Synthesis and characterisation of palladium(ii) complexes with hybrid phosphinoferrocene ligands bearing additional O-donor substituents

Article abstract of DOI:10.1039/c9nj00298g

While 1,1′-bis(diphenylphosphino)ferrocene (dppf) is widely used as a ligand in coordination chemistry and catalysis, its congeners with oxygen-containing functional groups have long been overlooked. Accordingly, we studied the coordination behaviour in Pd(ii) complexes of three phosphinoferrocene ligands bearing secondary O-donor groups, Ph₂PfcR, wherein R = CHO (1), Ac (2) and CMe₂(OH) (3), and fc = ferrocene-1,1′-diyl. Depending on the stoichiometry, reactions of 1-3 (L) with [PdCl₂(cod)] (cod = η²:η²-cycloocta-1,5-diene) produced the respective mono- and dipalladium complexes, trans-[PdCl₂(L-κP)₂] and trans-[PdCl(μ-Cl)(L-κP)]₂. Compound [PdCl(μ-Cl)(3-κP)]₂ was found to dehydrate readily, giving rise to [PdCl(μ-Cl)(Ph₂PfcC(Me)═CH₂-κP)]₂. Furthermore, ligands 1-3 cleaved [(L^NC)Pd(μ-Cl)]₂ (L^NC = 2-((dimethylamino-κN)methyl)phenyl-κC¹), yielding [(L^NC)PdCl(L-κP)], which were converted into the cationic complexes [(L^NC)PdCl(L)]X (L/X = 1/PF₆, 2/SbF₆, 3/PF₆). Compounds with ligands 1 and 2 were structurally authenticated as stable bis-chelate complexes. In contrast, the product featuring ligand 3 was rather unstable and converted into [(L^NC)Pd(AcOEt-κO)(3-κP)][PF₆] upon recrystallisation. Weak oxygen coordination was confirmed via reactions of [(L^NC)PdCl(L)]X with (PhCH₂NEt₃)Cl in which the parent chloride complexes were regenerated, and this was further corroborated by DFT computations. Our findings, pointing to hemilabile coordination of 1-3, are relevant for catalysis because de-coordination of the weaker binding O-donor moiety may open a vacant site for a substrate, thereby enhancing the catalytic properties of metal complexes with ligands of this type.

Full text of DOI:10.1039/c9nj00298g

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Synthesis and characterisation of palladium(II) complexes with
hybrid phosphinoferrocene ligands bearing additional O-donor
substituents†
Received 00th January 20xx,
Accepted 00th January 20xx
Petr Vosáhlo,a Jiří Schulz,a Karel Škoch,a Ivana Císařováa and Petr Štěpničkaa*
DOI: 10.1039/x0xx00000x
www.rsc.org/
While 1,1’-bis(diphenylphosphino)ferrocene (dppf) is widely used as a ligand in coordination chemistry and catalysis, its
congeners with oxygen-containing functional groups have long been overlooked. Accordingly, we studied the coordination
behaviour in Pd(II) complexes of three phosphinoferrocene ligands bearing secondary O-donor groups, Ph2PfcR, wherein R
= CHO (1), Ac (2) and CMe2(OH) (3), and fc = ferrocene-1,1′-diyl. Depending on the stoichiometry, reactions of 1-3 (L) with
[PdCl2(cod)] (cod = η2:η2-cycloocta-1,5-diene) produced the respective mono- and dipalladium complexes, trans-[PdCl2(L-
κP)2] and trans-[PdCl(µ-Cl)(L-κP)]2. Compound [PdCl(µ-Cl)(3-κP)]2 was found to dehydrate readily, giving rise to [PdCl(µ-
Cl)(Ph2PfcC(Me)=CH2-κP)]2. Furthermore, ligands 1-3 cleaved [(LNC)Pd(µ-Cl)]2 (LNC = 2-((dimethylamino-κN)methyl)phenyl-
κC1), yielding [(LNC)PdCl(L-κP)], which were converted into the cationic complexes [(LNC)PdCl(L)]X (L/X = 1/PF6, 2/SbF6,
3/PF6). Compounds with ligands 1 and 2 were structurally authenticated as stable bis-chelate complexes. In contrast, the
product featuring ligand 3 was rather unstable and converted into [(LNC)Pd(AcOEt-κO)(3-κP)][PF6] upon recrystallisation.
