Multinuclear Cu(I) Clusters Featuring a New Triply Bridging Coordination Mode of Phosphaamidinate Ligands

Article abstract of DOI:10.1021/acs.inorgchem.8b01422

Phosphabenzamidine [mes-NH-C(Ph)=P-mes) (1) and phosphaformamidine (mes-NH-CH=P-mes) (4) ligands have been synthesized and characterized. The conjugate bases of 1 and 4 coordinate by each bridging three Cu(I) ions, forming hexa- and tetranuclear clusters Cu₆[mes-N=C(Ph)-P-mes]₃Cl₄Li(THF)₂ (3) and Cu₄[mes-N=CH-P-mes]₄ (5), respectively. Both clusters have been fully characterized using ¹H NMR, ³¹P NMR, and X-ray crystallography. Complexes 3 and 5 exhibit a previously unknown coordination mode of phosphaamidinates, which are far less studied than their well-known amidinate counterparts.

Full text of DOI:10.1021/acs.inorgchem.8b01422

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Multinuclear Cu(I) Clusters Featuring a New Triply Bridging
Coordination Mode of Phosphaamidinate Ligands
Suresh C. Rathnayaka,† Sergey V. Lindeman,‡ and Neal P. Mankad*,†
†Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, United States
‡Department of Chemistry, Marquette University, 1414 W. Clybourn Street, Milwaukee, Wisconsin 53233, United States
S
* Supporting Information
ABSTRACT: Phosphabenzamidine [mes-NH-C(Ph)P-
mes) (1) and phosphaformamidine (mes-NH-CHP-mes)
(4) ligands have been synthesized and characterized. The
conjugate bases of 1 and 4 coordinate by each bridging three
Cu(I) ions, forming hexa- and tetranuclear clusters Cu6[mes-
NC(Ph)-P-mes]3Cl4Li(THF)2 (3) and Cu4[mes-NCH-
P-mes]4 (5), respectively. Both clusters have been fully
characterized using 1H NMR, 31P NMR, and X-ray
crystallography. Complexes 3 and 5 exhibit a previously
unknown coordination mode of phosphaamidinates, which are far less studied than their well-known amidinate counterparts
Chart 1. Metal Complexes of Phosphaamidinates
INTRODUCTION
■
Anionic amidinate ligands, [RNC(R′)NR]−, play many
important roles in molecular inorganic chemistry. Amidinates
have been utilized as chelating ligands in a variety of single site
and multimetal catalysis,1−9 including examples for polymer-
ization,1−4 hydrogenation,7 hydroamination,5 hydrosilylation,8
amidation,9 and CO2 fixation reactions.6 In addition to
catalysis, the Gordon group has employed amidinate
complexes of Ti, V, Mn, Fe, Co, Ni, Cu, Ag, and La as useful
precursors for atomic layer deposition.10 Furthermore, the
ability of amidinates to bridge metal ions has provided
opportunities to study metal−metal bonding.11−13,20
In contrast, the coordination chemistry of phosphaamidinate
analogues, [RNC(R′)PR]−, is less well-known since the initial
discovery of phosphorus−carbon multiple bond chemistry by
Issleib et al. in 1978,14 and only a few examples of metal
phosphaamidinate complexes are reported, including Pt(II)
and Rh(I) examples with N,P-chelation,14,17 a Tl(I) complex
with P,π-aryl-chelation,16 and a trinuclear-Au(I) complex with
μ2-P bridging.17 (Chart 1). The coordination chemistry of
phosphaamidinates thus far consists of chelation to single
metal sites or μ2-P bridging between two metal sites.
Phosphaamidinates possess different electronic properties to
that of amidinates, due to the softer P-donor atom and
different π-donor behavior of PC vs NC multiple bonds.
Moreover, additional coordination modes may be available to
phosphaamidinates that are unavailable to amidinates. The
different electronic features and coordination modes of
phosphaamidinates have the potential to complement the
established behavior of the prolific amidinate ligands, allowing
them to be versatile for designing organometallic architectures.
