Bifunctional Indenyl-Derived Receptors for Fluoride Chelation and Detection

Article abstract of DOI:10.1002/chem.201501547

Anion receptors based on a [CpFe(indenyl)] scaffold offer the possibility for the incorporation of adjacent Lewis acidic functions onto a six-membered carbocyclic framework, while at the same time retaining the colorimetric/electrochemical reporter mechanisms available to synthetically simpler ferrocene systems. Thus, [CpFe(indenyl)] systems featuring mutually ortho BMes₂ and PPh₂Me⁺ substituents (with either 4,5 or 5,6 regiochemistry) are accessible which are capable of cooperative fluoride ion fixation. Simultaneous binding at the borane and phosphonium centres can be established by spectroscopic, structural and computational approaches, and is responsible for the favourable thermodynamics associated with F^- uptake. Thus, in contrast to simple BMes₂ systems, the binding of fluoride is found to be more favourable than the uptake of cyanide (which interacts only with the borane Lewis acid). Moreover, in the case of a 4-(MePh₂P)-5-(Mes₂B)-7-Me-indenyl derivative, fluoride chelation is signalled not only by a large cathodic shift in the Fe^II/Fe^III potential (>500 mV in THF), but also by a distinct colour change from green (for the free receptor) to maroon for the adduct.

Full text of DOI:10.1002/chem.201501547

DOI: 10.1002/chem.201501547
Full Paper
&
Anion Receptors
Bifunctional Indenyl-Derived Receptors for Fluoride Chelation and
Detection
RØmi Tirfoin, Joseph A. B. Abdalla, and Simon Aldridge*[a]
Abstract: Anion receptors based on a [CpFe(indenyl)] scaf-
fold offer the possibility for the incorporation of adjacent
Lewis acidic functions onto a six-membered carbocyclic
framework, while at the same time retaining the colorimet-
ric/electrochemical reporter mechanisms available to syn-
thetically simpler ferrocene systems. Thus, [CpFe(indenyl)]
systems featuring mutually ortho BMes2 and PPh2Me+ sub-
stituents (with either 4,5 or 5,6 regiochemistry) are accessible
which are capable of cooperative fluoride ion fixation. Simul-
taneous binding at the borane and phosphonium centres
can be established by spectroscopic, structural and compu-
tational approaches, and is responsible for the favourable
thermodynamics associated with FÀ uptake. Thus, in contrast
to simple BMes2 systems, the binding of fluoride is found to
be more favourable than the uptake of cyanide (which inter-
acts only with the borane Lewis acid). Moreover, in the case
of a 4-(MePh2P)-5-(Mes2B)-7-Me-indenyl derivative, fluoride
chelation is signalled not only by a large cathodic shift in
the FeII/FeIII potential (>500 mV in THF), but also by a distinct
colour change from green (for the free receptor) to maroon
for the adduct.
Introduction
Such an approach, however, cannot easily be realised for
1,2-ferrocenediyl systems;[12,13] the geometric constraints of the
five-membered ring backbone in systems such as VII enforce
a B–B separation (ca. 3.68 Š) that is too wide to accommodate
fluoride in a bridging position.[12c,d] Given the value of the fer-
rocene unit as a robust electrochemical or colorimetric reporter
of anion binding events,[14] strategies have therefore been de-
veloped to circumvent this problem, including the incorpora-
tion of secondary Lewis acids in the 2-position, which feature
intrinsically longer EÀF bonds, consistent with fluoride chela-
tion (such as the silyl groups used in VIII and IX).[12d,15]
The realisation of receptor design strategies for the selective
binding of anions and neutral molecules is a critical step in the
development of detection protocols for such analytes.[1] The
detection, and ultimately sensing, of anions such as fluoride
has clear-cut societal benefits derived, for example, from moni-
toring drinking water quality, and from the detection of certain
classes of chemical warfare agent.[2] Within the field of receptor
design, Lewis acid systems featuring the chemically robust
BMes2 binding domain have recently been the subject of sig-
nificant research effort;[3] simple systems of this type, however,
typically suffer from competing responses from other small,
hard anions, such as cyanide.[3–6] To overcome such problems
of selectivity, the development of pre-organised arrays featur-
ing more than one binding site (with appropriate shape and
size) could be perceived as being beneficial.
However, given the widespread versatility of systems based
on a 1,2-disubstituted benzene backbone, in particular with re-
spect to the variety of Lewis acid combinations which can
A number of bifunctional boranes capable of chelating the
fluoride anion have been reported in the literature,[7–11] includ-
ing the 1,8-naphthalenediyl systems I–IV+,[7] developed origi-
nally by Katz and more recently by Gabba¨ı, and Piers’ highly
electron-deficient 1,2-diborylated benzene V,[8] together with
organometallic systems such as the cobaltocenium bis-boryl
complex VI, reported by Herberich and co-workers.[9]
[a] Dr. R. Tirfoin, Dr. J. A. B. Abdalla, Prof. S. Aldridge
Department of Chemistry, University of Oxford
Inorganic Chemistry Laboratory
South Parks Road, Oxford, OX1 3QR (UK)
Web: http://users.ox.ac.uk/~quee1989/
E-mail: Simon.Aldridge@chem.ox.ac.uk
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501547.
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readily be incorporated,[5j,16] a potentially more powerful strat-
egy involves the combination of ortho-phenylene scaffolds of
this type with a metallocene reporter. One way to accomplish
this, which offers conjugated electronic communication be-
tween binding and reporter functions, is through the develop-
ment of indenyl receptors functionalised at the 4–7 positions
(Figure 1). Thus, herein we report strategies for the synthesis of
systems of types X and XI, together with fluoride/cyanide
binding studies of two such systems for which
ERn =PPh2Me+.[17]
Scheme 1. Syntheses of dibromoindanone precursors 7a–9a. Key reagents
and conditions: a) Reflux in neat SOCl2, followed by addition of AlCl3 in di-
chloromethane.
With this in mind, the use of a precursor featuring a ‘blocking’
methyl group was targeted (e.g., 6), with the aim of achieving
selectivity in the Friedel–Crafts cyclisation leading to indanone
intermediate 9a (Scheme 1). Such chemistry can be successful-
ly employed and the closely related 4,5-dibromo-7-methylin-
denyl complex 9d subsequently synthesised on a 10–15 g
scale (ca. 30% overall yield from 6; Scheme 2).
Figure 1. Molecular design strategies employed in the current study.
Results and Discussion
Syntheses of bifunctional indenyl-based receptors
Initial attempts to synthesise ortho-disubstituted indenyl-based
Lewis acids revolved around the use of 5,6- and 6,7-dichloro-
1H-indenes (1 and 2)[18] and the corresponding (dichloroinde-
nyl)cyclopentadienyliron(II) complexes (3: 5,6-dichloro; 4: 4,5-
dichloro; see the Supporting Information). Despite the relative-
ly straightforward syntheses of both 3 and 4, attempts to in-
troduce the boryl function by the use of a lithiation/Mes2BF
electrophilic quench met with no success.[17] Previous reports
on the generation of benzyne species from ortho-dichloroben-
zenes, suggest that the elimination of lithium chloride from
(lithio)chlorobenzenes is extremely facile (even at very low
temperatures),[19] and this may account for the lack of success
in borylating either 3 or 4. With this in mind, and given the
greater thermal robustness reported for ortho-(lithio)bromo-
benzenes,[20] attention was shifted to the corresponding dibro-
moindenyl precursors (Schemes 1, 2, and 3).
Scheme 2. Syntheses of the dibromoindenyl complexes 7d–9d from the
corresponding indanones 7a–9a. Key reagents and conditions: a) NaBH4 in
MeOH/dichloromethane (1:1 v/v); b) 12 h at 1258C in toluene with p-TsOH;
c) KH then [FeCp(naphthalene)][PF6] in THF.
