Exploration of tin-catalyzed phosphine dehydrocoupling: Catalyst effects and observation of tin-catalyzed hydrophosphination

Article abstract of DOI:10.1016/j.ica.2014.07.002

The phosphine substrate scope in dehydrocoupling reactions catalyzed by Cp^?₂SnCl₂ (Cp^? = pentamethylcyclopentadienyl, 1) have been explored. Catalyst variants R₂SnX₂ (R = Cp^?, Ph; X = Cl, Me, Ph) were also tested, which revealed that activity is dependent on the Cp^? ligands as well as more electron withdrawing X ligands. Steric factors at the phosphine substrate are also important. Compound 1 was found to be a catalyst for hydrophosphination of styrene, 2,3-dimethylbutadiene, and diphenylacetylene with phenylphosphine, which is the first example of a p-block catalyst for hydrophosphination.

Full text of DOI:10.1016/j.ica.2014.07.002

Inorganica Chimica Acta 422 (2014) 141–145
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Exploration of tin-catalyzed phosphine dehydrocoupling: Catalyst
effects and observation of tin-catalyzed hydrophosphination
Karla A. Erickson a, Lily S.H. Dixon b, Dominic S. Wright b, Rory Waterman a,
⇑
a Department of Chemistry, University of Vermont, Burlington, VT 05405 0125, USA
b Department of Chemistry, Cambridge University, Cambridge CB2 1EW, UK
a r t i c l e i n f o
a b s t r a c t
The phosphine substrate scope in dehydrocoupling reactions catalyzed by Cp⁄2SnCl2 (Cp⁄ = pentamethyl-
cyclopentadienyl, 1) have been explored. Catalyst variants R2SnX2 (R = Cp⁄, Ph; X = Cl, Me, Ph) were also
tested, which revealed that activity is dependent on the Cp⁄ ligands as well as more electron withdrawing
X ligands. Steric factors at the phosphine substrate are also important. Compound 1 was found to be a cat-
alyst for hydrophosphination of styrene, 2,3-dimethylbutadiene, and diphenylacetylene with phenylphos-
phine, which is the first example of a p-block catalyst for hydrophosphination.
Article history:
Received 29 May 2014
Received in revised form 30 June 2014
Accepted 2 July 2014
Available online 14 July 2014
Dedicated to Prof. T. Don Tilley on the
occasion of his 60th birthday. We look
forward to many more years of his scientific
leadership, steadfast mentorship, and kind
friendship
Ó 2014 Elsevier B.V. All rights reserved.
Keywords:
Tin(IV) catalysis
Phosphine dehydrocoupling
Hydrophosphination
Main group catalysts
1. Introduction
compounds that engage in classically transition-metal-mediated
catalysis [1–4], including hydrogenation [7], hydrophosphination
Transition-metal catalysts are responsible for many powerful
reactions. However, due to increasing scarcity and price, main
group catalysts have become appealing as potential alternatives.
Main group catalysis is a burgeoning field that features several
examples of transformations that are equally efficient as those
with transition-metal complexes [1–4]. These are exciting develop-
ments as main group elements had been viewed as largely
unsuited towards catalysis except as Lewis acids [2]. This view
originates from the lack of readily accessible and reversible redox
reactivity under mild conditions as is known for many transition-
metal systems. For example, the d orbital energies of transition-
metals allows for facile reductive elimination and oxidative
addition reactions as well as potentially labile coordination of
ligands. However, there are many powerful reactions that do not
[8–12], hydrosilylation [13], dehydrocoupling [14], heterodehydro-
coupling [15–20], and hydroamination [21–24].
Recently, we reported on the dehydrogenation of amine bor-
anes with tin catalysts, which exhibits an unusual dependence of
mechanism on amine-borane substrate [17]. Those studies were
prompted by Wright and coworkers’ report of phosphine dehydro-
coupling catalyzed by a tin(IV) complex, Cp⁄2SnCl2 (Cp⁄ = pentam-
ethylcyclopentadienyl, 1), at 10 mol % catalyst loading (Table 1) [25].
