Exceptional coordination mode of unsaturated PNP ligands (Me 3Si)2C=PN(R)PPh2 with palladium and platinum dichlorides: insertion of phosphaalkene phosphorus atoms into metal-chlorine bonds

Article abstract of DOI:10.1002/ejic.200900236

Unique molecular bis-chelate complexes M[(Me₃Si) ₂C=P(Cl)N(l-Ada)PPh₂]₂ (1-Ada = 1-adamantyl; 3: M = Pt; 4: M = Pd) were isolated as crystalline solids from solutions that contain mixtures of products from, reac

Full text of DOI:10.1002/ejic.200900236

SHORT COMMUNICATION
DOI: 10.1002/ejic.200900236
Exceptional Coordination Mode of Unsaturated PNP Ligands
(Me3Si)2C=PN(R)PPh2 with Palladium and Platinum Dichlorides: Insertion of
Phosphaalkene Phosphorus Atoms into Metal–Chlorine Bonds
Daniela Lungu,[a] Constantin Daniliuc,[a] Peter G. Jones,[a] László Nyulászi,[b]
Zoltán Benkõ,[b] Rainer Bartsch,[a] and Wolf-W. du Mont*[a]
Dedicated to Professor Reinhard Schmutzler on the occasion of his 75th birthday
Keywords: Palladium / Platinum / Insertion / P-Chloroylides / Phosphaalkenes / Density functional calculations
Unique molecular bis-chelate complexes M[(Me3Si)2C=P(Cl)-
N(1-Ada)PPh2]2 (1-Ada = 1-adamantyl; 3: M = Pt; 4: M = Pd)
were isolated as crystalline solids from solutions that contain
mixtures of products from reactions of the metal dichlorides
with the P-phosphanylaminophosphaalkene (Me3Si)2C=PN-
(1-Ada)PPh2 (1). Attachment of chloride ions to PdII- and PtII-
(R,S) isomer 4a was isolated, whereas three of the four pos-
sible isomeric platinum complexes Pt[(Me3Si)2C=P(Cl)N(1-
Ada)PPh2]2 (3a–3c) were investigated crystallographically.
DFT calculations on model compounds indicate that the en-
hanced electrophilicity of the chelating phosphaalkene li-
gand 1 leads to chlorine migration from the metals to the
phosphaalkene phosphorus atom.
coordinated
1 leads to the novel anionic (alkylidene)-
(chlorido)(phosphanylamino)phosphanido ligand [(Me3Si)2-
C=P(Cl)N(1-Ada)PPh2]–, which contains a stereogenic phos-
phorus atom. With PdCl2(COD), only the centrosymmetric
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
Introduction
We are currently exploring properties of “hybrid” P-phos-
phanylaminophosphaalkene ligands (Me3Si)2C=PN(R)-
PRЈ2 (C) with the intention of combining features of cata-
lytically significant, small-bite-angle iminobis(phosphane)
“PNP” ligands RN(PRЈ2)2 (A)[1] with those of more re-
cently developed bidentate phosphaalkenes such as Yoshi-
fuji’s strongly π-accepting 1,2-diaryl-3,4-diphosphanylid-
ene-cyclobutene ligands DCPB-Y (B).[2,3] The electronega-
tive nitrogen substituent is expected to enhance the π-ac-
ceptor properties of the P=C bond in type C ligands.[4–6]
On subjecting this concept to experimental investigation
by reacting type C ligands with halides of nickel(II), palla-
dium(II) and platinum(II), in the case of PdII and PtII we
unexpectedly observed the crystallisation of 2:1 chelate
complexes of the composition MCl2[(Me3Si)2C=PN(R)-
PPh2]2, which proved to contain chlorine atoms covalently
bonded to phosphorus and not to palladium or platinum.[7]
In this paper, we report initial results of the characterisation
of the novel complexes M[(Me3Si)2C=P(Cl)N(1-Ada)-
PPh2]2 (1-Ada = 1-adamantyl; 3: M = Pt; 4: M = Pd), which
are unprecedented in phosphaalkene coordination chemis-
try.[5,6]
Results and Discussion
Synthetic experiments on type C ligands showed that
(Me3Si)2C=PCl fortuitously reacted with lithium imino-
phosphanides Li[Ph2PNR],[8] prepared in situ by the reac-
tion of the aminophosphanes Ph2PN(H)(1-Ada)[7] and
Ph2PN(H)tBu with LDA,[9] to give the type C P-(phos-
phanylamino)phosphaalkenes (Me3Si)2C=PN(1-Ada)PPh2
(1) and (Me3Si)2C=PN(tBu)PPh2 (2),[7] whereas the reac-
tion of the 1-aza-2-phosphaallyl anion [(Me3Si)2C=
PNtBu]–[10,11] with R2PCl led preferentially via P–P bond
formation to the undesired phosphanylphosphorane iso-
[a] Institut für Anorganische und Analytische Chemie der Techn-
ischen Universität Braunschweig,
Hagenring 30, 38106, Braunschweig, Germany
[b] Department of Inorganic and Analytical Chemistry, Budapest
University of Technology and Economics,
Szt Gellert ter 4, 1521, Budapest, Hungary
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.200900236.