Weak oxygen coordination was confirmed via reactions of [(LNC)PdCl(L)]X with (PhCH2NEt3)Cl in which the parent chloride
complexes were regenerated, and was further corroborated by DFT computations. Our findings, pointing to hemilabile
coordination of 1-3, are relevant for catalysis because de-coordination of the weaker binding O-donor moiety may open a
vacant site for a substrate, thereby enhancing the catalytic properties of metal complexes with ligands of this type.
remain unexplored. A similar situation is encountered for 1’-
11,13
Introduction
(diphenylphosphino)-1-(hydroxymethyl)ferrocene (3*
)
and
1’-(diphenylphosphino)-1-acetylferrocene (
2
; Scheme 1).14,15
Replacing one phosphine substituent in the extensively studied
1,1’-bis(diphenylphosphino)ferrocene (dppf)1 provides access
PPh2
PPh2
PPh2
PPh2
to
a
vast family of structurally diverse functional
Fe
Fe
Fe
phosphinoferrocene ligands (Scheme 1).2 In this regard,
compounds with chemically dissimilar donor moieties are
particularly relevant as hybrid ligands capable of hemilabile
coordination.3 In addition, such compounds can serve as
advanced synthetic building blocks. Among dppf analogues
bearing oxygen functions, research efforts have focused on 1’-
(diphenylphosphino)ferrocene-1-carboxylic acid (Hdpf),4 which
has been studied as a ligand in coordination compounds5 and
SDG
CO2H
Hdpf
dppf
PPh2
PPh2
PPh2
Fe
Fe
Fe
O
OH
CHO
R
R
3 (R = Me), 3* (R = H)
Me
1
2
Scheme 1. Dppf and analogous phosphinoferrocene ligands with oxygen donor
converted into new ligands, primarily phosphinoferrocene groups relevant to this study. SDG = secondary donor group
amides6 and oxazolines.7 In contrast, compounds containing
Considering this knowledge gap, we aimed to investigate the
chemically related aldehyde (CHO) and alcohol (CH2OH) groups
have been studied considerably less than Hdpf.8 For instance,
aldehyde 19,10,11 (Scheme 1) has been used to synthesize
phosphinoferrocene ligands,9,12 but its coordination properties
coordination behaviour of these archetypal hybrid O,P-ligands,
specifically in Pd(II) complexes16 with simple terminal (i.e.,
halide) and N,C-chelating auxiliary ligands. For such purpose,
we chose structurally related compounds, namely the
aforementioned aldehyde
1
, acyl derivative
2
, and an alcohol
(Scheme
a.Department of Inorganic Chemistry
Faculty of Science, Charles University
bearing 2-hydroxyprop-2-yl substituent, compound
3
Hlavova 2030, 128 40 Prague, Czech Republic
E-mail address: petr.stepnicka@natur.cuni.cz
1). Hence, in this contribution, we report the synthesis and
detailed structural characterisation by spectroscopic, X-ray
diffraction and DFT computational methods of Pd(II)
complexes accommodating these compounds as ligands.
† Electronic Supplementary Informaꢀon (ESI) available: complete experimental
details including those on structure determination and on DFT computations,
additional structural diagrams, copies of the NMR spectra (PDF), and Cartesian
coordinates of the DFT optimized structures (MOL2). CCDC 1882120-1882129. See
DOI: 10.1039/x0xx00000x
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Results and discussion
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DOI: 10.1039/C9NJ00298G
Aldehyde
1
was synthesized as previously reported.11
However, attempts to prepare
2
using a modified literature
method (Scheme 2), i.e., from acetyl chloride and 1’-
(diphenylphosphino)-1-lithoferrocene generated in situ from
bromide 4
and n-butyllithium,2a,10 instead of opening 1-phenyl-
1-phospha[1]ferrocenophane with phenyllithium, as originally
reported,14 reproducibly led to disappointingly low yields
(12%). Whilst searching for an alternative synthesis, we found
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that the yield of
with the corresponding Weinreb amide.17 Thus, lithiation of
followed by N-methoxy-N-methylacetamide addition gave rise
2 increases when replacing acetyl chloride
4
to
2
in 55% yield after simple chromatographic purification.