Phosphine dehydrocoupling reactions have been rarely
catalyzed by main group compounds [26], and a limited number
of transition-metal catalysts have been reported for the transfor-
mation [27,28]. Stoichiometric main group-mediated phosphine
dehydrocoupling is better known in the literature than catalytic
examples, and tin has been implicated in both [29,30].
require changes in the oxidation state of the metal (e.g.,
r
-bond
Tin-catalyzed dehydrogenative P–P bond formation was depen-
dent on the oxidation state of tin. Only tin(IV) showed catalytic
activity, whereas stoichiometric phosphine dehydrocoupling was
observed in reactions with a tin(II) complex, Cp⁄2Sn. It was proposed
that the redox instability of this and other Sn(II) complexes render
them non-catalytic [1,29–31]. Further evidence from isolated crys-
talline byproducts indicated that Cp⁄ was subject to protonation by
metathesis), and the possibility of using main group metals for
these redox-neutral processes has fueled interest in main group
catalysis [1,5,6]. Currently, there are many examples of main group
⇑
Corresponding author. Tel.: +1 802 656 0278; fax: +1 802 656 8705.
E-mail address: rory.waterman@uvm.edu (R. Waterman).
http://dx.doi.org/10.1016/j.ica.2014.07.002
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

142
K.A. Erickson et al. / Inorganica Chimica Acta 422 (2014) 141–145
Table 1
Table 2
Reported conversions of RPH2 to dehydrocoupled prod-
Results of the catalytic dehydrocoupling of new substrates, RR0PH, using 1a.
ucts using 1 [25]a.
Entry
R
R0
Conversion (%)
Major product (%)
R
Conversion (%)
1
2
3
4
5
Ph
H
H
Ph
Cy
Mes
80
47
41
40
34
PhPH–PHPh
dmpPH–PHdmp
Ph2P–PPh2
Cy2P–PCy2
Mes2P–PMes2
Cy
80
68
82
65
dmp
Ph
tBu
Fcb
Cyb
Mes
FcCH2
a
a
Conditions: 60 °C for 4 d in THF.
Fc = ferrocenyl, (C5H5)Fe(C5H4).
Conditions: 60 °C for 3 d in benzene-d6 10 mol % catalyst loading. Percent
b
conversion was determined through integration of an external standard (a glass
capillary solution of PPh3 in benzene-d6) by 31P{1H} NMR spectroscopy.
b
Cy = cyclohexyl.
substrate, and that Sn(IV) can be reduced to Sn(II), which is catalyst
deactivating. The proposed mechanism for this transformation is
similar to that hypothesized by Stephan for phosphine dehydrocou-
pling catalyzed by Cp⁄2ZrHꢀ3 (Scheme 1) [32].
While the oxidation state of tin played a tremendous role in cat-
alytic activity, ligand and substrate effects merited further study.
Additionally, the facile P–H activation displayed by 1 suggested
that further catalysis is possible, and hydrophosphination is a good
initial target transformation owing to its broad utility [33–40].
Herein, both efforts are described.
these reactions were monitored by 31P{1H} NMR spectroscopy,
and percent conversions were calculated by integration against
an external standard. These results display a similar trend to those
previously reported in that increased steric bulk of substrate leads
to decreased conversion [25]. For example, the dehydrocoupling of
PhPH2 goes further to completion than dmpPH2 under the same
conditions (Table 2, Entries 1 and 2). In the dehydrocoupling of
PhPH2 both diastereomers (rac and meso) are formed in almost
equal amounts. However, in the dehydrocoupling of dmpPH2 only
one diastereomer is observed (vide infra).
The activity of 1 towards secondary phosphines was probed,
and it was found that 1 gave lowered conversions as compared
to reactions with primary phosphines (Table 2, Entries 4–6). There
is no strong trend here. Lowered conversion to product is observed
with more sterically encumbered but electron rich Mes2PH
(Mes = 2,4,6-trimethylphenyl) in comparison to Ph2PH and Cy2PH.
The dialkylphosphine Cy2PH gives similar conversion to products
as Ph2PH. An apparent electronic dependence is inconsistent with
2. Results and discussion
2.1. Catalyst effects on phosphine dehydrocoupling
In the initial report of phosphine dehydrocoupling using Cp2⁄-
SnCl2 (1), the substrate scope consisted of primary alkyl phosphines.
Here, the activity of 1 towards other phosphine substrates was
explored with primary aryl phosphines, PhPH2 and dmpPH2,
(dmp = 2,6-dimesitylphenyl) as well as secondary aryl and alkyl
phosphines (R2PH, R = Ph, Cy (cyclohexyl), and Mes (mesityl). These
substrates were treated with 1 under reaction conditions similar to
those reported, which all resulted in H2 evolution, and the results
are summarized in Table 2.