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W.-W. du Mont et al.
SHORT COMMUNICATION
Scheme 1.
mers (Me3Si)2C=P(=NtBu)PR2.[10b,11,12] The molecular from usual cationic platinum bis-chelates of the type
structure of solid 1 was determined by X-ray crystallogra- {[PtL4]2+(X–)2}: there are covalent bonds between the phos-
phy.
phorus atoms of the hypothetical cationic moiety [Pt(1)2]2+
After adding a solution of ligand 1 in an aprotic solvent and the two chlorine atoms. The molecular structure of 3a
to an equimolar amount of the PtCl2 cyclooctadiene (COD) is shown in Figure 1. The related 1:1 reaction of ligand 1
complex suspended in dichloromethane, 31P NMR spectra with PdCl2(COD) provided a solution exhibiting two AX
of the orange solution indicate the complete consumption patterns in its 31P NMR spectrum [δ = 86 and 35 ppm,
of 1 [AX-pattern, δ = +373.5, +37.7 ppm, 2J(P,P) = J(P,P) = Ϯ28 Hz; δ = 76 and 32 ppm, J(P,P) = Ϯ38 Hz]
Ϯ12 Hz] and the formation, depending on the reaction con- and a few crystals of the solid pentane-solvated palladium
ditions, of up to three new species exhibiting AX-patterns complex 4a. Complexes 4a and 3a are isotypic.
that all appear in the chemical shift range +60 to +15 ppm
[initially an AX pattern with δ(31P) = 60.5 and 21.8 ppm, J
2
= Ϯ45 Hz, predominates], showing couplings J(P,P) and
1J(PtP). Free COD was detected by 1H NMR spectroscopy.
Sets of further weak 31P NMR signals are assigned to an
A2M2 pattern [δ = +71, +42; 2J(P,P) = Ϯ21 Hz], and tenta-
tively to higher-order multiplets, partially hidden by plati-
num satellites of the stronger signals. The signals do not
appear in the “P=C” range, but rather in a shift range typi-
cal of tetracoordinated phosphorus, suggesting an unusual
bonding mode of the P=C function of 1 with PtCl2. Further
characterisation of the species giving rise to AX-patterns,
which we tentatively assign to isomers of di-µ-Cl-bridged
1:1 complexes (Scheme 1), has not yet been achieved. Crys-
tallisation from pentane/dichloromethane provided a very
small amount of single crystals of 3a, but the major part of
the reaction products remained in the mother liquor, exhib-
iting the above-mentioned AX-patterns in the 31P NMR
spectrum. Redissolved crystals of 3a, however, exhibited in
their 31P NMR spectra the above-mentioned A2M2 pattern
with platinum satellites. This pattern is in accordance with
a trans-configured bis-chelate moiety [Pt(PPЈ)2], as in a 1:2
chelate complex of PtCl2 with ligand 1. When two equiva-
lents of ligand 1 were used to favour the formation of the
1:2 complex, ligand 1 and the coordinated species giving
rise to AX-patterns were consumed within several days ac-
cording to 31P NMR spectra, which showed, inter alia,
Figure 1. Molecular structure of 3a. Selected bond lengths (Å) and
angles (°): P1–Pt 2.3411(5), P2–Pt 2.2900(5), P1–C1 1.673(2), P1–N
1.7617(17), P2–N 1.6945(16), P1–Cl 2.1107(7), P1–N–P2 100.96(9).
Hydrogen atoms are omitted for clarity. Atoms are drawn as 50%
thermal ellipsoids.
