Cyclopalladation of a ferrocene acylphosphine and the reactivity of the C-H activated products

Article abstract of DOI:10.1039/d1dt00668a

Acylphosphines are an attractive subclass of phosphine ligands with specific reactivity and ligating properties. This study describes the synthesis of a ferrocene-based acylphosphine, FcC(O)PPh2 (1, Fc = ferrocenyl), the corresponding phosphine chalcogenides FcC(O)P(E)Ph2 (E = O, S and Se), and palladium(ii) complexes resulting from the orthometallation of 1, viz. [Pd(μ-X)(1 - H)]2 (2-X, where X = Cl, Br and I), which in turn serve as a convenient entry point to a range of monopalladium complexes. Thus, the cleavage of 2-X with phosphines, 4-(dimethylamino)pyridine and in situ-generated 1,3-dimesitylimidazolin-2-ylidene (L) provided complexes [PdX(L)(1 - H)] (3-5), which are typically obtained as single isomers by crystallisation but undergo spontaneous isomerisation in solution leading to equilibrium mixtures of cis and trans isomers. Upon treatment with sodium acetylacetonate (Na(acac)), 2-Cl was transformed into [Pd(acac)(1 - H)] (6), which reacted with (diphenylphosphino)acetic acid under proton transfer to give [Pd(Ph2PCH2CO2-κ2O,P)(1 - H)] (7), while the reaction with allylating agents produced the π-allyl complex [Pd(η3-C3H5)(1 - H)] (9). Compound [PdCl(PMe3)(1 - H)] was further used to prepare a series of diorganopalladium complexes [Pd(R)(PMe3)(1 - H)] (8; R = Me, 4-C6H4Me, 4-C6H4CF3, CHCHC6H4Me-4 and CCC6H4Me-4). All compounds were structurally characterised using spectroscopic methods and, in most cases, also by X-ray diffraction analysis. The structural data indicated the rigidity of the Pd(1 - H)+ fragment and revealed variations in the Pd-donor distances dictated by an interplay between the trans influence and steric demands of the ligands surrounding Pd(ii). Moreover, some compounds were studied by DFT to rationalise their isomerisation equilibria and electrochemical properties, which were examined by cyclic voltammetry.

Full text of DOI:10.1039/d1dt00668a

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Cyclopalladation of a ferrocene acylphosphine and
the reactivity of the C–H activated products†
Cite this: Dalton Trans., 2021, 50,
6232
Petr Vosáhlo,
Jiří Schulz,
Ivana Císařová
and Petr Štěpnička
*
Acylphosphines are an attractive subclass of phosphine ligands with specific reactivity and ligating pro-
perties. This study describes the synthesis of a ferrocene-based acylphosphine, FcC(O)PPh2 (1, Fc = ferro-
cenyl), the corresponding phosphine chalcogenides FcC(O)P(E)Ph2 (E = O, S and Se), and palladium(II)
complexes resulting from the orthometallation of 1, viz. [Pd(µ-X)(1 − H)]2 (2-X, where X = Cl, Br and I),
which in turn serve as a convenient entry point to a range of monopalladium complexes. Thus, the clea-
vage of 2-X with phosphines, 4-(dimethylamino)pyridine and in situ-generated 1,3-dimesitylimidazolin-2-
ylidene (L) provided complexes [PdX(L)(1 − H)] (3–5), which are typically obtained as single isomers by
crystallisation but undergo spontaneous isomerisation in solution leading to equilibrium mixtures of cis
and trans isomers. Upon treatment with sodium acetylacetonate (Na(acac)), 2-Cl was transformed into
[Pd(acac)(1 − H)] (6), which reacted with (diphenylphosphino)acetic acid under proton transfer to give [Pd
(Ph2PCH2CO2-κ2O,P)(1 − H)] (7), while the reaction with allylating agents produced the π-allyl complex
[Pd(η3-C3H5)(1 − H)] (9). Compound [PdCl(PMe3)(1 − H)] was further used to prepare a series of diorgano-
palladium complexes [Pd(R)(PMe3)(1 − H)] (8; R = Me, 4-C6H4Me, 4-C6H4CF3, CHvCHC6H4Me-4 and
CuCC6H4Me-4). All compounds were structurally characterised using spectroscopic methods and, in
most cases, also by X-ray diffraction analysis. The structural data indicated the rigidity of the Pd(1 − H)+
fragment and revealed variations in the Pd-donor distances dictated by an interplay between the trans
influence and steric demands of the ligands surrounding Pd(II). Moreover, some compounds were studied
by DFT to rationalise their isomerisation equilibria and electrochemical properties, which were examined
by cyclic voltammetry.
Received 27th February 2021,
Accepted 22nd March 2021
DOI: 10.1039/d1dt00668a
rsc.li/dalton
properties of the P–C(O) bond, which is prone to protolytic
cleavage and CO extrusion.4a,c,5 These seemingly “negative”
Introduction
While the first primary acylphosphine Cl3CC(O)PH2 was identi- aspects have nevertheless been demonstrated to open up new
fied as the product of the reaction between Cl3CC(O)Cl and opportunities for the applications of these compounds in
phosphane as early as 1846,1 its tertiary analogues RC(O)PR′2 phosphine synthesis via P–C bond-forming reactions,6 decar-
were reported only a century later, with acetyl phosphines bonylation7 and CvO bond reduction.8
3
Et2PAc2 and PhPAc2 representing the first characterised com-
In addition to their peculiar reactivity,9 acylphosphines are
pounds of their kind. Other well-defined acylphosphines RC unique phosphine ligands. Their electron-withdrawing acyl
(O)PR′2 soon followed, during the 1950s, along with reliable moiety reduces the σ-donor ability and enhances the
synthetic routes, making these compounds available for π-acceptor ability of the phosphorus atom,10 which has been
further studies.4 However, the chemistry and applications of advantageously used to design tailored ligands for catalysis.11
acylphosphines, which can be alternatively regarded as phos- Attractive coordination behaviour, including C–H bond acti-
phaamides, have long remained unexplored. This can be vation, has recently been reported for pincer-type bis(acylpho-
linked to their relatively low chemical stability reflecting the sphine) ligands,12 while cyclic diketo-phosphines13 comprising
C(O)PRC(O) units were studied as stereochemically distinct
ligands14 and prospective materials.15
Despite the renewed interest in acylphosphine ligands,
ferrocene-based compounds of this sort remain nearly unex-
plored, in sharp contrast to the widespread applications of
ferrocene phosphines in coordination chemistry and cataly-
sis.16 Ferrocene acylphosphines reported to date are limited to
phosphatriafulvene A (plus its dimer),17 and secondary acyl-
Department of Inorganic Chemistry, Faculty of Science, Charles University,
Hlavova 2030, 128 40 Prague, Czech Republic. E-mail: petr.stepnicka@natur.cuni.cz
†Electronic supplementary information (ESI) available: Complete experimental
details, additional structural diagrams and parameters, selected crystallographic
data (Table S1), and copies of the NMR spectra. CCDC 2063464–2063483. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d1dt00668a
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Compound 1 is relatively stable towards hydrolysis but
becomes slowly oxidised in solution. For instance, stirring the
solutions of 1 in 1 : 1 THF/MeOH and THF/EtOH mixtures,
under ambient conditions for 72 h, provided mixtures of 1 and
1O by partial oxidation of the dissolved phosphine. When a
THF/H2O mixture was employed under otherwise identical
conditions, the oxidation was complete (see the ESI, Fig. S1†).
