Cu(I) coordination by N,N,N’,N’-tetra(di-ortho-anisylphosphanylmethyl) ethylene and propylene diamine: First example of a sandwiched CuCl-tetramer

Article abstract of DOI:10.1016/j.ica.2020.120162

New chelating α-aminophosphanes were obtained by Mannich-type reactions between formaldehyde, bis(2-methoxyphenyl)phosphane and aliphatic primary diamines bearing either a C2 or C3 carbon bridge between the amine functionalities. The reaction conditions, and in particular, the reaction sequence was found to significantly depend on the length of the carbon bridge of the diamine. Both obtained α-aminophosphanes (i.e. N,N,N’,N’-tetra(di-ortho-anisylphosphanylmethyl)-1,2-diaminoethane (L¹) and N,N,N’,N’-tetra(di-ortho-anisylphosphanylmethyl)-1,3-diaminopropane (L²)) gave along with neocuproine cationic dinuclear Cu(I) complexes. The Cu(I) complex with L² as ligand was further converted by the aid of NiCl₂(DME) (DME = 1,2-dimethoxyethane) into a tetranuclear copper cluster showing a new type of CuCl-tetramer molecular structure. All synthesized compounds were characterized in solution by multinuclear NMR-, UV/Vis-spectroscopy, mass spectrometry and in the solid state by elemental analysis and some of them by single crystal structure analysis.

Full text of DOI:10.1016/j.ica.2020.120162

Inorganica Chimica Acta 516 (2021) 120162
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Cu(I) coordination by N,N,N’,N’-tetra(di-ortho-anisylphosphanylmethyl)
ethylene and propylene diamine: First example of a sandwiched
CuCl-tetramer
Johann Panna, Katharina Ehrmanna, Richard Pehna, Helena Roithmeyera, Wolfgang Viertla,
,
,*
Holger Kopackaa, Peter Brüggellera *, Werner Oberhauserb
a University of Innsbruck, Centrum for Chemistry and Biomedicine, Institute of General, Inorganic and Theoretical Chemistry, Innrain 80-82, 6020 Innsbruck, Austria
b Istituto di Chimica dei Composti Organometallici (CNR-ICCOM), Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy
A R T I C L E I N F O
A B S T R A C T
New chelating
α
-aminophosphanes were obtained by Mannich-type reactions between formaldehyde, bis(2-
Dedicated to Dr. Maurizio Peruzzini on the
occasion of his retirement
methoxyphenyl)phosphane and aliphatic primary diamines bearing either a C2 or C3 carbon bridge between
the amine functionalities. The reaction conditions, and in particular, the reaction sequence was found to
significantly depend on the length of the carbon bridge of the diamine. Both obtained α-aminophosphanes (i.e. N,
Keywords:
N,N’,N’-tetra(di-ortho-anisylphosphanylmethyl)-1,2-diaminoethane (L1) and N,N,N’,N’-tetra(di-ortho-anisyl-
phosphanylmethyl)-1,3-diaminopropane (L2)) gave along with neocuproine cationic dinuclear Cu(I) complexes.
The Cu(I) complex with L2 as ligand was further converted by the aid of NiCl2(DME) (DME = 1,2-dimethoxy-
ethane) into a tetranuclear copper cluster showing a new type of CuCl-tetramer molecular structure. All syn-
thesized compounds were characterized in solution by multinuclear NMR-, UV/Vis-spectroscopy, mass
spectrometry and in the solid state by elemental analysis and some of them by single crystal structure analysis.
Copper
Cluster
Aminophosphane
Tetradentate ligand
1. Introduction
ligands (i.e. chelating ligand forming a six-member metallacycle), with
both phosphorus atoms cis-coordinating to the same metal centre [4].
–
Aminophosphanes, featured by P N single bonds, are a very inter-
esting group of ligands, which coordinate essentially through the
phosphorus donor atom to soft metal centers (i.e. metals in a low
oxidation state) [1]. Recently, they found application in the synthesis of
metal phosphide nanocrystals [2] or quantum dots [3]. The coordina-
tion properties of aminophosphanes increase upon separation of both
Binuclear metal complexes with the same metal coordination are
accessible by introducing a n-alkyl chain in between the nitrogen atoms
of two α-di(phosphanomethyl)amine units [4]. In this respect, the most
studied ligand is N,N,N’,N’-tetra(diphenylphosphanylmethyl)ethyl-
enediamine (dppeda), which showed at least in the solid state (i.e. data
stemming from single crystal structure analysis) the coordination of only
the phosphorus donor atoms to Pd(II) [13,14], Ni(II) [15], Au(I) [16],
Ag [17] and Cu(I) [18–20]. Three-coordinated Cu(I) diphosphane
complexes have found applications as dopants in light-emitting diodes
(OLEDs) [21,22] and as precatalyst in the asymmetric hydroboration of
alkenes [23].
