A sterically congested 1,2-diphosphino-1′-boryl-ferrocene: Synthesis, characterization and coordination to platinum

Article abstract of DOI:10.1039/c9dt01921a

A new class of tritopic ferrocene-based ambiphilic compounds has been prepared by assembling diphosphino- and boryl-substituted cyclopentadienides at iron. The presence of five sterically demanding substituents on the ferrocene platform induces conformational constraints, as is apparent from XRD and NMR data, but does not prevent the chelating coordination to platinum. The Lewis acid moiety is pendant in both the free ligand and the platinum complex.

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DOI: 10.1039/C9DT01921A
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ARTICLE TYPE
A sterically congested 1,2–diphosphino–1'–boryl–ferrocene: synthesis,
characterization and coordination to platinum
Emmanuel Lerayer,a Patrice Renaut,a Julien Roger,a Nadine Pirio,a Hélène Cattey,a Paul Fleurat-Lessard,a
Maxime Boudjelel,b Stéphane Massou,c Ghenwa Bouhadir,b Didier Bourissou,*b and Jean-Cyrille Hierso*a,d
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000
A new class of tritopic ferrocene-based ambiphilic compound
has been prepared by assembling diphosphino- and boryl-
phosphino and boryl groups on ferrocene is tedious and
inherently complicated by compatibility/selectivity issues.
substituted cyclopentadienides at iron. The presence of five 50 Reacting pre-functionalized cyclopentadienides with iron
10 sterically demanding substituents on the ferrocene platform
induces conformation constraints, as apparent from XRD and
NMR data, but does not prevent chelating coordination to
platinum. The Lewis acid moiety is pendant in both the free
ligand and the platinum complex.
dichloride is more convergent and might be synthetically
attractive. This assembling methodology proved to be very
efficient for the preparation of polyphosphines such as the 1,2,1'-
triphosphino-ferrocenes H.12 In such tritopic compounds the two
55 bulky t-Bu groups exert conformational control and maintain the
phosphorus atoms in close proximity, resulting in unique
coordinating properties towards transition metals, as well as rare
“through-space” nuclear spin-spin couplings.13 Recently, we
extended this assembling strategy to the preparation of 1,1'-
60 diboryl ferrocenes I (and related cobaltocenes) from boryl-
substituted cyclopentadienides.14,15 These advances prompted us
to test the same approach for the preparation of hitherto unknown
1,2–diphosphino–1'–boryl–ferrocenes.
15 Ambiphilic compounds combine Lewis acids and Lewis bases on
a common platform.1 When strong donor-acceptor interaction
between these antagonist moieties is prevented sterically and/or
geometrically, their juxtaposition opens great opportunities in
small molecule activation and ultimately metal-free catalysis (so-
20 called frustrated Lewis Pair –FLP– catalysis),2 as well as in
coordination chemistry and metal/Lewis acid cooperativity.3
Various scaffolds have been used to build ambiphilic
compounds. In particular, C2 spacers have attracted great interest
due to the proximity they impart to the antagonist sites, while
25 enabling to tailor rigidity (cf. ortho-C6H4, CH=CH, CH2–CH2
bridges). Due to its unique and versatile properties, the ferrocene
platform is also very appealing and over the years, an increasing
variety of ferrocene-based ambiphilic compounds have been
described (Figure 1). Most studies have focused to date on
30 monophosphine-boranes, in which the relative position of the
phosphorus and boron atoms have been varied (see compounds
A-D).4-8 A few tritopic compounds have also been reported.
