Non-symmetric diphosphines based on the imidazole scaffold: An unusual group interchange involving Pd-CH3 and (imidazole)P-Ph cleavage

Article abstract of DOI:10.1039/c3dt53025f

Two regioisomeric, non-symmetric P^C2P^N-imidazoles, t-Bu₂PNCH=CHNC(PPh₂) (L1, P^C2 = PPh ₂, P^N = P(t-Bu)₂) and Ph₂PNCH= CHNC[P(t-Bu)₂] (L2, P^C

Full text of DOI:10.1039/c3dt53025f

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Non-symmetric diphosphines based on the
imidazole scaffold: an unusual group interchange
involving Pd–CH3 and (imidazole)P–Ph cleavage†
Pengfei Ai,a Andreas A. Danopoulos*a,b and Pierre Braunstein*a
Cite this: Dalton Trans., 2014, 43,
1957
Received 26th October 2013,
Accepted 25th November 2013
DOI: 10.1039/c3dt53025f
www.rsc.org/dalton
Two
regioisomeric,
non-symmetric
PPh2, PN
PC2PN-imidazoles,
P(t-Bu)2) and
t-
Ligands of the type 1-(di-t-butyl- or -aryl-phosphino)-imida-
zole, -imidazolium, and 1-(di-t-butylphosphino)-N-heterocyclic
carbene (NHC), with a P–N covalent bond, belong to the broad
class of aminophosphines10 and have only recently become
available,11 attracting interest as ligands and as intermediates
for the synthesis of imidazolium salts and NHCs.11b,d,12
Due to our long-standing efforts in the chemistry of
ligands with P–N bonds13 we set out to study the
chemistry of the chelating regioisomeric diphosphines
Bu2PNCHvCHNC(PPh2) (L1, PC2
=
=
Ph2PNCHvCHNC[P(t-Bu)2] (L2, PC2 = P(t-Bu)2, PN = PPh2), respect-
ively, show dramatic differences in the reactivity of the N-bound
phosphine group; the L2 isomer is extremely sensitive to P–N
bond cleavage by nucleophiles, and when coordinated to the PdCl
(Me) fragment it undergoes facile interchange of one PN phenyl
with the methyl originating from Pd.
Functional phosphine ligands of the type PRn(Het)3−n, R =
alkyl or aryl, Het = aza-heteroaryl, n = 0, 1, 2, are well studied
for Het = m-pyridyl1 (m = 2, 3, 4), but less so with other N-het-
eroaryls. PRn(m-pyridyl)3−n were used as ligands for the Pd-cat-
alysed alkoxycarbonylation of propyne.2 More recently 2-(di-
alkyl- or -aryl-phosphino)-1R-imidazole ligands (R = H, alkyl or
aryl) were employed for the hydration of alkynes,3 the isomeri-
sation of alkenes,4 and for carbonylative cross-coupling reac-
tions;5 in the cases reported, catalytic performances were
superior compared to non-heteroaryl analogues. Attempts to
gain insight into the role of the N-heteroaryl group have
pointed to its ability to be involved in the formation of small
bite angle (P, N)-chelates with potential hemilability,6 in intra-
molecular or intermolecular hydrogen bonding and to provide
a basic site facilitating proton transfer during catalysis.3c,7
Information on the donor characteristics of 2-(di-alkyl- or -aryl-
phosphino)-1R-imidazoles is scarce but supports similarities
(based on the electronic Tolman parameter) to analogous
PR2Ph ligands.8 Recently, complexes with the chelating flexible
1,2-bis-(2-diphenylphosphino-imidazolyl)-benzene and 1,2-bis-
t-Bu2PNCHvCHNC(PPh2) (L1) and Ph2PNCHvCHNC-[P(t-Bu)2]
(L2) shown in Scheme 1, which result from a swap of the PPh2
and P(t-Bu)2 donors between the 1- and 2-positions of the het-
erocycle. Such ligands offer a platform to explore rigid, chelat-
ing imidazole-based diphosphines, with one P–N and one P–C
bond and thus one less donating, more π-acidic P donor and a
more donating, less π-acidic P donor, respectively. There are two
previous reports on bidentate PC2PN-imidazoles11b,14 formed as
undesired products from the coordination of 2-dimethyl-
phosphanyl-imidazole on a W(0) carbonyl centre and the synthesis
of N-phosphanyl-NHCs. More recently, the synthesis of the bis-
(di-t-butyl) analogue of L1 and L2 has been reported.15
The high yielding and rational routes a and b (Scheme 1)
can provide L1 and L2 in gram quantities and are based on the
(2-diphenylphosphino-imidazolium)-benzene
reported.9
have
been
aLaboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS),
Université de Strasbourg, 4 rue Blaise Pascal, 67081 Strasbourg Cedex, France.
E-mail: danopoulos@unistra.fr, braunstein@unistra.fr
bInstitute for Advanced Study, USIAS, Université de Strasbourg, France
†Electronic supplementary information (ESI) available: Experimental details and
full characterisation of all compounds; crystal structure data for L1, L2, 1b, 2a,
2b and 2c. CCDC 968367–968372. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c3dt53025f
Scheme 1 The ligands L1 and L2 and their synthesis.
