Modular Design Strategy toward Second-Generation Tridentate Carbodiphosphorane N,C,N Ligands with a Central Four-Electron Carbon Donor Motif and Their Complexes

Article abstract of DOI:10.1021/acs.organomet.1c00231

The reaction of sym-bis(P-chlorodiphenyl)carbodiphosphorane (1) with difunctional nucleophiles leads to carbodiphosphoranes carrying two additional chelating N-donor functionalities. A proof of concept is demonstrated by the synthesis and characterization of sym-bis(3,5-dimethyl-1H-pyrazol-1-yl)carbodiphosphorane (CDP(3,5-MePz)2, 2) and sym-bis(pyridin-2-yloxy)carbodiphosphorane (CDP(O-2Py)2, 3). Due to their superbasic central two-/four-electron carbon donor functionality, these neutral ligands are electronically flexible to act as neutral six- or eight-electron donors, as pincer ligand templates, or as two geminally metal bridging ligands. Their potential to form mono- and dinuclear complexes involving two 6-ring or two 5-ring N,C-chelate ring motives has been explored. Complexes of 2 and 3 with fac-[M(CO)3] fragments (ls d6 M = Cr, Mo, W) were used as spectroscopic probes. They reveal a strong σ-donor and potential π-donor ability of the central carbon donor pushing electron density for enhanced M-CO back-bonding into the metal d orbitals. DFT calculations consolidate this observation. Dinuclear and multinuclear d10 Cu(I) complexes have been formed and structurally investigated upon treating these CDP ligands 2 and 3 with CuX (X = Cl, Br, I).

Full text of DOI:10.1021/acs.organomet.1c00231

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Article
Modular Design Strategy toward Second-Generation Tridentate
Carbodiphosphorane N,C,N Ligands with a Central Four-Electron
Carbon Donor Motif and Their Complexes
Marius Klein and Jörg Sundermeyer*
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ABSTRACT: The reaction of sym-bis(P-chlorodiphenyl)-
carbodiphosphorane (1) with difunctional nucleophiles leads to
carbodiphosphoranes carrying two additional chelating N-donor
functionalities. A proof of concept is demonstrated by the synthesis
and characterization of sym-bis(3,5-dimethyl-1H-pyrazol-1-yl)-
carbodiphosphorane (CDP(3,5‑MePz)2, 2) and sym-bis(pyridin-2-
yloxy)carbodiphosphorane (CDP(O-2Py)2, 3). Due to their
superbasic central two-/four-electron carbon donor functionality,
these neutral ligands are electronically flexible to act as neutral six-
or eight-electron donors, as pincer ligand templates, or as two
geminally metal bridging ligands. Their potential to form mono-
and dinuclear complexes involving two 6-ring or two 5-ring N,C-
chelate ring motives has been explored. Complexes of 2 and 3 with fac-[M(CO)3] fragments (ls d6; M = Cr, Mo, W) were used as
spectroscopic probes. They reveal a strong σ-donor and potential π-donor ability of the central carbon donor pushing electron
density for enhanced M−CO back-bonding into the metal d orbitals. DFT calculations consolidate this observation. Dinuclear and
multinuclear d10 Cu(I) complexes have been formed and structurally investigated upon treating these CDP ligands 2 and 3 with CuX
(X = Cl, Br, I).
Some time ago, the very strong four-electron σ- + π-donor
character of this neutral CDP carbon ligand was proven
experimentally in organometallic complexes of highly π acidic
d0-[ReO3]+ fragments.12 In contrast, Lewis acids without a
LUMO of π symmetry (similar to protons) address both the
HOMO and HOMO-1 of the CDP, preferentially forming
dinuclear complexes with two dative σ bonds of the bridging
four-electron-donor carbon atom. These versatile properties
make CDPs attractive as ligands in coordination chemistry.2−4
Common synthetic strategies for the synthesis of CDPs involve
deprotonation,1,13 dehydrohalogenation,14 or dehalogena-
tion11,15,16 of the corresponding precursors such as [HC-
(PR3)2]X, [H2C(PR3)2]X2, or [ClC(PR3)2]X. CDPs have also
been used as central building blocks in pincer ligands, thus
extending their potential as templates in organometallic
chemistry. Rhodium and platinum precursor complexes do
react with hexaphenylcarbodiphosphorane to yield cyclo-
metalated, formally dianionic C,C,C pincer complexes.17−21
INTRODUCTION
■
Hexaphenylcarbodiphosphorane was first synthesized in 1961.1
For many years, organometallic chemistry mainly focused on
this particular monofunctional carbodiphosphorane (CDP), a
neutral zwitterionic carbon donor with a donor capability of
potentially four electrons.2−5 Traditionally, the bonding in
CDPs has been discussed in terms of zwitterionic, double
ylidic, or donor−acceptor resonance forms with some
preference for the ylidic form. The other resonance form
suggesting a CDP bonding situation best described as a
donor−acceptor complex of two phosphines stabilizing a
formally zerovalent carbon atom of coordination number 2 has
been much promoted by Frenking and co-workers.6−10 On the
basis of their DFT analyses the central carbon atom can act as
an acceptor atom in its excited singlet (1D) state stabilized by
the σ-donating phosphine ligands. The two lone pairs
characteristic for such a carbon(0) complex or “carbone” are
differentiated into one of π symmetry, partially stabilized by
back-bonding into the two phosphine LUMOs of π symmetry,
while the other carbon lone pair of σ symmetry is the CDP
orbital typically addressed first for σ bonds toward metal
cations (or protons).11 Without a doubt, the Frenking carbone
model is an inspiration, especially if related phosphine (or L)
complexes of higher homologues of elemental carbon and
other elements are envisaged or analyzed by DFT calculations.