Acids and bases expectedly facilitated the C–P bond cleavage.
In a model experiment, 1 was cleanly converted into the ester
FcCO2Me, when treated with KOH (1 equiv.) in a THF/MeOH
(1 : 1) mixture at room temperature for 5 h.
Scheme 1 Ferrocene acylphosphines and related compounds.
Phosphine 1 also reacted with sulfur and selenium to give
the respective P-chalcogenides 1S and 1Se (Scheme 2). While
the selenation of 1 with grey selenium in refluxing chloroform
proceeded smoothly, the reaction with sulfur in hot toluene
required time optimisation to achieve full conversion without
pronounced decomposition.
phosphines B obtained by the protodesilylation of phosphaalk-
enes FcPvCR(OSiMe3) (Fc = ferrocenyl) and further converted
to diacyldiphosphines C (Scheme 1).18,19 In this context, we
have designed tertiary ferrocene acylphosphine 1.
Furthermore, the structural similarity between 1 and [(di-
methylamino)methyl]ferrocene (D),20 phosphine E 21 or
amides F 22 (Scheme 1), all of which undergo directed ortho-
palladation at the ferrocene unit, has also led us to focus on
the possible palladation of the new acylphosphine. In this con-
tribution, we report the preparation and structural characteris-
ation of acylphosphine 1, its successful orthopalladation
under mild conditions and extensive reactivity studies with the
orthometallated products.
Compound 1 displayed a singlet in its 31P NMR spectrum (δP
11.0) and signals due to the monosubstituted ferrocene unit
1
and the phenyl groups in the H NMR spectrum. The 13C NMR
signal of the carbonyl group was observed as a doublet (1JPC
=
34 Hz) at δC 213. The oxidation of the phosphine moiety led to a
down-field shift of the 31P resonances (δP: 16.4 for 1O, 36.8 for
1S, and 28.2 for 1Se) and a characteristic increase in the JPC
1
coupling constants.23 The JPSe coupling constant24,25 deter-
mined for 1Se (749 Hz) indicated that the basicity of the phos-
phorus atom is lower in 1Se than in FcPPh2 (FcP(Se)Ph2: δP
1
31.8, JPSe 733 Hz in CDCl3),26 in line with the electron-with-
drawing nature of the acyl substituent. The 13C NMR signals of
the carbonyl groups in 1E were up-field shifted with respect to
the parent phosphine. In contrast, the position of the IR bands
attributable to the carbonyl stretching modes in 1 and 1E varied
only marginally. Single crystals suitable for X-ray diffraction
were obtained for 1 and for all chalcogenides. The structures
and selected geometric parameters are presented in Fig. 1 and
Table 1, respectively.
The geometric parameters of the acylphosphine moieties in
1 and 1E were generally similar to those determined for 2,6-
(Ph2PC(O))2pyridine12b and Mes2PC(O)Ph (Mes = mesityl).27 In
the series, these parameters did not change significantly upon
oxidation and replacement of the P-bound chalcogen atom.
The ferrocene units adopted their usual geometry and were
only negligibly tilted (<4°). While the phosphaamide plane
{C11,P,O1} in 1 was twisted by 16.5(1)° with respect to its
bonding cyclopentadienyl ring C(1–5), a considerably smaller
twisting (≤5°) was observed in chalcogenides 1E (cf. the C2–
C1–C11–P1 torsion angles in Table 1). The PvE bond lengths
were similar to those in triphenylphosphine chalcogenides,28
but the orientation of the CvO and PvE bond dipoles varied
from nearly antiparallel in 1O, bearing the most electronega-
tive atom at phosphorus, to less diverted in 1S and 1Se.
Results and discussion
Synthesis of acylphosphine 1 and phosphine chalcogenides 1E
Acylphosphine 1 was synthesized by a reaction of freshly pre-
pared (chlorocarbonyl)ferrocene with diphenylphosphine in
diethyl ether in the presence of triethylamine as an HCl sca-
venger (Scheme 2). The compound was purified by column
chromatography and crystallisation, and was obtained as a
burgundy red, air-stable crystalline solid. The typical yields
ranged from 65 to 85% for the reactions performed at a
1–50 mmol scale; minor amounts (typically less than 5%) of
the corresponding phosphine oxide 1O were also isolated.
Palladation of 1 and reactions of the orthopalladated dimers
Metallation of 1 with Na2[PdCl4] (1 equiv.) in the presence of
sodium acetate (1.5 equiv.) as a base (in dry MeCN at 70 °C for
3 h) produced the orthopalladated dimer 2-Cl (Scheme 3) in
Scheme 2 Synthesis of acylphosphine 1 and phosphine chalcogenides 1E.
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Scheme 3 Palladation of 1 and the exchange of the bridging ligands in
2-Cl.
Fig. 1 Molecular structures of acylphosphine 1 and P-chalcogenides 1E
(for additional structural diagrams, see the ESI†).
The NMR spectra of 2-X clearly suggested the metalation of
the acyl-substituted ferrocene ring and the coordination of the
phosphine moiety (δP 50.5–53 for all three compounds and
their isomers); the νCvO bands were observed at lower frequen-
cies (≈1650 cm−1) with respect to free ligand 1.
Crystals suitable for structure determination were
obtained for 2-Cl·AcOEt and 2-Br·PhMe. In both cases, crys-
tallisation produced pure trans-(Rp,Sp) isomers, viz. trans-
(Rp,Sp)-2-Cl·AcOEt and trans-(Rp,Sp)-2-Br·PhMe, formally the
meso form, having inversion symmetry, which was preserved
even in the solid state (Fig. 2). Upon dissolving the crystals,
however, the compounds underwent isomerisation to
yield the same isomer mixtures as those resulting from
synthesis.