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Fig. 1 Thermal ellipsoid representation (30% probability level) of the structure of L1 (left) and L2 (right). Selected bond lengths (Å) and angles [°]:
for L1: P1–N1 1.763(1), N1–C1 1.389(2), C1–P2 1.828(1), C1–N2 1.321(2); N2–C1–N1 111.6(1), N2–C1–P2 126.1(1), N1–C1–P2 122.16(9), C1–N1–P1
122.17(9). For L2: P1–C1 1.828(3), C1–N1 1.321(3); C1–N2 1.396(4); N2–P2 1.751(2); N1–C1–N2 110.5(2), N1–C1–P1 128.4(2), N2–C1–P1 121.2(2),
C1–N2–P2 125.6(2).
reaction of PPh2Cl with C2 lithiated 1-(di-tert-butylphosphino)-
imidazole and 2-(di-tert-butylphosphino)-imidazole. An in-
direct, less convenient formation of L1 has recently been descri-
bed.11a Interestingly, attempted preparation of L2 by
deprotonation of 1-(diphenylphosphino)imidazole with n-BuLi
(in a sequence analogous to route a) led to the cleavage of the
Ph2P–Nimid bond and the formation of Ph2P(n-Bu) (identified
by 31P NMR spectroscopy: δ −16 ppm). This demonstrated the
weakness of the Ph2P–Nimid bond (compared to (t-Bu)2P–Nimid
)
and its susceptibility to the presence of strong nucleophiles.
Ligand L1 is stable in air, while L2 is very sensitive to both
water and oxygen. Their different behaviour is not mirrored by
major structural differences (e.g. P–N bond in L1 (1.763(1) Å)
and L2 (1.751(2) Å) (see Fig. 1).
Preliminary comparative studies of the coordination chem-
istry of L1 and L2 gave some unexpected results (see
Scheme 2).
All characterisation data point to the retaining of the inte-
grity of the basic ligand framework after complexation both in
solution and in the solid state (Fig. 2–4). The Pd centre in 2a
shows a typical distorted square planar coordination geometry;
Scheme 2 Synthesis of palladium complexes 1a/b and 2a/b/c. Reaction
conditions: (i) [PdCl2(cod)] or [PdCl(Me)(cod)], THF; (ii) THF or CH2Cl2,
room temperature, quantitative after 5 days.
the ligand bite angle is 89.04(1)°. Slight shortening of the complicated for 2b due to a rearrangement reaction described
Ph2P–N bond and reduction of the P2–N1–C1 angle are notice- below. Therefore, crystallisation of 2b had to be carried out at
able on coordination. There are significant differences −38 °C in the glove box. However, after successful isolation,
between the two Pd–P bond distances, [Pd–PPh2 (2.2070(4) vs. the solids 1b and 2b are air-stable. The structures of 1b and 2b
Pd–P(t-Bu)2) (2.2850(4) Å], but not between the two Pd–Cl are shown in Fig. 3 and 4.
bonds.
The coordination geometry around the Pd centre in both
Reaction of L1 and L2 with [PdCl(Me)(cod)] gave complexes complexes is distorted square planar; the ligand bite angles
1b and 2b (see ESI†). The two doublets in 31P NMR at δ 114.9 are 88.62(2)° and 89.41(3)°, respectively. In both cases, the
2+3
2+3
(d,
J
= 37.2 Hz, P(t-Bu)2) and 27.4 (d,
J
PP
= 37.2 Hz, chloride is located trans to the PPh2 group with Pd–Cl bond
PP
2+3
2+3
PPh2), 85.9 (d,
J
PP
= 35.4 Hz, PPh2) and 41.4 (d,
J
PP
=
distances of 2.3792(7) and 2.369(1) Å and the Pd–CH3 bond
35.4 Hz, P(t-Bu)2), respectively, in combination with the two distances of 2.091(2) and 2.121(3) Å, respectively. There are sig-
doublets assignable to the Pd–CH3, in the 1H NMR spectrum due nificant differences between the two Pd–P bond lengths in
3
to J-coupling of the methyl group protons with the P atoms are each structure, [Pd–P(t-Bu)2 2.361(1) Å and Pd–PPh2 2.1808(9) Å
diagnostic for complex formation. Attempts to obtain X-ray for 2b; Pd–P(t-Bu)2 2.2072(6) Å and Pd–PPh2 2.3632(6) Å
quality crystals were straightforward for 1b but were for 1b].
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Fig. 2 Thermal ellipsoid representation (30% probability level) of the
structure of 2a. Selected bond lengths (Å) and angles [°]: Pd1–P1
2.2850(4), Pd1–P2 2.2070(4), Pd1–Cl1 2.3507(3), Pd1–Cl2 2.3634(4);
P1–C1 1.831(2), C1–N1 1.378(2), C1–N2 1.316(2), N1–P2 1.721(1);
Cl1–Pd1–Cl2 91.12(2), P1–Pd1–P2 89.04(1), P1–Pd1–Cl1 97.56(2),
P2–Pd1–Cl1 173.34(2), P1–Pd1–Cl2 171.29(2), P2–Pd1–Cl2 82.28(1),
N1–C1–N2 111.7(1), N1–C1–P1 117.7(1), N2–C1–P1 130.5(1), C1–N1–P2
121.9(1), C1–P1–Pd1 103.66(5), N1–P2–Pd1 107.26(4).