The addition of two chloride ions to the phosphaalkene
overlapping signals of multiplets including the A2M2 phosphorus atoms of the hypothetical cationic moiety
pattern of 3a as a major product.
trans-[M(1)2]2+ (M = Pd, Pt) leads formally to two four-
The X-ray crystallographic structure determination con- coordinate stereogenic phosphorus atoms P(C)(N)(Cl)(M)
firmed that the solid 3a is a complex of the composition in the resulting molecular complexes. Complexes 3a and 4a
[PtCl2(1)2] (as a 1:1 pentane solvate) with tetracoordinated consist of centrosymmetric molecules with trans orientation
platinum surrounded by two trans-oriented P, P Ј-chelating of the two types of phosphorus atoms around palladium
ligands in a square-planar fashion, but is uniquely different and platinum; the chlorine atoms are anti oriented, one
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Phosphaalkene P Atom Insertion into Metal–Chlorine Bonds
Scheme 2. Relative energies of complexes related to isomers 3a–3d (R = Me3Si) for ligand 1Ј and for 1ЈЈ (in boldface) [(H3Si)2C=PN(Me)-
PRЈ2, RЈ: Me (1Ј), Ph (1ЈЈ)] in kcalmol–1 at the B3LYP/B//B3LYP/A level of theory (see ref.[15]).
above and one below the square plane (R,S). The alterna-
Under the criterion of bond lengths, the P=C(SiMe3)
tive (R)- or (S) configurations at the phosphorus atom, to- groups of 1 remain unaffected upon coordination [ligand 1:
gether with the intrinsic possibility of cis- or trans-P,PЈ-che- P=C 1.665(1) Å; complex 3a: P+–C– 1.673(2) Å; complex
late ligand orientations around square-planar platinum or 4a: P+–C– 1.667(3) Å]. The bond from the ylide phosphorus
palladium, mean that up to four stereoisomers 3a–3d (or to nitrogen, however, is lengthened to 1.7618(17) Å (3a) and
4a–4d) are possible (Scheme 2).
1.7618(17) Å (4a), relative to 1.746(1) Å in ligand 1, con-
Variation of the reaction conditions and of the solvent comitant with a shortening of the other P–N bond lengths
mixtures for crystallisation allowed us to isolate a few single [Ph2P–N; 1: 1.724(1); 3a: 1.6942(16) Å; 4a: 1.695(2) Å]
crystals of two further isomers, 3b and 3c (Scheme 2), of the upon coordination. The strong coordination of the Ph2P
platinum complex 3. Complex 3b (from CH2Cl2/CH3CN) is group to the metals [Pt–P 2.2900(5) Å; Pd–P 2.2996(8) Å]
another trans complex, but with syn orientation of the Cl will enhance the electron-withdrawing capacity of nitrogen
atoms, which leads to a chiral molecule that has no imposed towards the adjacent P=C moiety, which thereby becomes
symmetry. The insertion of the carbenoid phosphorus atom so Lewis acidic that phosphorus “satisfies its needs” by co-
into the Pt–Cl bonds must have involved an attack of the ordination with the chloride ion as a Lewis base. This oc-
two chlorine nucleophiles at the two electrophilic phospho- curs at the expense of P(ylide)–N and M–P(ylide) bond
rus atoms from the same side of the square plane. Complex strengths [3a: d(Pt–P) = 2.3410(5) Å; 4a: d(Pd–P) =
3c (from THF/pentane) contains the chelate ligands with a 2.3460(8) Å].
cis arrangement of the PPh2 groups and of the P=C func-
In order to understand the remarkable electrophilicity of
tions; one of its chlorine atoms is above and the other is type C PNP ligands, model calculations based on density
below the square plane (anti), i.e. the complex displays ap- functional theory have been performed: the electrophilicity
proximate C2-symmetry. Solid 3b and 3c exist as racemic index (ω) was obtained from vertical ionisation energies
mixtures of (R,R) and (S,S) enantiomers.
and electron affinities calculated at the (U)B3LYP/6-
Bond lengths and angles of 3b and 3c show no significant 311+G**//B3LYP/6-31+G* level of theory[15b] by using the
differences from those of 3a. In all three isomers, each bi- original definition reported by Parr et al.[16] The electrophi-
dentate ligand 1 unit has acquired one chlorine atom licity indices (ω) and the energies of the π*(C=P) antibond-
bonded to a (formerly) phosphaalkene phosphorus atom. ing orbitals (ε, these are the LUMOs, except for the Pt com-
This phosphorus atom acts as a σ-donor towards platinum plexes, where they are the LUMO+1s) of some compounds
and as a σ-acceptor towards the chlorine atom, i.e. the pho- are shown in Scheme 3.