1. n-BuLi
2. AcCl
12%
PPh2
Br
PPh2
Figure 1. UV-vis absorption spectra of compounds 1-3 as recorded in 1.0 mM
solutions in dichloromethane (optical path 1 cm)
Fe
Fe
Me
O
The crystal structure of
analysis (Figure 2). The cyclopentadienyl rings in the molecule
of are tilted by 1.37(9)° and adopt an intermediate
conformation which brings the substituents into the anti-
position ( = –165.9(1)°; τ is the torsion angle C1-Cg1-Cg2-C6,
2 was determined by X-ray diffraction
55%
4
2
1. n-BuLi
2. AcN(OMe)Me
2
Scheme 2. Synthesis of 2 by lithiation of bromide 4 and trapping of the lithiated
intermediate
τ
Alcohol
synthesized by lithiation of bromide
intermediate organolithium with dry acetone (42% yield).
Similarly to compound was isolated along with
3
, completing the series of ligands, was similarly
wherein Cg1 and Cg2 are the centroids of the cyclopentadienyl
rings C(1-5) and C(6-10), respectively, and C1 and C6 are the
pivotal atoms of the cyclopentadienyl rings). The C11=O bond
4
and by quenching of the
2
,
3
(1.220(2) Å) in
2 has a length identical to that in
acetylferrocene (1.222 Å)21 and lies in the plane of its parent
cyclopentadienyl ring (twist angle: 5.5(1)°). The P-C bond
lengths (P-C6 1.809(1) Å, and P-C12/C18 1.835(1) Å) compare
very well with those determined for FcPPh2.22
(diphenylphosphino)ferrocene (FcPPh2, Fc = ferrocenyl), which
results from unwanted protonolysis of the lithiated
intermediate.
PPh2
1. n-BuLi
PPh2
2. Me2CO
Fe
Fe
Me
42%
Me
OH
Br
4
3
Scheme 3. Synthesis of alcohol 3.
In their 1H and 13C NMR spectra, compounds
2 and 3 displayed
characteristic sets of signals attributed to the 1,1’-
disubstituted ferrocene unit. Resonances of the polar
substituents were observed at
201.94 (COMe) for , and at δH 1.43, δC 31.05 (CMe2) and δC
68.91 (CMe2) for
δH 2.31, δC 27.49 (COMe) and δC
2
3
. The presence of the phosphine moiety was
1
shown by the H and 13C (31P-coupled18) NMR signals of the
PPh2 group and by the 31P NMR resonances at δP ≈ –17. The IR
Figure 2. View of the molecular structure of acylphosphine
2
spectra of
1662/1655 cm–1 for acetylferrocene19) and a broad band due Synthesis and characterisation of Pd(II) complexes with
to
OH vibration at 3447 cm–1 with a weaker absorption at 3568 chloride supporting ligands. Mixing compounds
with
cm−1, respectively. Differences in the extent of conjugation, [PdCl2(cod)] (cod = cycloocta-1,5-diene) at a 2:1 ligand-to-
highlighted already by the colour of the compounds ( and
metal ratio led to bis(phosphine) complexes (Scheme 4).
are burgundy red, is orange brown), corresponded to the When the amount of ligand was reduced to one molar
positions of the low-energy bands in their UV-vis spectra equivalent, the reaction exclusively produced chloride-bridged
(Figure 1). Specifically, a weak ligand-field band at 440 nm in dipalladium(II) complexes
. In both cases, the ferrocene
the spectrum of the unsubstituted ferrocene20 was shifted to ligands coordinated in trans positions and as P-monodentate
459 and 454 nm for and , respectively, but remained donors. Such coordination is in accordance with the low
2 and 3 showed a strong ν
C=O band at 1663 cm–1 (cf.