Our group has been using formamidinates to stabilize
unusual tetranuclear copper sulfide clusters that resemble the
active site of nitrous oxide reductase.18,19 Formamidinates have
been shown to stabilize the 2Cu(I):2Cu(II) and 3Cu(I):
1Cu(II) states of a [Cu4S] model system, but they are unable
to stabilize the fully reduced 4Cu(I) state that would model
the active state of the enzyme. We hypothesized that the use of
phosphaamidinates with mixed P/N-donor atoms in place of
harder formamidinates would stabilize a larger range of redox
states for our clusters. In an attempt to test our hypothesis, we
discovered a new coordination mode of phosphaamidinates
that has not been reported before. Herein, we report the
synthesis and characterization of hexa- and tetranuclear copper
clusters featuring this triply bridging phosphaamidinate
coordination mode wherein a single phosphaamidinate unit
binds simultaneously to three Cu(I) ions.
Received: May 23, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b01422
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Scheme 1. Synthesis of Phosphabenzamidine 1 and Phosphabenzamidinate 2
Figure 1. Side (a) and bottom (b) view of the crystal structure of Cu6[mes-NC(Ph)-P-mes]3Cl4Li(THF)2. Only the core structure atoms are
shown as 50% probability thermal ellipsoids for clarity.
four Cl− anions and a Li+ cation were retained in the structure.
We have been unable to induce LiCl precipitation from 3
RESULTS AND DISCUSSION
■
The phosphabenzamidine 1 was synthesized by adopting the
synthetic routes reported in the literature,17,21 utilizing N-
through a variety of methods, and addition of Na2S simply
results in partial Li+/Na+ exchange as judged by NMR
benzylidyne-2,4,6-trimethylbenzenaminium triflate (C) and
spectroscopy and preliminary X-ray crystallography.
mesitylphosphane (D) (Scheme 1). The reaction conditions
drive a 1,3 hydrogen shift to afford 1a (∼80%) and 1b
(∼20%). Without purification, both 1a and 1b can be
transformed into corresponding lithium phospha-
benzamidinate 2 by deprotonation with n-butyllithium.
With 2 in hand, we were interested in studying its
coordination with Cu(I) ions. Cu(I)Cl was chosen as the
source of copper, with the expectation that precipitation of
LiCl salt would facilitate coordination. The reaction of 2 with 3
equiv of Cu(I)Cl in THF delivered an orange solid, which
showed distinct peaks for inequivalent mesityl CH3 groups in
the 1H NMR spectrum, possibly due to π−π stacking
interactions restricting rotation of the mesityl groups. The
crystal structure revealed this orange solid to be Cu6[mes-N
C(Ph)-P-mes]3Cl4Li(THF)2 (3) (Figure 1). To our surprise,
The solid-state structure of complex 3 consists of two
triangular Cu(I) clusters arranged on top of each other as a
hexacopper antiprism. The “bottom” Cu1···Cu2···Cu3 base is
capped by a Cl− ligand, and the “top” Cu4···Cu5···Cu6 base is
supported by three μ2-Cl− ligands that, in turn, are capped by a
Li+ ion. The Li+ ion has also two coordinated THF ligands.
The phosphabenzamidinate ligands are arranged in a regular
propeller-like chiral fashion, with their N atoms coordinating
the “bottom” 3Cu(I) base and their P atoms acting as μ2-
ligands bridging the “bottom” and “top” bases of the antiprism.
The Cu···Cu separations within the “bottom” base are shorter
(2.69−2.75 Å) than in the “top” (2.95−3.14 Å), and both are
shorter than Cu···Cu separations between the bases (∼3.3 Å
B
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for P-bridged Cu(I) ions and 3.6−3.8 Å between nonbridged
ones).
Many hexacopper clusters based on either Cu(I) or Cu(II)
are reported in the literature, featuring quite different
assemblies of Cu atoms (Figure 2).24−29 The hexacopper
NMR spectrum of the new species indicated restricted rotation
of the mesityl groups, similar to 3. However, the solubility of
the new species was poor in most of the available solvents,
hence making it is difficult to grow X-ray quality crystals or
analyze the structure by solution methods.