Landis and co-workers studied early on compounds E in which a
pendant benzoxaborolidine moiety was appended to a 1,1'-
35 diphosphino-ferrocene scaffold.9 Later on, the ferrocene and
ortho-phenylene spacers were combined by Emslie et al. to
prepare the diphosphine-borane F. The structure and reactivity of
ensuing platinum complexes were investigated, providing nice
examples of metal–Lewis acid cooperativity.10
PR2
BMes2
PPh2
R"
Ph
B
Fe
B(C6F5)2
P
Mes2
Fe
Fe
Fe
R"
P
Ph
Fe
BR'2
A
B
C
D
R = Ph, Mes, i-Pr
R'2 = Mes2, i-Pr2, (OMe)2, pin
R" = H, t-Bu
Me
PPh2
PPh2
Bpin
N
B
Ph2
PPh2
B
O
Fe
Fe
Fe
pinB
R
PPh2
P
t-Bu
E
F
G
R = Me, Ph, p-ClC6H4
PPh2
PPh2
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
PR2
PR2
BR2
Fe
Fe
Fe
t-Bu
BR'2
BR2
PR2
J
H
I
40
To take advantage of possible cooperativity between
neighbouring sites we became interested in ambiphilic
compounds featuring three or more functional groups directly
bonded to the ferrocene platform. To our knowledge, only one
such compound was reported to date: the 1-phosphino-1',3'-
R = Mes, Cy
R = Ph, i-Pr
This work
65
Fig. 1 Selected ferrocene-based (di)phosphine-boranes, poly-phosphines
and poly-boranes.
45 diboryl ferrocene G that was prepared by Clark and co-workers
In this context, we report here the preparation and full
via
direct
iridium-catalysed
C–H
borylation
of
characterization of a sterically congested compound J featuring
70 five bulky substituents on a ferrocene. Coordination of this
(diphenylphosphino)ferrocene.11 The stepwise introduction of
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diphosphine-borane to platinum is described. The conformations
of the free (P, P, B)-ambiphilic ligand (Figure 2) and of its
platinum complex are discussed based on X-ray crystallographic
data, along with their multi-nuclear NMR properties, in particular
with respect to “through-space” nuclear spin-spin couplings.
butyl groups. Steric congestion also induces some tilting of the
two Cp rings that are not exactly parallel [tilt ang. = 10.21(13) °].
With this conformation, the three heteroatoms are oriented in
the same direction, opposite to the tert-butyl groups. The two
45 phosphorus atoms are in quite different environments, the one
referred as P2 sitting closer to boron. The P2-C2-Ct1 bond angle
deviates from linearity by ca. 11° (Fig. 3 and Fig. S1)17 but this
distortion does not result in intramolecular P→B interaction. The
boron atom is in trigonal planar environment [sum of bond angles
50 of 359.1(2) °], and the P2…B is long at 4.239(2) Å. This value
falls in the low range of those observed in related
monophosphino-boryl-ferrocenes [P…B distances ranging from
4.106(3) to 6.731(3) Å].4,5 It is also noteworthy that the
orientation of the lone pairs at P1 and P2 differs from that usually
55 encountered in sterically congested 1,2-diphosphino-ferrocenes.12
Indeed, the P lone pairs typically point to each other in a
conformation prone to the chelating coordination of transition
metals, and to the existence of strong through-space JPP coupling
(TSJPP, see below).13 In the diphosphino-boryl-ferrocene 3, the
60 lone pair at P1 points towards P2, while the P2 lone pair is
oriented outwards making an angle of 45° with the P2-B axis.
Given this conformation and the steric congestion of the
ferrocene backbone, we were intrigued about the coordination
properties of the diphosphino-boryl-ferrocene 3. Are the two
65 phosphorus atoms accessible enough and does the framework
retains enough conformational freedom for 3 to behave as a
chelating ligand? To address these questions, 3 was reacted with
PtCl2(COD). The platinum complex 4 was thereby obtained in
quantitative yield (Figure 4). Despite the conformational
70 restrictions imposed by the five sterically demanding substituents,
compound 3 preserves enough conformational flexibility to
enable rotation around the Cp-P2 bond, allowing the chelating
coordination of phosphorus P1/P2 to platinum. The two Cp rings
5
PPh2
t-Bu
t-Bu Fe
PPh2
PPh2
1/ FeCl2
PPh2
t-Bu
Li
t-Bu
BMes2
Li
1
BMes2
2/
3
2
32% isol. yield
Fig. 2 Synthesis of the 1,2-diphosphino-1'-boryl-ferrocene 3 by direct
assembling of functionalized cyclopentadienides at iron.