donor atoms by a C1 or C2 carbon bridge, giving thus
α-[4,5] and
β-aminophosphanes [6], respectively. The most common synthetic
routes for α-aminophosphanes are the condensation reaction between an
amine, an oxo-compound and a secondary phosphane [4,7,8] and the
reaction between amines and hydroxymethylphosphanes [4,9,10].
Diphenylphosphanomethylamines are the most common
α-amino-
In order to contribute in the area of coordination chemistry of
phosphanes, which act as monodentate ligands (i.e. phosphorus atom is
coordinating, while the amine functionality can either be involved in
proton transfer reactions, which are beneficial for instance in the proton
reduction to hydrogen on a metal centre characterized by a low oxida-
tion state [11,12].) Bis(phosphanomethyl)amines act as bidentate
α-aminophosphanes, we synthesized two new P4N2-based ligands,
characterized by an ethyl and propyl carbon chain in between the ni-
trogen atoms and ortho-anisyl (i.e. 2-methoxyphenyl) units linked to the
phosphorus donor atoms. This latter phosphorus substituents have been
chosen for their potential to interact through the oxygen atom with the
* Corresponding authors.
E-mail addresses: Peter.Brueggeller@uibk.ac.at (P. Brüggeller), werner.oberhauser@iccom.cnr.it (W. Oberhauser).
https://doi.org/10.1016/j.ica.2020.120162
Received 28 October 2020; Accepted 23 November 2020
Available online 30 November 2020
0020-1693/© 2020 Elsevier B.V. All rights reserved.

J. Pann et al.
Inorganica Chimica Acta 516 (2021) 120162
metal centre, leading either to metal atom coordination [24,25] or to a
vacuum. The product can be purified from phosphane oxide applying
the procedure described for the synthesis of L1. Yield: 542.7 mg (71%).
Anal. Calcd. for C63H70N2O8P4 (1107.09): C, 68.35%; H, 6.37%; N,
2.53%. Found: C, 68.22%; H, 6.43%; N, 2.49%.
–
metal-assisted C O bond breaking of the methoxy unit [26–28]. In
addition, methoxy oxygen atoms of the ligand can be involved in
hydrogen bond interactions with the substrate in catalytic reactions
driving hence the catalytic activity and selectivity of the substrate
conversion [29,30].
1H NMR (300.0 MHz, CD2Cl2, ppm): δ = 1.65 (m, 2H, CHP2r), 2.83 (d,
3JHH = 6.1 Hz, 4H, CHP2r), 3.48 (d, 1JPH = 4.0 Hz, 8H, CHP2,N), 3.61 (s,
24H, OMe), 6.70–6.74 (m, 16H, HAr), 7.18 (m 16H, HAr). 13C{1H} NMR
3
2. Experimental
(75.0 MHz, CD2Cl2, ppm): δ = 22.0 (s, CHP2r), 52.0 (d, JPC = 9.0 Hz,
CHP2r), 53.0 (d, 1JPC = 4 Hz, CH2P,N), 55.0 (s, OCH3), 110.0 (s, CAr), 120.7
(d, 3JPC = 2.4 Hz, CAr), 129.0 (s, CAr), 134 (d, 2JPC = 7.0 Hz, CAr), 161.0
(d, 1JPC = 13.0 Hz, CAr). 31P{1H} NMR (121.5 MHz, CD2Cl2, ppm): δ =
-44.00 (s).
2.1. Materials and instruments
Neocuproine (2,9-dimethyl-1,10-phenanthroline), [Cu(MeCN)4]BF4
and [NiCl2(DME)] (DME = 1,2-dimethoxyethane) were purchased from
Sigma Aldrich, while extra dry solvents (i.e. stored over molecular
sieves) were purchased from Acros.
2.2.2. Synthesis of [Cu2(neocuproine)2(
μ
-L1)](BF4)2 (1a)
[Cu(MeCN)4]BF4 (17.16 mg, 0.06 mmol) was added to a solution of
L1 (30.0 mg, 0.03 mmol) in THF (10.0 mL). The obtained solution, which
became opaque after 15 min. was stirred at room temperature for 1.5 h.