Next, phosphaformamidine 4 was synthesized and applied in
place of 1. The previous synthetic route (Scheme 1) could not
be applied as multiple attempts to make the imidoyl chloride
that corresponds to (B) were unsuccessful. Hence, ethyl-N-
mesitylformimidate was synthesized22 and reacted with lithium
mesitylhydrophosphanide to afford 4 as a yellow green solid
(Scheme 2). Production of mesityl phosphane was also
observed as the lithium mesitylhydrophosphanide is basic
enough to deprotonate 4. However, an overnight long reaction
time, followed by the addition of NH4Cl, shifted the
equilibrium toward the formation of 4.
Figure 2. Different assembly types for hexanuclear Cu(I) and Cu(II)
clusters. Only core structures are shown for clarity, and only solid
lines represent a bond. Aqua ball - Cu(II); orange ball - Cu(I); red
ball - O; blue ball - N; gray ball - C; yellow ball - S; pink ball - P; green
ball - Cl; purple ball - I.
A small-scale reaction of 4 (0.0673 mmol) with
mesitylcopper in Et2O afforded 5 as an orange solid after 2
days. However, scaling up the reaction by 10 times required 8
days to complete. So, an alternative approach was applied by
first deprotonating 4 with sodium bis(trimethylsilyl)amide
(NaHMDS) and then adding Cu(MeCN)4PF6 in Et2O,
cluster type E consists of four 5-coordinate Cu(II) atoms
located at four alternative vertices of a central defective cubane
unit with two μ3- and two μ2-O atoms, and two satellite Cu(II)
atoms connect to the cubane via μ2-O atoms.24 The cluster
type F consists of two 6-coordinate Cu(II) trimer units bridged
by two nitrate ions,25 while G possesses a coplanar assembly of
5-coordinate Cu(II) atoms.26 Octahedral or distorted octahe-
dral assemblies are common for hexanuclear Cu(I) clusters as
evidenced by clusters H,27 I,28 and J.29 In contrast, the
arrangement of Cu(I) atoms in 3 defines an antiprism rather
than an octahedron.
We are interested in more reactive complexes with
coordinatively unsaturated Cu(I) ions, as precursors to
construct tetranuclear copper sulfide clusters. To attempt to
address this, 2 was reacted with 1 and 2 equiv of Cu(I)Cl, to
target a di- or trinuclear cluster with a 1:1 ratio of bidentate
phosphabenzamidinate to Cu(I). Interestingly, the 31P NMR
spectra showed formation of 3 and unreacted 2 in both
reactions, suggesting that the formation of 3 is highly favorable
regardless of metal:ligand stoichiometry in the reaction
mixture.
As confirmed by the crystal structure, the presence of μ2-Cl
ligands stabilizes 3 by balancing the charge and saturating the
Cu centers. Such coordination of halides is common, and a
similar coordination has been reported by Jessop and co-
workers in a phosphaamidine-Cu(I)Br cluster.23 So, we next
wondered whether a more reactive complex could be attained
by applying a halide-free copper source. Mesitylcopper was an
ideal candidate for this purpose, as it allows the simultaneous
deprotonation and coordination. To begin with, 1 was reacted
with 1 equiv of mesitylcopper at room temperature, and after 3
h, 1 was partially consumed and a new product was observed
by 31P NMR spectroscopy. The formation of mesitylene was
confirmed by 1H NMR spectroscopy. After purification, the 1H
1
resulting in rapid formation of 5. The H NMR spectrum of
5 exhibited restricted mesityl bond rotation similar to 3, and
the single crystal XRD confirmed the identity of 5 as Cu4(mes-
NCH-P-mes)4 (Figure 3).
The structure of 5 contains a tetrahedron of Cu(I) ions, with
the overall cluster having noncrystallographic S4 symmetry.
Each ligand caps one of the faces of the tetrahedron with N
atoms reaching the Cu atom opposite to the bridged edge at a
2.01 Å distance. All P−Cu bond lengths are in a narrow range
2.26−2.28 Å and have an average Cu···Cu separation 2.94 Å.