Fig. 4 Structure of 2b. Selected bond lengths (Å) and angles [°]:
Pd1–C24 2.121(3), Pd1–Cl1 2.369(1), Pd1–P1 2.361(1), Pd1–P2 2.1808(9),
C1–N2 1.324(5), C1–N1 1.377(5), C1–P1 1.824(4), N1–P2 1.736(3);
N2–C1–N1 111.4(3), N2–C1–P1 130.1(3), N1–C1–P1 118.5(3), C1–N1–P2
123.0(3), C1–P1–Pd1 101.8(1), N1–P2–Pd1 107.2(1), C24–Pd1–P2
84.5(1), C24–Pd1–P1 173.9(1), P2–Pd1–P1 89.41(3), C24–Pd1–Cl1
88.7(1), P2–Pd1–Cl1 172.78(4), P1–Pd1–Cl1 97.33(4).
assignable to the CH3 and the appearance of one doublet at δ
2
2.04 (d, JPH = 9.7 Hz). The structure of the molecule is given
in Fig. 5.
In 2c the Pd is adopting a square planar geometry (ligand
bite angle, 88.81(4)°). The chloride is still trans to P-phenyl
and the Pd–Cl bond is longer compared to 2b. The Pd–P
bonds in 2c are longer than those in 2b. There is no significant
difference between the Nimid–PPhMe and Nimid–PPh2 bond
lengths.
Although the electronic characteristics of the PN and PC are
not precisely known, it is reasonable to assume that the PC-
(t-Bu)2 is the strongest donor in the systems studied, and there-
fore should weaken in 2b the Pd–Me bond that is trans to it;
the rearrangement results in positioning the Ph (with stronger
Pd–Caryl) trans to the PC(tBu2). It also places the electron releas-
ing Me on the electron deficient (and therefore electrophilic)
PN centre. A relevant rearrangement occurring in a Rh-methyl
phosphine complex has been recently described,16 and the
implications of P–C/Pd–C bond cleavage/formation for homo-
geneous catalysis have been emphasised.17 Recently the
mechanistic diversity of the transition metal-mediated P–C/X
exchange has been reviewed.18 From the mechanistic scenario
proposed, the intramolecular nucleophilic attack on the elec-
trophilic PN is plausible with the current ligand system. Our
Fig. 3 Structure of 1b. Selected bond lengths (Å) and angles [°]:
Pd1–C24 2.091(2), Pd1–Cl1 2.3792(7), Pd1–P1 2.2072(6), Pd1–P2 2.3632(6),
C1–N2 1.317(3), C1–N1 1.378(3), C1–P1 1.817(2), N1–P2 1.749(2);
N2–C1–N1 112.7(2), N2–C1–P1 127.4(2), N1–C1–P1 119.8(2), C1–N1–P2
121.4(2), C1–P1–Pd1 106.36(8), N1–P2–Pd1 103.72(7), C24–Pd1–P1
88.73(8), C24–Pd1–P2 172.21(8), P1–Pd1–P2 88.62(2), C24–Pd1–Cl1
86.61(8), P1–Pd1–Cl1 172.57(3), P2–Pd1–Cl1 96.73(2).
Solutions of 2b in THF or CH2Cl2 undergo a facile experimental observations on Pd–Me/P–Ph interchange may
rearrangement (t1/2 ∼ 2 days at room temperature), in which have relevance to reaction pathways or catalysts deactivation
the methyl group bound to Pd exchanges with one of the Ph in e.g. cross-coupling reactions.
groups in PPh2 to give the new complex 2c cleanly and quanti-
A. A. D. thanks the CNRS for support, the Région Alsace,
tatively after 5 days (Scheme 2). This could be confirmed by the Département du Bas-Rhin and the Communauté Urbaine
the appearance in the 31P NMR of two new doublets at δ 72.1 de Strasbourg for a Gutenberg Excellence Chair (2010–2011)
(d, 2+3JPP = 35.3 Hz) and 42.0 (d, 2+3JPP = 35.3 Hz) and in the 1H and USIAS for a fellowship. We thank the CNRS, the MESR
NMR the disappearance of the original two doublets (Paris), the UdS, the China Scholarship Council (PhD grant to
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Pd1–Cl1 2.380(1), Pd1–P1 2.378(1), Pd1–P2 2.198(1), C1–N1 1.323(5),
C1–N2 1.375(5), C1–P1 1.820(4), C12–P2 1.802(4), N2–P2 1.729(4);
N1–C1 N2 111.7(4), N1–C1–P1 130.6(4), N2–C1–P1 117.7(3), C1–N2–P2
124.0(3), C1–P1–Pd1 102.1(2), N2–P2–C12 103.7(2), N2–P2–Pd1 106.9(1),
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