Neocuproine (12.47 mg, 0.06 mmol) was then added to the latter so-
lution, causing an instant color change to orange. The obtained sus-
pension was further stirred for 1 h and the yellow solid was separated by
filtration from solution and dried under vacuum. Yield: 42.0 mg (85%).
Anal. Calcd. for C90H92B2Cu2F8N6O8P4 (1810.30): C, 59.71%; H, 5.12%;
N, 4.64%. Found: C, 59.65%; H, 5.18%; N, 4.61%. Mass spectrometry:
[M]2+ found: 818,33; the isotopic shift pattern matches. Single crystals
of 1a, suitable for an X-ray structure analysis, were obtained by slow
evaporation of a dichloromethane solution of 1a in the presence of n-
hexane.
Schlenk technique was employed for all syntheses using Ar 5.0 as
inert gas atmosphere. NMR measurements were performed on a Bruker
Avance DPX 300 spectrometer. Elemental analyses were carried out
using a Perkin-Elmer Model 2400 C,H,N elemental analyzer. Mass
spectrometric analyses were performed on a Thermo Scientific Q Exac-
tive Orbitrap with an electrospray ion source in the positive mode.
2.2. Syntheses
2.2.1. Syntheses of N,N,N’,N’-tetra(di-ortho-anisylphosphanylmethyl)-1,2-
diaminoethane (L1) and N,N,N’,N’-tetra(di-ortho-
anisylphosphanylmethyl)-1,3-diaminopropane (L2)
1H NMR (300.0 MHz, CDCl3, ppm): δ = 2.34 (s, 12H, CH3), 2.94 (s,
24H, OCH3), 3.03 (s, 4H, CHE2t), 4.17 (s, 8H, CH2P,N), 6.54 (d, 3JHH = 6 Hz,
8H, HAr), 6.75 (t, 3JHH = 6 Hz, 8H, HAr), 7.16–7.29 (m, 16H, HAr), 7.52
(d, 3JHH = 8.3 Hz, 4H, HAr-neo), 7.95 (s, 4H, HAr-neo), 8.37 (d, 3JHH = 8.3
Hz, 4HAr-neo). 31P{1H} NMR (121.5 MHz, CDCl3, ppm): δ = -21.66 (s).
Synthesis of L1: A mixture of paraformaldehyde (78.80 mg, 2.62
mmol) and 1,2-diaminoethane (43.8 µL, 0.66 mmol), were heated in
toluene (5.0 mL) at 80 ◦C, obtaining a clear, colorless solution. Then bis
(2-methoxyphenyl)phosphane (680.0 mg, 2.76 mmol) was added to the
latter solution and stirring continued at the same temperature for 12 h. A
pale yellow reaction solution was obtained, which was analyzed by 31
P
2.2.3. Synthesis of [Cu2(neocuproine)2(
μ
-L2)](BF4)2 (1b)
{1H} NMR spectroscopy in order to prove the completeness of the con-
version. The reaction solution was concentrated to dryness at room
temperature obtaining a white foamy solid, which was triturated by
stirring in diethyl ether (10.0 mL). A suitable purification step of the
product (i.e. removing phosphane oxide) consisted in dissolving the
obtained product in a minimal amount of THF, followed by separation of
[Cu(MeCN)4]BF4 (34.48 mg, 0.11 mmol) was added to a solution of
L2 (60.69 mg, 0.05 mmol) in THF (20.0 mL) and continued to stir for 1.5
h at room temperature. Neocuproine (22.83 mg, 0.11 mmol) was then
added to the suspension, which immediately changed color to orange.
The latter suspension was further stirred for 1 h, followed by successive
separation of the precipitate from solution by filtration and drying of the
yellow solid by vacuum. Yield: 43.3 mg (21%). Anal. Calcd. for
the insoluble part by filtration (i.e. 0.22 μm PTFE filter) and then pre-
cipitation of the desired product upon addition of n-hexane to the
filtrate. The purified product precipitated as white solid. Yield: 560.08
mg (74%). Anal. Calcd. for C62H68N2O8P4∙4CH2Cl2 (1432.77): C,
55.33%; H, 5.35%; N, 1.96%. Found: C, 55.23%; H, 5.41%; N, 1.93%.
Single crystals with the composition L1∙4CH2Cl2 suitable for an X-ray
structure analysis were obtained by slow evaporation of a dichloro-
methane solution of L1 in the presence of n-hexane.
C
91H94B2Cu2F8N6O8P4 (1824.33): C, 59.91%; H, 5.19%; N, 4.61%.