ν
1-3
1
2
5
3
6
1
2
virtually unaffected in the spectrum of
3
(≈
443 nm).
propensity of the Pd(II) ions to coordinate hard oxygen
2 | J. Name., 2012, 00, 1-3
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donors.16 Thus, although the oxygen atoms in the functional coordination planes of 5a·2CH2Cl2 and 5b are eVxiaewctAlryticlpelOannlianer
substituents are properly positioned to form O,P-chelate rings due to the imposed symmetry, the cooDrdOiIn: 1a0t.i1o0n39g/Ce9oNmJ0e0tr2y98iGn
(see below), the Pd(II) ions preferably share the relatively 5c is distorted towards tetahedral (Cl-Pd-Cl
′ = 171.71(3)°, P-Pd-
softer chloride ligands, forming dimers = 167.65(2)°). The ferrocene units in 5a·2CH2Cl2 and 5b
6
.
P′
assume approximately eclipsed anticlinal conformations,1a
which divert their polar substituents from the Pd atom, and
the substituents are coplanar with their parent
cyclopentadienyl rings to maximise conjugation. In contrast,
the cyclopentadienyl rings in 5c are rotated to an intermediate
conformation (cf. the τ angles in Table 1), and the non-
conjugated C11-OH moieties are directed towards the chloride
ligands, forming O-H···Cl hydrogen bonds (O···Cl = 3.223(2) Å).
*** Figure 3 near here ***
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Table 1. Selected distances and angles for 5a·2CH2Cl2, 5b and 5c (in Å and deg).a
Parameter
Pd-Cl
5a·2CH2Cl2
2.2926(4)
2.3396(4)
86.49(1)c
4.2(1)
5b
5c
2.2971(5)
2.3308(5)
89.59(2)/ 89.52(2)
3.7(1)
2.2959(5)
2.3420(5)
88.31(2)c
1.5(1)
Pd-P
Cl-Pd-P
Cp1,Cp2
τ
Scheme 4. Synthesis of Pd(II) complexes 5 and 6 (cod = cycloocta-1,5-diene).
146.0(1)
1.206(3)
5.4(2)
147.6(2)
1.220(3)
2.1(2)
82.9(2)
Complexes
6
crystallise less readily than their monopalladium
, and the extended times required for their
C11-O
C11-O/Cp1b
1.430(3)
counterparts
5
20.2(2)
crystallisation may allow decomposition. For instance, the
attempted crystallisation of 6c produced mixtures of this
compound and a new complex 6d, resulting from dehydration.
Compound 6c can be prepared in non-halogenated solvents
(e.g., benzene, ethyl acetate or acetone) or in their HCl-free
halogenated alternatives. However, when using halogenated
solvents, the product must be isolated rapidly to avoid
dehydration promoted by traces of HCl. This observation was
corroborated by an independent experiment in which a tiny
a Definitions: Cp1 and Cp2 are the cyclopentadienyl rings C(1-5) and C(6-10), and
Cg1/Cg2 stand for their respective centroids. τ = torsion angle C1-Cg1-Cg2-C6.
c
b Angle at which the C11-O bond intersects the Cp1 plane. The adjacent
interligand angles sum up to 180° for symmetry reasons.
Of the dimeric complexes 6, only 6a·2CHCl3, 6b·CHCl3 and 6d
provided crystals suitable for X-ray diffraction analysis (Figure
3, Table 2). Although similar in the geometry around individual
Pd centres, the compounds differed in the arrangement of
their dipalladium cores. While the entire {Pd(µ-Cl)ClP}2
moieties in 6a·2CHCl3 and 6d are planar, owing to imposed
inversion symmetry, in 6b·CHCl3, the {PdCl3P} planes are tilted
by 41.93(3)° into a butterfly arrangement, similar to [Pd(µ-
drop of methanolic HCl was added to in situ generated 6c
,
causing a clean and rapid conversion of this complex into 6d
(within 30 min). Notably, complex 5c did not dehydrate upon
recrystallisation and reacted only sluggishly with HCl.
