 1.409(4), C(1)−N(3) 1.265(5), C(29)−N(4) 1.415(4), C(29)−N(5) 1.424(4), C(29)−N(6) 1.266(4), P(1)−P(2) 2.2251(14), N(1)−
P(1)−N(4) 108.28(15), N(2)−P(2)−N(5) 108.67(15), N(1)−C(1)−N(2) 111.7(3), N(4)−C(29)−N(5) 110.9(3); (b) P(1)−Cl(1) 2.1228(11),
P(2)−Cl(2) 2.1290(12), P(1)−N(1) 1.696(2), P(1)−N(4) 1.688(2), P(2)−N(2) 1.701(2), P(2)−N(4) 1.704(2), C(1)−N(1) 1.411(3), C(1)−
N(2) 1.416(3), C(1)−N(3) 1.270(3), N(1)−P(1)−N(4) 101.36(10), N(2)−P(2)−N(4) 100.52(10), N(1)−C(1)−N(2) 114.7(2).
12PCl−15PCl (Chart 5) were analyzed using density func-
tional theory (DFT) by determining their charge distributions,
Table 3. Atomic Partial Charges (δ) As Obtained from the
Natural Population Analysis of the Studied
Chlorophosphines
Chart 5. Compounds Studied Computationally
b
P
Cl
N
C/Si
Δδ(P−Cl)
a
a
a
a
a
5PCl
1.15
1.16
1.16
1.18
1.17
1.19
1.23
1.17
1.20
−0.36
−0.32
−0.33
−0.37
−0.38
−0.38
−0.45
−0.42
−0.53
−0.71
−0.68
−0.71
−0.73
−0.74
−1.09
−1.17
−0.68
−0.71
0.59
0.60
0.59
0.61
0.61
1.98
2.02
−0.08
−0.09
1.51
1.48
1.49
1.56
1.55
1.57
1.68
1.58
1.72
12PCl
13PCl
14PCl
15PCl
16PCl
17PCl
18PCl
19PCl
a
b
Average values. Absolute charge difference.
orbital structures, and relative P−Cl bond energies. For com-
parison, similar calculations were also performed for chlorophos-
phines incorporated in four-membered (PNSiN) and five-
membered (PNCCN) rings 16PCl−19PCl, which are
known to undergo facile metathesis.
within the molecular framework, thereby yielding the least polar
P−Cl bond. In contrast, the N-alkyl substituted variants 5PCl,
14PCl, and 15PCl display a less uniform charge distribution
and consequently a more ionic Pδ+−Clδ− interaction. These
results pinpoint N-alkyl derivatives of the target chlorophos-
phines as the most favorable candidates for halide abstraction.
The increased reactivity of the cyclohexyl derivative 5PCl to-
ward Me3SiOTf was also observed experimentally (vide supra).
A comparison of charge distributions calculated for 5PCl and
12PCl−15PCl with those of compounds for which halide
abstraction is reported to take place reveals, as expected, that
the P−Cl interaction is significantly more ionic in the latter
systems. The currently characterized chlorophosphines
16PCl−19PCl display equal and roughly 0.10 units greater
charge separation within the P−Cl bond as compared to the
identically N-substituted species with a P−N−C−N backbone
(see Table 3). Consequently, chlorophosphines with a P−N−
C−N backbone appear, by their nature, to be resilient to salt
Table 3 lists the atomic partial charges for the studied chloro-
phosphines as obtained from the natural population analysis
(NPA) of their Kohn−Sham electron densities.59 It is necessary
to stress that the absolute values of the calculated charges have
no physical meaning and it is only their relative magnitudes
which yield useful information about structure-induced changes
in the electron distribution. The atomic partial charges of
phosphorus and carbon in 5PCl and 12PCl−15PCl display
only small variations, which is in contrast to those calculated for
nitrogen and chlorine. These both show a more distinct de-
pendence on the electron withdrawing/donating nature of
N-substituents; most notably, the all-Ph derivative 12PCl has
the least charge concentrated on the electronegative elements
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Figure 5. Lowest unoccupied molecular orbitals of the studied N-heterocyclic phosphines and their orbital energies (eV, in parentheses).
metathesis. To analyze the origin of this phenomenon in detail,
we turned our attention to the frontier orbital structure of the
corresponding NHPs.
dissociation energy, which takes into account the energy gained
from relaxation of the fragment geometries upon bond break-
ing. However, when comparing bonding trends across multiple
similar systems, the snapshot-type picture given by the EDA
procedure is sufficient.