Received: April 10, 2021
Published: June 28, 2021
© 2021 The Authors. Published by
American Chemical Society
https://doi.org/10.1021/acs.organomet.1c00231
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Isolation of bis-ortho-lithiated Li2CDP22 allows transferring
this dianionic C,C,C pincer ligand to s-, p-, d-, and f-metal
centers.23 P,C,P pincer CDP complexes have been introduced
by Peringer24−28 and further investigated by Langer.29−31 For a
long time, they had only been characterized in the form of their
complexes. Only recently, the free ligand base has C(dppm)2
been isolated, characterized, and used in ligand transfer
reactions.32 In the field of N,C,N carbodiphosphorane pincer
ligands, the Zhu and Sundermeyer groups reported the
synthesis of CDP complexes incorporating two phosphorus-
bound 2-pyridyl donor functionalities.33,34 The free pincer
ligand was obtained following the Appel strategy,16 a radical
reaction of 2-Py-PPh2 and CCl4.33 Notably, several years
before, a mono-2-pyridyl-substituted carbodiphosphorane was
synthesized by Alcarazo et al. in order to investigate
heterobimetallic complexes; however, tridentate or pincer-
type CDP ligands were not considered or realized.35 Recently,
the Gessner group has reported the coordination capability of
an bis-P-piperidino-substituted CDP ligand.36a They stated
that the N-donor character toward d10-Zn2+ is either
nonexistent or rather weak. Both the electron-withdrawing
character of the phosphonio group attached to the proposed
N-donor atom and an unfavorable bite defined by potentially
two annelated four-membered chelate rings hinder this ligand
from being a privileged tridentate donor. In order to design
new pincer type N,C,N ligands via a construction kit and a
different strategy from that which we previously reported, we
decided to study the reaction of ambident N,N- and N,XO-
anion building blocks with the known sym-bis(P-
chlorodiphenyl)carbodiphosphorane (CDP-Cl2) as a P-electro-
phile.37
bis(2-pyridyl)tetraphenylcarbodiphosphorane CDP(2Py)2, the
new N-functional CDPs 2 and 3 both do not show any
photoluminescence33 or triboluminescence.38 As expected,
31P{1H} NMR spectra display singlets for 2 (8.9 ppm, C6D6)
and for 3 (23.1 ppm, CD2Cl2), a trend deviating from the
parent hexaphenylcarbodiphosphorane (4.3 ppm, C6H5Cl)39
or CDP(2Py)2 (−5.6 ppm, C6D6).33 However, our observation
is in accord with the relatively high group electronegativity of
the introduced N- and O-anions substituting electronegative P-
chloro or P-fluoro substituents in sym-CDP-Cl2 (1) (22.7 ppm,
C6D6) and sym-CDP-F2 (43.2 ppm, C6D6).40
Since no crystal structure of the free ligands 2 and 3 could
be obtained so far by applying various crystallization
techniques, DFT calculations at the PBE-D3(BJ)/def2-
TZVPP level of theory were performed in order to gain a
deeper insight into the gas-phase structures of 2 and 3 (see
Figures S-87−S-94). A quantum mechanical analysis reveals
that both structures show the arrangement of two coplanar
phenyl rings attached to different phosphonio groups. The
other two phenyl rings point with their ortho hydrogen atoms
toward the electron-rich central carbon atom with its formally
two electron lone pairs. This phenomenon of stabilization by
π-stacking and weak intramolecular C···Hortho interactions has
also been observed33,41 and discussed33,42 for other CPDs. The
repulsive nature of the substituents and carbon lone pairs
forces the 3,5-dimethyl-1H-pyrazol-1-yl groups of 2 and the
pyridin-2-yloxy groups of 3 into an anti conformation. Not
surprisingly, DFT calculations propose that the HOMO-1 can
be interpreted as a lone pair orbital of σ symmetry and the
HOMO as one of π symmetry in both 2 and 3 (Figure 1). This
is supported by a natural bond orbital (NBO) analysis, where
the atomic partial charge of the central carbon q(C) was
calculated at the PBE-D3(BJ)/def2-TZVPP level of theory.
For 2 q(C) is −1.41 e and for 3 q(C) is −1.38 e.
In accord with previously reported proton affinities (PA) of
carbodiphosphoranes,10,33 DFT calculations on 2 give PA
values of 282.6 and 186.5 kcal/mol for the first and second
protonations, whereas the corresponding values are 287.6 and
192.3 kcal/mol for 3. Upon the first and second protonations
of the neutral CDPs the negative charge q(C) becomes less
negative: the q(C) value of [2H]+ is −1.33 e, that of [2H2]2+ is
−1.17 e, that of [3H]+ is −1.36 e, and that of [3H2]2+ is
−1.19 e. The DFT results clearly show that considerable
electron density is located at the imidazole and pyridine N-
donor atoms at a bite angle perfectly suitable for a double-
chelating bonding situation. Therefore, we anticipated that, in
contrast to Gessner’s bis(amino)-CDP36a and our tetra- and
hexaamino-CDPs,34b we might have generated new tridentate
or even pincer-type CDP ligands.
