Reactions of coordinated hydroxymethylphosphines with NH-functional amines: The phosphorus lone pair is crucial for the phosphorus Mannich reaction

Article abstract of DOI:10.1039/c1dt10586h

Non-coordinated hydroxymethylphosphines react readily with primary and secondary amines by the phosphorus Mannich reaction. To determine if this reactivity can be used to synthesize phosphine macrocycles, trans-Fe(DHMPE) ₂Cl₂ (DHMPE = 1,2-bis(dihydroxymethylphosphino)ethane) was prepared and reacted with various amines. However, no phosphorus Mannich reactivity was observed. In order to understand why no reactions occurred, the Mannich reactivity of the borane-coordinated hydroxymethylphosphines DHMPE·2BH₃ and Ph₂PCH₂OH·BH ₃ was investigated. These borane-coordinated phosphines also did not undergo the phosphorus Mannich reaction. These results suggest that the lone pair of electrons on the phosphorus atom is essential for the phosphorus Mannich reaction to occur, and therefore it is not possible to use this reaction in a templated synthesis of phosphine macrocycles. It is speculated that the mechanism of the phosphorus Mannich reaction may involve a methylenephosphonium intermediate, analogous to an iminium in the standard Mannich reaction. X-ray crystal structures of trans-Fe(DHMPE)₂Cl₂ and DHMPE·2BH₃ are also presented. Both crystal structures display an extended hydrogen-bonding network in the solid state. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt10586h

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PAPER
Reactions of coordinated hydroxymethylphosphines with NH-functional
amines: the phosphorus lone pair is crucial for the phosphorus Mannich
reaction†
Charles D. Swor, Kyle R. Hanson, Lev N. Zakharov and David R. Tyler*
Received 5th April 2011, Accepted 14th June 2011
DOI: 10.1039/c1dt10586h
Non-coordinated hydroxymethylphosphines react readily with primary and secondary amines by the
phosphorus Mannich reaction. To determine if this reactivity can be used to synthesize phosphine
macrocycles, trans-Fe(DHMPE)2Cl2 (DHMPE = 1,2-bis(dihydroxymethylphosphino)ethane) was
prepared and reacted with various amines. However, no phosphorus Mannich reactivity was observed.
In order to understand why no reactions occurred, the Mannich reactivity of the borane-coordinated
hydroxymethylphosphines DHMPE·2BH3 and Ph2PCH2OH·BH3 was investigated. These
borane-coordinated phosphines also did not undergo the phosphorus Mannich reaction. These results
suggest that the lone pair of electrons on the phosphorus atom is essential for the phosphorus Mannich
reaction to occur, and therefore it is not possible to use this reaction in a templated synthesis of
phosphine macrocycles. It is speculated that the mechanism of the phosphorus Mannich reaction may
involve a methylenephosphonium intermediate, analogous to an iminium in the standard Mannich
reaction. X-ray crystal structures of trans-Fe(DHMPE)2Cl2 and DHMPE·2BH3 are also presented.
Both crystal structures display an extended hydrogen-bonding network in the solid state.
the appropriate primary amines (or ammonia) and a suitable
Introduction
metal template (Scheme 1). This property was first discovered
Water-soluble phosphines are useful ligands for aqueous catalysis,
by Jeffery in 2000 while investigating a red coloration obtained
aqueous/organic biphasic catalysis,1–7 biochemical8 and medicinal
when aqueous solutions of (P(CH2OH)4)2SO4 (a biocide) were
applications,4,9–13 and electroless metal plating.14 Of the water-
soluble phosphines, hydroxymethylphosphines (RnP(CH2OH)3-n
added to oil deposits containing FeS.42 Such self-assembly has
been further developed by Burrows and colleagues.43,44 The self-
assembled molecules are presumed to be generated by a series
of phosphorus Mannich reactions between hydroxymethylphos-
phines and ammonia. We hypothesized that if this process could
be better understood and harnessed then it could lead to a
variety of easily synthesized polyphosphine ligands and their
complexes, including macrocyclic phosphine complexes (eqn (3)).