Combination of the reagents in deuterated solvent resulted in
bright yellow solutions, which gradually became colorless as prod-
ucts formed. A fine colorless precipitate was also observed in all
reactions that could not be definitively identified. The progress of
r
-bond metathesis [5]. though a trend has not truly been
identified based on two substrates (Ph, Mes) alone. Likewise, the
products of the dehydrocoupling of secondary phosphines (e.g.,
R2P–PR2) appear to discount an
pathway [6].
a-phosphinidene elimination
This supposition was buttressed through the dehydrocoupling of
dmpPH2. In some stoichiometric systems, the formation of
dmpP = Pdmp has been considered indicative of the condensation
of two phosphinidene fragments [40,41]. Here, it appears that
a-phosphinidene elimination does not occur. No products of an
apparent phosphinidene elimination such as a diphosphine are
observed, and instead, a resonance in the 31P{1H} NMR spectra at
d = ꢀ101 ppm with JPH = 227 Hz is observed that is tentatively
assigned as dmpPH–PHdmp based on similarity to Mes⁄PH–PHMes⁄
and MesPH–PHMes [42–44].
This broader scope of phosphine substrates indicates that steric
factors play a role in the efficiency of the catalysis. A second area of
investigation was ligand effects at the catalyst. Three other Sn(IV)
Table 3
The effect of catalyst, L2SnL02, on phosphine dehydrocoupling to
products (% conversion)a.
Compound
L
L0
Conversion (%)
1
2
Cp⁄
Cp⁄
Cp⁄
Ph
Cl
80
33
73
1
Me
Ph
Cl
3
4b
4c
Ph
Cl
2
a
Conditions: 10 equiv. PhPH2 in benzene-d6 at 60 °C. Percent
conversion was determined through integration of an external
standard (a glass capillary solution of PPh3 in benzene-d6) by
31P{1H} NMR spectroscopy.
b
ꢁ20 equiv tBuPH2 in THF.
Scheme 1. Proposed catalytic cycle for phosphine dehydrocoupling using
1
c
ꢁ12 equiv. o-(PH2)2C6H4.
adapted from reference 30.

K.A. Erickson et al. / Inorganica Chimica Acta 422 (2014) 141–145
143
compounds, Cp⁄2SnR2 (R = Me, 2; Ph, 3) and Ph2SnCl2 (4), were used
to dehydrocouple primary phosphines. The percent conversion of
these reactions were compared to those achieved in reactions cata-
lyzed by 1 under the same conditions (Table 3).
dehydrocoupling (PhHP–PHPh, [PPh]4, and [PPh]6) as well as PhPH2
were observed. These dehydrocoupling products are similar to
stoichiometric reactions of tin compounds with phosphines in the
presence of base [1,29–31]. The formation of (PhPH)2 has been
reported in similar reactions between lithiated phosphine and
Sn(II) complexes and was an indicator that Sn(II) phosphinidiide
cagwwwes formed [31]. When Ph2PLi was reacted with 1, only
Ph2P–PPh2 was observed.
Main group hydrides have been implicated as important
reactive intermediates in several main group catalytic reactions
[1,17,45,46]. To attempt to observe hydrides or other catalyst spe-
cies, 119Sn{1H} NMR spectra were obtained for phosphine dehydro-
coupling reactions. Unfortunately, no signal from Sn-containing
species could be detected. This situation may result from multiple
tin species in the reaction mixture at relatively low concentrations
and is consistent with the difficulty in observing new 31P{1H} NMR
resonances.
The more bulky but also more electron withdrawing phenyl
ligands of compound 3 resulted in slightly lowered percent conver-
sions to products as compared to those of catalyst 1 (73% and 80%
conversion, respectively). The significantly less bulky methyl
ligand in complex 2 is more electron donating, and the reaction
with PhPH2 resulted in significantly lowered product conversions
than those of both 1 and 3. In the dehydrocoupling of 2, loss of
Cp⁄ was observed in the 1H NMR spectrum, which is also observed
in reactions of 1 and 3 with phosphine. Methane loss could not be
definitively observed due to multiple resonances region of meth-
ane in the 1H NMR spectra. Thus, methane may have been
obscured. Despite apparent protonation of ligands, clear spectro-
scopic evidence for a tin phosphido intermediate could not be
accrued.