RESULTS AND DISCUSSION
■
CDP-Cl2 (1) can be converted to sym-bis(3,5-dimethyl-1H-
pyrazol-1-yl)carbodiphosphorane (CDP(3,5‑MePz)2, 2) and
sym-bis(pyridin-2-yloxy)carbodiphosphorane (CDP(O-2Py)2,
3) by reaction with 2 equiv of potassium 3,5-dimethyl-1H-
pyrazolate and sodium-2-pyridinolate, respectively. After 18 h
at room temperature in THF the conversions were complete,
as monitored by 31P{1H} NMR. Light green CDP(3,5‑MePz)2
(2) was isolated in 77% yield and gray CDP(O-2Py)2 (3) in
close to quantitative yield in their free ligand base forms
(Scheme 1). In sharp contrast to the previously described sym-
Scheme 1. Synthesis of the Novel Tridentate N,C,N
Carbodiphosphorane Ligands 2 and 3
In order to evaluate the coordinative potential of these tri- or
tetradentate six- or eight-electron-donor ligands, 2 and 3 were
reacted with [M(CO)3(NCMe)3] (M = Cr, Mo, W) in THF.
The products fac-[M(CO)3(CDP(3,5‑MePz)2)] (4a, 5, and 6)
Figure 1. Kohn−Sham orbitals of the HOMO-1 (a) and HOMO (b) of 2 and the HOMO-1 (c) and HOMO (d) of carbodiphosphorane 3
calculated for the optimized S0 state geometry (isovalue 0.05). Calculations were performed at the PBE-D3(BJ)/def2-TZVPP level of theory.
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Scheme 2. Synthesis of Novel Carbodiphosphorane Carbonyl Complexes of sym-Bis(3,5-dimethyl-1H-pyrazol-1-
yl)carbodiphosphorane (2) and sym-Bis(pyridin-2-yloxy)carbodiphosphorane (3)
and fac-[M(CO)3(CDP(O-2Py)2)] (7−9) were isolated in
Table 1. Selected Bond Distances (Å) and Angles (deg) for
4a, 5, 7, and 8
good yields of between 54% and 85% (Scheme 2).
The Cr and Mo complexes 4a, 5 and 7, 8 were crystallized
via layering a solution in dichloromethane or chloroform with
n-pentane. The results of single-crystal XRD analyses are
displayed in Figure 2.
4a
5
7
8
M−C1
2.220(3)
2.175(4)
2.173(4)
1.823(5)
1.810(4)
1.813(5)
1.173(5)
1.185(4)
1.167(5)
1.624(5)
1.653(5)
134.8(2)
2.339(4)
2.318(5)
2.300(4)
1.938(6)
1.940(5)
1.928(6)
1.172(7)
1.178(6)
1.181(7)
1.632(6)
1.649(6)
136.2(3)
2.264(2)
2.190(2)
2.2408(18)
1.829(2)
1.813(2)
1.835(2)
1.176(3)
1.168(3)
1.180(3)
1.647(2)
1.646(2)
129.0(1)
2.371(2)
2.320(2)
2.351(2)
1.937(2)
1.928(2)
1.947(2
1.172(2)
1.179(2)
1.170(2)
1.647(2)
1.643(2)
129.9(1)
M−N1
M−N2
M−C2
M−C3
M−C4
C2−O1
C3−O2
C4−O3
C1−P1
C1−P2
P1−C1−P2
2.3713(17) Å (8, Mo). This is consistent with the presence of
M−C single bonds, without any π-dative contribution, as
expected for such 18-valence-electron metal centers without
matching metal-centered LUMOs and with respect to the
differences in covalent radii of Cr (1.39 Å) and Mo (1.54 Å).43
While the P1−C1−P2 angle increases in the more rigid 5-ring
chelate structures of 2 rising 134.8(2)° (4a, Cr) to 136.2(3)°
(5, Mo), a less pronounced increase is registered in the more
flexible 6-ring chelate structures of 3 from 129.01(13)° (7, Cr)
to 129.91(11)° (8, Mo). For the same metal, the less
constrained annelated 6-ring chelates display shorter M−C1
bonds and the more constrained 5-ring chelates longer M−C1
bonds. The trend to shorter M−C1 bonds correlates with a
tendency for slightly longer M−N bonds to both pyridyl sp2
donor atoms in 7 and 8 in comparison to pyrazolyl sp2 donor
atoms in 4a and 5. Due to the additional methyl groups at the
3,5-dimethyl-1H-pyrazol-1-yl units and a much more con-
strained situation in tridentate chelate complexes of 2, the
coplanar π-stacked phenyl rings previously observed in the free
ligand base deviate from coplanarity and experience less π
stacking (Figure 2a). However, less constrained pyridin-2-yloxy
complexes 7 and 8 with their smaller P1−C1−P2 angles
display such pairs of nearly coplanar phenyl rings in the
crystalline solid state: interplanar angles of both phenyl rings
are as follows: 4a, 22.15°; 5, 24.52°; 7, 9.65°; 8, 9.89° (Figure
2b). For details see Figures S-83−S-86 in the Supporting
Information.
Figure 2. XRD molecular structures of (a) fac-[Cr(CO)3(CDP-
(
3,5‑MePz)2)] (4a) and (b) fac-[Cr(CO)3(CDP(O-2Py)2)] (7). The
isostructural fac-[Mo(CO)3(CDP(3,5‑MePz)2)] (5) and fac-[Mo-
(CO)3(CDP(O-2Py)2)] (8) are displayed in Figure S-80 of the
Supporting Information. Hydrogen atoms and solvent molecules have
been omitted for clarity; thermal ellipsoids are set at 50% probability.
For details see the Supporting Information.