Cl)Cl(Ph2PfcPO3Et2-κP)]2·H2O23 (tilt angle: 40.91(5)°; fc
ferrocene-1,1 -diyl). Although such tilting is not
unprecedented, the majority 75%) of structurally
=
Complexes
phosphinoferrocene ligands in their H NMR spectra but could
be clearly distinguished by their 31P NMR shifts (
: δP 15;
δP
30-32).23 In addition, the 13C{1H} NMR spectra of the
bis(phosphine) complexes
displayed signals of 31P-coupled
5 and 6 exhibited similar sets of signals due to the
1
′
a
5
≈
6:
(
≈
≈
characterised [Pd(µ-Cl)Cl(PR3-κP)]2 complexes (PR3 is a non-
chelating phosphine) are planar or nearly planar (tilt angles
<2°).26 A moderate twisting may indeed occur when steric
effects come into play,27 but a tilting as high as by 75° could be
enforced via π···π interactions of aromatic substituents.28
The geometric parameters of 6a·2CHCl3, 6b·CHCl3 and 6d are
generally similar to the parameters determined for [Pd(µ-
Cl)Cl(Ph2PfcPO3Et2-κP)]2. In particular, the Pd-Cl(terminal)
5
carbon atoms as apparent triplets arising from virtual coupling
spin systems of the type 13C(A)-31P(X)-Pd-31P(X
′)-
in the AXX
′
24
12C, which is not observed in complexes
6.
All bis(phosphine) complexes, 5a·2CH2Cl2, 5b and 5c, were
structurally characterised by X-ray diffraction analysis. Their
molecular structures are shown in Figure 3 and the selected
geometric parameters are outlined in Table 1. Compounds
5a·2CH2Cl2 and 5b crystallise with the symmetry of the triclinic
space group P−1 and with the complex molecules lying over
the inversion centres. Conversely, compound 5c crystallises in
the space group C2/c with the two-fold axis perpendicular to
the coordination plane and passing through the Pd atom.
bonds in
6 are shorter than those involving the bridging
chloride ligand, and the Pd-Cl(bridge) bonds with the
phosphine moiety in the trans position are elongated
(approximately by 0.1 Å) compared to the Pd-Cl(bridge) bonds
located trans to the other chloride ligand, reflecting the
stronger trans influence of the phosphine donors.29 Trans
The Pd-donor distances determined for the 5-type complexes
influence also affects the Pd-P bonds in
shorter than those in complexes
The ferrocene units in 6a·2CHCl3, 6b·CHCl3 and 6d exert a
larger tilting (up to 7°) than those in . In 6a·2CHCl3, the
6, which are ≈0.1 Å
are similar along the series and match the bond lengths
determined for complex with the non-functionalised ligand,
trans-[PdCl2(FcPPh2)2].25 The interligand angles in 5a-c depart
from the ideal 90° only marginally. However, while the
5
.
5
ferrocene substituents are located in roughly anti positions,
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and the C11=O bonds remain in the planes of their bonding in turn suggests that the oxygen donor moieties are relatively
View Article Online
DOI: 10.1039/C9NJ00298G
cyclopentadienyl rings. Conversely, in 6b·CHCl3 and 6d, the weakly coordinated.
ferrocene substituents are rotated closer, and the C=O or C=C
bonds are twisted by 10-20°.30
*** Figure 4 near here ***
8
Table 2. Selected distances and angles for 6a·2CHCl3, 6b·CHCl3 and 6d (in Å and deg).a
9
Parameter
Pd-Clt
6a·2CHCl3
2.2740(5)
(Cl1)
6b·CHCl3
6d
10
11
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2.2847(9)c
2.2795(9)
(Cl4)d
2.2841(9)
(Cl1)
(Pd1)
2.4391(8)c
(Cl2)
Pd-Clb
Pd-Clb
2.3199(5)
(Cl2)
2.3273(9)
(Cl2)d
2.329(1)
(Cl2)
2.4317(5)
(Cl2′)
2.3077(9)
(Cl3)c
2.4091(9)