The complex molecules in the structures of trans-(Rp,Sp)-2-
Cl·AcOEt and trans-(Rp,Sp)-2-Br·PhMe are very similar.29 The
palladium and its four ligating atoms are coplanar (the
maximum deviations from the mean plane are 0.068(3) Å and
0.115(4) Å, respectively), as reflected in the values of the τ4
index,30 which are close to zero (0.09 and 0.06, respectively).
The five-membered metallacycles are also planar (within 0.1 Å)
and give rise to the smallest interligand angle (ca. 81°).
Notably, the conversion of 1 into 2-X leaves P–C and CvO
90% yield. The compound is a mixture of four isomers (see the
ESI, Fig. S2†), which differ in the mutual positioning of the
ferrocene units (syn/anti) and donor moieties (cis/trans), in
turn corresponding to two racemic and two meso forms (see
Fig. S3†). Such an assignment is consistent with the obser-
vation that 2-Cl is cleaved into single monopalladium products
as racemic mixtures of the (Sp) and (Rp) isomers (vide infra).
Notably, the direct metalation of 1 with palladium(II)
acetate in toluene at room temperature or at 60 °C was also
possible but resulted in mixtures of palladated complexes and
additional minor products, presumably Pd-acetate complexes.
Although we were able to suppress the by-product formation
by adding sodium acetate and to convert the major product
into better defined 2-Cl by adding LiCl in methanol, the
former route, producing 2-Cl directly, was more efficient than
the approach employing Pd(OAc)2.
The bridging chloride ligand in 2-Cl was smoothly replaced
with heavier halides upon treatment with NaX in acetonitrile
(Scheme 3). Complexes 2-Br and 2-I were also obtained as mix-
tures of four isomers, similar to 2-Cl (Fig. S2†).
Table 1 Selected distances and angles for 1 and chalcogenides 1E (E = O, S, and Se)
Parametera
1
1O (E = O2)
1S (E = S1)
1.9505(7)
1Se (E = Se1)
P1vE
P1–C11
P1–C12/18
C11–O1
P1–C11–O1
E–P1–C11–O1
Tilt
n.a.
1.489(1)
1.881(1)
1.804(1)/1.802(1)
1.228(2)
115.5(1)
−154.9(1)
3.79(8)
1.6(2)
2.1035(7)
1.897(2)
1.814(2)/1.814(2)
1.225(2)
116.4(1)
1.884(2)
1.834(1)/1.825(1)
1.225(2)
118.6(1)
n.a.
0.93(9)
9.4(2)
1.893(2)
1.811(1)/1.814(2)
1.223(2)
116.8(1)
−112.6(1)
2.86(9)
2.1(2)
−114.4(1)
3.3(1)
C2–C1–C11–P1
−174.7(1)
a Definitions: Fe–C is the range of the Fe–C(1–10) bond lengths, and tilt denotes the dihedral angle of the least-squares cyclopentadienyl planes.
n.a. = not applicable.
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Fig. 2 View of the complex molecule in the structure of trans-(Rp,Sp)-
2-Cl·AcOEt (the structural diagram of trans-(Rp,Sp)-2-Br·PhMe is avail-
able in the ESI†). Selected distances and angles (in Å and °) for trans-(Rp,
Sp)-2-Cl·AcOEt [trans-(Rp,Sp)-2-Br·PhMe]: Pd1–X1 2.3997(8) [2.5132(5)],
Pd1–X1’ 2.4230(9) [2.5360(5)], Pd1–P1 2.2070(8) [2.2114(9)], Pd1–C2
1.996(3) [2.003(3)], P1–Pd1–C2 80.78(8) [81.1(1)], C2–Pd1–X1 94.65(8)
[94.4(1)], X1–Pd–X1’ 86.38(3) [87.19(2)], P1–Pd–X1’ 98.32(3) [97.78(3)],
Pd1–X1–Pd1’ 93.62(3) [92.81(2)], C11vO1 1.215(3) [1.213(4)], P1–C11
1.893(3) [1.890(3)], C1–C11–P1 107.5(2) [107.3(2)], P1–C12 1.808(3)
[1.811(4)], P1–C18 1.825(3) [1.836(3)]. The primed atoms are generated
by crystallographic inversion; X = Cl or Br.
Scheme 4 Cleavage of the dimeric complexes 2-X with Lewis bases
(Cy = cyclohexyl, dmap = 4-(dimethylamino)pyridine, SIMes·HCl = 1,3-
dimesityl-4,5-dihydroimidazolium chloride, and Mes = mesityl).
The presence of the PR3 ligand in 3–5 was clearly indicated
by an additional 31P NMR resonance and by the splitting of
the 31P NMR signals into doublets, with markedly different
2JPP coupling constants of the cis and trans isomers31 (Fig. 3).
The resonances due to the CvO groups in the 13C NMR
spectra were only slightly affected (δC 208 for cis-3a–5a and
trans-3b/3c) and were split into double doublets via inter-
actions with the two phosphine ligands. Surprisingly, IR spec-
troscopy provided only limited diagnostic information on 3–5
because the spectra of pure isomers and the respective isomer
mixtures (in the 400–4000 cm−1 range) were nearly identical
and did not change after replacing the halide co-ligand.
bond lengths virtually unchanged but affects the angles
around C11: the P1–C11–C1 angle decreases by ca. 12° whilst
the P1–C11–O1 (by ca. 10°) and C1–C11–O1 (by <2°) angles
widen concomitantly. The phosphaamide planes {C11,O1,P1}
in the molecules of 2-Cl and 2-Br are twisted by 7.1(3)° and 5.6
(4)° with respect to their parent cyclopentadienyl rings, and
the ferrocene units are tilted by 4.6(2)° and 4.4(3)°.
Trimethylphosphine addition to a dichloromethane solu-
tion of 2-Cl caused an immediate cleavage of the starting
dimer, producing the air-stable, burgundy red monopalladium
complex 3a as a mixture of cis and trans-P,P isomers in a
72 : 28 ratio (equilibrium ratio in CDCl3; Scheme 4). 2-Br and
2-I behaved similarly (Scheme 4), but the isomer ratio of the
iodide complex 5a differed from those of the chloride and
bromide analogues (cis : trans-4a = 72 : 28, cis : trans-5a = 68 : 32
in CDCl3). The pure cis isomers of 3a–5a were reproducibly iso-
lated by the crystallisation of the mixture from ethyl acetate/
hexane and structurally characterised. In solution, however,
they isomerised (vide infra), producing equilibrium mixtures
with the same compositions as those resulting from synthesis.
As shown by the 3a → 4a conversion, the bis-phosphine com-
plexes are also accessible by halogen exchange.