All isostructural complexes crystallize in the triclinic space
group P1 with two units in their unit cells. As has already been
̅
observed for related CDP(2Py)2 ligand complexes,33 the
central atoms are coordinated in a slightly distorted facial
configuration. This is due to a strong thermodynamic trans
effect of the π-acidic carbonyl ligands directing the much less
π-acidic pincer donor atoms into a facial array. Both 3,5-
dimethyl-1H-pyrazol-1-yl donor functionalities are part of five-
membered chelate rings and both pyridin-2-yloxy units are part
of six-membered chelate rings with respect to the central
carbon donor. Selected bond distances and angles for 4a, 5, 7,
and 8 are displayed in Table 1. As expected, the M−C1, M−N
and M−CO distances increase when the 3d metal Cr in 4a and
7 is replaced by the 4d metal Mo in 5 and 8. For example, the
prominent M−C1 distance increases from 2.220(3) Å (4a, Cr)
to 2.339(4) Å (5, Mo) and from 2.264(2) Å (7, Cr) to
In attempts to crystallize 4a from DCM/pentane for an
extended period of several weeks, a small crop of single crystals
of different habitus in comparison to 4a was separated and
analyzed by XRD analysis and IR. The results unveiled a
tetracarbonyl complex of the composition [Cr(CO)4(CDP-
(
3,5‑MePz)2)] (4b) with one coordinated and one dangling
pyrazolyl ligand (Figure 3). Small quantities of 4b might have
formed either upon partial decomposition of 4a with release of
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a shoulder is typically observed. The DFT calculations (PBE-
D3(BJ)/def2-TZVPP) support the experimental results and
suggest two asymmetric stretching modes between 1829 and
1853 cm−1 and one symmetric stretching vibration between
1908 and 1919 cm−1.
The experimental spectra and corresponding vibrational
modes are presented in Figures S-43−S-66 in the Supporting
Information. An example of a comparison of the experimental
and calculated IR spectra is displayed in Figure S-46.
The calculated vibrations are marginally shifted to higher
wavenumbers in comparison to the experimental results. As has
already been observed for the N,C,N-CDP complex fac-
[Cr(CO)3(CDP(2Py)2)],33 the asymmetric CO stretching
vibrations are shifted to lower wavenumbers in comparison
to [Cr(CO)6] (2003 cm−1),44 indicating that the two 3,5-
dimethyl-1H-pyrazol-1-yl or pyridin-2-yloxy groups are rea-
sonable but less efficient π-acceptor ligands in comparison to
carbonyl groups. In summary, the imidazolyl and pyridyl
carbodiphosphoranes 2 and 3 perform as strong C-σ-donor,
medium-strong N-σ-donor, and weak N-π-acceptor ligands.
The ligand’s HOMO, formally the carbone electron pair of π
symmetry, remains as a nonbonding ligand-centered orbital in
these types of carbonyl metal complexes 4−9. This HOMO is
partially stabilized by back-bonding into antibonding orbitals
of both phosphonio groups, as evidenced by a shorter average
P−C bonding distance in comparison to typical P−C single
bonds in pentavalent tetracoordinate phosphonium groups
such as Ph4P+ (P−C = 1.799(5) Å).45
Figure 3. Formula and XRD molecular structure of the side or
decomposition product cis-[Cr(CO)4(CDP(3,5‑MePz)2)] (4b). Hydro-
gen atoms and solvent molecules have been omitted for clarity;
thermal ellipsoids are given at 50% probability. For details see the
Supporting Information.
CO or by the presence of traces of [Cr(CO)4(MeCN)2] in the
[Cr(CO)3(MeCN)3] starting material. [Cr(CO)4(CDP-
(
3,5‑MePz)2)] (4b) shows an additional asymmetric stretching
vibration due to an additional terminal CO group. Crystallo-
graphic data, bond distances, and a DFT analysis of the
calculated CO frequencies of 4b are displayed in Figures S-47,
S-48, and S-95 and Table S-4 in the Supporting Information.
The complexes 4−9 were investigated via IR spectroscopy.
The results of ATR-FT-IR spectra of finely ground solid
samples were compared to vibrational spectra in the CO
stretching region simulated by DFT calculations (Table 2).
Experimentally, each of the complexes 4−9 shows three CO
stretching vibrations between 1739 and 1915 cm−1. However,
the two asymmetric stretching vibrations are superimposed and
Our interest in CDP d10-Cu(I) complexes arose because
compounds of the type [(CuX)2(CDP(2Py)2)] (X = Cl, Br, I)
turned out to be fluorescence emitters.32 For this reason we
investigated reactions of 2 and 3 with copper halides CuX. Our
attempts to synthesize mononuclear copper(I) complexes of 2
and 3 failed. Instead, dinuclear complexes were isolated in poor
yields from equimolar reactions. However, the complexes
[(CuI)2(CDP(3,5‑MePz)2)] (10) and [(CuX)2(CDP(O-2Py)2)]
(11−13, X = Cl, Br, I) were obtained in good yields (66−
77%) when 2 or 3 was reacted with 2 equiv of the copper(I)
halide in THF (Scheme 3). All of these compounds were
obtained as single crystals by layering a solution of the sample
in dichloromethane or chloroform with n-pentane. Figure 4
displays a selection of two representatives.
Table 2. Calculated and Experimentally Observed C−O
a
Stretching Vibrations of 4−9
[(CuI)2(CDP(3,5‑MePz)2)] (10) crystallizes in triclinic space
wavenumber (cm−1
)
̅
group P1 with two units in the unit cell. The result of the XRD
4a
5
structure refinements reveals that each of the two ylidic carbon
lone pairs bind one trigonally coordinated copper ion of d10
electronic configuration. The coordination sphere is com-
plemented by one N donor of the 3,5-dimethyl-1H-pyrazol-1-
yl ligand and by the iodide ligand. The Cu1−Cu2 distance of
2.543(2) Å is twice the size of the covalent radius of Cu(I)
(1.27 Å)46 and shorter than the van der Waals radius of Cu
(1.4 Å).47 This suggests that a weak metal−metal interaction
might complement a distorted quasi-pseudotetrahedral copper
coordination sphere. On the other hand, the short Cu−Cu
distance can be traced back to this ligand system with two
strong geminal Cu−C ylide interactions.32 On the other hand,
the XRD molecular structures of pyridine-2-yloxy-substituted
representatives 11−13 display much longer distances in the
range Cu···Cu 3.266−3.608 Å, which definitely are longer than
any strong metal−metal interaction. Obviously, both the type
of halide ion and the flexibility of the six-membered in
comparison to the more rigid five-membered chelate triggers
these d10−d10 metal distances.