)
are especially attractive because of their straightforward prepara-
tion from PH-functional phosphines (or PH3) and formaldehyde
(eqn (1)).15–21 The hydroxymethylphosphine ligands are easily func-
tionalized by using the phosphorus Mannich reaction with NH-
functional amines (eqn (2)).22–31 Phosphine ligands with ancillary
amino groups are useful because they can provide basicity in the
secondary coordination sphere of metal complexes and because
complexes with such ligands can activate small molecule substrates
as both Lewis acids and Lewis bases.32–41 It is interesting to note
that the aminomethylphosphines generated by these reactions are
generally air-stable, a rare trait among phosphine ligands.
(1)
(2)
A useful property of hydroxymethylphosphines is their ability
to self-assemble into multidentate ligands when combined with
University of Oregon, Eugene, OR 97403. E-mail: dtyler@uoregon.edu
† Electronic supplementary information (ESI) available. CCDC reference
numbers 821007–821008. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt10586h
Scheme 1 Self-assembling metal-phosphine complexes generated by the
phosphorus Mannich reaction.42–44
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In this paper, we report the results of our investigation on the
phosphorus Mannich reaction using the DHMPE ligand (1,2-
bis(dihydroxymethylphosphino)ethane) and its metal complexes.
In addition to demonstrating the synthetic possibilities and
limitations of the phosphorus Mannich reaction, the results have
implications for the mechanism of the reaction.
(3)
Fig. 1 ORTEP plot of trans-Fe(DHMPE)2Cl2, showing intramolecular
hydrogen bonds. Ellipsoids are drawn at 50% probability. Some hydrogen
atoms have been omitted for clarity.
Results and discussion
Synthesis of trans-Fe(DHMPE)2Cl2
In order to investigate the feasibility of the reaction in eqn
(3) as a route to phosphine macrocycle complexes, the trans-
Fe(DHMPE)2Cl2 complex was synthesized and characterized. The
reaction of 2 equiv. of DHMPE with 1 equiv. of FeCl2·4H2O
in THF gave, within seconds, a deep green solution. After
stirring for five minutes, a lime-green solid precipitated, leaving
behind a pale red solution. The solid is insoluble in all but
strongly hydrogen-bond-accepting solvents (pyridine, DMF, and
DMSO), and spectroscopic and X-ray analysis showed it to be
1
trans-Fe(DHMPE)2Cl2 (eqn (4)). The 31P{ H} NMR spectrum in
DMSO showed a single sharp peak at +73.2 ppm, indicative of
a trans-FeCl2P4 coordination geometry in the complex. Crystals
suitable for X-ray diffraction were grown by removing an aliquot of
the reaction mixture immediately after mixing the reactants and al-
lowing the product to quiescently precipitate from solution. X-ray
crystallography confirmed the structure as trans-Fe(DHMPE)2Cl2
(Fig. 1). The coordination geometry is pseudo-octahedral, with
the iron and equatorial phosphines exactly coplanar, and the
molecule is centrosymmetric (Ci symmetry). The chelate angle
of the DHMPE ligand is 84.5◦, and the axial chloride ligands are
tilted 3.2◦ from orthogonal to the equatorial plane. Selected bond
lengths are shown in Table 1.
Fig. 2 Packing of trans-Fe(DHMPE)2Cl2, showing the three-dimensional
hydrogen-bonded network.
˚
Table 1 Selected bond lengths (A) and angles (deg) for trans-
Fe(DHMPE)2Cl2
Fe(1)—P(1)
Fe(1)—P(2)
Fe(1)—Cl(1)
2.2484(7)
2.2312(7)
2.3626(7)
P(1)—Fe(1)—P(2)
P(1)—Fe(1)—Cl(1)
P(2)—Fe(1)—Cl(1)
84.48(2)
89.27(2)
86.93(2)
bonding network in the crystal structure, which may account for
the product’s insolubility in all but the most strongly hydrogen-
bond-accepting organic solvents.
(4)
Reactivity of DHMPE complexes with primary amines
Two intramolecular hydrogen bonds are present in the molecule:
an OH ◊ ◊ ◊ O bond between two hydroxymethyl groups attached
to the same phosphorus atom, and an OH ◊ ◊ ◊ Cl hydrogen bond
between each chloro ligand and one of the hydroxyl groups on
the other phosphorus atom. In addition, examination of the
crystal packing revealed four intermolecular hydrogen bonds (Fig.