The syntheses of the requisite dibromo-1H-indene precur-
sors can be achieved using a similar Friedel–Crafts acylation/re-
duction/dehydration protocol to that previously reported for
the corresponding monofunctional 7-bromo-1H-indene
(Schemes 1 and 2 and the Supporting Information).[17] The
only issue encountered here is the lack of selectivity during
the cyclisation of 5 using aluminium chloride (Scheme 1); this
protocol generates the isomeric 5,6- and 6,7-dibromoinda-
nones (7a[21] and 8a) from which indenyl complexes 7d and
8d can be derived by subsequent reduction/dehydration/com-
plexation (Scheme 2). In the case of 7a, fractional crystallisa-
tion from the mixture of isomers 7a and 8a yields a spectro-
scopically pure material that can be used in subsequent steps.
Recovery of 8a from the supernatant solution proved much
more difficult to achieve in usable yield and this route was
therefore not viable for the bulk synthesis of the 4,5-dibro-
moindenyl complex 8d for further studies (4% overall yield).
All three dibromoindenyl complexes 7d, 8d and 9d were
isolated as dark purple crystalline materials and characterised
both by standard spectroscopic methods and single crystal X-
ray diffraction (Scheme 3 and the Supporting Information).
From systems 7d and 9d, which are accessible in bulk quanti-
ties, introduction of boryl Lewis acid functions was attempted
in stepwise fashion. Thus, treatment of 7d with one equivalent
of nBuLi at À1208C followed by addition of Mes2BF yields the
corresponding 5-bromo-6-dimesitylborylindenyl complex 7e
(Scheme 3). The crude product can be purified by air-free
column chromatography and obtained as a red solid in ap-
proximately 20% yield. The relatively low yield reflects—at
least in part—the fact that Mes2BF is a relatively bulky electro-
phile. Thus, as seen with related cyclopentadienyl nucleophiles,
a major portion of the bromo(lithium)indenyl intermediate is
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Scheme 3. Borylation of dibromoindenyl complex 7d to give 7e. Key re-
agents and conditions: a) nBuLi then Mes2BF at À1208C in THF/Et2O (1:1 v/
v). Molecular structures of 7d (left) and 7e (right) as determined by X-ray
crystallography. Thermal ellipsoids set at the 50% probability level; mesityl
groups shown in wireframe format and H atoms omitted for clarity. Selected
bond lengths [Š] and angles [8] for 7e: B17ÀC18 1.571(3), B17ÀC27 1.581(3),
B17ÀC3 1.566(3); C3-B17-C18 122.23(18), C3-B17-C27 115.81(18), C18-B17-
C27 121.84(18), C2-C3-B17 126.73(19).
not quenched when the temperature is raised, but protonated
instead.[12c,d]
The formation of 7e is implied by 11B NMR spectroscopy,
with a shift typical of a triarylborane being observed in CDCl3
solution (dB =77 ppm).[3] Moreover, the structure of 7e in the
solid state was confirmed crystallographically, with a number
of structural features implying that the steric hindrance around
the boron centre is greater than that found in related com-
pounds such as 1,2-fc(BMes2)Br. Thus, the mesityl rings in 7e
are aligned essentially perpendicular to the indenyl plane (7e:
88.78 and 87.48, cf. 1,2-fc(BMes2)Br: 51.28 and 81.58) and the
C2-C3-B17 angle [126.7(2)8] is somewhat wider than expected
in order to accommodate the large BMes2 unit ortho to the
bromine atom.[12c,d] Additional implications of this degree of
steric hindrance become apparent in the next synthetic step. A
range of reaction conditions was tested for the incorporation
of the second BMes2 unit, but in our hands these were unsuc-
cessful. Moreover, the high degree of steric loading might in
fact explain the absence of any 1,2-(BMes2)2 benzene deriva-
tives in the literature, with 1,2-fc(BMes2)2 presumably being ac-
cessible due to the greater separation between ortho substitu-
ents enforced by a five (vs. six) membered carbocyclic backbo-
ne.[12c,d]
Scheme 4. Syntheses of bifunctional boryl/phosphonium Lewis acids [7h]+
and [9h]+. Key reagents and conditions: a) tBuLi then PPh2Cl in THF/Et2O
(1:1 v/v) at À1208C; b) tBuLi then Mes2BF in THF at 258C; c) excess MeI;
d) nBuLi then Ph2PCl in THF/Et2O (1:1 v/v) at À1208C; e) nBuLi then Mes2BF
in THF at À788C; f) excess CF3SO3Me.
methylation at phosphorus would be a viable synthetic strat-
egy (Scheme 4).
Thus, as seen in the synthesis of related 1,2-boryl/phosphino
benzenes,[16b] the reactions of Ph2PCl with the lithiates generat-
ed in situ from nBuLi and either 7d or 9d at À1208C, are
shown to represent efficient and practical routes to 7 f and 9 f
(in yields of 83% and 37%, respectively) and these (bromo)-
phosphino indenyl derivatives can be purified without the
need for column chromatography. In the case of 9 f, the appar-
ent regioselectivity in the lithiation process is high, with no
evidence being obtained for the formation of the alternative 5-
phosphino derivative in anything other than trace amounts (as
1
revealed by in situ H NMR measurements). In a similar fashion,
With this in mind, the incorporation of alternative secondary
Lewis acid functions was targeted. Previous studies have
shown that ortho boryl/silyl and boryl/phosphonium combina-
tions (among others) can be incorporated onto a benzene skel-
eton,[16] and given the additional electrostatic component to
anion binding implicit in the latter, we therefore targeted in-
lithiation of 7 f/9 f followed by reaction with Mes2BF yields 7g/
9g, which can also be purified merely by washing/extraction
procedures rather than necessitating chromatography. In the
final step, [7h]+ and [9h]+ can be synthesised by a simple
methylation process; introduction of a large excess of methyl
iodide or methyl trifluoromethanesulfonate generates the
target boryl/phosphonium species [7h]I and [9h][CF3SO3] in
almost quantitative yields.
+
denyl systems featuring ÀBMes2 and ÀPR3 functions with 4,5
and 5,6 regiochemistries. Moreover, given that Ph2PCl has been
shown to be a much more reactive electrophile towards aryl-
lithium species than Mes2BF,[12a] we hypothesised that sequen-
tial installation of phosphine/boryl functions, followed by
At each step the intermediate species have been character-
ised by standard spectroscopic and analytical techniques, and
by X-ray crystallography. Formation of indenylphosphine inter-
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mediates 7 f and 9 f is signalled by the appearance of a reso-
nance in the respective 31P NMR spectra at dP =À4.14 and
À0.17 ppm (cf. PPh3, dP =À6.0 ppm) and the molecular struc-
ture of both compounds has been confirmed crystallographi-
cally (Figure 2). The subsequent step (lithiation/Mes2BF
Figure 3. Fluxional interconversion between rotamers in 7g.
of the BMes2 unit. For both systems, a propeller-like geometry
is in evidence for the borane unit; in the case of 9g, the mesi-
tyl rings are almost perpendicular to the indenyl ligand (83.1
and 89.28) and the angle between the least-squares planes of
the cyclopentadienyl and the indenyl unit deviates by approxi-
mately 5.68 from co-planarity. The P-C-C-B torsion angles in 7g
and 9g differ somewhat (22.58 and 4.08 respectively), as do the
intramolecular B–P distances (7g: 3.205 Š; 9g: 3.063 Š), pre-
sumably as a result of the closer proximity of the metallocene
and PPh2 units in 9g.
In the final synthetic step, the target cations [7h]+ and
[9h]+ can be obtained by methylation of the phosphine func-
tion, a reaction which can be monitored by multinuclear NMR
spectroscopy. The respective 31P signals are shifted downfield
(to dP =23.4 and 24.4 ppm for [7h]+ and dP =17.5 ppm for
[9h]+) and both cations are also discerned in the correspond-
ing positive ion ESI mass spectra. The presence of the [CuI4]2À
counter ion in the lattice of [9h]2[CuI4] can presumably be
traced back to the iodomethane reagent which contains
copper as a stabiliser.