calcd
exptl
1829, 1847, 1908
1739, 1762, 1884
calcd
exptl
1835, 1848, 1914
1746, 1889
6
calcd
exptl
1831, 1847, 1908
1746, 1890
7
calcd
exptl
1841, 1852, 1913
1749, 1774, 1890
8
calcd
exptl
1839, 1854, 1919
1748, 1777, 1893
9
calcd
exptl
1836, 1853, 1914
1802, 1868, 1915
a
For more details see Table S-1 and further Supporting Information.
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Scheme 3. Synthesis of Novel Carbodiphosphorane Copper(I) Complexes of sym-Bis(3,5-dimethyl-1H-pyrazol-1-
yl)carbodiphosphorane (2) and sym-Bis(pyridin-2-yloxy)carbodiphosphorane (3)
C1 from 1.970(5) Å (11) to 2.018(4) Å (13). The inverse
trend is observed for both N−CuX distances decreasing for
Cu1−N1 from 2.297(5) Å (11) to 2.199(4) Å (13) and for
Cu2−N2 from 2.326(4) Å (11) to 2.192(3) Å (13). These
trends reflect the increasing affinity of Cu(I) ions in the order
zwitterionic carbon > anionic halide > neutral sp2 nitrogen.
The bonds of strongly Cu(I) affine ligands are less influenced
by the regime of the other less Cu(I) affine ligands competing
for covalent bond shares. The increased electron density of the
Cu(I) ions in 11−13 can be monitored by NBO analysis. The
atomic partial charges q(Cu) of the Cu atoms of
[(CuCl)2(CDP(O-2Py)2)] (11) are 0.79 and 0.80 e, those of
the Cu atoms of [(CuBr)2(CDP(O-2Py)2)] (13) are 0.77 and
0.77 e, and those of the Cu atoms of [(CuI)2(CDP(O-2Py)2)]
(13) are 0.74 and 0.74 e.
The difference in the crystallographic data of 11 (P21/c, Z =
2) and the related 12 and 13 (P21/c, Z = 8) has an unexpected
reason: much to our surprise, the unit cell of the crystal
containing the molecular complex [(CuCl)2(CDP(O-2Py)2)]
(11) does contain an independent [(CuCl)8(CDP(O-2Py)2)]
molecule cocrystallizing with 11 in a 1:1 ratio. Its molecular
structure is displayed in Figure 5, and selected bonding
Figure 4. XRD molecular structures of the representative dinuclear
complexes (a) [(CuI)2(CDP(3,5‑MePz)2)] (10) and (b)
[(CuCl)2(CDP(O-2Py)2)] (11). Related analytical results for
[(CuBr)2(CDP(O-2Py)2)] (12) and [(CuI)2(CDP(O-2Py)2)] (13)
are given in Figure S-82 in the Supporting Information. Hydrogen
atoms and solvent molecules have been omitted for clarity; thermal
ellipsoids are set at 50% probability.
Complexes 11−13 crystallize in the monoclinic space group
P21/c with two units (11) or eight units (12, 13) in their unit
cells. Representative distances and angles are presented in
Table 3. A noteworthy structural feature is the trend in
elongation of both geminal C−CuX bonds with the increasing
effective ionic radius of the halide ligand X:48 namely, Cu1−C1
increases from 1.976(5) Å (11) to 1.995(4) Å (13) and Cu2−
Figure 5. X-ray crystal structure of [(CuCl)8(CDP(O-2Py)2)2]
cocrystallized with [(CuCl)2(CDP(O-2Py)2)] (11). Hydrogen
atoms and solvent molecules have been omitted for clarity; thermal
ellipsoids are given at 50% probability. For details see the Supporting
Information. Bond lengths (Å): P3−C36 1.698(5), P4−C36
1.703(5), C36−Cu3 1.957(5), C36−Cu4 1.985(5), Cl3−Cu6
2.394(2), Cl4−Cu5 2.315(2), Cu5−Cl5A 2.226(3), Cu5−Cl6
2.196(2), Cu6−Cl5A 2.215(3), Cu6−Cl6′ 2.203(2). Bond angles
(deg): P3−C36−P4 118.3(3), Cu3−C36−Cu4 119.4(3), C36−
Cu3−Cl3 163.9(2), C36−Cu4−Cl4 157.6(2), Cu3−Cl3−Cu6
85.3(6), Cu4−Cl4−Cu5 100.4(7), Cu5−Cl6−Cu6 127.0(9), Cu5−
Cl5A−Cu6 107.3(1).