Found: C, 59.85%; H, 5.23%; N, 4.57%. Mass spectrometry: [M]2+
found: 824,23; the isotopic shift pattern matches.
1H NMR (300.0 MHz, CDCl3, ppm): δ = 1.77 (m, 2H, CHP2r), 2.32 (s,
12H, CH3), 2.89 (s, 24H, OCH3), 3.00 (m, 4H, CHP2r), 4.04 (s, 8H, CH2P,N),
6.49 (d, JHH = 8.1 Hz, 8H, HAr), 6.76 (t, JHH = 7.4 Hz, 8H, HAr),
7.16–7.27 (m, 16H, HAr), 7.48 (d, 3JHH = 8.32 Hz, 4H, HAr-neo), 7.94 (s,
4H, HAr-neo) 8.37 (d, 3JHH = 8.28 Hz, 4H, HAr-neo). 31P{1H} NMR (121.5
MHz, CH2Cl2, ppm): δ = –22.57 (s).
3
3
1H NMR (300.0 MHz, CD2Cl2, ppm): δ = 3.12 (s, 4H, CHE2t), 3.60 (d,
2JPH = 4.0 Hz, 8H, CH2P,N), 3.72 (s, 24H, OCH3), 6.81–6.88 (m, 16H, HAr),
7.24–7.30 (m, 16H, HAr). 13C{1H} NMR (75.0 MHz, CD2Cl2, ppm): δ =
53.0 (t, 3JPC = 9.0 Hz, CH2Et), 55.0 (dd, 1JPC = 10.0 Hz, 3JPC = 4.0 Hz, CHP2,
N), 56.0 (s, OCH3), 111.0 (s, CAr), 121.0 (d, 2JPC = 2.0 Hz, CAr), 126.0 (d,
2.2.4. [Cu4(μ2-Cl)4(
μ
-H4L2-κ1-P:κ1-P’:κ1-P’’:κ1-P’’’)2](BF4)4 (2)
2
1JPC = 16.0 Hz, 4C, CAr), 130.0 (s, CAr), 134.0 (d, JPC = 7.0 Hz, CAr),
1b (20.41 mg, 0.01 mmol) was dissolved in dichloromethane (10.0 mL)
and [NiCl2(DME)] (24.09 mg, 0.11 mmol) was added. The obtained yellow
suspension was stirred overnight giving a pinkish-red solution, which was
filtered and used for a gas phase diffusion crystallization with n-hexane.
Colorless and red crystals stemming from 2 and [Ni2Cl4(neocuproine)2],
respectively, were obtained. Yield (2) 10 mg (59%). Anal. Calcd. for
C126H144B3.33 Cl4.67Cu4F13.33N4O19P8∙1.41CH2Cl2∙2.78ClCH2CH2Cl⋅3H2O
(3376.10): C, 47.31%; H, 4.89%; N, 1.66%. Found: C, 47.23%; H, 4.93%; N,
1.57%. Mass spectrometry: Fragments, Cu(L2) [1169.19]; Cu2Cl(L2)
[1267.11]; Cu3Cl2(L2) [1365.01] (see ESI Figs. S14-S16). Single crystals with
the composition [2H4](BF4)3.33Cl0.67∙1.41CH2Cl2∙2.78ClCH2CH2Cl⋅3H2O
suitable for an X-ray structure analysis were obtained by slow evaporation of
a dichloromethane/1,2-dichloroethane (v/v = 1:1) solution of 2 in the
presence of n-hexane. This crystallization was only successful under an
161.0 (d, 1JPC = 13.0 Hz, CAr). 31P{1H} NMR (121.5 MHz, CD2Cl2, ppm): δ
= -44.14 (s).
Synthesis of L2: Paraformaldehyde (82.92 mg, 2.76 mmol) and bis(2-
methoxyphenyl)phosphane (680.0 mg, 2.76 mmol) were stirred in a
deaerated 1:1 (v.v) THF/MeOH solvent mixture (5.0 mL) at room tem-
perature for 20 h. The completeness of the reaction was controlled by
31P{1H} NMR spectroscopy. Due to the volatility of formaldehyde, it
might be necessary to add paraformaldehyde in order to complete the
reaction. To the clear solution was added 1,3-diaminopropane (58.0 μL,
0.69 mmol) and the resulting solution stirred for 4 h. Afterwards, the
obtained yellow solution was concentrated to dryness, giving a white
foamy solid, which was triturated upon stirring in diethyl ether (10.0
mL). The solid product was then separated by filtration and dried under
2

J. Pann et al.
Inorganica Chimica Acta 516 (2021) 120162
atmosphere of air, since water molecules are present in the crystal lattice.