(Cl3)d
2.421(1)
(Cl2′)
Pd-P
2.2224(5)
2.2268(8)
(P1)
2.2354(8)
(P2)
2.2292(8)
P-Pd-Clt
P-Pd-Clb
Clt-Pd-Clb
Clb-Pd-Clb
87.82(2)
(Cl1)
88.17(3)
(P1/Cl1)c
93.49(3)
(P1/Cl3)c
93.23(3)
(Cl1/2)c
85.08(3)
(Cl2/3)c
7.0(2) (Fe1)
−77.2(2)
(Fe1)
88.57(3)
(P2/Cl4)d
96.93(3)
(P2/Cl2)d
89.13(3)
(Cl4/3)d
85.34(3)
(Cl2/3)d
3.0(2) (Fe2)
79.1(2)
90.51(3)
(Cl1)
96.35(2)
(Cl2)
94.32(3)
(Cl2)
89.68(2)
(Cl1/2′)
86.77(2)
(Cl2/2′)
5.2(1)
90.48(3)
(Cl1/2′)
84.84(3)
(Cl2/2′)
5.0(2)
Scheme 5. Synthesis of (LNC)Pd(II) complexes
7 and their transformations
Cp1,Cp2
τ
−160.0(1)
−87.6(2)
(Fe2)
According to the NMR spectra, complexes 7-8 are exclusively
C11-O
1.213(2)
1.4(1)
1.222(6)
(Fe1)
1.218(5)
(Fe2)
1.318(7)e
19.6(3)e
formed as trans-P,N isomers, wherein the ligands with the
strongest trans influence occupy cis positions.31 This was
C11-O/Cp1b
14.2(3)
(Fe1)
8.9(3) (Fe2)
1
indicated by the splitting of the H and 13C NMR resonances
due to the CH2NMe2 moieties into characteristic 31P-coupled
doublets.32 The coordination of the phosphine moieties and
carbonyl groups was inferred from the low-field 31P NMR
a The parameters are defined as for complexes 5 (see footnote to Table 1). Clt and
Clb are terminal and bridging chloride ligands. Angle at which the C11-O bond
b
intersects the Cp1 plane. c Pd = Pd1. d Pd = Pd2. Parameters involving the C11-
e
signals (7a-c: δP 32-33, 8a
the νC=O bands to lower energies (8a: 1600 cm−1, 8b
1598/1582 cm−1). A similar shift was observed for the νC=O
band of
(1652 cm−1 vs. 1742 cm−1 for AcOEt in CCl433). The IR
and NMR spectra further attested the presence of the counter
anions. Finally, the ESI+ MS spectra of and displayed
fragments attributable to the cations [(LNC)Pd(L)]+ (L =
).
Unfortunately, no such direct evidence was obtained for 8c
The spectra indeed showed signals due to P-coordinated (δP
/8b: δP ≈ 29) and from the shifts of
C24 double bond rather than the C11-O bond.
:
Synthesis and characterisation of (LNC)Pd(II) complexes
Subsequent experiments focused on Pd(II) complexes with a 2-
[(dimethylamino-κN)methyl]phenyl-κC1 (LNC) ancillary ligand
and primarily aimed to engage the oxygen donor groups in
coordination (Scheme 5). The first reactions in which the
dimeric precursor [(LNC)Pd(µ-Cl)]2 was cleaved with
9
7
8
1-3
.
3
stoichiometric amounts of ligands 1-3 afforded complexes 7a-c
−
31.8) and the signature of the PF6 anion but were less
conclusive on the role of the CMe2(OH) group. The most
striking feature observed in the IR spectrum of 8c was a sharp,
medium-intensity band at 3524 cm−1, which replaced the
in quantitative yields. However, the subsequent removal of Pd-
bound chloride ligands expected to provide cationic O,P-
chelate species proved less straightforward. While the reaction
between 7b and Ag[SbF6] produced chelate complex 8b
,
broad and composite νOH bands observed in the spectra of
and 7c. In addition, the NMR signals assigned to the CMe2(OH)
moiety shifted when converting 7c into 8c 7c 8c: CMe2 δH
3
similar reactions with 7a and 7c resulted in oxidation
(presumably of the ferrocene ligands) and decomposition.
Replacing Ag[SbF6] with Tl[PF6] circumvented this problem and
(
/
1.46/1.58, δC 31.61/32.16, and CMe2 δC 68.66/72.83) and the
signal due to CMe2 in 8c was split into a doublet, indicating
coordination of the alcohol function, though perhaps
made it possible to isolate stable complexes 8a and 8c
.