In dehydrocoupling reactions with 4, it is apparent that the Cp⁄
ligand is essential for catalytic dehydrocoupling and leads to the
conclusion that there are not only steric but electronic effects at
2.2. Hydrophosphination of alkenes and alkynes
Facile P–H activation at tin suggested that other bond-forming
reactions to phosphorus may be possible. Hydrophosphination is
a natural choice because of its broad importance to a variety of
fields [33–40,47]. The heightened reactivity of 1 with primary ver-
sus secondary phosphines governed the choice to explore this sub-
strate. Initial experiments were designed using PhPH2 with a series
of unsaturated organic substrates and catalytic amounts of 1 to
probe the viability of tin-catalyzed hydrophosphination.
t
play. In the original report, BuPH2 was dehydrocoupled with 1 in
68% conversion, however when 4 is used as a catalyst, the catalytic
activity is drastically reduced. While 4 has reduced steric hindrance
around the metal center as compared to 1, it is hypothesized that
lower activity results from the ligand being less susceptible to pro-
tonation, which is a barrier to generating the active tin catalyst.
From these results, there are several inferences that can be
made. First, the activity of the catalyst appears to be driven by
the electronics of the ligand. The chloro ligand appears to be nec-
essary for catalytic activity in Sn(IV) complexes as 2 and 3 are both
less active than 1. Compound 3 does exhibit greater catalytic activ-
ity than 2, which is consistent with more electron-withdrawing
phenyl substituents. Observation of C5Me5H during catalysis sug-
gests that protonation of the Cp⁄ is a common if not necessary step
to initiate phosphine dehydrocoupling, though the observation
that Ph2SnCl2 is catalytically inactive shows that Cp⁄ is essential,
even if it is lost. The greater observed catalytic activity for 1 as
compared to 2 and 3 may be consistent with the stabilization of
the tin(IV) center imparted by the more electronegative Cl ligands
(hence the greater catalyst lifetime and turnovers).
Some efforts were made to observe a Sn–P intermediate through
31P{1H} NMR spectroscopy of stoichiometric reactions. Phosphido
reagents (PhPHLi and Ph2PLi) were added to cold benzene solutions
of 1, and upon mixing, NMR spectra were collected. These reactions
were allowed to further react at ambient temperature under mon-
itoring by 31P{1H} NMR spectroscopy. These experiments did not
result in the formation of any new intermediate that could be dis-
cerned by either 31P{1H} or 119Sn{1H} NMR spectroscopy. Instead,
upon addition of PhPHLi, immediate formation of the products of
Styrene, 1-hexene, 2,3-dimethylbutadiene, diphenylacetylene,
and phenylacetylene were chosen as substrates of interest. Classic
hydrophosphination substrates that are Michael acceptors were
deliberately shunned in these studies to potentially identify inser-
tion-based reactivity. In initial studies, these reactions were
probed using 10 mol % catalyst loading in benzene-d6 at 60 °C
and with one equiv. of PhPH2 for ca. 18 h after which 1H, 31P{1H},
and 13C NMR spectra were collected. In the 13C NMR spectrum,
the absence of diagnostic sp2 carbon resonances is noted, indicat-
ing the unsaturated substrate is fully reduced within the limits of
detection. The presence of characteristic 31P{1H} NMR resonances
demonstrated that the hydrophosphination of styrene and 2,3-
dimethylbutadiene had occurred. Roughly half of diphenylacety-
lene was hydrophosphinated as well. 1-Hexene was inert under
these preliminary reaction conditions, and further attempts to
optimize this substrate were not pursued. The hydrophosphination
reaction of phenylacetylene under these conditions afforded very
low conversions to product and also suffered from low selectivity.
Many unidentified resonances were observed in the 31P{1H} NMR
spectrum. No additional efforts towards optimization were made.
In the hydrophosphination of styrene, a mixture of secondary and
tertiary phosphine products (PhCH2CH2PHPh and (PhCH2CH2)2PPh,
Table 4
Product distribution of hydrophosphination reactions (relative %)a.
Substrate
Mono- hydrophosphination
Bis-hydrophosphination
PhHP–PHPh
% conv. of substrate
Rxn time (h)
Styreneb
73 [48]
56
83 [48]
99
21 [48]
40
0
0
0
0
0
0
0
2
4
17
1
100
44
55
29
54
100
100
100
100
0
25
50
30
ꢁ10
5
18
5
18
18
18
18
18
18
Styrenec
2,3-Dimethylbutadieneb
2,3-Dimethylbutadienec
1-Hexene
0
Diphenylacetyleneb
Diphenylacetylenec
Diphenylacetylened
Phenylacetyleneb
19 E, 37 Z [47]
34 E, 45 Z
24 E, 16 Z
14 E, 32 Z [49]
a
b
c
Reactions in benzene-d6, heated at 60 °C. Relative percent yield determined through 31P{1H} NMR spectroscopy.