The two opposite nonbridged Cu atoms are elongated to a
distance of 3.13 Å.
The literature includes reports of a variety of tetranuclear
Cu(II) and Cu(I) assemblies. The Cu(II) structures include
chairlike K,30 linear L,31 tetrahedral M,32 and planar N33
geometries (Figure 4). Most of them are held together by
bridging O or N atoms. Tetrahedral or distorted tetrahedral
assemblies are common for tetranuclear Cu(I) clusters, yet
different arrangements are possible, as evidenced by P35 where
four planar Cu(I) atoms are bridged by carboxylate ligands
above and below the plane. Both O and Q display tetrahedral
arrangements of Cu(I) atoms. Each face of O is occupied by a
μ3-I atom, and each edge of Q is bridged by a μ2-Se atom.34,36
Complex 5 also resembles a tetrahedron of Cu(I) ions and is
similar to O, with each face capped by a phosphaformamidi-
nate ligand.
Even in the absences of halides, 5 still has μ2-bridging
phosphorus atoms, implying that the triply bridging coordina-
tion behavior of phosphaamidinates is intrinsic. To our
Scheme 2. Synthesis of N-((Mesitylphosphanylidene)methyl)-2,4,6-trimethylaniline
C
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Figure 3. Top (a) and side (b) view of the crystal structure of Cu4[mes-NCH-P-mes]4. Only the core structure atoms are shown as 50%
probability thermal ellipsoids for clarity.
Chart 2. Coordination Modes of Deprotonated NN, NP,
and NS Ligands
Figure 4. Different assembly types for tetranuclear Cu(I) and Cu(II)
clusters. Only core structures are shown for clarity, and only solid
lines represent a bond. Aqua ball - Cu(II); orange ball - Cu(I); red
ball - O; blue ball - N; gray ball - C; yellow ball - S; pink ball - P; green
ball - Cl; purple ball - I; beige ball - Se.
knowledge, this coordination mode has not been observed
previously in the phosphaamidinate literature. Analogous
behavior is not seen in formamidinates, as the nitrogen lone
pairs are resonance delocalized and thus unavailable to bridge
two metals at a single N atom (Chart 2). In conjugate bases of
1 and 4, there is apparently less delocalization due to poor
overlap of the 3p orbitals of P with the 2p orbitals of C, making
the phosphorus lone pairs available for an extra coordination to
a third metal center.
However, phosphaamidinates do not coordinate to Au(I) in
a similar fashion. The Lammertsma group reported two
coordination complexes of phosphaamidinates with Rh(I) and
Au(I).17 In the Rh(I) complex, the phosphaamidinate ligand
chelates to a single Rh(I) center, whereas the Au(I) complex
displays μ2-P, bridging without any N-coordination (Chart 1
(ii), (iv)). This behavior is likely driven by the preference of
Au(I) to have linear coordination geometry, and so
phosphaamidinate ligands are more likely to exhibit triply
bridging coordination behavior with Cu ions (Chart 2). A 2D
network of Cu(II) atoms supported by a μ3-bridging tridentate
carboxylate has been reported,37 and the mode of coordination
is attributed to the electronic effects of the substituents. In
addition, the reaction of Cu(O2CPh)2·2H2O with a mixture of
(py)2CO and NaN3 produced a hexanuclear Cu(II) cluster
supported by (py)2CO22− and (py)2C(OH)O− anionic ligands
having μ2-O bridging two Cu atoms and each nitrogen
coordinating to a different Cu atom.24 The (2-(diphenylphos-
phanyl)phenyl)amide ligand (vii) is an example of a 1,1,4-N,P
ligand, stabilizing J (Figure 4) but with μ2-N rather than μ2-P
bridges. Benzoxazole-2-thiolate (viii) possesses a very similar
coordination to that of phosphaamidinates discussed here.
Ligand (viii) acts as 1,1,3-S,N ligand stabilizing I (Figure 4)
with μ2-S bridging to two Cu(I) atoms and N coordinating to a
different Cu(I) atom. The extra coordination in both (vii) and
(viii) has been generated at the site of deprotonation (amide
and thiolate groups, respectively), whereas the deprotonation
of the amine group in 1 and 4 leads to the formation of a N
C bond and creates an extra coordination at P.