Compounds 8a and 8b could be crystallised and were thus
structurally authenticated by X-ray diffraction. The formulation
of 8c was only based on spectroscopic data and on elemental
analysis. Repeated attempts to crystallise 8c failed, except
when using ethyl acetate/hexane, thereby producing a few
1
hemilabile as indicated by broadening of the H and 13C NMR
signals. Here, it must be noted that fully characterized Pd(II)
complexes with coordinated phosphine and alcohol functions
crystals of [(LNC)Pd(AcOEt-κO)(
3-κP)][PF6] (9).
remain extremely rare.
A
search in the Cambridge
Crystallographic Database revealed only one such compound
for which a crystal structure was determined, namely a
Notably, compounds 8a-c could be transformed back to their
precursors 7a-c by adding a chloride source (Scheme 5), which
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(LNC)Pd(II) complex with O,P-chelating monoterpenic
View Article Online
DOI: 10.1039/C9NJ00298G
phosphinoalcohol.34
The crystal structures of 8a, 8b and 9 are shown in Figure 5.
Relevant structural parameters are presented in Table 3.
Among these compounds, 8b crystallized as the solvate
8b·1/4CHCl3 with disordered solvent and with two structurally
independent but practically identical formula units (Figure
S10).
6
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The palladium atoms in 8a, 8b and 9 have identical donor sets
in terms of the donor atoms and the type of the donor
moieties. Their coordination environments are twisted due to
varying Pd-donor distances35 and steric demands of the
chelating ligands. The LNC ligands expectedly form the smallest
interligand angles (82-83°), while the most opened are the
angles associated with the O,P-chelating ligands (97-100°) in
8a and 8b or, alternatively, the P-Pd-C angle in 9, where steric
Scheme 6. Gibbs free energies for the solvolysis of O,P-chelate (LNC)Pd(II) cations
with ligands 1-3
crowding between the phenyl rings occurs.
Chelate coordination significantly distorts the molecules of the
ferrocene ligands in 8a and 8b. The donor substituents are
rotated towards each other albeit without notable tilting of
the ferrocene scaffold (tilt angles < 3°). In addition, the pivotal
C-C(O) bonds are deflected from the planes of their bonding
cyclopentadienyl rings (Table 3) and the C=O bonds are
rotated. No such distortion is observed at the phosphinylated
Conclusion
The results reported in this paper illustrate differences in the
behaviour of structurally analogous phosphinoferrocene
ligands bearing oxygen functional groups. In complexes with
soft Pd(II) ions, the phosphine group serves as the primary
coordination site, while the weaker-binding secondary O-
cyclopentadienyl rings. In contrast, the ferrocene ligand in
9
adopts an open conformation with the substituents located in
roughly anti positions.
donor moieties differentiate the ligands. Ligands
1 and 2
*** Figure 5 near here ***
bearing formyl and acetyl substituents can be easily converted
from the P-monodentate into an O,P-chelating form when a
vacant coordination site is generated at palladium (in the
absence of better donors). These ligands give rise to smaller
and, because of a limited mobility of their conjugated C(O)R
substituents, also more rigid chelate rings, making the chelate
Table 3. Selected distances and angles for 8a, 8b·1/4CHCl3 and 9 (in Å and deg).a
Parameter
8a
8b·1/4CHCl3
(molecule 1)
2.269(1)
2.175(3)
2.004(4)
2.135(3)
98.19(8)
92.0(1)
8b·1/4CHCl3
(molecule 2)
2.266(1)
2.176(3)
2.006(4)
2.141(3)
99.93(9)
91.3(1)
9
Pd-P
Pd-O
2.263(1)
2.153(5)
1.999(6)
2.118(4)
97.2(1)
92.4(2)
82.7(2)
88.8(2)
2.1(4)
2.2745(5)
2.173(1)
2.005(2)
2.139(2)
92.76(4)
95.58(6)
82.75(7)
88.78(6)
4.8(1)
complexes reasonably stable. For ligand
3, an analogous
Pd-C
reaction affords a product whose NMR spectra indicate labile
coordination of the alcohol function. However, alcohols are
harder (less polarisable) donors than aldehydes and ketones
and, therefore, coordinate Pd(II) less willingly. The results from
DFT calculations correspond to experimental observations,
indicating an easy de-coordination of the hydroxyl group.