Table 3. Selected Bond Distances (Å) and Angles (deg) for
10−13
a
a
10
11
12
13
Cu1−C1
Cu2−C1
Cu1−N1
Cu2−N2
Cu1−X1
Cu2−X2
Cu1−Cu2
C1−P1
2.061(1)
2.075(1)
2.032(9)
2.028(1)
2.449(1)
2.261(2)
2.543(2)
1.694(1)
1.688(1)
120.6(6)
1.976(5)
1.970(5)
2.297(5)
2.326(4)
2.164(1)
2.163(2)
3.266
1.691(5)
1.701(5)
118.3(3)
1.973(6)
1.990(6)
2.254(5)
2.233(6)
2.282(1)
2.282(1)
3.414
1.697(6)
1.695(6)
118.7(4)
1.995(4)
2.018(4)
2.199(4)
2.192(3)
2.485(3)
2.459(6)
3.608
1.687(4)
1.695(4)
119.0(2)
distances and angles are given in the caption. The cocrystal-
lized product is best rationalized as the eight-membered ring
[Cu4(μ2-Cl)4] stabilized by two Cu,Cu′-bridging neutral
Cl,Cl′-donor ligands 11 and [3(CuCl)2], respectively (see
Figure 5). It seems that the bite of electron-rich terminal
chlorido functionalities in molecular 11 perfectly fits the
C1−P2
P1−C1−P2
a
Only one of the two individual molecules in the unit cell were
considered for comparisons. This does not affect the discussed trend.
For more information see SI.
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stabilization of each copper atom of the supramolecular 8-ring
array [Cu4(μ2-Cl)4] by 2 × 2 dative (3)Cu−Cl→Cu bonds.
This interaction leads to more or less trigonally coordinated
d10-Cu(I) ions of coordination number 3 and to Cl−Cu−Cl
angles, allowing the central eight-membered ring structure to
be built up in a chair conformation. It is plausible that this
supermolecule of composition [(CuCl)8(CDP(O-2Py)2)2] had
formed upon the reaction of molecular complex 11 with an
excess of [CuCl]x.
copper complexes display a significant shift to lower wave-
lengths: 12 and 13 exhibit nearly the same shape and maxima
at 278 and 277 nm, and chloride complex 11 is even further
shifted to lower wavelengths (263 nm).
In order to gain a better understanding of redox processes,
the representative complex [(CuI)2(CDP(O-2Py)2)] (13) was
studied via cyclic voltammetry versus ferrocene in DMSO at
room temperature (c = 5.153 mmol/L, scan rate 100 mV/s).
The CV plot is displayed in Figure 7, and redox potentials are
The structural chemistry of Lewis base adducts of [CuCl]x is
extraordinarily rich. Similar [Cu4(μ2-Cl)4] cores can exist
either in a “tube” form49−52 or in a “step” or “twist-chair”
form.53−57 Some of the latter forms exist with two linear and
collinear Cl−Cu−Cl units with angles close to 180°.58−62 The
eight-membered Cu4Cl4 unit can be part of a “drumlike” Cu6X6
unit.63 In [(CuCl)8(CDP(O-2Py)2)2] the central Cu4Cl4 core
reveals an unusual motif with a plane defined by six atoms (4 ×
Cu and 2 × Cl: Cu5, Cu5′, Cu6, Cu6′ and Cl6, Cl6′) and only
two out-of-plane chloride ions Cl5 and Cl5′. The out-of-plane
chloride ions are formally induced by two neutral copper-
bridging Cl,Cl′-ligands 11.
Furthermore, the series of Cu(I) halide complexes 11−13 of
CDP(O-2Py)2 (3) were analyzed using UV/vis spectroscopy.
The presence of an electron-rich oxa group at the pyridine
functionality seems to quench photoluminescence (PL). 11−
13 are PL-silent in contrast to corresponding PL-active Cu(I)
halide complexes of CDP(2Py)2 with its pyridine-centered
LUMO of low energy.32 Table 4 summarizes the absorption
maxima and the extinction coefficients; correlated spectra are
displayed in Figure 6.
Figure 7. Plots of the cyclic voltammetry of [(CuI)2(CDP(O-2Py)2)]
(13) in DMSO at room temperature (c = 5.153 mmol/L) with
TBAPF6 (50 mmol/L) as electrolyte at a scan rate of 100 mV/s. The
red line shows the first cycle. Potentials are plotted against the
ferrocene/ferrocenium redox couple. A platinum working electrode
(diameter 0.25 mm) and a platinum counter electrode were used, as
well as a silver/silver sulfide reference electrode. The dashed line
shows the plot with a switching potential of −0.25 V.
Table 4. Absorption Maxima and Extinction Coefficients of
3 and the Corresponding Complexes 11−13
λmax (nm)
ε (L mol−1 cm−1
)
3
306
263
278
277
2.415 × 103
1.069 × 104
5.920 × 103
8.511 × 103
11
12
13
Table 5. Redox Processes Observed in the CV of
[(CuI)2(CDP(O-2Py)2)] (13)
oxidation process
oxidation potential (V)
a
−0.43
b
c
d
0.14
0.50
reduction process
reduction potential (V)
a′
b′
d′
−0.89
−0.55
0.31
given in Table 5. The plot shows four oxidation potentials in
the anodic section of the cyclic voltammogram: a−d. At the
same time only three corresponding redox couples are
observed in the cathodic part of the spectrum, which can be
assigned to the oxidation potentials a′, b′, and d′. Oxidation
process c is an irreversible process. The red line of the CV plot
of [(CuI)2(CDP(O-2Py)2)] (13) describes the first cycle of the
measurement. With a starting voltage of −0.50 V the formation
of the peaks a and b cannot be observed unless they undergo
the reduction processes a′ and b′.
Figure 6. Experimental UV/vis spectrum of (CDP(O-2Py)2)] (3,
black) and the corresponding complexes 11 (red), 12 (green), and 13
(blue).
While ligand 3 shows an absorption maximum at 306 nm
with an extinction coefficient of 2.415 × 103 L mol−1 cm−1, the
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Figure 8. Redox processes of [(CuI)2(CDP(O-2Py)2)] (13) simulated via DFT calculations and the corresponding Kohn−Sham orbitals (isovalue
0.05). Calculations were performed at the PBE-D3(BJ)/def2-TZVPP level of theory. In the case of [13]2− and [13]2+ the triplet states lower in
energy in comparison to singlet states are presented. The blue/red color code in the formula 13 indicates where oxidation (red) and reduction
(blue) take place. The turquoise/orange coding represents unoccupied and red/blue coding illustrates occupied Kohn−Sham orbitals.