sphaalkene phosphorus atom has inserted in a carbene-like
On the basis of these results, the known strong electron-
fashion into the Pt–Cl bond. Carbene-like behaviour of un- acceptor behaviour of the π*(C=P) unit[4] is remarkably in-
charged two-coordinate phosphorus is common in the creased by the silyl groups (the LUMO energy drops). The
chemistry of iminophosphanes RP=NRЈ, but it is unusual effect is enhanced by attaching phenyl groups on the σ3,λ3-
in phosphaalkene chemistry.[6] The 1,1-addition of M–Cl P instead of methyl groups. Further remarkable enhance-
units to an uncharged two-coordinate phosphorus atom is ment of the electrophilicity is achieved for the entire ligand
without precedence. The formation of tetracoordinated by the metal complexation. It is interesting to note that the
phosphonium centres as parts of P-chloro-P-metallaylide electrophilicity of the Pt-complexed ligands surpasses that
functions (N)(Cl)(M)P(+)–C(–)(SiMe3)2 in 3a–3c and 4a by of some typical electrophiles such as formaldehyde [H2CO,
M–Cl addition to two-coordinate phosphorus can be con- ω = 1.127 eV, (U)B3LYP/6-311+G**], borane [BH3, ω =
sidered as the reverse reaction of 1,1-eliminations on or- 1.693 eV, (U)B3LYP/6-311+G**] and boron trifluoride
ganic P-chloroylides R2P(Cl)=CHRЈ leading to phos- [BF3, ω = 1.654 eV, (U)B3LYP/6-311+G**].[17]
phaalkenes RP=CHRЈ. Lewis acids are known to catalyse
In addition to the electrophilicity indices, the Fukui func-
such eliminations by the formation of cationic species tions[18] can be applied[19] to understand the site reactivity
[R2P=CHRЈ]+.[13,14]
of molecules. The Fukui function for nucleophilic attack
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W.-W. du Mont et al.
SHORT COMMUNICATION
i.e. anionotropy offers an alternative to the concept of hemi-
labile ligands. Anionotropic σ-acceptor mechanisms in-
volving metallaylides with reactive P–Cl bonds should also
be taken into consideration for explaining some peculiar
observations in the coordination chemistry of low-coordi-
nated phosphorus ligands,[5b,20–23] which have hitherto been
ascribed to their excellent π-accepting capacity.
Experimental Section
1: Lithium diisopropylamide (1.65 mL 2  in THF, 3.3 mmol) was
added dropwise at –40 °C to (1-adamantylamino)diphenylphos-
phane (1.1 g, 3.3 mmol) dissolved in dry THF (30 mL). The reac-
tion mixture was allowed to reach room temperature and stirred
for about 2 h. After removal of all volatiles in vacuo, the residue
was dissolved in THF (30 mL), and this solution was added drop-
wise to a stirred solution of (Me3Si)2C=PCl (1.07 g, 3.3 mmol) in
THF (30 mL) at –40 °C. The mixture was warmed slowly to room
temperature and stirred for 2 h, the solvent was pumped off, and
the residue was redissolved in pentane. After removal of LiCl by
filtration, pentane was evaporated to leave behind a brown oil.
Crystallisation from THF/pentane (at room temp.) afforded 1 as
pale yellow crystals. Yield: 95% (1.6 g); m.p. 105 °C. C29H43NP2Si2
(523.78): calcd. C 66.5, H 8.27, N 2.67; found C 65.8, H 8.33, N
2.85. MS (EI): m/z (%) = 523 (4) [M]+, 450 (100) [M – SiMe3]+,
335 (15) [M – C=P(SiMe3)2]+, 135 (16) [M – (SiMe3)2C=PN-
PPh2]+. 1H NMR (200.1 MHz, C6D6): δ = 7.7–7.6 (m, CH, Ph),
7.08–6.9 (m, CH, Ph), 2.3 (br. s, 6 H, 3 CH2, Ada), 1.9 (br. s, 3 H,
Scheme 3. Electrophilicity indices (ω, in eV, B3LYP/6-311+G**//
B3LYP/6-31+G*[15b]) and the π*C=P Kohn–Sham orbital energies
(ε, in eV, B3LYP/6-31G*//B3LYP/6-31+G*) for various PNP model
compounds.