2, Table 2), three of which are OH ◊ ◊ ◊ O hydrogen bonds and
one of which is an (admittedly weak) OH ◊ ◊ ◊ Cl bond. These
hydrogen bonds form an extended, three-dimensional hydrogen-
The phosphorus Mannich reaction between hydroxymethylphos-
phines and amines42–44 suggests that macrocyclic phosphines
might be synthesized by reacting Fe(DHMPE)2Cl2 with the
appropriate primary amine, as shown in eqn (3). To investigate
this possibility, Fe(DHMPE)2Cl2 was reacted with a variety of
amines (methylamine, n-butylamine, t-butylamine) in a range of
solvents (ethanol, pyridine, THF, DMF). In all cases, however,
2+
the major product was the homoleptic Fe(DHMPE)3 complex,
1
as indicated by 31P{ H} NMR spectroscopy (d = 93.3 (s)) and
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Table 2 Bond distances and angles for hydrogen bonds in trans-Fe(DHMPE)2Cl2
X-H—Y angle (◦)
Strength
˚
˚
X-H—Y distance (A)
X---Y distance (A)
O(4)–H ◊ ◊ ◊ O(3) (intramolecular)
O(2)–H ◊ ◊ ◊ Cl(1) (intramolecular)
O(3)–H ◊ ◊ ◊ O(2) (intermolecular)
O(1)–H ◊ ◊ ◊ O(4) (intermolecular)
O(4)–H ◊ ◊ ◊ O(1) (intermolecular)
O(2)–H ◊ ◊ ◊ Cl(1) (intermolecular)
2.64(3)
2.19(2)
1.76(2)
1.87(2)
1.91(2)
3.06(2)
3.132(3)
3.031(2)
2.673(3)
2.726(3)
2.728(3)
3.498(3)
116(3)
154(3)
164(4)
165(3)
149(3)
112(3)
Weak
Moderate
Moderate
Moderate
Moderate
Weak
Symmetry transformations used to generate equivalent atoms: #1 -x, -y + 1, -z + 1, #2 -x + 1/2, y + 1/2, -z + 3/2, #3 x + 1/2, -y + 1/2, z - 1/2, #4
-x - 1/2, y + 1/2, -z + 3/2
+
ESI-MS (+733 (FeCl(DHMPE)3 )).‡ No nitrogen-containing
products were formed, as indicated by elemental analyses.
The phosphorus Mannich reaction was also attempted on
of borane-dimethylsulfide (Scheme 3). Ph2PCH2OH·BH3 has
previously been reported,47 but DHMPE·2BH3 is new. After
treatment of DHMPE with BH3·Me2S, 31P NMR of the initial
reaction mixture showed two broad signals at +1.6 ppm and
+25.6 ppm, of roughly equal intensity. The peak at +25.6 ppm
corresponds to DHMPE·2BH3; presumably the peak at +1.6
ppm corresponds to the singly-protected DHMPE·BH3. After
evaporation of the solvent, only the +25.6 ppm peak was present.
DHMPE·2BH3 is air stable as a solid for up to two weeks. Single
crystals of DHMPE·2BH3 were grown by allowing a solution of
the compound in ethanol to evaporate overnight. X-ray crystallog-
raphy showed that the molecule is centrosymmetric (Ci symmetry;
20
the previously reported complexes Ni(DHMPE)2Cl2 and trans-
Ru(DHMPE)2Cl2.46 These complexes were refluxed with n-
butylamine in either water or absolute ethanol, and the reactions
were monitored by 31P NMR. In both cases, the complexes showed
no reactivity toward the amine, even after 2 weeks under reflux
(Scheme 2).
˚
Fig. 3). The P–B bond is 1.907 A, an intermediate length for a
phosphine-borane bond.48 Like FeCl2(DHMPE)2, DHMPE·2BH3
contains an extended intermolecular hydrogen-bonded network;
however, in this case, the network is two-dimensional (Fig. 4,
Table 3).
Scheme 3 Synthesis of DHMPE·2BH3 and Ph2PCH2OH·BH3.
Scheme 2 Summary of reactions of DHMPE complexes with primary
amines.
Synthesis and Mannich reactivity of borane-protected phosphines
To understand the absence of reactivity of hydroxymethylphos-
phine complexes toward the phosphorus Mannich reaction,
the reactions of free and borane-protected hydroxymethylphos-
phines with secondary amines were studied. The phosphine-
borane adduct of DHMPE, as well as the adduct of
diphenyl(hydroxymethyl)phosphine, Ph2PCH2OH, were chosen as
substrates for this study. (The Ph2PCH2OH ligand was chosen
because it would react in a simple 1 : 1 stoichiometry with
secondary amines.) The phosphine-borane adducts were prepared
by reaction of each phosphine with stoichiometric quantities
Fig. 3 ORTEP plot of DHMPE·2BH3. Ellipsoids are drawn at 50%
probability. Some hydrogen atoms have been omitted for clarity.