The processes a/a′ and b/b′ were further investigated
individually by reducing the switching potential to −0.25 V
(dashed line in Figure 7). In this case the reaction can
tentatively be considered as a quasi-reversible two-electron-
ransfer process. On consideration that at the start of the cycle
only the neutral compound 13 is present, both processes can
be considered as reversible reduction waves of 13 forming
[13]2−. In the following oxidation cycle [13]2− is reoxidized to
the neutral species 13. Therefore, a can be considered as the
oxidizing process [13]2− → [13]− and a′ as the reducing
process [13]− → [13]2−, while b is the inverse [13]− → 13
and b′ is 13 → [13]−. This leads to the conclusion that the
reaction can be considered as a quasi-reversible redox reaction
in the potential range of −1.75 to −0.25 V. In the extended
range −1.75 to +0.75 V, the an oxidation process c is observed
followed by another oxidation d. While d shows a
corresponding reduction process d′, c does not show a
corresponding reduction process and seems to be irreversible.
In order to gain a deeper understanding of these redox
processes, the frontier orbitals of 13 were calculated using DFT
at the PBE-D3(BJ)/def2-TZVPP level of theory. The results
are displayed in Figure 8 (see also Figure S-78). The DFT
calculations reveal that the electron density of HOMO and
HOMO-1 is delocalized over seven atoms of both copper
atoms and their direct donor atoms: the central ylidic C atom
as well as the two Cu−I units, and to a smaller extent both sp2
N donor atoms. At the same time the LUMO and LUMO+1
can be considered as being ligand based, located mainly at both
phosphino groups including their aryl carbon backbone with a
higher population at the pyridyl groups. On consideration that
oxidation takes place at the HOMO and reductive electron
transfer occurs into the LUMO, the calculations suggest that
the redox processes a/a′ and b/b′ refer to ligand-based
processes, which are typical for noninnocent ligands.64 When it
is taken into account that the stepwise reduction of 13
addresses the LUMO, a singly occupied molecular orbital
(SOMO) with a duplet character (Figure 8) is formed. This
reduction reaction 13 → [13]− refers to process b′ in the CV
experiment. Once again, the SOMO is mainly of ligand
character and is located on the phenyl and pyridyl groups and
not at copper. By further reduction of [13]− to [13]2− either a
singlet or a triplet state can be formed. The calculations show
that the triplet state is energetically slightly more stable;
therefore, two SOMOs are formed. Both SOMOs are strongly
phenyl and pyridyl based and also show only a very small
contribution of both copper atoms. This process can be related
to process a′ and describes [13]− → [13]2−. The experimental
results of the CV measurements suggest that these reductions
are reversible. On the other hand, the oxidation seems to be
irreversible. As was mentioned, the HOMO is spread over the
central C−(Cu−I)2−Npy unit. An oxidation leads to [13]+,
2
where the positive charge is equally spread all over this seven-
atom unit. This can be illustrated by the duplet character of
SOMO of [13]+ that is formed (Figure 8). As the SOMO
shows high symmetry, the existence of the mixed-valent
[(CuIII)(CuII)(CDP(O-2Py)2)]+ is unlikely. This consider-
ation is supported by an NBO analysis, where the natural
charges of the copper atoms are almost equal: q(Cu1) = 0.85 e
and q(Cu2) = 0.85 e. Further oxidation of [13]+ → [13]2+
generates a triplet state with two SOMOs; the corresponding
singlet state is 53.6 kJ/mol less favorable. While the
energetically marginally lower lying SOMO displays a strong
C−(Cu−I)2 character, the Kohn−Sham orbitals of the second
SOMO are spread over the complete C−(Cu−I)2−Npy unit.
2
Therefore, the oxidation processes can be considered as
electron transfer occurring not only from both Cu(I) metal
centers but also as a process that affects the whole C−(Cu−
I)2−Npy2 unit. This complicates a distinct interpretation of the
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Bis(pyridin-2-yloxy)carbodiphosphorane (3). A 2.00 g portion of
1 (4.41 mmol, 1.00 equiv) and 1.03 g of sodium-2-pyridinolate (8.83
mmol, 2.00 equiv), prepared from 2-hydroxypyridine and NaH in
THF, were suspended in 15 mL of THF and stirred for 18 h at rt. The
solvent was removed under reduced pressure, and the residue was
washed with DEE (2 × 10 mL) and with n-pentane (2 × 10 mL) and
dried under reduced pressure (1 × 10−3 mbar) to form the desired
product 3 as a gray powder (2.69 g, 4.41 mmol, quantitative). 31P{1H}
processes c, d, and d′ of the CV measurements. Additionally,
we have to consider that the calculations evaluate the gas-phase
environment and that the solvent DMSO used in CV should
have an effect upon oxidizing the neutral compound 13 to a
cation and dication. A summation of both the calculations and
CV measurements suggests that ligand 3 can act as a
noninnocent ligand and the redox behavior can be considered
as quasi-reversible in the potential range of −1.75 to −0.25 V.
This feature is of interest for designing transition-metal
complexes or catalysts with potentially redox active sites.