1H NMR (300.0 MHz, CD2Cl2, ppm): δ = 3.24 (s, 24H, OMe), 3.36 (s,
24H, OMe), 4.35 (br. s, 8H, CHP2,N), 4.79 (br. s, 8H, CHP2,N), 6.25 (t, J =
7.4 Hz, 8H), 6.59 (d, J = 8.8 Hz, 8H), 6.80 (t, J = 7.0 Hz, 8H), 6.90 (d, J
= 7.9 Hz, 8H), 6.98 (s, 8H), 7.16 (t, J = 6.9 Hz, 16H), 7.51 (t, J = 7.2 Hz,
8H), 10.41 (br, s, 4H, NH+). 31P{1H} NMR (121.5 MHz, CD2Cl2, ppm): δ
= -17.9 (s).
SHELXTL-NT V6.1 and SHELXL-2014/7. Final refinements on F2 were
done with anisotropic thermal parameters for all non-hydrogen atoms in
all cases except for one disordered solvent carbon atom in the case of 2.
Furthermore, in 2 site sharing occurs between one BFꢀ4 anion (1/3 site
occupation) and a chloride anion (2/3 site occupation). During crys-
tallization 2 is completely protonated as a result of the strong basicity of
the four amines in 2 in only very slightly wet solvents. The hydrogen
atoms were included using a riding model with isotropic U values
depending on the Ueq of the adjacent carbon atoms. In the case of sub-
stantially disordered solvent molecules the hydrogen atoms on these
solvent molecules have been omitted.
2.3. Operando 31P{1H} NMR experiments
2.3.1. Reaction of paraformaldehyde with bis(ortho-methoxyphenyl)
phosphane
CCDC numbers of the cif files of the crystal structures reported in this
paper: 2,038,636 (L1⋅4CH2Cl2), 2,038,637 (1a), and 2,038,638 (for
2∙1.41CH2Cl2∙2.78ClCH2CH2Cl⋅3H2O). These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
Bis(ortho-methoxyphenyl)phosphane (1.3601 g, 5.5232 mmol, 1.05
eq.) was dissolved in a 1:1 (v:v) THF/MeOH solvent mixture. To the
clear solution, paraformadehyde (157.0 mg, 5.228 mmol, 1.0 eq.) was
added giving a lightly opaque solution. A homogenized NMR sample
was withdrawn and the operando 31P{1H} NMR measurement were
carried out. The results of these measurements are summarized in Fig. 2.
3. Results and discussion
3.1. Ligand syntheses
2.3.2. Conversion of 1b into 2
1b (2.87 mg, 1.5 µmol) was dissolved in 1,2-dichloroethane (1.25
mL) giving a yellow solution to which [NiCl2(DME)] (3.50 mg, 16 µmol)
was added. The resulting suspension was quantitatively transferred into
a 5 mm NMR tube and 31P{1H} NMR spectra were acquired at time in-
tervals shown in Fig. 5.
The
α
-aminophosphane ligand L1, characterized by two 2-methoxy-
phenyl (An) substituents at each phosphorus donor atoms (Scheme
1A) was tried to synthesize following the same synthesis protocol re-
ported for the known phenyl-analogue (i.e. 65 ◦C, toluene as solvent, 6 h
reaction time) but unfortunately without success [14]. Due to the higher
steric crowding of the 2-methoxyphenyl compared to phenyl sub-
stituents a mixture of mono-, di-, tri- and tetra-substituted ethylenedi-
amines were obtained, even after applying higher reaction temperatures
(90 ◦C) and longer reaction time (up to one week).
2.4. Crystallography
Prior to the XRD measurements, the single crystals were handpicked
using a Leica Wild M10 microscope with vertical illumination and a
polarising filter. Single-crystal X-ray diffraction analyses of L1∙4CH2Cl2,
1a, and 2∙1.41CH2Cl2∙2.78ClCH2CH2Cl⋅3H2O were performed on a
Bruker D8 diffractometer with incoatex-microfocus-channel, a multi-
layer-monochromator and the new CMOS-technology from Bruker (D8
Quest). Cell refinement, data reduction and the empirical absorption
correction were performed using Apex III and Saint-V 8.34 A pro-
grammes. All structure determination calculations were realised using
The Synthesis of L1 (Scheme 1A) was achieved following a classical
Mannich-type reaction, employing the secondary phosphane as CH-
acidic compound. “In situ” conducted control NMR experiments
confirmed the absence of an iminium intermediate, which is in line with
the basic Mannich mechanism proposed by Cummings and Shelton [31].