The isomer ratios32 in solution (under equilibrium) were
partly controlled by steric factors as spatially demanding phos-
phine ligands are expectedly destabilised in a position adja-
cent to the bulky PPh2 moiety (in complexes 3a–c, the amount
The reactions of 2-Cl with triphenyl- and tricyclohexyl-
phosphine proceeded similarly, albeit through the preferential
formation of trans-P,P isomers (the cis : trans ratios were 22 : 78
for 3b and ≈10 : 90 for 3c in CDCl3), which were isolated in
good yields by crystallisation. However, as observed for the
PMe3-complexes 3a–5a, the trans isomers gradually converted
into isomer mixtures in solution (consequently, chromato-
graphy could not be used to isolate pure isomers).
Fig. 3 31P{1H} NMR (162 MHz, CDCl3) spectrum of cis/trans-3a.
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of the trans-P,P isomer increased with the cone angle23 of the
terminal phosphine ligand, albeit not linearly) and further
affected by the trans influence (phosphines and hydrocarbyl
ligands exert a strong trans influence33 and destabilise each
other when occupying mutually trans positions34). The analysis
of the crystallisation residues showed that both isomers are
present in the mother liquors in ratios identical to those of the
equilibrium mixtures. This suggests that the isomeric bis-
phosphine complexes co-exist in a dynamic equilibrium,
which supplements the easier-to-crystallise isomer when it is
depleted from a solution during crystallisation. The equili-
brium also depends on the solvent properties because the
isomers differ substantially in their dipole moments (cis >
trans; see Table S3†) and, hence, in their solvation.31
DFT computations performed for model compounds 3a, 4a
and 5a (Table 2) reproduced the experimental observations by
energetically favouring the cis-P,P isomers in all cases. The
estimated energy differences between the isomers were never-
theless small (≤3 kcal mol−1 at 298 K), which in turn corres-
ponds with the fact that both isomers can coexist in solution.
The inclusion of solvation effects by using the PCM model35
for CHCl3 changed the energy values but not the overall trend.
In addition, the kinetics of the isomerisation process was
studied by NMR spectroscopy for several solvents and 3a–5a as
the representatives. It was found that complexes containing
heavier halide ligands undergo faster isomerisation (Fig. 4).
The half-times of the isomerisation reactions determined in
CDCl3 at 25 °C were approximately 11 h for 3a, 7 h for 4a, and
3.5 h for 5a.
Fig. 4 Kinetic traces of the isomerisation reaction of cis-3a–5a in
CDCl3 at 25 °C.
Isomerisation of cis-3a proceeded at slower rates in haloge-
nated solvents (CDCl3 and 1,2-C2D4Cl2) than in acetone-d6 and
particularly in acetonitrile-d3 and benzene-d6 (Fig. 5). Even
this trend is reflected in the results of DFT calculations, which
show a smaller free energy difference between cis- and trans-3a
in benzene (ΔG = Gcis − Gtrans = −1.7 kcal mol−1) than in Fig. 5 Time profiles of cis-3a isomerisation in different solvents, at
chloroform and acetone (−2.1 and −2.6 kcal mol−1, respect-
ively; note: non-deuterated solvents were considered for the
25 °C. Isomerisation in 1,2-C2D4Cl2 is the slowest, reaching a 70 : 30 cis :
trans equilibrium ratio within approximately 72 h.
calculations at 298 K). Adding free trimethylphosphine
(10 mol% of PMe3 as a 1 M THF solution) accelerated the iso-
merisation reaction in CDCl3. The yield of this reaction (PhCH2NEt3)Cl) changed the reaction mechanism, as
exceeded 90% in 20 min after the PMe3 addition, reaching suggested by the different (sigmoidal) shape of the kinetic
equilibrium within 3 h (the kinetic trace is not included in profile. The acceleration of the isomerisation processes by
Fig. 5). Conversely, the addition of free chlorides (10 mol% both polar solvents and “free” PMe3 is in line with the associ-
ative mechanisms proposed for the cis–trans isomerisation of
d8 square-planar complexes.36
The structures of cis-3a·21AcOEt, cis-4a, and cis-5a·12AcOEt are
shown in Fig. 6 and in the ESI.† Differing only in the halide
Table 2 DFT calculated free energy differences, ΔG = Gcis − Gtrans, for
the isomers of the bis-phosphine complexes 3a, 4a and 5a (in kcal
ligand, the structures of 3a–5a are quite similar (Table 3). The
coordination spheres in the complexes are planar, as indicated
by τ4 parameters of ≈0.1 (the palladium and its ligating atoms
remain coplanar within ca. 0.1 Å). A small distortion can be
illustrated by the angle subtended by the {Pd1,P1,C2} and
{Pd1,P2,X1} half-planes of 3.60(5)° for cis-3a, 8.26(7)/7.58(7)°
for cis-4a (two independent molecules), and 7.31(5)° for cis-5a.
For steric reasons, the orthometallated acylphosphine is
associated with the smallest interligand angle, while the adja-
mol−1 a
)
Complex
3a
4a
5a
Isolated molecule
CHCl3
−1.5
−2.1
−1.7
−2.5
−2.3
−3.1
a Calculated at the PBE0/def2-TZVP:sdd(Pd,Fe,I)-D3 level of theory. The
solvent effect was approximated by using the polarised continuum
model; for further details, see the ESI.† Negative values indicate a
lower energy for the cis-P,P isomer.
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Fig. 7 View of the complex molecule in the structure of trans-3b·PhMe
(the structural diagram of trans-3c is available in the ESI†).
Fig. 6 View of the complex molecule in the structure of cis-3a·21AcOEt
(structural diagrams of cis-4a and cis-5a·21AcOEt are available in the
ESI†).