‡ Fe(P2)32+-type products are the known decomposition products of
Fe(P2)2Cl2 complexes.45
In order to study the phosphorus Mannich reaction of free-
and coordinated-phosphines, NMR-scale reactions of Et2NH
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Table 3 Bond distances and angles for hydrogen bonds in DHMPE·2BH3
˚
˚
X–H ◊ ◊ ◊ Y distance (A)
X ◊ ◊ ◊ Y distance (A)
X–H ◊ ◊ ◊ Y angle
Strength
O(1)–H ◊ ◊ ◊ O(2)
O(2)–H ◊ ◊ ◊ O(1)
1.764(17)
1.80(2)
2.670(2)
2.684(2)
174(2)
163(3)
Moderate
Moderate
Symmetry transformations used to generate equivalent atoms: #1 -x + 1, -y, -z + 2 #2 x, y + 1, z #3 -x + 1/2, y - 1/2, -z + 3/2
Scheme 4 Reactions of hydroxymethylphosphines with diethylamine.
Fig. 4 Packing of DHMPE·2BH3 showing the two-dimensional hydro-
gen-bonded network.
on the phosphorus atom is essential for the phosphorus Mannich
reaction to occur, and therefore it is not possible to use this
reaction to functionalize coordinated phosphines. It has previously
been assumed that this reaction proceeds by initial decomposition
of the hydroxymethylphosphine, followed by a typical Mannich
pathway involving reaction of the amine and formaldehyde to form
an iminium, followed by nucleophilic attack of the phosphine.51
However, no mechanistic evidence was given to support this
assumption aside from observation of the decomposition of the
hydroxymethylphosphine. Indeed, it is unclear why coordination
of the phosphine would prevent such a mechanism: coordinated
hydroxymethylphosphines do lose formaldehyde, which can then
react with the amine in solution. If the coordinated PH-functional
phosphine is insufficiently nucleophilic to attack the iminium,
coordination would prevent this mechanism; however, this is
unlikely because metal coordination often increases phosphine
nucleophilicity (following deprotonation of the phosphine), and
this would allow the reaction to proceed.52 Instead, an alternative
mechanism may be envisioned (Scheme 5) where the hydrox-
ymethylphosphine eliminates hydroxide to form an electrophilic
methylenephosphonium ion (analogous to an iminium ion in
a typical Mannich reaction53), followed by nucleophilic attack
of the amine. Note that the non-bonded phosphorus lone pair
is required to form the P C p-bond in this mechanism. This
mechanism is also supported by the previously reported reaction
of aminomethylphosphines with secondary phosphines to form
methylenebisphosphines according to eqn (5).49 Mannich-type
transformations involving methylenephosphonium intermediates
are summarized in Scheme 6.
with Ph2PCH2OH or DHMPE were carried out, as were re-
actions with the corresponding phosphine-boranes. The reac-
tions were monitored by 1H and 31P NMR. Treatment of
DHMPE with 4 equiv. Et2NH resulted in complete conversion
to (Et2NCH2)2PCH2CH2P(CH2NEt2)2 in 8 h. In the case of
Ph2PCH2OH, the reaction proceeded over the course of 36 h to
give Ph2PCH2NEt2.§ In the case of Ph2PCH2OH·BH3, the only
product was Ph2PCH2NEt2, apparently formed by deprotection
of a small amount of the starting material. Over time, more
Ph2PCH2NEt2 was slowly formed, presumably due to continuing
slow deprotection of the starting material, which was then able to
react with the amine. It is important to note, however, that the
Mannich-reacted phosphine-borane Ph2PCH2NEt2·BH3 was not
observed. Likewise, DHMPE·2BH3 showed no reaction with the
amine after 2 days. These results are summarized in Scheme 4. The
conclusion from these experiments is that the phosphine-borane
adducts do not undergo the Mannich reaction with amines and
that any Mannich-type product likely results from deprotection of
the phosphine-borane.