Figure 2. Molecular structures of 7 f/9 f (upper) and 7g/9g (lower) as deter-
mined by X-ray crystallography. Thermal ellipsoids set at the 50% probability
level; phenyl groups shown in wireframe format and H atoms omitted for
clarity. Selected bond lengths [Š] and angles [8]: 7 f: C3ÀP4 1.839(2); C3-P4-
C5 100.3(1), C3-P4-C11 103.3(1), C5-P4-C11 100.9(1); 9 f: C11ÀP12 1.841(2);
C11-P12-C13 105.9(1), C11-P12-C19 102.9(1), C13-P12-C19 100.5(1). For key
bond lengths and angles for 7g and 9g, see Table 1.
quench) results in the appearance of broad signals at dB =74
and 76 ppm in the respective 11B NMR spectra, consistent with
the presence of an uncomplexed triarylborane function in 7g
and 9g, respectively.[3] The incorporation of the Lewis acidic
boryl function perturbs slightly the 31P NMR signals, leading to
an upfield shift in each case (e.g., 9 f: dP =À0.2 ppm; 9g: dP =
À5.8 ppm). Interestingly, compound 7g features two distinct
peaks in the solution-phase 31P NMR spectrum at room tem-
perature (dP =À6.5, À9.5 ppm) in an approximate ratio of 1:2,
which coalesce at approximately 858C. As is evident in the
crystal structure of 7g, the high steric bulk of the mutually
ortho-orientated PPh2 and BMes2 substituents enforces a non-
negligible torsion angle along the B-C-C-P chain. As a conse-
quence, two limiting conformations are conceivable, in which
either the PPh2 or the BMes2 function is displaced out of the
indenyl plane on the face occupied by the CpFe unit
(Figure 3). Of these, one (A) corresponds to the conformer
seen in the solid state, with the other (B) presumably accessi-
ble through concerted rotation of the aryl groups associated
with both the phosphine and borane components.[22] In the
case of 9g, only one 31P signal is observed, which we associate
with conformational ‘locking’ enforced by the closer proximity
of the 4-PPh2 group to the CpFe unit.
The transformation of 7g and 9g into the respective phos-
phonium species leads to significant changes in the solid-state
structures (Figure 4 and Table 1). The introduction of the
methyl group leads to enhanced steric bulk in the P compo-
nent, resulting in an increase in the intermolecular B–P dis-
tance (by 0.278 and 0.368 Š for 7g/[7h]+ and 9g/[9h]+ re-
spectively) and widening of the C-C-P and C-C-B angles. The
B–P separations are also very similar to that found in the
ortho-phenylene derived system [1,2-C6H4(BMes2)(PPh2Me)]+
([XII]+, 3.494(3) Š).[16b] In each case, however, the boron centres
Figure 4. Molecular structures of the cationic components of [7h]I (left) and
[9h]2[CuI4] (right) as determined by X-ray crystallography. Thermal ellipsoids
set at the 50% probability level; mesityl and phenyl groups shown in wire-
frame format, and H atoms, counter anions and dichloromethane solvate
molecules omitted for clarity. For key bond lengths and angles, see Table 1.
The structural characterisation of boryl phosphines 7g and
9g confirms the increased steric hindrance upon assimilation
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+43 mV respectively for the iron-centred oxidation event (vs.
Fc/Fc+). In addition, an irreversible reduction wave centred at
approximately À2300 mV is provisionally assigned the B·À/B
redox couple.[23] The corresponding redox events for [7h]+ and
[9h]+ (obtained for the trifluoromethylsulfonate salts rather
than the parent iodide compounds to eliminate anion-centred
electrochemical events) were measured in acetonitrile solution
at +575/+576 mV and À1980/À1480 mV, respectively. The
magnitudes of the shifts observed for each redox process on
methylation are consistent with the formation of a cationic
species,[5l] and with the corresponding changes in orbital ener-
gies (see the Supporting Information). Thus for 7g/[7h]+ the
respective HOMO energies are À3.98/À6.58 eV, and the corre-
sponding LUMOs found at À2.40/À5.13 eV.
Table 1. Selected bond lengths (Š) and angles (8) for novel borylphos-
phine and borylphosphonium species, and the corresponding data for
the related (known) system [1,2-C6H4(BMes2)(PPh2Me)]+ ([XII]+).[16b]
Parameter
7g
[7h]+
9g
[9h]+
[XII]+
1.577(2)
1.571(2)
1.569(2)
359.95
3.205
22.48
116.1(1)
123.2(1)
1.575(4)
1.581(5)
1.572(4)
359.2
3.483
18.78
1.584(3)
1.575(3)
1.578(3)
359.62
3.063
4.01
114.8(1)
124.2(2)
1.599(14)
1.587(15)
1.550(15)
359.7
3.431
0.91
1.562(4)
1.577(4)
1.585(4)
359.7
3.494(3)
16.55
d(BÀCaryl
)
ÆaCBC
d(BÀP)
B-C-C-P torsion
aP-C-C
123.1(2)
129.3(3)
126.0(7)
131.0(9)
124.4(2)
130.1(3)
aB-C-C
remain trigonal planar, (ÆCBC =359.28 and 359.78 for [7h]+ and
[9h]+, respectively), consistent with little, if any, interaction of
the borane function with the counter anion.
The binding capabilities of the receptors 7g/9g and [7h]+/
[9h]+ can be monitored by multinuclear NMR spectroscopy
(11B, 19F and 31P NMR). Thus, phosphine–borane receptors 7g
and 9g were exposed to cyanide or fluoride in dry THF or
chloroform, but only on addition of cyanide was a binding
event detected. For both isomers, the 11B NMR spectrum exhib-
its a new resonance at dB ꢀÀ12 ppm, characteristic of a dimesi-
tylborane–cyanide adduct.[5l] Addition of a large excess of fluo-
ride in dry chloroform (e.g., four equivalents of [nBu4N]F·3H2O
or [S(NMe2)3][Me3SiF2]), however, does not seem to quaternize
the Lewis acidic boron, and an 11B NMR spectrum displaying
a broad signal at dB ꢀ77 ppm is observed in each case, togeth-
er with free fluoride in the 19F NMR spectrum.[3] The absence of
a peak envelope for the fluoride adduct detected by negative-
ion ESI-MS further suggests that fluoride is not bound by
either 7g or 9g, even in dry aprotic solvents. By contrast, the
corresponding spectra obtained in the presence of cyanide dis-
play the molecular ion peak and expected isotopic distribu-
tions for both [7gÀCN]À and [9gÀCN]À. This selectivity can be
explained by the lower intrinsic Lewis basicity of fluoride in
comparison to cyanide, in combination with the high steric
loading provided by adjacent dimesitylborane and diphenyl-
phosphine functions, which provides an enthalpic disincentive
to the change in coordination number at boron from three to
four. Consistent with such observations, recent studies have
shown that the selective binding of cyanide over fluoride at
BMes2 receptors can be engineered by the incorporation of en-
hanced steric bulk adjacent to the borane binding site.[5d,17]
The molecular structures of [nBu4N][7gÀCN] and [nBu4N]
[9gÀCN] were determined by single X-ray crystallography
(Figure 6). The binding of cyanide in each case is accompanied
by pyramidalisation of the boron centre (ÆCBC =340.78 and
340.58 for [7gÀCN]À and [9gÀCN]À respectively), by elongation
of the BÀC bonds (by at least 0.09 Š), and by widening of the
intramolecular BÀP separation (Table 2). The B-CN bond length
in each case falls within the range of previously reported cya-
nide adducts ([7gÀCN]À: 1.625(2) Š; [9gÀCN]À: 1.626(7) Š, cf.
[FcBMes2ÀCN]À: 1.639(4) Š).[5f] The long contacts between
phosphorus and the cyanide guest (3.102 Š for [7gÀCN]À,
3.240 Š for [9gÀCN]À), together with the marginal shifts in the
respective 31P NMR spectra on CNÀ binding, argue however
against any interaction between the phosphorus atom and the
bound cyanide.
Anion binding by bifunctional borane/phosphonium species
The scope for the indenyl systems outlined above to act as
anion receptors has been probed by computational and elec-
trochemical means. Thus, DFT calculations were carried out on
7g, [7h]+, 9g and [9h]+ in order to probe issues of electronic
structure relating to their potential abilities as anion hosts.