Table 4 Selected interatomic distances and angles of trans-3b·PhMe,
trans-3c, trans-3d and trans-3e·21PhMe (in Å and °)
Table 3 Selected bond distances and angles of cis-3a·21AcOEt, cis-4a,
and cis-5a·21AcOEt (in Å and °)
Param
trans-3b·PhMe trans-3c
trans-3d
trans-3e·12PhMe
X
P2
P2
N1
C31
Pd1–P1
Pd1–C2
Pd1–X
Pd1–Cl1
P1–Pd1–C2
C2–Pd1–X
X–Pd1–Cl1
2.2824(6)
2.020(2)
2.3246(6)
2.3656(6)
82.94(5)
93.82(5)
92.92(2)
2.2728(5) 2.2250(5) 2.2619(7)
cis-4a a
2.032(2)
2.3618(5) 2.114(2)
2.001(2)
2.009(3)
2.080(3)
Param
cis-3a·12AcOEt mol 1
mol 2
cis-5a·12AcOEt
2.3666(5) 2.3732(6) 2.3902(8)
83.63(5)
97.17(5)
94.54(2)
85.50(2)
82.48(5)
92.73(7)
89.99(5)
96.10(2)
82.86(8)
95.8(1)
94.36(8)
87.18(2)
Pd1–P1
Pd1–C2
Pd1–X1
Pd1–P2
P1–Pd1–C2 81.83(5)
C2–Pd1–X1 91.42(5)
2.2206(5)
2.041(2)
2.3647(5)
2.3368(5)
2.2290(7)
2.2221(7)
2.045(2)
2.4894(5)
2.3396(8)
81.49(7)
91.58(7)
89.33(2)
97.15(3)
2.2417(4)
2.059(2)
2.6543(3)
2.3448(5)
81.90(4)
91.13(4)
89.76(1)
96.72(2)
2.047(2)
2.5010(5)
2.3442(7)
81.18(7)
92.06(7)
90.23(2)
96.69(2)
Cl1–Pd1–P1 92.01(2)
X1–Pd1–P2
P2–Pd–P1
87.38(2)
99.37(2)
Pd1–P1 distances than cis-3a. Conversely, the Pd1–Cl1 bond
lengths are virtually identical, while the Pd1–P2 separations
vary with the nature of the phosphine ligands PR3. Another
difference in cis-3a–5a is found in the interligand angles,
affected by the trans positioning of the bulky phosphine
a The numbering scheme adopted for molecule 2 of cis-4a is essentially
the same the molecules are distinguished by the first digit.
cent P1–Pd1–P2 angle is the widest. The parameters of the donors (Table 4). Even in trans-3b·PhMe and trans-3c,
acylphosphine moiety are similar to those of the dimeric pre- however, the P1–Pd1–C2 angle remains the most acute.
cursors 2-Cl and 2-Br and suggest unperturbed ferrocene Furthermore, as indicated by the angles between the coordi-
units.
nation half-planes {Pd1,P1,C2} and {Pd1,P2,Cl1} of 15.77(5)°
DFT calculations performed for the cis and trans isomers of in the molecule of trans-3b and of 15.36(5)° in trans-3c, the
3a–5a closely reproduced the experimental structural data coordination spheres of the Pd atoms are more twisted than
(Table S4†) and revealed interesting trends as well.37 In par- in cis-3a–5a (the τ4 indices are 0.17 in trans-3b and 0.19 in
ticular, they suggest that the change from a cis to a trans trans-3c accordingly).
arrangement is associated with an elongation of Pd–C(ferroce-
Complex 2-Cl did not observably react with triphenyl-
nyl) and with a shortening of Pd-PPh2 bonds, which corres- phosphine sulfide, dimethylsulfoxide or dimethylsulfide,
ponds to a strong trans influence of the phosphine ligands while the N,N,N′,N′-tetramethylurea addition resulted in the
(PR3 > X). Similarly, the Pd-PMe3 distances in the cis isomer formation of two new complexes, presumably cis and trans
are longer than those in the respective trans isomer, while the isomers, but we were unable to isolate them. Even so, the reac-
opposite holds true for the Pd–X distances due to the differ- tion with lutidine afforded two new compounds, which did
ences in the trans influence between the ferrocenyl and phos- not crystallize. In the reaction with 4-(dimethylamino)pyridine
phine ligands (C− > PR3).
(dmap) as a stronger base, 2-Cl was cleanly cleaved, yielding
These trends are reflected in the structures of trans- the stable complex 3d as a 13 : 87 mixture of cis- and trans-P,N
3b·PhMe and trans-3c (Fig. 7 and Table 4), which contain isomers (Scheme 4). Crystallisation from ethyl acetate or from
different phosphine ligands. As predicted by DFT calcu- ethyl acetate/hexane produced pure trans-3d, which rapidly iso-
lations, the trans isomers display shorter Pd1–C2 and longer merised in solution. Lastly, 2-Cl addition to in situ-generated
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1,3-dimesitylimidazolin-2-ylidene (SIMes, Scheme 4) gave
complex 3e, which was isolated as a pure trans-P,carbene
isomer. In this case, the selective formation of the trans
isomer and its isomerisation stability can be explained by the
steric bulk of the carbene ligand and also by the strong trans
influence exerted by the carbene (NHC) ligand38 (see also the
discussion of structural parameters below).
The NMR spectra of trans-3d displayed the signals of the
palladated acylphosphine (δP 47.4; cis-isomer: δP 48.7) and of
the coordinated pyridine ligand, namely a singlet due to the
NMe2 substituent (δH 3.10, δC 39.29) and resonances attribu-
table to the substituted pyridine ring. The 13C NMR signal
assigned to the CvO moiety was observed as a doublet (1JPC
34 Hz) at δC 208. The presence of the carbene ligand in trans-
3e was also confirmed by the NMR spectra. In this case, the
13C resonances of the carbene and carbonyl carbon atoms at
2
1
δC 210 (d, JPC 143 Hz) and δc 208 (d, JPC 27 Hz) were of par-
ticular diagnostic value. The 31P NMR signal of trans-3e was
observed at δP 36.7.
The structures of trans-3d and trans-3e·12PhMe are shown in
Fig. 8, and the relevant geometric parameters are outlined in
Table 4. The coordination sphere in the dmap complex trans-
3d is bent along the C2⋯Cl1 diagonal, as indicated by the P1–
Pd1–N1 angle of 167.57(5)° (N.B. the Cl1–Pd1–C2 angle is
173.05(5)°). The pyridine ring is rotated by 49.70(7)° with
respect to the mean coordination plane and, because of its
weak trans influence, the complex displays the shortest Pd1–P1
distance among the structurally analogous compounds 3b–e.
The imidazolin-2-ylidene ring in the structure of trans-
3e·12PhMe is also rotated from the coordination plane, by 76.9
(1)°, most likely due to the steric demands of the bulky SIMes
ligand, as already indicated by the widening of the C31–Pd1–
C2/Cl1 angles. The chemically distinct Pd–C bonds in 3e have
similar lengths (Δd ≈ 0.07 Å) and do not depart from the
typical values.21,39
Fig. 8 View of the complex molecules in the structure of trans-3d and
trans-3e·21 PhMe.
In another experiment, 2-Cl was treated with sodium acetyl-
acetonate in acetonitrile to produce the air-stable, deep red
bis-chelate complex
6 in 75% yield after crystallisation
(Scheme 5). This compound reacted cleanly and rapidly with
(diphenylphosphino)acetic acid to give compound 7 through
the proton transfer and chelate coordination of the formed
(diphenylphosphino)acetate anion.40 Complex 7 was isolated
as a pure trans isomer after crystallisation. Similarly to other
bis-phosphine complexes, however, it isomerised in solution,
resulting in a 15 : 85 mixture of cis and trans isomers (in
CDCl3).