The 31P{1H} spectrum of L1 acquired in CD2Cl2 showed a singlet at
ꢀ 44.14 ppm, confirming the chemical equivalence of all four phos-
phorus donor atoms. In addition, the 1H NMR spectrum acquired in the
same solvent exhibited one singlet for the equivalent methylene
hydrogen atoms of the N,N bridging carbon atoms centered at 3.12 ppm,
and a doublet at 3.60 ppm with a 2JPH of 4.0 Hz for four equivalent N,P
methylene bridges. The solid state structure of L1⋅4CH2Cl2 confirmed the
atom connectivity (Fig. 1).
Attempts to synthesize L2 by applying an analogous synthesis pro-
tocol used for L1, failed. In accordance with published results obtained
from reaction of 1,3-diaminopropane with formaldehyde, which leads to
hexahydroxypyrimidine and its derivatives [32], we confirmed their
formation by acquiring 1H NMR spectra during the reaction of para-
formaldehyde and 1,3-diaminopropane.
In order to overcome the formation of 1,3-diazinanes, we condensed
first the aldehyde (paraformaldehyde) with the secondary phosphane to
give the corresponding hydroxymethylenphosphane as intermediate
(Scheme 1A).
Indeed, this latter type of compounds has been generally used as
useful synthon in aminophosphane synthesis [33–36]. A sequence of 31
P
{1H} NMR spectra acquired under operando experimental conditions
show the conversion of HP(An)2 (singlet at ꢀ 72.80 ppm) into HOCH2P
(An)2 (singlet at ꢀ 27.80 ppm) (Fig. 2).
The second step of the L2 synthesis was the reaction of HOCH2P(An)2
with 1,3-diaminopropane in a THF/MeOH solvent mixture. 31P{1H} and
1H NMR data acquired for L2 in CD2Cl2 are in agreement with the
assigned molecular structure (Scheme 1A). Isolated L1/2 are reasonable
air-stable, while aerated solutions of either latter ligands yielded a
mixture of oxides, whose formation was easily catalyzed in the presence
of Cu(I) ions.
Fig. 1. ORTEP plot of L1⋅4CH2Cl2, showing only selected labels for the asym-
metric unit. Solvent molecules and hydrogen atoms have been omitted for
clarity and the thermal ellipsoids are shown at 50% probability level. Selected
bond distances (Å) and angles (◦) for L1: N(1)-C(2) = 1.471(2), N(1)-C(1) =
1.473(2), N(1)-C(3) = 1.476(2); C(2)-N(1)-C(1) = 111.40(15), C(2)-N(1)-C(3)
= 110.26(15), C(1)-N(1)-C(3) = 109.60(15).
3

J. Pann et al.
Inorganica Chimica Acta 516 (2021) 120162
Fig. 2. Sequence of 31P{1H} NMR spectra acquired in a 1:1 (v:v) THF/MeOH solvent mixture at room temperature during the reaction of paraformaldehyde with
HP(An)2.
3.2. Cu(I) complex syntheses
0.388 and 0.785 Å, respectively, from the plane defined by P(1), P(2), C
(2) and C(3).
Both ligands L1 and L2 reacted with [Cu(I)(MeCN)4]BF4 in THF in the
presence of neocuproine at room temperature to give the corresponding
cationic dinuclear Cu(I) complexes 1a and 1b, respectively (Scheme 1B).
The successful coordination of L1/2 by Cu(I) yielding two independent
Cu(I) coordination spheres characterized by the cis-coordination of two
phosphorus donor atoms of L1/2 and two nitrogen atoms of neocuproine
is proved in solution by a singlet in the corresponding 31P{1H} NMR
spectrum at ꢀ 21.66 (1a) and –22.57 ppm (1b) and by the integral of the
aromatic hydrogen atoms from the corresponding 1H NMR spectrum.
The molecular structure in the solid state has been proven by conducting
a single crystal X-ray structure analysis at least for compound 1a (Fig. 3).