The redox behavior of both 3 and 5 was studied using cyclic
voltammetry. Complex 3 displayed few redox events that are
not well-defined, and surprisingly, complex 5 was redox
innocent (see the Supporting Information, page S9) despite
being supported by electron rich phosphorus σ-donors. In
addition, due to our interest in constructing tetranuclear
copper-sulfide clusters, we added several sulfur-atom donor
D
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mixture was stirred for 10 min at −78 °C, and the cooling bath was
removed. The mixture was further stirred for 30 min, while allowing it
to warm up to the room temperature. Triethylamine (252 μL, 1.80
mmol) was added, and the color of the solution changed from red-
orange to yellow. The mixture was stirred for 15 min, and volatiles
were removed under vacuum, leaving a yellow-orange oily residue. It
was purified (under normal atmosphere) by filtering over a pad of
neutral, activated alumina eluting with pentane. Volatiles were
removed under vacuum, leaving (1) as a yellow solid. The gram
scale synthesis gave (1) as a yellow sticky product which solidified in
weeks. Reaction conditions yield 1 as a mixture of 1a (∼80%) and 1b
(∼20%). Total yield: 0.3506 g, 72%. 1H NMR of 1a (500 MHz,
CDCl3) δ 7.34 (d, J = 8.2 Hz, 2H), 7.21 (t, J = 7.3 Hz, 1H), 7.14 (t, J
= 7.5 Hz, 2H), 6.99 (s, 2H), 6.67 (s, 2H), 6.17 (s, 1H), 2.59 (s, 6H),
2.29 (s, 3H), 2.15 (s, 3H), 2.04 (s, 6H). 31P NMR of 1a (162 MHz,
CDCl3) δ 61.74 (s), 1b (162 MHz, CDCl3) −74.51 (d, J = 241.1 Hz).
m/z theoretical: 373.20, 374.20, 375.20. Found 374.3, 375.3, 376.3
Synthesis of Bis(tetrahydrofuran)lithium mesityl((mesityl-
phosphanylidene)(phenyl)methyl)amide (2).
reagents to 5. Unfortunately, no reaction was observed under
the conditions we examined, indicating the inert nature of 5
(which is even air-stable).
CONCLUSION
■
In summary, phosphaamidine ligands 1 and 4 have been
synthesized and characterized. The corresponding phosphaa-
midinates coordinate with Cu(I)Cl and Cu(MeCN)4PF6,
respectively, making complexes 3 and 5. As evidenced by the
crystal structures, phosphaamidinates coordinate to Cu(I) in
triply bridging fashion with the help of μ2-bridging P-donor
atoms. This observation exemplifies the different coordination
behavior of phosphaamidinates compared to that of isoelec-
tronic amidinates. Neither 3 nor 5 exhibits any well-defined
redox chemistry; nevertheless, we believe the new coordination
mode for phosphaamidinate ligands will enhance their
utilization in building organometallic architectures.
EXPERIMENTAL SECTION
■
General Remarks. Unless otherwise mentioned, all the syntheses
were done in a glovebox filled with N2 or using standard Schlenk line
techniques. All the chemicals purchased from a commercial source
were used without further purification. Solvents were dried using a
glass contour solvent system built by Pure Process Technology, LLC.
Deuterated solvents that were packed under Ar were stored in 3-Å
molecular sieves without further degassing. Compounds A, B, C, D,
and E were synthesized by adopting synthetic routes reported in the
literature.15,16,21 Caution! Synthesis of D involves quenching LiAlH4 with
H2O. Extreme care should be taken when adding the first few drops of
H2O, as the quenching is highly exothermic and H2 is evolved. Any waste
or glassware containing phosphane D should be properly cleaned as
phosphanes are highly smelly.