Scheme 5 Synthesis of acetylacetonate complex 6 and its conversion
into 7 (Hacac = acetylacetone).
The 31P NMR spectra of 6 and trans-7 displayed the
expected signals, namely a singlet δP 42.5 for 6 and a pair of
doublets at δP 10.3 and 34.0 (2JPP = 376 Hz) for trans-7. The H at δC ≈ 206 (1JPC = 38 and 29 Hz for 6 and trans-7, respectively),
NMR spectra were consistent with the proposed structures, while the FTIR spectra displayed νCvO bands near 1640 cm−1
showing signals due to the acetylacetonate ligand (Me: δH 1.99, (for 6, the νCO bands attributable to the acetylacetonate ligand
2.13/δC 28.03, 28.15; CH: δH 5.45/δC 99.60; and CvO: δC were observed at 1581 and 1519 cm−1).
1
187.07) for 6 and resonances attributable to the methylene (δH
The molecular structures of 6 and trans-7 are shown in
3.51 and 3.55/δC 39.03) and carboxylate groups (δC 178.75, dd Fig. 9. The coordination spheres in both complexes are planar,
with JPC = 15 and 4 Hz) for trans-7. The 13C NMR signals due to as evidenced by the dihedral angles of the coordination half-
CvO of the orthometallated ligand were identified as doublets planes (1.4(1)° and 9.5(1)°, respectively; see Table 5) and by
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Scheme 6 Synthesis of diorganopalladium complexes 8.
in reagent-grade benzene in the presence of potassium tert-
butoxide. The reaction, which was performed in air for 24 h,
yielded the product as a mixture of cis and trans isomers
(≈87 : 13), whose crystallisation afforded a pure cis isomer.41
Compound 8a is a deep red, air-stable solid, which can be
stored for months under ambient conditions but gradually
decomposes in halogenated solvents and on common chroma-
tographic media.
The 31P NMR signal of the orthopalladated acylphosphine
in cis-8a (δP 38.1) was high-field shifted with respect to cis-3a,
while the position of the PMe3 resonance changed only slightly
(δP −18.6), and the 2JPP coupling constant (26 Hz) corroborated
the cis-P,P arrangement. The signals due to the PdMe group
were observed at δH 0.68 (dd, JPH = 8.5, 6.6 Hz) and δC 1.00 (dd,
JPC = 93, 10 Hz).
Fig. 9 Views of the molecular structures of 6 and trans-7.
Table 5 Selected interatomic distances and angles of 6 and trans-7 (in
Å and °)
Parameter
6
trans-7
The arylation of 3a with p-tolylboronic acid (Scheme 6) pro-
ceeded easily at room temperature in benzene/water using
K2CO3 as a base (full and clean conversion within 1 h).
Complex 8b was more stable than 8a and was isolated by crys-
tallisation and chromatography, resulting in a pure cis isomer,
which did not isomerise in CDCl3. An analogous reaction with
4-(trifluoromethyl)boronic acid affording cis-8c required a
longer reaction time (2 h) to reach completion.
X/Y
O2/O3
P2/O3
Pd1–P1
Pd1–C2
Pd1–X
2.1999(9)
1.976(3)
2.068(2)
2.076(2)
81.6(1)
91.2(1)
91.42(9)
95.80(7)
2.3083(9)
2.002(3)
2.2799(9)
2.077(2)
82.75(9)
97.34(8)
83.63(6)
96.75(6)
Pd1–Y
P1–Pd1–C2
C2–Pd1–X
X–Pd1–Y
Y–Pd1–P1
Under similar conditions, the reaction of 3a with (E)-(4-
methylstyryl)boronic acid resulted in incomplete conversion;
therefore, we resorted to alkylation in pure benzene using an
excess of both KOt-Bu and boronic acid. The crude product
was adsorbed on Celite, and cis-8d was eluted with diethyl
ether and dichloromethane and further purified by crystallisa-
tion from hot hexane. The compound decomposes on silica
gel, which precludes the removal of 3a (if present) by chrom-
atography; decomposition was also observed in CDCl3, leading
to 3a.
the τ4 indices (0.07 and 0.08). The six-membered chelate ring
in the structure of 6 is planar and shows a minimal differen-
tiation in the C–O and C–C bond lengths. For trans-7, the ring
formed by the phosphinocarboxylate ligand is also essentially
planar and associated with an acute interligand angle (N.B.
both in-ring angles in trans-7 are smaller than the other
angles). In addition, the Pd1–P1 bond is significantly length-
ened when comparing 6 to trans-7, in line with the “antisym-
biosis” of the two phosphine donors.33
Lastly, the η1-alkynyl complex 8e was prepared by treating
3a with 4-ethynyltoluene and KOt-Bu in reagent-grade benzene
under ambient conditions. Crystallisation afforded cis-8e,
Synthesis of organopalladium(II) complexes
Further experiments were aimed at the synthesis of diorgano- which gradually isomerised in solution. The equilibrium ratio
palladium(II) complexes by replacing the chloride ligand in 3a of cis- and trans-8e determined in C6D6 was 42 : 58 (for the
(mixture of isomers) with suitable “alkylating” agents corresponding kinetic curves, see Fig. S4†). This observation
(Scheme 6). Thus, the methylpalladium(II) complex 8a was matches the DFT computations, which further underscores the
obtained upon treating 3a with methylboronic acid (5 equiv.) importance of solvation effects: the calculated ΔG values, ΔG =
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Gcis − Gtrans, were −0.4 kcal mol−1 in a vacuum and 1.1 kcal
mol−1 in benzene at 298 K (N.B. the small energy difference is
in line with the comparable equilibrium concentrations of the
isomers). In CDCl3, cis-8e underwent simultaneous isomerisa-
tion and decomposition, slowly regenerating complex 3a
(Fig. S5†).