Moreover, to put these results in perspective, analogous calcu-
lations at the same level of theory were performed on [XII]+
and [1,2-fc(BMes2)(PPh2Me)]+ ([XIII]+; see the Supporting Infor-
mation).[16b] Not unexpectedly, the formation of the respective
phosphonium cations by methylation of the phosphine func-
tion leads to significant stabilisation of the LUMO and
LUMO+1 (e.g., by ca. 3.0 eV for 9 and by ca. 2.7 eV for 7); the
absolute energies of these orbitals are also very similar to
those calculated for [XII]+ and [XIII]+. In addition, whereas the
parent neutral systems 7g and 9g feature Lewis acid character
that is focused at the borane function, significant contributions
to low lying orbitals are centred on the phosphonium function
in both [7h]+ and [9h]+. Thus, in the case of [9h]+, for exam-
ple, the LUMO is still B-centred, but the LUMO+1 features sig-
nificant character centred on the PÀPh units (Figure 5). A simi-
lar orbital picture is found for the ortho-phenylene-based
cation [XII]+ (see the Supporting Information).
Cyclic voltammetry measurements on 7g and 9g were also
carried out (in THF solution), yielding values of +166 and
Figure 5. Contour plots of the LUMO and LUMO+1 for [9h]+ (density iso-
values=0.03).
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Figure 6. Molecular structures of the cationic components of [nBu4N][7gÀ
CN] (left) and of [nBu4N][9gÀCN] (right). Thermal ellipsoids set at the 50%
probability level; mesityl and phenyl groups shown in wireframe format,
and H atoms and tetrabutylammonium counter ions omitted for clarity. Se-
lected bond lengths [Š] and angles [8]: [nBu4N][7gÀCN]: B24ÀC8 1.661(2),
B24ÀC25 1.663(2), B24ÀC34 1.667(2), B24ÀC43 1.625(2); C8-B24-C25
119.62(13), C8-B24-C34 110.01(13), C25-B24-C34 111.04(13), C9-C8-B24
124.17(14), C8-C9-P11 119.15(12), B24-C43-N44 178.57(17), B24-P11 3.349(2),
B24-C8-C9-P11 19.3; [nBu4N][9gÀCN]: B24ÀC9 1.677(6), B24ÀC25 1.669(7),
B24ÀC36 1.678(7), B24ÀC34 1.626(7); C9-B24-C25 120.9(4), C9-B24-C36
106.5(3), C25-B24-C36 113.1(3), B24-C34-N35 174.9(4), C9-C10-P11 121.6(3),
C10-C9-B24 128.9(4), B24-P11 3.456(5), B24-C9-C10-P11 21.0.
Figure 7. Molecular structures of 7hÀCN/9hÀCN (upper) and 7hÀF/9hÀF
(lower). Thermal ellipsoids set at the 50% probability level; mesityl and
phenyl groups shown in wireframe format, and H atoms and dichlorome-
thane solvent molecules omitted for clarity. For key bond lengths and
angles, see Table 2.
Table 2. Selected bond lengths (Š), angles (8) for anionic and zwitterionic
cyanide/fluoride adducts.
spectively). A very similar coupling constant (24.3 Hz) was re-
ported for the fluoride-chelated system XIIÀF.[16b]
[7gÀ
[9gÀ
7hÀCN 9hÀCN 7hÀF
9hÀF
CN]À
CN]À
Single crystals suitable for X-ray crystallography were ob-
tained for all four zwitterionic adducts 7hÀCN, 9hÀCN, 7hÀF
and 9hÀF (Figure 7); selected bond lengths and angles are in-
cluded in Table 2. The four structures each display a tetra-coor-
dinate boron centre featuring either a bound cyanide or fluo-
ride ion. Interestingly, for both receptors, the binding of cya-
nide increases the intramolecular B–P distance (e.g., from
3.483(4) to 3.5594(18) Š for [7h]+, and from 3.430(12) to
3.529(2) Š for [9h]+); by contrast, fixation of fluoride leads to
a marked decrease in the B–P separation (to 3.335(3) and
3.295(2) Š for 7hÀF and 9hÀF, respectively). Moreover, both
fluoride adducts show P–F distances [2.6756(15) and
2.6936(15) Š, respectively] which fall within the sum of the rel-
evant Van der Waals radii (3.27 Š).[24] That these contacts are
structurally significant is consistent with the observed contrac-
1.661(2) 1.677(6) 1.663(2) 1.663(3) 1.657(3) 1.644(3)
1.663(2) 1.669(7) 1.656(2) 1.666(3) 1.663(4) 1.652(3)
1.667(2) 1.678(7) 1.663(2) 1.673(3) 1.649(3) 1.654(3)
d(BÀCaryl
)
ÆaCBC
d(BÀP)
d(BÀX)
340.7
340.5
341.29 340.32 341.15 340.09
1.625(2) 1.626(7) 1.624(2) 1.626(3) 1.478(3) 1.477(2)
3.349
19.29
3.455
21.03
3.559
12.26
3.529
19.54
3.335
11.96
3.295
16.6
B-C-C-P torsion
aP-C-C
119.2(1) 121.6(3) 126.5(1) 124.9(1) 123.4(2) 121.8(2)
124.2(1) 128.9(4) 127.1(1) 129.9(2) 121.6(2) 124.2(2)
aB-C-C
Binding studies were also conducted on the cationic recep-
tors [7h]+ and [9h]+, revealing contrasting behaviour to the
parent systems 7g/9g. Addition of [nBu4N][CN]·4H2O can be
monitored by 11B NMR spectroscopy, which confirms the for-
mation of a cyanide adduct, with a shift to dB =À13 ppm
being observed for both 7hÀCN and 9hÀCN. In contrast to
the behaviour of the charge-neutral systems, however, treat-
ment of receptors [7h]+ and [9h]+ with one equivalent of
fluoride (as either [S(NMe2)3][Me3SiF2] or [nBu4N]F·3H2O) yields
the respective fluoride adducts. Upfield shifts to dB ꢀ7 ppm
are observed in each case, consistent with the formation of
a fluoroborate,[3] and, additionally, broad new signals in the
corresponding 19F NMR spectra are also measured (at dF =
À149.0 and À145.1 ppm for 7hÀF and 9hÀF, respectively). Evi-
dence for the chelation of the fluoride ion between borane
and phosphonium Lewis acids is provided by 31P NMR spec-
troscopy. In both cases, a downfield shift from dP ꢀ24 ppm to
29 ppm is accompanied by the splitting of the resonance into
a doublet (1J(F,P)=26.0 and 22.9 Hz for 7hÀF and 9hÀF, re-
1
tion in the B–P distance in each case, and with the J(F,P) cou-
pling observed in the respective 31P NMR spectra. Gabba¨ı and
co-workers report a similar P–F contact [2.666(2) Š] for the
fluoride adduct XIIÀF, together with a close-to-linear F-P-Cphenyl
angle [176.4(1)8] and
a marked elongation of the P-Ph
bond.[16b] Similar geometric features can also be discerned
here, for example, a(F-P-Cphenyl)=179.0, 172.38 for 7hÀF and
9hÀF, respectively. In the case of XIIÀF these spectroscopic/
structural observations have been attributed to the presence
of a lp(F)!s*(CÀPh) interaction, and a similar scenario can be
postulated for 7hÀF and 9hÀF. Consistently, AIM (atoms in
molecules)[25] calculations reveal a bond path between P9 and
F26 for 7hÀF (Figure 8), with an electron density, 1(r), of 2.13”
10À2 ebohrÀ3 at the bond critical point, very similar to that re-
ported for XIIÀF (2.05”10À2 ebohrÀ3).
Chem. Eur. J. 2015, 21, 11813 – 11824
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Table 3. Fluoride and cyanide binding affinities determined for [7h]+,
[9h]+ and related receptors in THF solution.
Receptor
log K(F)
log K(CN)
Reference
FcBMes2
6.06(0.06)
6.36(0.10)
6.28(0.03)
7.84(0.38)
8.09(0.26)
6.55(0.13)
7.01(0.21)
6.97(0.39)
7.28(0.13)
6.81(0.04)
[12d]
[17a]
[17b]
This work
This work
[CpFe{4-(Mes2B)-indenyl}]
[CpFe{5-(Mes2B)-indenyl}]
[7h]+
[9h]+
Figure 8. Atoms in molecules analysis of the B-F-P unit in 7hÀF, with key
bond paths and critical points depicted.