[f+(r), r is the vector of the point of interest] was calculated
as the electron density difference between the anion formed
by the addition of an electron to the ligand and the neutral
molecule. This density distribution for a model ligand is
shown in Figure 2, and as can be seen, the nucleophilic at-
tack occurs at the P=C bond, especially at the dicoordi-
nated phosphorus.
4
3 CH, Ada), 1.5 (br. s, 6 H, 3 CH2, Ada), 0.2 (d, JH,P = 3 Hz,
SiMe3), 0.0 (s, SiMe3) ppm. 13C NMR (50.3 MHz, C6D6): δ = 190.5
[d, 1J(C,P) = 98.8 Hz, C=P], 139.7 (d, 1JC,P = 17.7 Hz, C-P), 135.0
3
(m, CH, Ph), 128.6 (s, CH, Ph), 128.1 (d, JC,P = 6.5 Hz, CH, Ph),
2
3
60.6 (d, JC,P = 22.7 Hz, C-P, Ada), 46.2 (d, JC,P = 11.5 Hz, 3
3
CH2, Ada), 36.6 (s, 3 CH2, Ada), 31 (s, 3 CH, Ada), 3.6 (d, JC,P
= 17.18 Hz, SiMe3), 2.9 (d, JC,P = 3.5 Hz, SiMe3) ppm. 31P NMR
3
(80.1 MHz, C6D6): δ = 373.5 (d, 2JP,P = 12 Hz, P=C), 37.7 (d, 2JP,P
= 12 Hz, PPh2) ppm.
3a: A solution of 1 (0.77 g, 1.47 mmol) in dichloromethane (20 mL)
was added dropwise (2 hours) to a solution of Pt(COD)Cl2
(0.275 g, 0.73 mmol) in dichloromethane (20 mL). The red solution
was stirred for a further 5 d. The solution was concentrated to
about 20 mL under reduced pressure, and the product was precipi-
tated as an orange solid by addition of pentane (40 mL). After
Figure 2. Representation of the Fukui function for nucleophilic at-
tack [f+(r)] in the case of (H3Si)2C=PN(Me)PMe2 calculated at the
(U)B3LYP/6-311+G**//B3LYP/6-31+G* level of theory.
We are currently investigating whether the AX patterns
in the 31P NMR spectra of solutions delivering solid 3a–3c washing with pentane (40 mL) and drying in vacuo, an orange
powder was obtained. Crystallisation from dichloromethane/pen-
tane at room temperature provided a small crop of orange single
crystals of 3a. C58H86Cl2N2P4PtSi4 (1313.57): calcd. C 53.03, H
6.60, N 2.13; found C 51.48, H 6.99, N 2.04. 31P NMR (80.1 MHz,
and 4a are attributable to mixtures of diastereomers of the
chloride-bridged dimers, and further DFT calculations are
being carried out in order to comprehend the mechanism
of the P atom insertion and the formation of complexes.
To understand and to control the migration of anionic
ligands from the metal to unsaturated PNP ligands (“anion-
otropy”), we are presently engaged in tuning type C ligands.
Current experiments show that ligand 2 behaves just like
ligand 1, but, by using the N-supermesityl-bridged ligand
(Me3Si)2C=PN(2,4,6-tBu3C6H2)PPh2, 31P NMR spectro-
scopic data of the PtCl2 complex are in agreement with a
“normal” 1:1 chelate.[7] When anion migration in 1:1 com-
plexes of type C ligands with d8 metal ions can be made
reversible by appropriate choice of metal salts and ligands,
chelating unsaturated type C PNP ligands may create vac-
ant coordination sites for coordination of substrates by re-
versibly hosting the neighbouring anions (such as chloride),
2
1
CD2Cl2) (trans/anti isomer 3a): δ = 71.3 (t, JP,P = 21, JP,Pt
2112 Hz), 41.9 (t, JP,P = 21, JP,Pt = 2940 Hz) ppm.
=
2
1
CCDC-731434 (1), -731435 (3a), -731436 (3b), -731437 (3c) and
-731438 (4) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): Total energies (in hartrees) and geometrical parameters in Car-
tesian coordinates (in Å) calculated at the B3LYP/6-31+G* level of
theory for selected compounds.
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Received: March 12, 2009
Published Online: June 3, 2009
Eur. J. Inorg. Chem. 2009, 2901–2905
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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