1
NMR (101 MHz, CD2Cl2): δ/ppm 23.1 (s). H NMR (300 MHz,
CD2Cl2): δ/ppm 8.05 (dd, 3,5JH,H = 2.1, 4.3 Hz, 2 H, H1.), 7.82−7.72
(m, 8 H, H7.), 7.41−7.19 (m, 14 H, H3/H8/H9), 6.92−6.84 (m, 4 H,
H2/H4). 13C{1H} NMR (75 MHz, CD2Cl2): δ/ppm 148.1 (s, C1),
138.7 (s, C3), 131.9 (t, JC,P = 5.6 Hz, C7), 130.4 (s, C9), 128.2 (t, JC,P
=
CONCLUSION
6.8 Hz, C8), 119.5 (s, C2), 115.7 (s, C4). APCI+/HRMS (CH2Cl2):
m/z (%) calcd for [C35H28P2O2N2]H+ 571.1704, found 517.1702.
UV/vis (CH2Cl2): c = 169.1 μm, λmax = 306 nm, ε = 2.415 × 103 L
mol−1 cm−1.
■
A new generation of carbodiphosphoranes incorporating two
chelating N-donor functionalities was introduced. A novel
synthesis strategy, the reaction of P-electrophile CDP-Cl2 (1)
with ambident pyrazolate and 2-pyridinolate anions, has been
established in order to isolate sym-bis(3,5-dimethyl-1H-
pyrazol-1-yl)carbodiphosphorane CDP(3,5‑MePz)2 (2) and
sym-bis(pyridin-2-yloxy)carbodiphosphorane CDP(O-2Py)2
(3) in their free ligand base forms. In contrast to N,C,N
pincer ligands incorporating a central NHC carbon donor,
neutral CDP ligands 2 and 3 are more flexible, both in the
number of donated electrons of their zwitterionic carbon atom,
a σ-, σ,π- or 2σ-electron pair donor, and in their ability to adapt
to M−L configurations other than only meridional: e.g., also
facial. For a first insight into their coordination capability, we
chose d6-[M(CO)3] fragments (M = Cr, Mo, W) forming
octahedral complexes of the type fac-[M(CO)3(CDP-
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00231.
■
sı
Experimental procedures, IR spectra, UV/vis spectra,
CV measurements, elemental analysis results, crystal
data tables, and details and results of DFT calculations
(PDF)
Cartesian coordinates of calculated structures (XYZ)
Accession Codes
CCDC 1976467, 1976473, 1976475, 1976477−1976478, and
1976486−1976489 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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(
3,5‑MePz)2)] (4−6) and fac-[M(CO)3(CDP(O-2Py)2)] (7−
9) and d10-[CuX] fragments (X = Cl, Br, I) forming dinuclear
complexes [(CuI)2 (CDP(3 , 5 ‑ M e Pz)2 )] (10) and
[(CuX)2(CDP(O-2Py)2)] (11−13) next to the unexpected
cocrystallized octanuclear supramolecular complex
[(CuCl)8(CDP(O-2Py)2)]. The characterization focused on
XRD, IR, and UV−vis studies. The ligand properties,
vibrational modes of the carbonyl complexes, and electro-
analytical results of a representative CV of 13 were modeled by
DFT calculations.
AUTHOR INFORMATION
Corresponding Author
■
Jörg Sundermeyer − Fachbereich Chemie and
Wissenschaftliches Zentrum fur Materialwissenschaften,
̈
EXPERIMENTAL SECTION
Philipps-Universität Marburg, 35043 Marburg, Germany;
orcid.org/0000-0001-8244-8201; Email: jsu@staff.uni-
marburg.de
■
General Considerations. Reactions were carried out under an
inert atmosphere using standard Schlenk techniques. Moisture- and
air-sensitive substances were stored in a conventional nitrogen-flushed
glovebox. sym-Bis(P-chlorodiphenyl)carbodiphosphorane (1) was
synthesized via a modified procedure adapted from Appel et al.37
Details of this modification, of all synthetic procedures, analyses, and
characterizations, of DFT calculations, and of analytical and
spectroscopic instruments used are given in the Supporting
Information.
Author
Marius Klein − Fachbereich Chemie and Wissenschaftliches
Zentrum fur Materialwissenschaften, Philipps-Universität
̈
Marburg, 35043 Marburg, Germany
Complete contact information is available at:
Ligand Syntheses and Characterization. Bis(3,5-dimethyl-1H-
pyrazol-1-yl)carbodiphosphorane (2). A 3.00 g portion of 1 (6.62
mmol, 1.00 equiv) and 1.77 g of potassium 3,5-dimethyl-1H-pyrazol-
1-yl (13.2 mmol, 2.00 equiv), prepared from KH and 3,5-
dimethylpyrazole in THF, were suspended in 50 mL of THF at
−78 °C and stirred for 18 h while the mixture was warmed to rt. The
solvent was removed under reduced pressure, and the residue was
washed with DEE (2 × 10 mL) and with n-pentane (2 × 10 mL) and
dried under reduced pressure (1 × 10−3 mbar) to form the desired
product 2 as a light green powder (2.93 g, 5.01 mmol, 77%). 31P{1H}
NMR (101 MHz, C6D6): δ/ppm 8.9 (s). 1H NMR (300 MHz, C6D6):
δ/ppm 7.92.7.84 (m, 8 H, H2), 7.05−6.98 (m, 12 H, H3/ H4), 5.58 (s,
2H, H5), 2.29 (s, 6H, H7), 2.09 (s, 6H, H6). 13C{1H} NMR (75 MHz,
C6D6): δ/ppm 149.7, 146.8, 136.7, 135.9, 135.1, 132.7, 130.3, 109.2,
13.9. APCI+/HRMS (CH2Cl2) m/z (%) calcd for [C35H34P2N4]H+
573.2337, found 573.2323.
https://pubs.acs.org/10.1021/acs.organomet.1c00231
Notes
The authors declare no competing financial interest.
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