Solutions of the 4-tert-butyl-1,2-bis(diphenylphosphino)-
10 cyclopentadienide lithium salt 1 (a well-known and useful
precursor for polyphosphino-ferrocenes such as H)12 and 3-tert-
butyl-1-dimesitylborylcyclopentadienide lithium salt 2 (used to
prepare 1,1'-diboryl-metallocenes I)14,15 in tetrahydrofuran were
successively added to FeCl2 at –50°C (Fig. 2). Conditions were
15 varied, but a mixture of ferrocenes was systematically formed.†
Nevertheless, this route gave direct access to the targeted 1,2-
diphosphino-1'-boryl-ferrocene 3 ferrocene in a reasonable 32%
isolated yield. Simple work-up followed by crystallization
afforded 3 as a purple air-stable crystalline powder. The structure
20 of 3 was analysed by X-ray diffraction (Figure 3, racemate,
centrosymmetric space group P21/n).
are quasi perfectly eclipsed in
4
[the dihedral angle
75 P2…Ct1…Ct2…B = –72.14(18) °]. The boron center shifts
further away from P2 [P2…B = 5.294(7) Å] and sits far from the
Pt coordination sphere. The Lewis acid moiety does not interact
with the metal center, as observed in previously reported
diphosphine-borane Pt complexes,10,18 but remains pendant.
t-Bu
t-Bu
t-Bu
BMes2 Ph2P
PPh2
P2
PPh2
Fe
t-Bu
Ph2P
P1
BMes2
Cl
Cl
Pt
Ph2P
Fig. 3 Molecular views of the 1,2-diphosphino-1'-boryl-ferrocene 3 (top
25 and side views, thermal ellipsoids at 50% probability, Ct1 = C1-C2-C3-
C4-C5 and Ct2 = C6-C7-C8-C9-C10 centroids). Hydrogen atoms omitted
for clarity. Selected bond length (Å) and angles, torsions (°): Fe-Ct1 =
1.6645(10); Fe-Ct2 = 1.6816(10); P1-C1 = 1.827(2); P2-C2 = 1.824(2);
B-C6 = 1.545(3); Ct1-Fe-Ct2 = 173.87(5); P1-C1-Ct1 = 179.46(15); P2-
PPh2
PtCl2(COD)
DCM, rt
t-Bu
Mes2B
3
Fe
t-Bu
4
30 C2-Ct1
= 168.56(16); B-C6-Ct2 = 166.42(19); P1-Ct1-Ct2-B = –
120.62(6); P2-<Ct1-Ct2-B = –41.76(6); B…P1 = 6.132(2); B…P2 =
4.239(2). Bottom right: P-lone pairs (green) visualized by ELF analysis.
80 Fig. 4 Synthesis and molecular structure of the 1,2-diphosphino-1'-boryl-
ferrocene platinum dichloride complex 4. Thermal ellipsoids at 50%
probability, same numbering as for 3. Hydrogen atoms and
dichloromethane molecules omitted for clarity. Selected bond length (Å)
and angles, torsions (°): Fe-Ct1 = 1.670(3); Fe-Ct2 = 1.689(3); P1-C1 =
85 1.802(6); P2-C2 = 1.783(6); B-C6=1.562(9); P1-Pt = 2.2185(15) ; P2-Pt =
2.2406(15) ; Pt-Cl1 = 2.3587(15); Pt-Cl2 = 2.3387(14); Ct1-Fe-Ct2 =
177.71(14); P1-C1-Ct1 = 170.6(5); P2-C2-Ct1 = 168.8(5); B-C6-Ct2 =
155.9(6); Cl2-Pt-Cl1 = 88.60(5); Cl1-Pt-P1 = 92.41(5); P1-Pt-P2 =
85.47(5); P2-Pt-Cl2 = 93.21(5); P1-Ct1-Ct2-B = –132.64(18); P2-Ct1-
90 Ct2-B = –72.14(18); B…P1 = 6.555(7); B…P2 = 5.294(7).
The diphosphine-borane 3, bearing five bulky groups on the
35 ferrocene backbone, is among the most hindered
polyfunctionalized ferrocenes reported to date.5,12,16 To optimize
the spatial distribution of the substituents, sterically hindered
ferrocenes classically adopt a staggered12a,c,d or an eclipsed12b
conformation. In compound 3, the two cyclopentadienyl (Cp)
40 rings are about eclipsed with a cisoid arrangement of the two tert-
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The difference in energy between the conformations of 1,2–
diphosphino–1'–boryl–ferrocene in the free ligand 3 and in the Pt
complex 4 was assessed by DFT.† Accordingly, rotating the P2
Such a dynamic phenomenon was reported by Erker et al. for the
1-PMes2-2-(CH2)2B(C6F5)2]-ferrocene D.8a In compound D, the
closed form dominated at low temperature with a 31P NMR signal
lone pair to enable chelation of platinum (conformation 3*, see 55 shifted downfield by ca. 40 ppm compared to the open form.