Mechanistic speculation
The experiments above show that coordination of a phosphine to
BH3 or to a metal in a complex inhibits the phosphorus Mannich
reaction. An important conclusion is that the lone pair of electrons
§ Note that both Ph2PCH2NEt2 and (Et2NCH2)2PCH2CH2P(CH2NEt2)2
have been previously reported;49,50 however, they were previously synthe-
sized by adding formaldehyde and Et2NH to the primary or secondary
phosphine in a single synthetic step.
(5)
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spectrometer. Mass spectra were obtained using a Thermo
Finnigan LCQ Deca XP Plus ESI Mass Spectrometer. Elemental
analyses were performed by Robertson Microlit Laboratories.
X-ray crystallography
Diffraction intensities were collected at 173(2) K on a Bruker
Apex CCD diffractometer using Mo-Ka radiation l = 0.71073
A.55 Space groups were determined based on systematic absences.
˚
Absorption corrections were applied by SADABS.56 Structures
were solved by direct methods and Fourier techniques and refined
on F2 using full matrix least-squares procedures. All non-H atoms
were refined with anisotropic thermal parameters. H atoms in
both structures were found on the residual density maps and
refined with isotropic thermal parameters. All calculations were
performed by the Bruker SHELXTL (v. 6.10) package.57
Scheme 5 Suggested mechanism for the phosphorus Mannich reaction.
The classic Mannich reaction is also shown for comparison. Protonation
and deprotonation steps have been omitted for clarity.
Synthesis of trans-Fe(DHMPE)2Cl2
A solution of 0.82 g (4.12 mmol) FeCl2·4H2O in 60 mL of ethanol
was added to a solution of 1.77 g (8.27 mmol) DHMPE in
100 mL ethanol, giving an immediate dark green solution. After
stirring for a few minutes, a bright green precipitate formed.
When the precipitate was allowed to settle, a pale red solution
was present. The solid was filtered through a glass frit, rinsed
with ethanol, and dried under vacuum overnight. Yield 2.13 g
1
(93%). 31P{ H} NMR (solvent): d 73.2 (s). 1H NMR (DMSO-d6):
d 2.40 (s, 4H, PCH2CH2P), 4.20 (m, 8H, PCH2OH)), 4.87 (s, 4H,
Scheme 6 Summary of phosphorus Mannich transformations via a
methylenephosphonium intermediate.
1
CH2OH). 13C{ H} NMR (DMSO-d6): d 17.4 (s, PCH2CH2P),
56.1 (s, PCH2OH). Anal. Calcd. for C12H32Cl2FeO8P4: C, 25.97;
H, 5.81. Found: C, 26.23; H, 5.90. Single crystals suitable for
X-ray diffraction were grown by removing an aliquot of the
reaction mixture and allowing the product to precipitate overnight.
Crystallographic Data: C24H64Cl4Fe2O16P8, M = 1110.01, 0.09 ¥
0.07 ¥ 0.04 mm, T = 173(2) K, monoclinic, space group P21/n, a◦=
Conclusions
The experiments reported above suggest that the lone pair of
electrons on the phosphorus atom is essential for the occurrence
of the phosphorus Mannich reaction. In light of these results,
the self-assembly reactions illustrated in Scheme 1 may occur
by a combination of two reactions: a) non-templated reactions
between free phosphines and amines, generating multidentate
ligands which then coordinate to the metal template, and b)
templated reactions on partially-formed multidentate ligands,
where one or more phosphorus atoms remain coordinated or
where one or more metal-phosphine bonds break in order to react
with the amine. As with most self-assembly processes, the multistep
reactions involved in generating these ligands are most likely in
equilibrium with many other Mannich products. The observed
self-assembled products may be thermodynamically favored or
they may be kinetic “traps”, inert to substitution due to the strong
binding of the multidentate ligand but not necessarily the most
thermodynamically stable assembly possible.
˚
˚
˚
7.7855(4) A, b = 13.0809(7) A, c = 10.4136(6) A, b = 92.393(1) ,
3
-3
˚
V = 1059.61(10) A , Z = 1, Z¢ = 0.25, Dc = 1.740 Mg m , m =
1.303 mm-1, F(000) = 756, 2qmax = 54.00◦, 11961 reflections, 2428
independent reflections [Rint = 0.0494], R1 = 0.0355, wR2 = 0.0729
and GOF = 1.073 for 1974 reflections (188 parameters) with I >
2s(I), R1 = 0.0500, wR2 = 0.0800 and GOF = 1.073 for all 2428
reflections, max/min residual electron density +0.477/-0.290 e
3
˚
A .