ꢁ2 equiv. PhPH2.
1 equiv. PhPH2.
d
ꢁ2 equiv. PhPH2, reaction heated at 75 °C.

144
K.A. Erickson et al. / Inorganica Chimica Acta 422 (2014) 141–145
Table 5
respectively) were formed, demonstrating that styrene is the limit-
ing reagent. In the hydrophosphination of diphenylacetylene, some
unknown products were also observed in the reaction mixtures in
addition to a mixture of E and Z isomers of PhCH = CPhPHPh. Like-
wise, in the hydrophosphination reaction with 2,3-dimethylbutadi-
ene, several unidentifiable products were formed. However, by
increasing the equivalents of PhPH2 to approximately two, signifi-
cant improvements in selectivity was observed in these reactions.
These reactions were monitored closely through 31P{1H} NMR spec-
troscopy where it was found that styrene and 2,3-dimethylbutadi-
ene required only ꢁ5 h to reach completion at 60 °C. In optimized
reactions, the only byproduct of significant quantities are phosphine
dehydrocoupling products (Table 4).
The preferential formation of the Z isomer was particularly
interesting in the hydrophosphination of the acetylenes at 60 °C
using either one equiv or excess amounts of PhPH2. This isomer
was the minor product in other reported catalytic examples of
hydrophosphination [47]. A brief foray into the study of how these
two isomers form led to the discovery that the temperature of the
reaction has a large effect. In reactions at 75 °C, the E isomer
predominates in the hydrophosphination of diphenylacetylene.
Further study into the origin of regioselectivity, selectivity, and
optimization of catalysis are under investigation. Nevertheless,
these initial results are exciting. Alkaline earth elements are known
and effective catalysts for hydrophosphination [8–12], but to the
best of our knowledge, this is the first example of a p-block
element as a catalyst for this reaction.
Catalytic dehydrocoupling of phosphines using Sn(IV) catalysts.
Catalyst (mg, mmol)
Substrate (mg, mmol)
1, (6.2 mg, 0.013 mmol)
1, (8.2 mg, 0.018 mmol)
1, (7.2 mg, 0.016 mmol)
1, (4.2 mg, 0.009 mmol)
1, (5.4 mg, 0.012 mml)
2, (4.2 mg, 0.010 mmol)
3, (4.1 mg, 0.008 mmol)
4, (10 mg, 0.031 mmol)
PhPH2 (14 mg, 0.12 mmol)
Ph2PH (37 mg, 0.20 mmol)
dmpPH2 (47 mg, 0.13 mmol)
Cy2PH (20 mg, 0.021 mmol)
Mes2PH (34 mg, 0.13 mmol)
PhPH2 (14 mg, 0.12 mmol)
PhPH2 (12 mg, 0.11 mmol)
tBuPH2 (50 mg, 0.55 mmol)
Table 6
Catalytic hydrophosphination of unsaturated organic substrates.
Catalyst
Substrate
PhPH2
1 (4.5 mg,
diphenylacetylene (20 mg,
0.11 mmol)
2,3-dimethylbutadiene (28 mg,
0.34 mmol)
20 mg,
0.18 mmol
56 mg,
0.51 mmol
28 mg,
0.009 mmol)
1 (12 mg,
0.026 mmol)
1 (5.4 mg,
0.012 mmol)
1 (11 mg,
0.023 mmol)
1 (4.7 mg,
0.01 mmol)
styrene (16 mg, 0.16 mmol)
0.25 mmol
63 mg,
1-hexene (23 mg, 0.27 mmol)
0.57 mmol
phenylacetylene (15 mg, 0.15 mmol) 25 mg,
0.23 mmol
or standard Schlenk techniques. Benzene-d6 was degassed and
dried over NaK alloy. Anhydrous THF-d8 was used as received.
1H, 31P{1H}, and 119Sn{1H} NMR spectra were recorded on a Bruker
Ascend 500 MHz NMR spectrometer. Reported 1H NMR resonances
are referenced to residual solvents (benzene-d6 = d 7.16 ppm,
THF-d8 = d 1.72 or 3.58 ppm). All chemicals were either synthe-
sized from literature methods or purchased from commercial sup-
pliers and dried by conventional means. For pertinent NMR
spectra, see Supporting information (see Tables 5 and 6).