9hÀF in acetonitrile solution, reflecting the conversion of an
electron-withdrawing borane function to a strongly electron-
donating fluoroborate. Fixation by [9h]+ is also accompanied
by a noticeable (and intuitive) colour change. Thus, for in-
stance ‘free’ [9h]+ is green (lmax =596 nm), whereas the fluo-
ride adduct is maroon (lmax =531 nm, Figure 9). [7h]+ also un-
dergoes a colour change upon fixation of fluoride, although
the change is not so marked; thus the free receptor is a dark
brown (lmax =445 nm), whereas the adduct is pale red (lmax
ꢀ493 nm).
Measurements of the binding affinities of 7g/9g and [7h]+
/[9h]+ for both cyanide and fluoride were carried out by UV/
Vis spectroscopy titration experiments. The neutral systems 7g
and 9g were shown to have no affinity for fluoride on the
basis of multinuclear NMR and mass spectroscopy. However,
cyanide addition led to a change of colour from burgundy to
pale red in each case, which in principle offers a basis for the
spectroscopic determination of KCN. However cyanide uptake
appears to be kinetically very slow. Thus, on addition of one
equivalent of the anion to solutions of either 7g or 9g in dry
THF, equilibrium was still not reached even after 60 min (see
the Supporting Information), thereby rendering accurate deter-
mination of binding constants extremely challenging. The slow
kinetics of binding can be attributed to the increased steric
loading provided by the diphenylphosphine unit ortho to the
BMes2 binding domain; a similar effect, in which positioning of
a sterically bulky non-interacting substituent ortho to the pri-
mary Lewis acid has been shown to lead to slow anion uptake,
has been reported for 1,2-fc(BMes2)2.[12c,d] By contrast, cationic
receptors [7h]+ and [9h]+ are much more labile towards the
binding of either cyanide or fluoride, and sequential addition
of alicquots of either anion can be monitored by UV/Vis spec-
troscopy. The resulting data can be fitted to a 1:1 binding iso-
therm (see the Supporting Information). The derived binding
affinities are summarised in Table 3, along with the corre-
sponding results for related monodentate receptors.
Figure 9. Changes in the absorption in the visible region of the spectrum on
addition of fluoride to a solution of [9h][CF3SO3] in THF.
Conclusion
Consideration of the data outlined in Table 3 shows the fol-
lowing: i) In general, indenyl-derived systems show slightly
greater fluoride and cyanide affinities than their cyclopenta-
dienyl counterparts, presumably due to greater arene conjuga-
tion, and consequent lowering of the LUMO energy;[3,12d,17] ii)
despite the cationic charge carried by both [7h]+ and [9h]+,
no uniform increase in cyanide affinity is seen over simple
charge-neutral systems, presumably as a result of a compensa-
tory steric disincentive to binding brought about by having
the bulky phosphonium substituent ortho to the BMes2 bind-
ing domain; iii) significant increases in fluoride ion affinity are
seen for both [7h]+ and [9h]+ as a consequence of the acces-
sibility of a pre-organised cooperative B/P binding geometry
for the monatomic FÀ ion.
Indenyl-functionalised receptors offer the possibility for incor-
poration of adjacent Lewis acidic domains onto a six-mem-
bered carbocyclic framework, while retaining many of the ad-
vantages of synthetically simpler ferrocene systems in terms of
available reporter mechanisms. Although the incorporation of
adjacent BMes2 groups has proven impossible in our hands,
presumably on steric grounds, systems featuring mutually
ortho BMes2 and PPh2Me+ functions are accessible. These re-
ceptors are shown to be capable of cooperative fluoride ion
fixation, involving simultaneous binding at the borane and
phosphonium centres, and leading to enhanced thermody-
namics for FÀ uptake. Thus, in contrast to simple BMes2-con-
taining systems, FÀ binding is found to be more favourable
than that of cyanide (which interacts only with the borane
Lewis acid). Moreover, in the case of the 4-(MePh2P)-5-(Mes2B)-
7-Me-indenyl derivative [9h]+, fluoride chelation is accompa-
nied by a marked colour change from green for the free recep-
tor to maroon for the adduct.
From a detection perspective, the complexation of the fluo-
ride or cyanide ion by [9h]+ can be monitored by using the
electrochemical potential of the iron centre. Thus a large
cathodic shift of À590 mV is observed between [9h]+ and
Chem. Eur. J. 2015, 21, 11813 – 11824
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flections; max. and min. residual electron densities 1.10 and
À0.56 eŠÀ3; CCDC reference: 1059132.
Experimental Section
General procedures
(4-Diphenylphosphino-5-dimesitylboryl-7-methyl-indenyl)cyclo-
penta-dienyliron(II) (9g): To a solution of 9 f (2.00 g, 3.90 mmol) in
THF (20 mL) at À788C was slowly added a solution of nBuLi
(4.1 mL of a 1.9m solution in hexane, 7.79 mmol). The reaction mix-
ture was stirred for 1 h at À788C, then a solution of Mes2BF
(2.09 g, 7.79 mmol) in THF (ca. 10 mL) was added, and the reaction
mixture warmed to room temperature over 12 h. Volatiles were
then removed under reduced pressure, and the crude solid
washed with hexane (3 ” 10 mL) and then extracted with dichloro-
methane (ca. 10 mL). The solution was concentrated and crystals
suitable for X-ray crystallography were obtained on cooling to
À258C (0.84 g, 31% yield). 1H NMR (400 MHz, CDCl3, 258C): d=
1.99 and 2.16 (b s, each 6H, ortho-Me of Mes), 2.20 and 2.34 (s,
each 3H, para-Me of Mes), 2.46 (s, 3H, Me), 3.52 (s, 5H, Cp), 4.00 (t,
3J(H,H)=2.3 Hz, 1H, C2H), 4.45 (d, 3J(H,H)=1.5 Hz, 1H, C1H), 4.89
Manipulations of air-sensitive reagents were carried out in a glove-
box, or by means of Schlenk-type techniques involving the use of
a dry argon or a nitrogen atmosphere. HPLC grade solvents were
purified, dried and degassed prior to use by a commercial available
Braun Solvent-Purification System (SPS 500). NMR solvents CDCl3
(vac transfer, stored over molecular sieves) (CD3)2SO (distilled from
CaH2, stored over molecular sieves) and C6D5Br (distilled from CaH2,
stored over molecular sieves) were pre-dried before use. The
known compounds [CpFe(naphthalene)][PF6] and Mes2BF were pre-
pared according to literature procedures.[26] The syntheses of
bromo precursors 7a–e, 8a–d and 9a–d, together with dichloroin-
dene and -indenyl systems 1–4 are given in the supporting infor-
mation.[18] MeI, MeSO3CF3 and Ph2PCl were used as supplied (Sigma
1
Aldrich). H and 13C NMR spectra were measured on a Bruker AVII
500 FT-NMR or Varian Mercury VX-300 spectrometer; 1H and
13C NMR spectra were calibrated using the residual proton or natu-
ral abundance 13C resonances of the solvent; 11B and 19F spectra
were referenced with respect to Et2O·BF3 and CFCl3, respectively.
Mass spectra were measured by the EPSRC National Mass Spec-
trometry Service, Swansea University, and elemental microanalysis
by London Metropolitan University.