Table 6 Selected interatomic distances and angles for cis-8a-b,d-e (in
Å and °)
Param
cis-8a
cis-8b
2.2923(5)
2.061(2)
2.3003(5)
2.048(2)
81.77(5)
88.94(6)
88.11(5)
101.15(2)
cis-8d a
2.293(1)
2.056(4)
2.297(1)
2.056(4)
82.3(1)
90.3(2)
86.7(1)
100.94(4)
cis-8e b
2.2658(7)
2.042(2)
2.308(2)
2.019(3)
81.78(7)
92.2(1)
Pd1–P1
Pd1–C2
Pd1–P2
Pd1–C31
P1–Pd1–C2
C2–Pd–C31
C31–Pd1–P2
P2–Pd1–P1
2.2883(5)
2.055(2)
2.2970(5)
2.113(2)
82.59(5)
90.15(7)
90.00(5)
96.85(2)
As observed for cis-8a, cis-8b-e displayed a pair of doublets
in their 31P NMR spectra, at δP 38–46 for the orthopalladated
acylphosphine and at δP −22.5–−18.5 for the PMe3 ligand, with
85.23(8)
100.89(5)
1
2JPP in the 26–29 Hz range. In H and 13C NMR spectra, they
showed characteristic signals due to the acylphosphine and σ-
hydrocarbyl ligands. Furthermore, the structures of complexes
cis-8a–b,d–e were corroborated by X-ray diffraction analysis
(Fig. 10 and Table 6).
a Additional data: C31–32 1.325(6), Pd1–C31–C32 123.7(3), C31–C32–
C33 128.4(4), Pd1–C31–C32–C33 −179.9(3). b Data for the dominating
orientation of the disordered phosphine groups. Additional para-
meters: C31–C32 1.201(4), Pd1–C31–C32 174.5(2), C31–C32–C33 178.3
(3).
The coordination spheres around Pd(II) in these com-
pounds are planar (within <0.1 Å in the entire series) but angu-
larly distorted because of the unlike steric demands of the
ligating moieties. While the P1–Pd1–P2 angle is the largest
interligand angle in all molecules (particularly in the com-
pounds containing bulkier hydrocarbyl ligands), the angle
associated with the orthopalladated acylphosphine represents
the other extreme. The variation in the Pd-donor separations is
marginal, except for the Pd–R bond length, which is longer in
cis-8a than in the complexes featuring conjugated hydrocarbyl
ligands, among which cis-8e displays the shortest Pd–R dis-
tance. For steric reasons, the aryl groups in the structures of
cis-8b and cis-8d are oriented perpendicularly to the coordi-
nation plane comprising Pd1 and its four ligating atoms; the
respective dihedral angles are 89.04(6)° and 87.0(2)°.
Scheme 7 Synthesis of η3-allyl complex 9 (the compound is a mixture
of isomers differing in the orientation of the π-allyl moiety).
To expand the series of complexes with hydrocarbyl
ligands, we decided to synthesize the η3-allyl complex 9
(Scheme 7). The reaction of 2-Cl with allylmagnesium bromide
in THF performed first only gave 2-Br as the product of halide
exchange. Gratifyingly, an analogous reaction with allyltributyl-
tin led to the formation of the desired compound, albeit rather
slowly (full conversion after 8 h in CDCl3 at room temperature;
heating the reaction mixture resulted in decomposition).
Moreover, the product was difficult to separate from the
reagent (used in excess) and side products. Nevertheless, treat-
ing the crude product with a fluoride salt, followed by crystalli-
sation, afforded reasonably pure 9 in 35% yield.
Fig. 10 Views of the molecular structures of cis-8a, cis-8b, cis-8d and cis-8e.
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To avoid problems with product isolation, the organotin indicating the conformational rigidity of the [Pd(1 − H)]+ frag-
reagent was ultimately replaced with the respective pinacol ment, which stems from the small size of the metallacycle, its
borate (Scheme 7). The reaction in benzene/water in the pres- conjugated nature and the steric properties of the ortho-fused
ence of K2CO3 led to a disappointingly low yield (10% after 1 h ferrocene moiety and of the PPh2 group (in turn, these geo-
at room temperature), which was markedly improved when metric constrains result in the distortion of the bond angles
using KOt-Bu in benzene (≈90% after 4 h). When using THF around the acyl carbon atom C11).
and KOH as the base, 2-Cl was fully converted into 9 within
4 h at room temperature (N.B. KOt-Bu also ensued in full con-
version but the product was less easily purified). Even under
The electrochemical behaviour of the representative com-
these conditions, however, a final crystallisation step was
Electrochemistry
pounds42 was studied by cyclic voltammetry at a glassy carbon
required to remove by-products. This additional step resulted
electrode in dichloromethane containing Bu4N[PF6] as the sup-
in a significant loss of the product, which is highly soluble in
porting electrolyte. In the accessible potential range, acylpho-
common solvents including hexane (the isolated yields were
sphine 1 underwent a single reversible redox transition
typically 30%, while less pure 9 can be obtained in a practically
(Fig. 12). The redox change was controlled by diffusion, as
indicated by the linear increase in the anodic peak current
quantitative yield by simply filtering and evaporating the reac-
tion mixture).
According to the NMR analysis, complex 9 is a mixture of
two isomers (δP 46.6 and 45.9), in an approximately 60 : 40
1
2
with the square root of the scan rate (ipa ∝ ν in the 10–500 mV
s−1 range) and was attributed to the one-electron oxidation
centred at the ferrocene unit. Such an assignment is consistent
with the results of DFT computations indicating that the
HOMO orbital is localised predominantly at the ferrocene unit
ratio, differing in the orientation of the allyl moiety. The solu-
tion data correspond with the solid-state structure of 9
(Fig. 11), wherein the η3-allyl moiety is disordered over two
and, mainly, that the change in the electron density from 1 to
positions (refined occupancies: 57 : 43) mutually related by a
180°-rotation along the Pd-allyl axis (see the ESI, Fig. S26†).
sively limited to the ferrocene iron (Fig. S27†). The oxidation
The allyl group is tilted by ca. 60° with respect to the {Pd1,P1,
1+, ρ(1) − ρ(1+), mapped at the geometry of 1,43 is almost exclu-
was observed at 0.28 V vs. the ferrocene/ferrocenium reference
C2} plane; the parameters of the Pd-acylphosphine fragment
(Table 7), consistently with the presence of the electron-with-
do not depart from the respective values discussed above.
drawing phosphaamide substituent that makes the electron
The wealth of structural data collected for the Pd(II) com-
removal more difficult.
The redox response of 1O and 1S was virtually identical, but
the redox waves were shifted to 0.35 V and 0.33 V, respectively,
plexes with orthometallated 1 (altogether 16 compounds)
called for a more detailed inspection. The comparison of
selected geometric parameters (Table S2†) reveals a minor vari-
ation in Pd1–P1 and Pd1–C2 distances dictated by the trans-
influence and steric hindrance of the other ligands completing
the coordination sphere. Conversely, the geometry of the
reversible, albeit somewhat broadened redox wave at 0.32 V
metallated acylphosphine remains largely unchanged, thus
due to the increased electron-withdrawing character of the
ferrocene substituent (cf. Hammett σp constants:44 PPh2 0.19,
P(O)Ph2 0.53, and P(S)Ph2 0.47). Selenide 1Se also displayed a
(Fig. 12). This wave was followed by an irreversible redox
process at a more positive potential (0.75 vs. ferrocene/ferroce-
Fig. 11 View of the molecular structures of 9 showing only the more
populated position of the disordered π-allyl moiety. Selected distances
and angles (in Å and °): Pd1–P1 2.2625(5), Pd1–C2 2.040(2), P1–Pd1–C2
83.16(5), Pd1–C31A/B 2.184(6)/2.136(9), Pd1–C32A/B 2.174(4)/2.155(6),
Pd1–C33A/B 2.180(7)/2.250(9), C31A–C32A–C33A 120.7(5), C31B–
C32B–C33B 122.0(7), C1–C11–P1 109.3(1); A and B refer to the two
orientations of the allyl group (see the ESI†).