n-Butyllithium (12.5 mL of 1.6 M in hexanes, 19.9 mmol) was
added dropwise to a solution of (1) (4.9544 g, 16.6 mmol) in THF
(25 mL) at −78 °C. The color of the mixture was changed from
yellow to orange to dark brown. The resulting mixture was allowed to
warm up to room temperature and stirred for a further 10 min. The
volatiles were removed under vacuum, leaving a brown residue, which
was dissolved in a minimum amount of THF and kept at −20 °C to
afford (2) as an orange solid. It was filtered and washed with cold
pentane. The filtrate was completely evaporated and redissolved in a
minimum amount of THF and kept at −20 °C to afford a second
crop. A third crop was obtained in a similar way. Total yield: 5.3342 g,
61%. 1H NMR (400 MHz, THF-d8) δ 7.21−7.12 (m, 2H), 6.86−6.79
(m, 3H), 6.75 (s, 2H), 6.41 (s, 2H), 3.64−3.59 (m, 4H; THF), 2.40
(s, 6H), 2.28 (s, 6H), 2.21 (s, 3H), 1.99 (s, 3H), 1.79−1.74 (m, 4H;
THF). 31P NMR (162 MHz, THF-d8) δ 25.55 (s).
Instrumentation. 31P NMR spectra were recorded at ambient
temperature using a Bruker Avance DPX-400 MHz instrument. All
1
the H NMR spectra were recorded at ambient temperature using
either Bruker Avance DPX-400 or Bruker Avance DPX-500 MHz
instruments. Mass analyses were performed with an Advion
ExpressionL CMS mass spectrometer using APCI+ mode. Elemental
analyses were performed by the Midwest Microlab, LLC, in
Indianapolis, IN. X-ray crystallography data of complexes 3 and 5
were collected at the X-ray Structural Laboratory at Marquette
University (Milwaukee, WI) using a SuperNova, Dual, Cu at home/
near, Atlas diffractometer [Cu Kα (λ = 1.54184)] at 100.15 K. The
structure was solved with the Olex2 structure solution program using
Charge Flipping and refined with the ShelXL refinement package
using Least Squares minimization. Electrochemical data of both 3 and
5 (approximately 1 mmol in THF) were collected in a classic three-
electrode system: a Pt working electrode, a Pt counter electrode, and a
Ag/Ag+ reference electrode, using a WaveNow USB Potentiostat from
Pine Research Instrumentation.
Synthesis of Cu6[mes-NC(Ph)-P-mes]3Cl4Li(THF)2 (3). A
solution of (2) (0.1014 g, 0.19 mmol) in THF (3 mL) was added
dropwise to a suspension of CuCl (0.0575 g, 0.58 mmol) in THF (5
mL) over 10 min. The reaction mixture immediately changed to
yellow and to cloudy orange at the end of the slow addition. It was
stirred at room temperature for 1 h and filtered through a pad of
Celite. The filtrate was completely evaporated, leaving a yellow solid.
It was added with DCM (6 mL) and filtered through a pad of Celite
to remove insoluble LiCl. The Celite pad was washed with DCM (1
mL × 2). The filtrate was completely evaporated, leaving (3) as a
yellow-orange solid. Yield: 0.0890 g, 77%. X-ray quality crystals were
1
grown by cooling a saturated solution of (3) in THF at −20 °C. H
NMR (400 MHz, CDCl3) δ 6.97 (s, 2H), 6.82 (t, J = 7.2 Hz, 1H),
6.75 (t, J = 7.5 Hz, 2H), 6.65 (s, 1H), 6.62 (s, 1H), 5.86 (s, 1H), 5.62
(s, 1H), 3.77 (s, 4H; THF), 3.03 (s, 3H), 2.60 (s, 3H), 1.99 (s, 3H),
1.98 (s, 3H), 1.97 (s, 3H), 1.86 (s, 8H; THF), 1.19 (s, 3H). 31P NMR
(162 MHz, CDCl3) δ −61.95 (s). Anal. Calcd For C83H97Cu6Cl4-
LiO2P3: C, 55.64; H, 5.46; N, 2.35. Found: C, 54.92; H, 5.12; N, 2.04.
(the error of % C was greater than the limit of satisfactory due to the
solvent coordination).