Overall, the differences in the affinity of donor groups present
Pd-N
P-Pd-O
P-Pd-C
C-Pd-N
N-Pd-O
Cp1,Cp2
τ
81.9(2)
82.3(2)
88.1(1)
86.7(1)
2.6(3)
2.6(3)
54.8(5)
1.233(8)
14.6(4)
60.4(3)
60.3(3)
159.1(2)
1.231(3)c
n.a.
in hybrid phosphine ligands3
1-3 towards palladium(II) may
C=O
C1-C(O)/Cp1b
1.241(6)
12.1(3)
1.240(5)
9.2(3)
result in hemilabile coordination and facilitate the cleavage of
Pd-O bonds by competing donors. This was shown by the
a
b
The parameters are defined as for complexes 5 (see footnote to Table 1).
c
e
reactions of the O,P-chelate complexes
8
with a chloride
and,
Angle between the pivotal C1-C11 bond and the Cp1 plane.
coordinated ethyl acetate.
C=O bond in the
source, which cleanly regenerated chloride complexes
7
previously, also by the reaction of [(LNC)PdCl(Ph2PfcCO2Me-κP)]
with AgClO4 in acetonitrile, which produced the cationic
complex [(LNC)Pd(MeCN-κN)(Ph2PfcCO2Me-κP)]ClO4 instead of
any O,P-chelate.36 Despite lowering the stability of the formed
species, however, the presence of the weakly bonded O-donor
groups can positively affect catalytic properties of the
complexes. In particular, de-coordination of the weaker
binding moiety can open a vacant site for the substrate and
thus assist the catalytic process. Once the catalytic
transformation is over and the product is released from the
DFT computations. Differences in the behaviour of the
complex cations [(LNC)Pd(L)]+ (L =
) were further analysed by
1-3
DFT theoretical calculations at the M06/def-TZVP:sdd(Fe,Pd)
level of theory by modelling the reactions of the cations
[(LNC)Pd(L-κ2O,P)]+ with ethyl acetate. An inspection of the
Gibbs free energies (in both gas phase and chloroform) already
showed that while the conversion of [(LNC)Pd(
[(LNC)Pd( -κP)(AcOEt-κO)]+ is slightly exergonic, similar
processes involving cations [(LNC)Pd(L-κ2O,P)]+, wherein L =
and , are energetically unfavourable (Scheme 6).
3
-κ2O,P)]+ into
3
1
2
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coordination sphere, the secondary donor group can bind
again thereby stabilising the catalytic intermediate.
View Article Online
DOI: 10.1039/C9NJ00298G
34, 1942; g) P. Štěpnička, M. Zábranský and I. Císařová, J.
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14 I. R. Butler and W. R. Cullen, Can. J. Chem., 1983, 61, 147.
15 It is worth noting that compound 2 is an isomer of the
Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This work was supported by the Czech Science Foundation
(project no. 15-11571S). Computational resources were
provided by the CESNET LM2015042 and the CERIT Scientific
Cloud LM2015085 (programme Projects of Large Research,
Development, and Innovations Infrastructures).
Notes and references
Palladium(II):
Phosphorus
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27 Replacement of 4-tolyl substituents (R) in [Pd(µ-Cl)Cl(PR3-
κP)]2 with 2-tolyl substituents induces bending of the planar
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and P. Štěpnička, Dalton Trans., 2015, 44, 3092.
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J. Schulz, T. Klemann, U. Siemeling and I. Císařová,
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921.
,
Figure 3. Views of the complex molecules in the structures of 5a·2CH2Cl2, 5b and 5c
.
Figure 4. Views of the complex molecules in the structures of 6a·2CHCl3,6b·CHCl3 and 6d
.
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Figure 5. View of the complex cations in the structures of 8a, 8b·1/4CHCl3 and 9.
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DOI: 10.1039/C9NJ00298G
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Graphical abstract entry
7
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Coordination of phosphinoferrocene ligands with O-donor substituents to Pd(II)
is affected by the nature of the oxygen functions and may results in hemilabile species.
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