Fig. 12 Representative cyclic voltammograms of 1 (the red line) and
1Se (the black line) recorded at a glassy carbon disc electrode and 0.1 V
s
−1 scan rate in dichloromethane.
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Table 7 Summary of the electrochemical dataa
Compound
E°′ [V]
Additional redox events
1
1O
1S
1Se
cis-3a
cis-4a
cis-5a
cis-8a
cis-8b
cis-8c
cis-8d
cis-8e
0.28
0.35
0.33
0.32
0.15
0.15
0.15
−0.01
0.01
0.05
0.02
0.08
None
None
None
Epa = 0.75 V (ir)
Epa ≈ 1.3 V (ir)b
Epa ≈ 1.2 V (ir)b
Epa ≈ 1.0 V (ir)
Epa = 0.89 V (ir), Epc ≈ 0.2 V (ir)
Epa = 0.90 V (ir), Epc ≈ 0.2 V (ir)
Epa ≈ 1.13 V (ir), Epc ≈ 0.3 V (ir)
Epa ≈ 0.76 V (ir), Epc ≈ 0.2 V (ir)
Epa = 1.03 V (ir), Epc ≈ 0.2 V (ir)
a E°′ was determined as an average of the anodic and cathodic peak
potentials, E°′ = 1(Epa + Epc). The potentials are quoted relative to the
2
ferrocene/ferrocenium reference. ir = irreversible. Presumably due to
high resistance, the peak separations of 85 mV for decamethyl-
ferrocene and ≈100 mV for the analysed compounds exceeded the
theoretical value (58 mV at room temperature). For experimental
details, see the ESI.† b Approximate value.
Fig. 14 Cyclic voltammograms of cis-8a recorded at a glassy carbon
disc electrode and 0.1 V s−1 scan rate in dichloromethane. The second
scan is shown as a dashed line.
nium; the peak potential at a 0.1 V s−1 scan rate is given), and
a broad, composite, reductive wave developed upon repeated
scanning.
order: Cl > Br > I. During the reverse sweep, additional reduc-
tive waves emerged, attributable to the species generated by
the second oxidation and follow-up processes.
The redox response of complexes 8 was also generally
similar (Fig. 14): the compounds underwent a reversible oxi-
dation at potentials even lower (E ≈ 0 V) than those of cis-3a,
in line with the presence of anionic and predominantly σ-
donor hydrocarbyl ligands. When the scan range was extended
towards more positive potentials, an additional irreversible
wave was observed. The second oxidation was associated with
chemical reactions that generated redox-active species, which
in turn gave rise to additional waves during the reversed
(reductive waves) and repeated (oxidative waves) scans. These
peaks were often broad and convoluted (Fig. 14).
Complexes cis-3a–5a uniformly displayed a reversible redox
transition at 0.15 V (Fig. 13), which can again be ascribed to
the ferrocene-centred oxidation based on the DFT compu-
tations (see Fig. S27†). A similar decrease in the ferrocene/fer-
rocenium redox potential upon metallation, corresponding to
an easier oxidation has been described for the orthopalladated
ferrocene imines45 and for a ferrocene amide.46 An additional
irreversible redox wave was observed at more positive poten-
tials, already at the onset of base electrolyte decomposition. In
contrast to the first wave, the potential of the second irrevers-
ible oxidation, tentatively attributed to palladium oxidation,47
varied with the halide ligand, decreasing in the following
Conclusions
In this contribution, we have reported the synthesis and
detailed structural characterisation of the ferrocene-based, ter-
tiary acylphosphine ligand 1. Based on our systematic reactivity
studies, the positioning of the phosphine moiety and the
acidity of the ferrocene CH groups render 1 susceptible to
selective C–H bond activation, resulting in the dimeric P,C-
chelate palladium(II) complexes 2-X as a mixture of isomers.
Complexes 2-X readily undergo further transformations (viz.
dimer cleavage and ligand substitution), thereby providing a
range of complexes featuring the [Pd(1 − H)]+ fragment. Due to
the spatial properties of the palladacycle fused onto the ferro-
cene unit, particularly its small size and conjugated nature,
the [Pd(1 − H)]+ fragment is rigid and, consequently, structu-
rally invariant. However, while the geometry of [Pd(1 − H)]+
remains unchanged, the Pd-donor distances and interligands
change with changing steric demands and trans influence of
the other ligands. The latter factors also control the equili-
brium between the cis and trans isomers of the monopalla-
Fig. 13 Cyclic voltammograms of cis-5a recorded at a glassy carbon
disc electrode and 0.1 V s−1 scan rate in dichloromethane. The second
scan is shown as a dashed line.
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dium complexes [PdL(X)(1 − H)] (L = neutral ligand, X =
anionic ligands), which typically have similar energy and,
hence, comparable abundance in the equilibrium mixtures.
Thanks to the high chemical stability of these compounds,
many of the reported synthetic transformations can be per-
formed without excluding air and moisture.
(b) H. Fernández-Pérez, J. Benet-Buchholz and A. Vidal-
Ferran, Chem. – Eur. J., 2014, 20, 15375–15384. See also:
(c) S. Gowrisankar, H. Neumann, A. Spannenberg and
M. Beller, Organometallics, 2014, 33, 94–99.
9 Acylphosphine ligands may arise in the coordination
sphere in reactions of carbonyl or carbonylate complexes.
These routes, however attractive, are not relevant to this study.
10 (a) E. Lindner and D. Hübner, Chem. Ber., 1983, 116, 2574–
2590; (b) R. A. Baber, M. L. Clarker, A. G. Orpen and
D. A. Ratcliffe, J. Organomet. Chem., 2003, 667, 112–119.
11 (a) S. Gowrisankar, C. Federsel, H. Neumann, C. Ziebart,
R. Jackstell, A. Spannenberg and M. Beller, ChemSusChem,
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Czech Science Foundation
(project no. 19-09334S).
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