3
(d, J(H,H)=1.5 Hz, 1H, C3H), 6.64 (b s, 2H, meta-CH of Mes), 6.71
(s, 1H, C6H), 6.81 (s, 2H, meta-CH of Mes), 6.94–7.31 ppm (overlap-
ping m, 10H, Ph); 13C{1H} NMR (100.6 MHz, CDCl3, 258C): d=19.8
(Me), 21.2 and 21.4 (para-Me of Mes), 23.7 (b s, ortho-Me of Mes),
59.6 (C3), 65.1 (C1), 67.9 (Cp), 70.9 (C2), 88.6 (C9), 91.8 (C8), 124.6
(C6), 127.1 and 127.6 (meta-CH of Ph), 127.9 and 128.0 (para-CH of
Ph), 128.1 (C7), 128.2 (b s, meta-CH of Mes), 132.7 (d, 2J(C,P)=
2
19.2 Hz, ortho-C Ph), 133.9 (d, J(C,P)=17.1 Hz, ortho-CH Ph), 134.4
and 136.0 (ipso-quaternary C Ph), 138.9 (para-quaternary C of Mes),
140.9 (b s, meta-CH of Mes), 141.1 (C4), 147.0 (b s, C5), 159.5 ppm
(ipso-quaternary C of Mes); 11B{1H} NMR (128 MHz, CDCl3, 258C):
d=76 ppm (b s); 31P{1H} NMR (162 MHz, CDCl3, 258C): d=
À5.8 ppm (b s); MS (EI+): m/z (%): 667.2 ([M-CH3]+), isotopic pat-
tern correct for C44H41BFeP; E1/2 = +43 mV (FeII/FeIII) vs Fc/Fc+ with
0.1m [nBu4N][PF6] in THF; UV/Vis (THF): lmax (e, molÀ1 LÀ1 cmÀ1)=
351 (5560), 531 nm (1340); crystallographic data: C45H44BFeP; Mr =
682.48; monoclinic; space group P21/c; a=20.2118(2), b=
8.13040(10), c=22.8385(2) Š; b=108.4989(11)8; V=3559.13(7) Š3;
Z=4; T=150 K; l=1.54180 Š. 20753 reflections collected; 7344
independent [R(int)=0.042] used in all calculations, with 433 re-
fined parameters; GOF on F2 =0.9877. R1 =0.0435; wR2 =0.1119 for
observed unique reflections [I>2s(I)] and R1 =0.0483; wR2 =0.1166
Syntheses of new compounds
(4-Diphenylphosphino-5-bromo-7-methyl-indenyl)cyclopentadie-
nyliron(II) (9 f): To a solution of 9d (0.24 g, 0.614 mmol) in a mix-
ture of THF and Et2O (20 mL, 1:1 v/v) at À1208C was added a solu-
tion of nBuLi (4.02 mL of a 1.6m solution in hexane, 6.43 mmol).
The reaction mixture was stirred for 30 min at À1208C, then a solu-
tion of PPh2Cl (1.14 mL, 6.14 mmol) in a mixture of THF and Et2O
(10 mL, 1:1 v/v) at À788C added to the red solution. After stirring
for a further 2 h at À1208C and warming to room temperature
over 12 h, volatiles were removed under reduced pressure. The
crude solid was washed with hexane (3 ” 10 mL) then extracted
with dichloromethane (ca. 10 mL). The solution was concentrated
and crystals suitable for X-ray crystallography were obtained on
cooling to À258C (0.46 g, 37% yield). 1H NMR (400 MHz, CDCl3,
for all unique reflections; max. and min. residual electron densities
À3
0.63 and À0.45 eŠ
.
3
258C): d=2.50 (d, J(H,H)=0.9 Hz, 3H, Me), 3.59 (s, 5H, Cp), 3.89
(apparent t, 3J(H,H)=2.6 Hz, 1H, C2H), 4.10 (dd, 3J(H,H)=2.6 Hz,
(4-Methyldiphenylphosphonium-5-dimesitylboryl-7-methylinde-
nyl)cyclo-pentadienyliron(II) trifluoromethylsulfonate ([9h]
4J(H,H)=0.9 Hz, 1H, C1H), 4.80 (dd, J(H,H)=2.6 Hz, J(H,H)=0.9 Hz,
3
4
4
1H, C3H), 6.95 (d, J(H,P)=3.2 Hz, 1H, C6H), 7.27 (m, 3H, CH of Ph),
[CF3SO3]): To a solution of 9g (0.50 g, 0.732 mmol) in chloroform
(10 mL) was added excess CF3SO3Me (0.83 mL, 7.33 mmol), and the
reaction mixture stirred at room temperature for 12 h. Volatiles
were removed under reduced pressure, and single crystals suitable
for X-ray crystallography were obtained from a solution in THF by
7.48 (m, 5H, CH of Ph), 7.78 ppm (tt, 3J(H,H)=7.5 Hz, 4J(H,H)=
1.5 Hz, 2H, para-CH of Ph); 13C{1H} NMR (100.6 MHz, CDCl3, 258C):
d=19.2 (Me), 60.0 (C3), 64.7 (C1), 68.7 (Cp), 71.1 (C2), 86.2 (C9),
92.4 (C8), 126.9 (C6), 128.4 and 128.6 (CH of Ph), 128.9 (meta-CH of
Ph), 129.3 (d, 1J(C,P)=36.0 Hz, C4), 130.3 (C5), 133.1 (CH of Ph),
134.1 (para-CH Ph), 136.9 (CH of Ph), 144.6 ppm (C7); 31P{1H} NMR
(162 MHz, CDCl3, 258C): d=À0.2; elemental analysis (%) calcd for
C27H22BrFeP·(CH2Cl2)0.2: C 61.62, H 4.26; found: C 61.78, H 4.66 (sol-
vent content verified by NMR); MS (EI+): m/z (%): 512.0 (100),
514.0 (95); accurate mass calcd for C27H22BrFeP: 511.9994 (M+, 56Fe/
79Br isotopomer), 513.9976 (M+, 81Br isotopomer; meas.: 511.9986,
513.9961, isotopic pattern correct for C27H22BrFeP; crystallographic
1
layering with heptane (0.62 g, 97% yield). H NMR (400 MHz, CDCl3,
258C): d=1.78 (b s, 3H, para-Me of Mes), 1.95 (b s, 6H, ortho-Me
of Mes), 2.19 (b s, 6H, ortho-Me of Mes), 2.32 (s, 3H, para-Me of
Mes), 2.56 (s, 6H, PMe and Me of C7), 3.78 (s, 5H, Cp), 4.23 (s, 1H,
C3H), 4.30 (t, 3J(H,H)=2.6 Hz, 1H, C2H), 5.15 (s, 1H, C1H), 6.75,
6.77, 6.80 and 6.82 (b s, each 1H, meta-CH of Mes), 6.88 (d,
4J(H,P)=3.3 Hz, 1H, C6H), 7.18 (dd, 3J(H,P)=13.1 Hz, 3J(H,H)=
7.4 Hz, 2H, ortho-CH of Ph#1), 7.44 (td, 3J(H,H)=8.1 Hz, 4J(H,P)=
3.7 Hz, 2H, meta-CH of Ph#1), 7.65 (td, 3J(H,H)=7.4 Hz, 5J(H,P)=
1.3 Hz, 1H, para-CH of Ph#1), 7.76 (td, 3J(H,H)=7.4 Hz, 4J(H,P)=
3.6 Hz, 2H, meta-CH of Ph#2), 7.85 ppm (m, 3H, para and ortho-CH
of Ph#2); 1H NMR (500 MHz, CD2Cl2, À808C, major isomer 55%):
d=1.83, 1.95 and 1.99 (s, each 3H, ortho-Me of Mes), 2.27 (s, 3H,
para-Me of Mes), 2.42 (s, 3H, ortho-Me of Mes), 2.53 (d, 2J(H,P)=
10.9 Hz, 3H, PMe), 2.55 (s, 3H, Me of C4), 3.76 (s, 5H, Cp), 4.01 (s,
¯
data: C27H22BrFeP; Mr =513.20; triclinic; space group P1; a=
9.6042(3), b=10.5337(5), c=11.2465(4) Š; a=92.027(3), b=
95.965(3), g=107.132(3)8; V=1078.80(7) Š3; Z=2; T=150 K; l=
1.54180 Š. 12199 reflections collected; 4507 independent [R(int)=