Synthesis of DHMPE·2BH3
2.5 mL of 2 M BH3Me2S in THF (5.0 mmol) was added
to a suspension of DHMPE (0.500 g, 2.34 mmol) in 30 mL
THF. The mixture was stirred vigorously under nitrogen for
30 min. The solvent was removed under reduced pressure, and
the product was recrystallized in air from 95% EtOH. The
white microcrystalline solid was dried under vacuum. Yield:
Experimental section
1
0.352 g (62%). 31P{ H} NMR (DMSO-d6): d 21.3 (br). 1H
Materials and methods
NMR (D2O): d 0.6 (q, 3H, BH3), 1.92 (d, 4H,PCH2CH2P),
1
Unless otherwise noted, all experimental procedures were
performed under an N2 atmosphere, using standard
Schlenk and glovebox techniques. DHMPE,20 Ph2PCH2OH,54
4.05 (s, 8H,PCH2OH). 13C{ H} NMR (DMSO-d6): d 11.1 (d,
PCH2CH2P), 54.2 (d, PCH2OH). 11B NMR (D2O): d -42.0 (s).
Anal. Calcd. for C6H22B2O4P4: C, 29.80; H, 9.17. Found: C,
29.97; H, 9.17. Single crystals suitable for X-ray crystallographic
analysis were grown by evaporating an ethanolic solution in
air overnight. Crystallographic Data: C6H22B2O4P2, M = 241.80,
Ph2PCH2OH·2BH3,47 RuCl2(DHMPE)2,46 and Ni(DHMPE)2Cl2
20
were prepared by previously published methods. NMR spectra
were obtained on a Varian/Unity 300 or Varian/Unity 500 NMR
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0.27 ¥ 0.12 ¥ 0.02 mm, T = 173(2) K, monoclinic, space group
by 31PNMR, butbecause 31Psignals arenotreliablefor integration,
the reaction progress was quantified by 1H NMR.
˚
˚
˚
C2/c, a =◦18.092(3) A, b = 6.2042(11) A, c = 11.757(2) A, b =
3
-3
˚
100.688(3) , V = 1296.8(4) A , Z = 4, Z¢ = 0.5, Dc = 1.238 Mg m ,
m = 0.323 mm-1, F(000) = 520, 2qmax = 54.00◦, 6768 reflections, 1408
independent reflections [Rint = 0.0549], R1 = 0.0484, wR2 = 0.1126
and GOF = 1.058 for 1138 reflections (108 parameters) with I >
2s(I), R1 = 0.0631, wR2 = 0.1224 and GOF = 1.058 for all 1408
Acknowledgements
This material is based on work supported by the National Science
Foundation under Grant Nos. DGE-0742540 and CHE-0809393.
3
˚
reflections, max/min residual electron density +1.153/-0.301 e A .
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Reactions of trans-Fe(DHMPE)2Cl2 in water
DHMPE (0.1072 g, 0.500 mmol) in 10 mL H2O was added to
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stirring. As the complex dissolved, the solution turned maroon,
then slowly turned orange over 20 min. After 24 h, the color of
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the solution stabilized as yellow-orange. 31P{ H} NMR revealed a
1
series of peaks at +30–95 ppm, including a sharp singlet at +93.3
ppm (Fe(DHMPE)3)2+.
Reaction of trans-Fe(DHMPE)2Cl2 with n-butylamine
n-BuNH2 (0.56 g, 7.6 mmol) was slowly added dropwise to a
suspension of trans-Fe(DHMPE)2Cl2 (0.5165 g, 0.906 mmol) in
30 mL NaCl-saturated H2O under heavy stirring. As n-BuNH2 was
added, the trans-Fe(DHMPE)2Cl2 dissolved, resulting in a maroon
solution. After addition of 8 equiv. n-BuNH2 and stirring for 5
min, no more undissolved trans-Fe(DHMPE)2Cl2 was present.
The solution turned orange after stirring for an additional 20 min.
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conversion to the aminomethylphosphine was monitored by H
NMR by following the appearance of PCH2N methylene signals
(2.9–3.1 ppm), which are significantly different than the PCH2OH
methylene signals (3.8–4.1 ppm). Conversion was also monitored
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