Table 4 lists the calculated EDA interaction energies along
with their division into individual contributions from Pauli re-
pulsion (EPauli) and electrostatic (Eelstat) and orbital interactions
(Eorb). The trend in Eint values confirms that the P−Cl bond is
markedly stronger in all chlorophosphines with a P−N−C−N
backbone as compared to systems previously reported to
undergo salt metathesis. The only exception are the fully alkyl
substituted 5PCl and 15PCl whose interaction energy equals
that of 19PCl. This is in accord with experimental observations,
which demonstrated the increased reactivity of 5PCl over other
halophosphines studied in the present work. Changes in the
Figure 5 shows the lowest unoccupied molecular (Kohn−
Sham) orbitals (LUMOs) and orbital energies of NHPs with
different backbones but identical N-substituents. This orbital is of
particular interest in the study of the nature of bonding in
equivalent chlorophosphines as it interacts directly with an elec-
tron pair from the halide anion to form the P−Cl bond. Although
the overall morphology of the orbitals in Figure 5 is rather similar,
a π-type molecular orbital (MO) with a major contribution from
the pz atomic orbital (AO) of phosphorus and a smaller admixture
of AOs from the two flanking nitrogen nuclei, their orbital energies
are vastly different with the LUMO of our target cations residing
by far the lowest. Frontier MO theory based arguments predict
that NHPs based on the P−N−C−N backbone should be the best
electron acceptors in the series and form the most covalent P−X
bonds. The observed differences in the charge distributions of the
investigated halophosphines can therefore be correlated with the
differing orbital characteristics of the corresponding NHPs. This
holds not only for systems shown in Figure 5 but also for other
chlorophosphines examined in the current work (see SI). In
addition, we note that the morphology of the LUMO is con-
sistent with the possibility to affect the details of the P−Cl in-
teraction by changing the exocyclic substituents from σ- (alkyl) to
π-type (aryl).
percentage of Eorb from the total attractive interactions (Eorb
+
Eelstat) show that as the P−Cl interaction weakens it also
becomes more electrostatic in nature. These results are fully in
accord with the picture gleaned from the calculated charge
distributions and LUMO energies. A correlation analysis on the
calculated P−Cl charge difference (Table 3) and the percentage
of covalent (orbital-type) character in the P−Cl bond (Table 4)
yields a linear relationship with a correlation coefficient R2 =
0.93 (Figure 6a). An equally good linear regression (Figure 6b)
is obtained if the LUMO energies of NHPs (see SI) are plotted
against the EDA interaction energies (Table 4).
Investigation of the unoccupied orbitals of the studied chloro-
phosphines offers a rationale for the role of bpy in capturing the
targeted NHP as a base-stabilized cation. As shown in Figure 7
for 12PCl, both of its two lowest unoccupied orbitals have
suitable morphologies to accept electron density from a
coordinating Lewis base. A geometry optimization for 12PCl
with bpy was run and led to the formation of an adduct with a
slightly (0.05 Å) longer P−Cl bond as compared to 12PCl.
This is consistent with the antibonding nature of the orbitals in
Figure 7, which in turn implies a weakened P−Cl interaction in
the adduct. Consequently, an EDA calculation conducted for
the bpy complex of 12PCl yields a P−Cl interaction energy
of −482 kJ mol−1 which is significantly less than that ob-
tained for free 12PCl and similar to that found for chloro-
phosphines with either PNSiN or PNCCN
backbones (Table 4).