0.026] used in all calculations, with 272 refined parameters; GOF
on F2 =1.0156; R1 =0.0310; wR2 =0.0842 for observed unique re-
flections [I>2s(I)] and R1 =0.0312; wR2 =0.0844 for all unique re-
Chem. Eur. J. 2015, 21, 11813 – 11824
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3
1H, C3H), 4.23 (s, 1H, C2H), 5.17 (s, 1H, C1H), 6.68, 6.76 and 6.82 (s,
5.8 Hz, each 2H, ortho-CH of Ph), 7.21 and 7.31 (t, J(H,H)=7.3 Hz,
3
3
4H, meta-CH of Mes), 6.96 (s, 1H, C6H), 7.02 (dd, J(H,P)=12.9 Hz,
each 2H, meta-CH of Ph), 7.91 ppm (t, J(H,H)=6.1 Hz, 2H, para-CH
3J(H,H)=7.3 Hz, 2H, ortho-CH of Ph), 7.38 (t, 3J(H,H)=7.5 Hz, 2H,
meta-CH of Ph), 7.58 (dd, 3J(H,P)=13.4 Hz, 3J(H,H)=7.3 Hz, 2H,
of Ph); 13C{1H} NMR (126 MHz, CDCl3, 258C): d=13.6 (CH3 of
[nBu4N]), 19.5 (CH2 of [nBu4N]), 20.7 and 20.8 (ortho-Me of Mes),
23.9 (CH2 of [nBu4N]), 26.2 (para-Me of Mes), 26.4 (Me bound to
C4), 57.5 (C3), 58.2 (NCH2 of [nBu4N]), 65.1 (C1), 67.6 (C2), 67.8 (Cp),
86.1 (C9), 95.1 (d, 2J(C,P)=5.4 Hz, C8), 125.8 (d, 1J(C,P)=27.0 Hz,
3
ortho-CH of Ph), 7.72 (t, J(H,H)=7.5 Hz, 2H, meta-CH of Ph), 7.79
3
(m, 1H, para-CH of Ph), 7.88 ppm (t, J(H,H)=7.3 Hz, 1H, para-CH
1
of Ph); H NMR (500 MHz, CD2Cl2, À808C, minor isomer 45%): d=
4
1.89, 1.90 and 2.05 (s, each 3H, ortho-Me of Mes), 2.27 (s, 3H, para-
quaternary C of Ph), 126.6 (d, J(C,P)=4.5 Hz, para-CH of Ph), 127.3
2
Me of Mes), 2.28 (d, J(H,P)=11.0 Hz, 3H, PMe), 2.31 (s, 3H, ortho-
(d, 4J(C,P)=4.2 Hz, para-CH of Ph), 128.4 (C7), 128.5 (meta-CH of
2
Me of Mes), 2.50 (s, 3H, Me of C4), 3.71 (s, 5H, Cp), 4.18 (s, 1H,
C3H), 4.29 (s, 1H, C2H), 5.15 (s, 1H, C1H), 6.60 (s, 1H, meta-CH of
Mes), 6.82 (s, 1H, C6H), 6.86 (s, 1H, meta-CH of Mes), 7.13 (dd,
3J(H,P)=12.6 Hz, 3J(H,H)=8.0 Hz, 2H, ortho-CH of Ph), 7.49 (t,
3J(H,H)=7.5 Hz, 2H, meta-CH of Ph), 7.69 (t, 3J(H,H)=7.5 Hz, 2H,
meta-CH of Ph), 7.77 (m, 1H, para-CH of Ph), 7.82 (dd, 3J(H,P)=
Mes), 131.1 (para-quaternary C of Mes), 133.4 (d, J(C,P)=18.3 Hz,
2
ortho-CH of Ph), 133.2 (d, J(C,P)=18.2 Hz, ortho-CH of Ph), 134.2
(d, J(C,P)=16.7 Hz, meta-CH of Ph), 135.5 (C6), 138.1 (d, J(C,P)=
16.9 Hz, meta-CH of Ph), 142.1 (d, J(C,P)=25.3 Hz, C4), 142.9 and
143.3 (ortho-quaternary C of Mes), 147.4 (b s, CN bound to B),
152.1 ppm (b s, ipso-quaternary C of Mes), 169.8 ppm (b s, C5);
11B{1H} NMR (128 MHz, CDCl3, 258C): d=À13 ppm; 31P{1H} NMR
(162 MHz, CDCl3, 258C): d=À12.3 ppm; MS (ESIÀ): m/z (%): 708.3
(100%); accurate mass calcd for [C46H44BFeNP]À: 708.2657 (MÀ, 11B/
56Fe isotopomer); meas.: 708.3428, isotopic pattern correct for
3
3
1
3
3
13.9 Hz, J(H,H)=7.8 Hz, 2H, ortho-CH of Ph), 7.90 ppm (t, J(H,H)=
7.3 Hz, 1H, para-CH of Ph); 13C{1H} NMR (126 MHz, CDCl3, 258C):
d=15.0 (1J(C,P)=57 Hz, PMe), 20.4 (Me), 21.1, 21.3, 23.9 and 24.2
(Me of Mes), 61.0 (C1), 64.5 (C3), 68.4 (Cp), 74.2 (C2), 87.9 (C9), 92.4
(C8), 114.2 (d, 1J(C,P)=90.0 Hz, C4), 119.2 (CF3SO3), 119.6 (d,
1J(C,P)=50.0 Hz, ipso-quaternary C of Ph), 126.0 (C6), 129.4 (ortho-
quaternary C of Mes), 130.0 and 130.1 (meta-CH of Mes), 130.2
(meta-CH of Ph), 130.3 (meta-CH of Mes), 130.4 (meta-CH of Ph),
[C46H44BFeNP]À;
elemental
analysis
(%)
calcd
for
C62H80BFeN2P·(CH2Cl2)0.4: C 76.09, H 8.27; found: C 75.75, H 8.66
(solvent content verified by NMR); UV/Vis (THF): lmax, (e,
molÀ1 LÀ1 cmÀ1)=531 nm
(1060);
crystallographic
data:
2
132.2 (b s, ortho-CH of Ph), 132.6 (d, J(C,P)=11.0 Hz, ortho-CH of
C62H80BFeN2P; Mr =950.96; monoclinic; space group P21; a=
11.3694(1), b=19.0026(1), c=12.1698(1) Š; b=90.0233(6)8; V=
2629.26(3) Š3; Z=2; T=150 K; l=1.54180 Š; 26989 reflections col-
lected; 10492 independent [R(int)=0.027] used in all calculations,
with 604 refined parameters; GOF on F2 =1.0009; R1 =0.0787;
wR2 =0.1880 for observed unique reflections [I>2s(I)] and R1 =
0.0799; wR2 =0.1891 for all unique reflections; max. and min. resid-
ual electron
4
Ph), 134.2 and 134.5 (d, J(C,P)=2.8 Hz, para-CH of Ph), 141.3 and
142.4 (each para-quaternary C of Mes), 150.2 (C7), 160.2 ppm (b s,
ipso-quaternary C of Mes), C5 bound to the boron atom not found;
11B{1H} NMR (128 MHz, CDCl3, 258C): d=75 ppm; 31P{1H} NMR
(162 MHz, CDCl3, 258C): d=17.5 ppm; 19F{1H} NMR (376 MHz,
CDCl3, 258C): dF =À77.8 ppm; MS (ESI+): m/z (%): 697.3 (100%),
698.3 (48%); accurate mass calc. for [C47H50BFeP]+: 697.2861 (M+,
11B/56Fe isotopomer); meas.: 697.2876, isotopic pattern correct for
[C47H50BFeP]+; E1/2 =À1302 mV (B·À/B) vs Fc/Fc+ with 0.1m [nBu4N]
[PF6] in THF, +576 mV (FeII/FeIII) and À1479 mV (B·À/B) vs Fc/Fc+
with 0.1m [nBu4N][PF6] in acetonitrile; UV/Vis (THF): lmax (e,
molÀ1 LÀ1 cmÀ1)=340 (24250), 596 nm (1070); although [9h]
[CF3SO3] could not be obtained as single crystals suitable for X-ray
diffraction, crystals of the corresponding [CuI4]2À salt could be ob-
tained from the corresponding reaction with methyl iodide in the
presence of adventitious copper (present as a stabiliser in the MeI);
crystallographic data: C92H94B2CuFe2I4P2; Mr =1966.18; monoclinic;
space group P21/c; a=12.9526(3), b=44.3256(11), c=16.6123(3) Š;
b=106.822(2)8; V=9129.5(4) Š3; Z=4; T=150 K; l=1.54180 Š;
18770 reflections collected; 18770 independent [R(int)=0.081]
used in all calculations, with 928 refined parameters; GOF on F2 =