UV–Vis spectra of 1a and 1b obtained from measurements carried out in
1,2–dichloroethane show a broad band at 420 nm for both complexes
with an extinction coefficient of 8000 (1a) and 6000 L molꢀ 1 cmꢀ 1 (1b)
(Fig. S1). These values are in a typical range for MLCT transitions of Cu
Compound 1b reacted in CH2Cl2 in the presence of excess
[NiCl2(DME)] to give the Cu(I) cluster 2 (Scheme 1C) as crystalline
compound along with the known dimer Ni(II) compound of the formula
[Ni2Cl4(neocuproine)] (i.e. a single crystal structure analysis and a
UV–Vis spectrum, Fig. S2, confirmed its formation [37,38]). Compound
2, which exhibited a low solubility in organic solvents, showed one
singlet at ꢀ 17.9 ppm in the 31P{1H} NMR spectrum, while the 1H NMR
spectrum showed two sets of NMR signals assigned to two different L2
ligands. Interestingly no interaction between aromatic hydrogen atoms
and the metal centers, as shown by other sterically demanding
2–methoxyphenylphosphine based PNP complexes, was observed [39].
In addition, a broad 1H NMR signal at 10.41 ppm (Fig. S11), was
assigned to the hydrogen atoms directly bond to the nitrogen atoms (i.e.
ammonium units) (Scheme 1C) [40].
The
single
crystal
X-ray
analysis
of
(I)N2P2-compounds. Further at 280 nm an intense
the neocuproine ligands is observable.
π
→
π
* transition of
2∙1.41CH2Cl2∙2.78ClCH2CH2Cl⋅3H2O (Fig. 4) is in agreement with
NMR spectroscopic evidences, showing two completely protonated L2
ligands sandwiching a CuCl-tetramer. The extra positive charge intro-
duced by four ammonium units belonging to two L2 ligands are
compensated by the presence of BFꢀ4 and Cl- anions. Four Cu(I) centers
bridged by chlorides define the main plane of the molecule (i.e. devia-
tion of Cu(I) and Clꢀ from the plane defined by Cu(1)–Cu(4): Cu(1)
0.005, Cu(2) ꢀ 0.005, Cu(3) ꢀ 0.005, Cu(4) 0.005, Cl(1) ꢀ 0.475, Cl(2)
0.517, Cl(3) 0.481, Cl(4) ꢀ 0.523 Å). Each Cu(I) atom, which shows a
tetrahedral coordination environment (i.e. the P-Cu-P bond angles are in
the range between 134.71(3)◦ and 125.07(3)◦, while the Cl-Cu-Cl bond
angles are in the range between 102.84(3)◦ and 98.44(3)◦), is coordi-
nated by two bridging chloride atoms and two phosphorus atoms
belonging to two different protonated L2 ligands, which are localized
The crystal structure of 1a shows a slightly distorted tetrahedral
coordination of both equivalent Cu(I) metal centers, as proved by the
dihedral angle between the N–Cu–N and P–Cu–P planes of 83.4◦. The
Cu–N bond distances of 2.073(3) and 2.093(3) Å are slightly longer
compared to that of a related mononuclear-cationic Cu(I)-neocuproine
complex, which bears a chelating
α-aminophosphane, featured by
phenyl-groups at the phosphorus donor atoms (i.e. 2.046(3) and 2.076
(3) Å) [22]. In fact, the higher steric encumbrance of ortho-anisyl sub-
stituents compared to phenyl ones at the phosphorus atoms leads to two
significantly different Cu-P bond distances of 2.236(1) and 2.288(1) Å.
The conformation of the six-membered Cu(I) metallacycle can be best
described with a boat conformation, where Cu(1) and N(1) deviate by
4

J. Pann et al.
Inorganica Chimica Acta 516 (2021) 120162
Scheme 1. Syntheses of L1/2, 1a/b and 2.
Fig. 3. ORTEP plot for 1a. Hydrogen atoms and 2-methoxy phenyl atoms except for the ipso carbon atom have been omitted. The thermal ellipsoids are shown at
50% probability level. Selected bond distances (Å) and angles (◦): Cu(1)-P(1) = 2.236(1), Cu(1)-P(2) = 2.288(1), Cu(1)-N(2) = 2.093(3), Cu(1)-N(3) = 2.073(3), P
(1)-Cu(1)-P(2) = 104.15(4), N(2)-Cu(1)-N(3) = 81.06(12).
above and below the Cu4-plane and almost perpendicularly orientated to
each other. The hydrogen atoms bonded to the four nitrogen atoms of
both ligands, were localized by difference Fourier analysis and each of
them point towards a bridging chloride atom of the CuCl-tetramer core,
showing H..Cl distances ranging from 2.234 to 2.425 Å, which are
typically in the range of hydrogen bonds [40].