5
ESI) was found to cost in energy about 2.6 kcal mol-1.
Steric congestion and conformational restraints are likely to
contribute to the broadness of the P2 NMR signal. Indeed,
hindered rotation around the Fc–B bond of 3 was apparent from
Polyphosphino-ferrocenes featuring proximal phosphorus
atoms such as triphosphine H (Fig. 2) display rather strong
nuclear spin couplings operating “through-space” (TS).12,13 This
1
the inequivalence of the mesityl groups at boron in the H NMR
non-classical coupling prompted us to have a closer look at the 60 spectrum (Fig. S4). Comparison with a less sterically congested
10 NMR features of the diphosphine-borane 3 and of its platinum
diphosphino-boryl-ferrocene 3' (Fig. 2) featuring a B(i-Pr)2
instead of BMes2 group, corroborated this view. In this case, a
sharp 31P NMR signal was conversely observed for P2.†
complex 4. The 31P NMR spectrum of diphosphine-borane 3 in
CDCl3 solution exhibits two signals, consistent with two
inequivalent phosphorus atoms, but in a puzzling pattern. As
Since the lone pair of P2 points towards the vicinity of the
depicted in Figure 5, an apparent doublet is found at –23.8 ppm (J 65 boron atom, we wondered about the existence of non-bonding
15 60 Hz), which is attributed to the phosphorus atom P1 by
31P-11B nuclear spin coupling which might also contribute to
broaden the P2 NMR signal.22 In the absence of visible 11B
signal,23 a series of 31P{1H, 11B} spectra were recorded by
varying the 11B decoupling frequency from +100 to –100 ppm
=
comparison with analogous trifunctionalized polyphosphino-
ferrocenes.12 The second phosphorus P2 is characterized by a
broad signal at –18.2 ppm with a width at half-height of 167 Hz.
To confirm the existence of spin coupling, the 31P NMR spectrum 70 (Fig. S5). These decoupling experiments did not result in any
20 was recorded at various frequencies (243, 202 and 121 MHz).
The same value of J = 60 Hz was retained for P1. The signal for
P2 remained broad but the 60 Hz coupling became
distinguishable at low frequency (Fig. 5, bottom).
sharpening of the P2 NMR signal, discarding the hypothesis of
TS 31P-11B spin coupling in 3.
Also noteworthy is the modification of the 31P NMR spectrum
upon coordination of 3 to platinum (Figure 6). Consistent with
75 the crystallographic structure of 4, the eclipsed conformation of
the ferrocene backbone together with the cis-chelation of the two
PPh2 groups to platinum places the BMes2 moiety further away
from P2. Accordingly, complex 4 displays two well-resolved 31P
NMR signals at close chemical shifts (22.0 and 21.6 ppm). The
80 JP1P2 spin-spin coupling constant is very much decreased (12.0 Hz
for 4), as expected from the quaternization of the phosphorus
atoms that no longer allows lone pair-conveyed spin-spin
interaction.13
25 Fig. 5. 31P NMR of the diphosphino-boryl-ferrocene 3 in CDCl3 at 243,
202 and 121 MHz (CDCl3, 300 K).