3. Conclusions
The goal of this work was to determine whether the activity of
Sn(IV) catalysts towards phosphine dehydrocoupling could be
enhanced through tuning the electronic and steric properties of
the ligands. In addition, the expansion of the phosphine substrate
scope was sought to afford additional insight into the mechanism.
Finally, the activity of tin compounds towards P–H activation was
applied to hydrophosphination.
4.2. Catalytic phosphine dehydrocoupling reactions
Analysis of the products formed through the dehydrocoupling
of bulky primary and secondary phosphines seems to support the
catalytic cycle proposed by Wright and coworkers [25]. By explor-
ing the phosphine substrate scope, it is apparent that increased
steric bulk diminishes activity. This was observed in reactions with
complex 1 and bulky primary phosphines as well as secondary
All reactions were conducted using a J-Young type polytetra-
fluoroethylene (PTFE)-valved NMR tube in benzene-d6 or THF-d8.
After the addition of reagents, an initial NMR spectrum was
obtained. The solution was frozen and the headspace was evacu-
ated. This was repeated at regular intervals during the course of
the reaction to remove H2. After thawing, the NMR tube was
heated at 60 °C. The yellow reaction mixture gradually turned clear
and resulted in the formation of colorless precipitates. All NMR
spectra were collected at 25 °C.
phosphines. These results help to discount an
mechanism and generally support the hypothesis of a mechanism
that is more similar to a process involving -bond metathesis or
a elimination-like
r
1,2-addition across a multiple bond. Further improvements in
PhPH2 dehydrocoupling were not achieved using other Sn(IV) cat-
alyst precursors. Readily labile Cp⁄ ligands and electronegative
ligands (Cl) appear to provide optimal catalytic activity.
Styrene, diene, and alkyne moieties were successfully hydropho-
sphinated with 1 and ꢁ2 equiv. of PhPH2. These reactions proceeded
at mild temperatures and resulted in good selectivity for the mono-
hydrophosphinated products using diphenylacetylene, styrene and
2,3-dimethylbutadiene, which justifies greater exploration the
alkene and alkyne substrate scope as well as optimization of these
initial results. In these preliminary studies some reaction conditions
seemed to influence the E to Z ratios in alkyne hydrophosphination.
4.3. Synthesis of Cp⁄2SnMe2 (2)
A solution of Cp⁄2SnCl2 (100 mg, 0.22 mmol) in hexanes (2 mL)
was charged in a scintillation vial and cooled to ca ꢀ30 °C. Methyl
lithium (0.3 mL of a 1.6 M solution in hexanes) was subsequently
added to this solution, resulting in a color change from yellow to
clear and the formation of a white precipitate. This reaction was
allowed to stir at room temperature for 1 h. Afterwards, the mixture
was filtered through a plug of glass fiber paper. The residual solvent
was removed under vacuum yielding a fine white powder (91 mg,
0.22 mmol) in quantitative yield. The formation of this product
was confirmed through comparison of 1H and 13C{1H} NMR spectra
with literature assignments [50].
4. Experimental considerations
4.1. General considerations
4.4. Synthesis of Cp⁄2SnPh2 (4)
All manipulations were performed under a dried nitrogen atmo-
sphere with dry, oxygen-free solvents using an M. Braun glovebox
A solution of Ph2SnCl2 (0.38 g, 0.0011 mol) in hexane was
cooled to –30 °C. Freshly prepared Cp⁄Li (0.31 g, 0.0022 mol)

K.A. Erickson et al. / Inorganica Chimica Acta 422 (2014) 141–145
145
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4.5. Catalytic hydrophosphination reactions
All reactions were conducted using a J-Young type polytetra-
fluoroethylene (PTFE)-valved NMR tube in benzene-d6. Substrate
and phosphine were mixed together prior to the addition of cata-
lyst as a solid. After the addition of reagents to the NMR tube,
the reaction mixture was heated to 60 °C for ca 18 h. NMR spectra
were obtained at 25 °C.
Acknowledgements
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This work was supported by the U. S. National Science
Foundation (NSF, CHE-0747612 and CHE-1265608 to R.W.) and
the E. U. (Advanced Investigator Award, D.S.W.). Particular acknowl-
edgement is made for an International Collaborations in Chemistry
supplement (US NSF) that fueled this collaborative investigation.
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