The conformation of (CuCl)-tetramer found in solid state is mainly
determined by the nature of the phosphane ligands used. In Scheme 2
are sketched the different known (CuCl)4 conformers (i.e. result of a
Cambridge data base search). The cubane-like-structure of (CuCl)-
5

J. Pann et al.
Inorganica Chimica Acta 516 (2021) 120162
tetramer (Scheme 2A) is mainly stabilized by monodentate phosphane,
phosphinite and phosphonite ligands [41–45], and by chelating phos-
phanes [40] and phosphaalkenes [46]. In solution equilibria between
cubane and the thermodynamically more stable chair conformation
(Scheme 2B) was observed. The latter (CuCl)-tetramer conformation
(Scheme 2B) was found to be stabilized by two bis(diphenylphosphanyl)
methane ligands, coordinating on both sides of the (CuCl)-tetramer chair
[47,48]. (CuCl)4 with a ring structure were stabilized upon coordination
to the appropriate chelating ligands and three different examples of
possible ligand coordination to the (CuCl)-tetramer were reported so far
(Scheme 2C-F) [49–51].
All the above mentioned phosphane-stabilized (CuCl)-tetramers
(Scheme 2) have in common a Cu(I) to phosphorus atom ratio of 1:1,
while in 2∙1.41CH2Cl2∙2.78ClCH2CH2Cl⋅3H2O a 1:2 ratio stabilizes a
completely cyclic (CuCl)-tetramer, which is forced in between two
protonated L2 ligands.
In order to gain more information concerning the conversion of 1b
into 2 in the presence of an excess of NiCl2(DME), we acquired 31P{1H}
NMR spectra in 1,2-dichloroethane under operando reaction conditions
(Fig. 5). As a result, a clean transformation of 1b (31P{1H} NMR singlet
at –22.1 ppm) into 2 (31P{1H} NMR singlet at ꢀ 17.9 ppm) has been
observed.
Fig. 4. ORTEP plot of 2∙1.41CH2Cl2∙2.78ClCH2CH2Cl⋅3H2O, showing only the
core of the cluster. Hydrogen atoms except those bonded to the nitrogen atoms
have been omitted and the thermal ellipsoids are shown at 50% probability
level. Selected bond distances (Å) and angles (◦): Cu(1)-P(1) = 2.2401(9), Cu
(1)-P(6) = 2.2423(9), Cu(2)-P(3) = 2.2774(9), Cu(2)-P(5) = 2.2777(9), Cu(3)-P
(2) = 2.2695(9), Cu(3)-P(8) = 2.2655(9), Cu(4)-P(4) = 2.2798(9), Cu(4)-P(7)
= 2.2772(9), Cu(1)-Cl(1) = 2.4885(9), Cu(1)-Cl(3) = 2.4388(8), Cu(2)-Cl(1) =
2.4510(9), Cu(2)-Cl(2) = 2.4463(9), Cu(3)-Cl(3) = 2.4274(8), Cu(3)-Cl(4) =
2.5062(8), Cu(4)-Cl(2) = 2.4335(8), Cu(4)-Cl(4) = 2.4624(8), P(1)-Cu(1)-P(6)
= 134.71(3), P(3)-Cu(2)-P(5) = 133.75(3), P(2)-Cu(3)-P(8) = 131.45(3), P(4)-
Cu(4)-P(7) = 125.07(3), Cl(1)-Cu(1)-Cl(3) = 98.80(3), Cl(1)-Cu(2)-Cl(2) =
102.84(3), Cl(3)-Cu(3)-Cl(4) = 101.11(3), Cl(2)-Cu(4)-Cl(4) = 98.44(3).
4. Conclusion
Two new bis-chelating α-aminophosphanes, featured by an ethylene
or propylene bridge in between the nitrogen donor atoms and ortho-
anisyl substituents at the phosphorus donor atoms, have been synthe-
sized and completely characterized. Both ligands coordinate Cu(I) ions
giving the corresponding dinuclear Cu(I) complexes, featured by a
tetrahedral metal coordination geometry and two completely indepen-
dent Cu(I) coordination spheres. The dimer Cu(I) complex featured by a
propylene bridge between the nitrogen atoms was successfully
Fig. 5. 31P{1H} NMR spectra acquired in 1,2-dichloroethane at different reaction times for the conversion of 1b into 2.
6

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Inorganica Chimica Acta 516 (2021) 120162
Scheme 2. Different solid state conformations observed for coordinated (CuCl)-tetramers.
transformed into a cyclic CuCl-tetramer cluster which showed an
absolutely new type of CuCl-tetramer coordination (i.e. (CuCl)4-core
sandwiched by two protonated and orthogonally orientated
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2020.120162.
α
-aminophosphanes).
Declaration of Competing Interest
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