The magnitude of the JP1P2 coupling is clearly very strong for a
covalent “through-bond” (TB) 3J coupling, suggesting some
30 “through-space” (TS) spin coupling, in line with the relatively
short internuclear distance d(P1…P2) = 3.871(2) Å observed in
the XRD of 3.19 The contribution of a J “through-space”
component was further supported by the solvent-dependence of
the JP1P2 coupling noticed when replacing chloroform for toluene
35 (from 60 to 44 Hz, see Fig. S2). TS couplings have been
authenticated in various types of constrained molecules,13,20
including functionalized ferrocenes, and the pathways for such
spin-spin couplings have been analysed theoretically.21 The JP1P2
coupling displayed by 3 falls in the lower range of those reported
40 for polyphosphino-ferrocenes,12c where couplings as high as 98
Hz have been found,12b while values up to 180 Hz have been
P1
21.6 ppm
P2
22.0 ppm
2JP1P2 = 12.0 Hz
P2
P1
P2 P1
1JP2-Pt = 3654 Hz
1JP1-Pt = 3621 Hz
38 36 34 32 30 28 26 24 22 20 18 16 14 12 10
f1 (ppm)
85 Fig. 6. 31P NMR of the diphosphino-boryl-ferrocene platinum complex 4
in CD2Cl2 (121 MHz, 300 K). Note that the large JPPt couplings of 3621
Hz (P1) and 3654 Hz (P2) are typical of a square-planar PtCl2 complex in
8
6
1
predicted
for
model
compounds
such
as
cis-
cis geometry.24
(H2P)CH=CH(PH2).21a
The origin of the P2 signal broadness was intriguing, and 31P
90
In conclusion, the diphosphine-borane 3, a new type of tritopic
ferrocene, was prepared by reacting functionalized
cyclopentadienides with FeCl2. The presence of five sterically
demanding substituents, including a BMes2 moiety, on the
ferrocene core induces conformational constraints, but does not
45 NMR analysis of 3 in the range 325–220 K (Fig. S3) did not
evidence major changes of the two 31P NMR signals in terms of
TS
chemical shift and
J
coupling (60 ± 3 Hz). The coupling
P1P2
became apparent at P2 below 260 K, but the signal remained
broad (1/2 = 80–170 Hz). This variable-temperature analysis
50 ruled out the hypothesis of an equilibrium between open and
closed forms as the result of an intramolecular P→B interaction.
95 prevent chelating coordination of the two phosphorus atoms to
platinum to give complex 4. The presence of “through-space”
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10 (a) B. E. Cowie and D. J. Emslie, Chem. Eur. J., 2014, 20, 16899; (b)
B. E. Cowie and D. J. H. Emslie, Organometallics 2015, 34, 4093.
11 S. E. Wright, S. Richardson-Solorzano, T. N. Stewart, C. D. Miller, K.
70 C. Morris, C. J. A. Daley and T. B. Clark, Angew. Chem., Int. Ed. 2019,
58, 2834.
12 (a) V. V. Ivanov, J.-C. Hierso, R. Amardeil and P. Meunier,
Organometallics 2006, 25, 989; (b) R. V. Smaliy, M. Beaupérin, H.
Cattey, P. Meunier, J. Roger, H. Doucet, Y. Coppel and J.-C. Hierso,
75 Organometallics 2009, 28, 3152; (c) S. Mom, M. Beaupérin, D. Roy, S.
Royer, R. Amardeil, H. Cattey, H. Doucet and J.-C. Hierso, Inorg. Chem.
2011, 50, 11592; (d) D. Roy, S. Mom, D. Lucas, H. Cattey, J.-C. Hierso
and H. Doucet, Chem.–Eur. J., 2011, 17, 6453.
spin nuclear couplings in these (P, P', B)-functionalized
ferrocenes has been analysed. Future work will seek to generalize
this assembling approach to other densely functionalized
ferrocene-based ambiphilic compounds with the ultimate goal of
triggering reactivity and ligand behaviour thanks to the redox-
properties of the ferrocene moiety.4b,25
This work was supported by the ANR (ANR-11-INTB-1008
MENOLEP), CNRS, Université de Bourgogne, Université de
Toulouse, Conseil Régional de Bourgogne through the plan
10 d’actions régional pour l’innovation (PARI–3MIM and PARI–
CDEA) and the fonds européen de développement regional
(FEDER) programs. Calculations were performed using HPC
resources from DNUM CCUB (Centre de Calcul de l’Université
de Bourgogne).
5
13 J.-C. Hierso, Chem. Rev. 2014, 114, 4838.
80 14 E. Lerayer, P. Renaut, S. Brandès, H. Cattey, P. Fleurat-Lessard, G.
Bouhadir, D. Bourissou and J.-C. Hierso, Inorg. Chem. 2017, 56, 1966.