Synthesis of N-((Mesitylphosphanylidene)(phenyl)methyl)-
2,4,6-trimethylaniline (1).
Synthesis of N-((Mesitylphosphanylidene)methyl)-2,4,6-tri-
methylaniline (4).
A solution of 2,4,6-trimethylphenylphosphane (0.2500 g, 1.64
mmol) in DCM (2 mL) was added dropwise to a suspension of N-
benzylidyne-2,4,6-trimethylbenzenaminium trifluoromethanesulfonate
(0.6101 g, 1.64 mmol) in DCM (2.5 mL) at −78 °C. The reaction
n-Butyllithium (1.6 M solution in hexane, 4.9 mL, 7.86 mmol) was
added dropwise over 15 min to a solution of 2,4,6-trimethylphenyl-
phosphane (1.0000 g, 6.57 mmol) in Et2O (45 mL) at −78 °C. The
E
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resulting mixture was stirred for 30 min at the same temperature,
followed by another 30 min outside the cold well, allowing it to warm
up to room temperature to afford lithium 2,4,6-trimethylphenyl-
hydrophosphanide as a yellow cloudy solution. It was added dropwise
over 30 min to a solution of ethyl N-mesitylformimidate (1.2569 g,
6.57 mmol) in Et2O (10 mL) at −78 °C. The resulting mixture was
stirred for 30 min at the same temperature and allowed to warm up to
room temperature overnight. The resulting yellow solution was added
with solid NH4Cl (0.3515 g, 6.57 mmol) and stirred for 30 min and
then filtered through a pad of Celite to remove the insoluble LiCl.
The filtrate was completely evaporated, leaving a yellow solid which
was added with pentane (20−25 mL) and filtered off using a coarse
frit and washed with pentane (2 mL × 6) to afford (4) as a yellow-
green solid. (If any unreacted ethyl N-mesitylformimidate or 2,4,6-
trimethylphenylphosphane is present, the yellow solid which results
from complete evaporation of the filtrate will appear as wet and sticky.
In that case, washing with pentane will remove any unreacted
materials and the byproducts, leaving (4) as a dry solid.) Yield: 1.0161
NMR spectra, mass spectra, electrochemistry data
(PDF)
Accession Codes
CCDC 1845805 and 1845806 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: npm@uic.edu.
ORCID
Neal P. Mankad: 0000-0001-6923-5164
1
g, 52%. H NMR (400 MHz, C6D6) δ 7.63 (dd, J = 44.1, 13.9 Hz,
1H), 6.88 (s, 2H), 6.54 (s, 2H), 2.61 (s, 6H), 2.15 (s, 3H), 2.05 (s,
3H), 1.89 (s, 6H). 31P NMR (162 MHz, C6D6) δ 54.43 (d, J = 44.2
Hz). m/z theoretical: 297.16, 298.17, 299.17. Found: 298.4,299.4,
300.4.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Synthesis of Cu4[mes-NCH-P-mes]4 (5) Using Mesitylcop-
per. A solution of mesitylcopper (0.1342 g, 0.73 mmol) in Et2O (11
mL) was added dropwise over 20 min to a solution of (4) (0.2184 g,
0.73 mmol) in Et2O (12 mL). The reaction mixture was well capped
and stirred at 40 °C for 3 h. The 31P NMR of an aliquot showed the
complete consumption of (4) to multiplets around −55 to −80 ppm.
The reaction mixture was further stirred for 8 days at 40 °C, resulting
in a yellow precipitate. The precipitate was collected by filtration and
redissolved in benzene and filtered off through a pad of Celite to
remove a brown color byproduct (possibly decomposed Cu from
mesitylcopper). The resulting yellow-orange filtrate was completely
evaporated, leaving 5 as a yellow-orange solid. Yield: 0.1728 g, 65%.
1H NMR (400 MHz, C6D6) δ 8.38 (s, 1H), 6.87 (s, 1H), 6.84 (s,
The National Institutes of Health (R01 GM116820) provided
financial support.
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1
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01422.
F
DOI: 10.1021/acs.inorgchem.8b01422
Inorg. Chem. XXXX, XXX, XXX−XXX

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