Given that our target cations appear to be much better
electron acceptors than other known NHPs, the details of the
P−X interaction in the corresponding halophosphines should
be inferable not only from atomic partial charges but also from
calculated bond strengths. We examined the nature of the P−Cl
bond in compounds 5PCl and 12PCl−15PCl with the help of
energy decomposition analysis (EDA) which partitions the
bonding interaction between a cationic NHP and a chloride
anion into physically meaningful components (see Table 4).60
Table 4. Results from Energy Decomposition Analyses of
a
P−Cl Bonding in the Studied Chlorophosphines
b
b
EPauli
Eelstat
Eorb
Eint
5PCl
947
1065
1021
909
910
654
869
774
932
−875 (57%)
−941 (56%)
−924 (56%)
−883 (58%)
−870 (58%)
−726 (64%)
−827 (60%)
−801 (62%)
−873 (58%)
−651 (43%)
−754 (44%)
−717 (44%)
−628 (42%)
−619 (42%)
−402 (36%)
−551 (40%)
−511 (38%)
−639 (42%)
−580
−630
−621
−602
−579
−475
−509
−539
−580
12PCl
13PCl
14PCl
15PCl
16PCl
17PCl
18PCl
19PCl
The collective results from computational investigations en-
able us to conclude that the exceptionally good electron ac-
ceptor properties of the target NHP cations render the cor-
responding halophosphines reluctant toward halide abstraction,
unless there is additional stabilization from a Lewis base. The
P−X bonds are not only stronger but also more covalent than
in analogous systems known to undergo salt metathesis. Con-
sequently, triflate-based reagents show no or only limited
reactivity with 1PCl−5PCl, most likely simply due to increased
energy required to break the P−Cl bond.
a
b
Energies are reported in kJ mol−1. Percentage of total attractive
interactions given in parentheses
It should be noted here that the instantaneous interaction
energy (Eint) given by the EDA is not (the negative of) bond
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Article
Figure 6. Correlation between (a) the covalent contribution and the charge difference in the P−Cl bond of the studied chlorophosphines, and (b)
correlation between the LUMO energies of the studied NHPs and the EDA P−Cl interaction energies of the studied chlorophosphines.
Figure 7. Two of the lowest unoccupied molecular orbitals of 12PCl.
University of Western Ontario, the Academy of Finland, the
Technology Industries of Finland Centennial Foundation, and
CONCLUSION
■
In summary, 1PCl−4PCl and 1PBr represent the first examples
of N-heterocyclic chlorophosphines supported by (dianionic)-
guanidinate ligands. The experimentally observed reluctance to
form the corresponding N-heterocyclic phosphenium cations was
rationalized by computational studies. Analysis of the charge
distributions, orbital structures, and relative P−Cl bond
energies of the computationally studied compounds gives cor-
roborating evidence of the strong P−X bond and strong Lewis
acidity of the cationic species. Reactivity studies of 1PCl
demonstrate the high degree of strain induced by the four-
membered ring, which leads to chemically and thermally in-
duced carbodiimide elimination, as well as a novel ring expan-
sion by insertion of chloro(imino)phosphine into a P−N bond
of 1PCl. These results demonstrate the electronic nature and
alluring reactivity of diaminochlorophosphines restricted in a
four-membered ring.
the University of Jyvaskyla for financial support. We also thank
̈
̈
CSCThe IT Center for Science in Espoo, Finland, for their
support in providing computational resources. Dr. C.D. Martin
and J.T. Price are also thanked for collection of X-ray diffraction
data.
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ASSOCIATED CONTENT
■
S
* Supporting Information
CIF data for 3, 1PCl-4PCl, 1PBr, 6Al, 9, 10, and 11; solid-state
structures of 3, 4, 7, 8, and 10; selected H NMR spectra;
optimized structures; and LUMO energies of computationally
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AUTHOR INFORMATION
■
(12) Momming, C. M.; Otten, E.; Kehr, G.; Frohlich, R.; Grimme, S.;
̈
̈
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pragogna@uwo.ca
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