15 For pioneering work on non-substituted borylcyclopentadienides, see:
G. E. Herberich and A. Fischer, Organometallics 1996, 15, 58.
16 For hexasubstituted polyphosphino- and polyamino-ferrocenes, see:
85 (a) R. V. Smaliy, M. Beaupérin, A. Mielle, P. Richard, H. Cattey, A. N.
Kostyuk and J.-C. Hierso, Eur. J. Inorg. Chem. 2012, 1347; (b) M.
Beaupérin, R. Smaliy, H. Cattey, P. Meunier, J. Ou, P. H. Toy and J.-C.
Hierso, Chem. Commun. 2014, 50, 9505; (c) F. Allouch, N. Dwadnia, N.
V. Vologdin, Y. V. Svyaschenko, H. Cattey, M.-J. Penouilh, J. Roger, D.
90 Naoufal, R. Ben Salem, N. Pirio and J.-C. Hierso, Organometallics 2015,
34, 5015; (d) N. Dwadnia, J. Roger, N. Pirio, H. Cattey, R. Ben Salem
and J.-C. Hierso, Chem. Asian J. 2017, 12, 459.
17 The boron center noticeably deviates out the plane of the Cp ring [B–
C(6)–Ct(2) = 166.42(19)°]. This alleviates the steric congestion induced
95 by the bulky mesityl groups. No intra or intermolecular -stacking
interactions between phenyl and/or mesityl groups were detected in the
solid-state structure.
18 S. Bontemps, M. Sircoglou, G. Bouhadir, H. Puschmann, J. A. K.
Howard, P. W. Dyer, K. Miqueu and D. Bourissou, Chem. Eur. J., 2008,
100 14, 731.
15 Notes and References
a Institut de Chimie Moléculaire de l’Université de Bourgogne (ICMUB-
UMR CNRS 6302), Université de Bourgogne Franche-Comté, 9, avenue
b
Alain Savary, 21078 Dijon, France ;
Laboratoire Hétérochimie
Fondamentale et Appliquée (UMR 5069), Université de Toulouse, CNRS,
c
20 118 route de Narbonne, 31062 Toulouse cedex 9, France. Institut de
Chimie de Toulouse (FR 2599). 118 Route de Narbonne, 31062 Toulouse
d
Cedex 09 (France) ;
Institut Universitaire de France (IUF), 103,
Boulevard Saint Michel, 75005 Paris Cedex, France
†Electronic Supplementary Information (ESI) available: Experimental
25 details and analytical data, NMR data and X-ray crystallographic data.
CCDC 1902394 (3) and 1902395 (4). Computational details, ELF and
AIM analysis, relative energy for conformers 3 and 3* (Figs. S6-S8). For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/x0xx00000x
30
19 For a consistent correlation between “geometric parameters” and
“through-space” nucleus spin-couplings, see: J.-C. Hierso, A. Fihri, V. V.
Ivanov, B. Hanquet, N. Pirio, B. Donnadieu, B. Rebière, R. Amardeil and
P. Meunier, J. Am. Chem. Soc., 2004, 126, 11077 and ref. 13.
105 20 (a) B. E. Cowie, F. A. Tsao and D. J. H. Emslie, Angew. Chem. Int.
Ed., 2015, 54, 2165; (b) V. Rampazzi, J. Roger, R. Amardeil, M.-J.
Penouilh, P. Richard, P. Fleurat-Lessard and J.-C. Hierso, Inorg. Chem.,
2016, 55, 10907; (c) G. Feng, G.; M. P. Conley and R. F. Jordan,
Organometallics, 2014, 33, 4486.
1 (a) G. Bouhadir, A. Amgoune and D. Bourissou, Adv. Organomet.
Chem. 2010, 58, 1; (b) Frustrated Lewis Pairs I and II, Topics in Current
Chemistry, D. W. Stephan and G. Erker Eds.; Springer Heidelberg New
York Dordrecht London, 2013; Vol. 332 and 334; (c) F.–G. Fontaine and
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Dalton Transactions
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A unique tritopic (P, P, B)-ferrocene ambiphile with antagonist DOfuI: 1n0.c10t3i9o/Cn9sDT01921A
directly attached to the metallocene backbone is described.