1.0007. R1 =0.0898; wR2 =0.2204 for observed unique reflections
9hÀF: A solution of [9h][CF3SO3] (250 mg, 0.374 mmol) in THF
(5 mL) was stirred with [nBu4N]F·3H2O (113 mg, 0.411 mmol) for 6 h
at room temperature. Volatiles were removed from the resulting
red solution under reduced pressure and the residue extracted
with diethyl ether (ca. 5 mL). The volatiles were removed under re-
duced pressure and the byproduct [nBu4N][CF3SO3] was then pre-
cipitated from dichloromethane by the addition of diethyl ether at
À258C. Crystals of 9hÀF suitable for X-ray crystallography were
then obtained from solution in THF by layering with heptane
1
(47 mg, 18% yield). H NMR (400 MHz, CDCl3, 258C): d=1.67 (b s,
3H, ortho-Me of Mes), 1.90 (b s, 9H, ortho-Me of Mes), 2.16 (s, 6H,
para-Me of Mes), 2.18 (d, 2J(H,P)=14.6 Hz, 3H, PMe), 2.41 (s, 3H,
Me of C7), 3.77 (s, 5H, Cp), 3.82 (s, 2H, C2H and C3H), 4.82 (s, 1H,
C1H), 6.49, 6.57 and 6.62 (each s, 4H, meta-CH of Mes), 7.12 (s, 1H,
3
[I>2s(I)] and R1 =0.1127; wR2 =0.2353 for all unique reflections;
C6H), 7.15 (b s, 2H, ortho-CH of Ph#1), 7.36 (t, J(H,H)=6.5 Hz, 2H,
À3
meta-CH of Ph#1), 7.52 (t, 3J(H,H)=6.8 Hz, 1H, para-CH of Ph#1),
7.72 (t, 3J(H,H)=7.7 Hz, 1H, para-CH of Ph#2), 7.76 (t, 3J(H,H)=
7.2 Hz, 2H, meta-CH of Ph#2), 7.81 ppm (d, 3J(H,H)=7.2 Hz, 2H,
ortho-CH of Ph#2); 13C{1H} NMR ( 126 MHz, CDCl3, 258C): d=18.5
max. and min. residual electron densities 3.13 and À2.47 eŠ
.
[nBu4N][9gÀCN]: A solution of 9g (0.25 g, 0.366 mmol) in THF
(10 mL) was stirred with [nBu4N][CN]·4H2O (0.11 g, 0.403 mmol) for
24 h at room temperature. Volatiles were removed from the result-
ing red solution under reduced pressure and the residue extracted
with diethyl ether (ca. 10 mL). After removal of the solvent under
reduced pressure, crystals suitable for X-ray crystallography were
obtained from a solution in chloroform by layering with heptane
(0.10 g, 28% yield). 1H NMR (500 MHz, CDCl3, 258C): d=0.94 (t,
3J(H,H)=7.2 Hz, 12H, CH3 of [nBu4N]), 1.27 (b sextet, 3J(H,H)=
7.1 Hz, 8H, CH2 of [nBu4N]), 1.44 (m, 8H, CH2 of [nBu4N]), 1.98 (s,
3H, Me bound to C7), 2.08 (s, 6H, ortho-Me of Mes), 2.14 (s, 6H,
1
2
(dd, J(C,P)=64.4 Hz, J(C,F)=19.2 Hz, PMe), 20.0 (s, Me of C7), 20.9
(s, para-Me of Mes), 24.2 and 24.9 (b s, ortho-Me of Mes), 59.7 (C1),
63.9 (C2), 68.9 (Cp), 69.5 (b s, C3), 86.3 (d, J(C,P)=8.5 Hz, C9), 94.3
3
(d, 2J(C,P)=18.8 Hz, C8), 108.5 (d, 1J(C,P)=97.2 Hz, C4), 116.8 (d,
1J(C,P)=101.5 Hz, ipso-quaternary C of Ph), 128.6, 128.7, 129.1 and
129.3 (meta-CH of Mes), 129.5 (d, 3J(C,P)=11.2 Hz, meta-CH of
Ph#1), 129.6 (d, 3J(C,P)=12.5 Hz, meta-CH of Ph#2), 132.0 (d,
2J(C,P)=9.2 Hz, ortho-CH of Ph#2), 132.5 (d, 3J(C,P)=8.0 Hz, C6),
132.6 and 132.7 (d, J(C,P)=7.6 Hz, para-CH of Ph of #1 and #2,
133.0 and 133.2 (para-quaternary C of Mes), 133.3 (d, 2J(C,P)=
11.5 Hz, ortho-CH of Ph#1), 140.8 and 142.9 (b s, ortho-quaternary
C of Mes), 143.7 (C7), 153.1 (b s, ipso-quaternary C of Mes),
183.8 ppm (b s, C5); 11B{1H} NMR (128 MHz, CDCl3, 258C): d=
3
para-CH3 of Mes), 2.28 (s, 6H, ortho-Me of Mes), 2.94 (t, J(H,H)=
7.9 Hz, 8H, NCH2 of [nBu4N]), 3.42 (s, 5H, Cp), 3.68 (s, 1H, C2H),
4.27 (s, 1H, C1H), 4.62 (s, 1H, C3H), 6.36 and 6.69 (s, each 2H,
meta-CH of Mes), 6.91 (s, 1H, C6H), 7.00 and 7.05 (d, 3J(H,H)=
Chem. Eur. J. 2015, 21, 11813 – 11824
www.chemeurj.org
11821
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
6.0 ppm; 31P{1H} NMR (162 MHz, CDCl3, 258C): d=21.3 ppm
(1J(F,P)=22.9 Hz); 19F NMR (376 MHz, CDCl3, 258C): dF =
À145.1 ppm (b s); E1/2 =À13 mV (FeII/FeIII) vs Fc/Fc+ with 0.1m
[nBu4N][PF6] in acetonitrile; UV/Vis (THF): lmax (e, molÀ1 LÀ1 cmÀ1)=
531 nm (1185); crystallographic data: C46H47BFFeP; Mr =716.51;
monoclinic, space group P21/n; a=17.1968(2), b=12.0044(1), c=
18.0916(2) Š; b=101.5347(12)8; V=3659.35(7) Š3; Z=4; T=150 K;
l=1.54180 Š. 21019 reflections collected; 7549 independent
[R(int)=0.067] used in all calculations, with 497 refined parameters;
GOF on F2 =0.9907. R1 =0.0550; wR2 =0.1422 for observed unique
reflections [I>2s(I)] and R1 =0.0599; wR2 =0.1496 for all unique re-
flections; max. and min. residual electron densities 0.85 and
À0.89 eŠÀ3. CCDC reference: 1059120.
Determination of binding constants
Binding constants were determined by titration using UV/Vis spec-
tra measured in tetrahydrofuran solution. A stock solution of the
receptor was prepared (typically 20 mg in 20 mL THF), from which
exactly 3 mL was transferred to a UV cuvette. A first absorption
spectrum of the free receptor was measured and then aliquots of
a solution of [nBu4N]F·3H2O or [nBu4N][CN]·4H2O were added. Fur-
ther alicquots of these solutions were added until a total of at
least 2.5 equivalents was reached. The binding constants were typ-
ically fitted to a 1:1 binding isotherm taking dilution into account,
using the REACTLAB software package.[28]
9hÀCN: A solution of [9h][CF3SO3] (250 mg, 0.732 mmol) in THF
(5 mL) was stirred with [nBu4N]F·3H2O (243 mg, 0.878 mmol) for 6 h
at room temperature. Volatiles were removed from the resulting
red solution under reduced pressure and the residue extracted
with diethyl ether (ca. 5 mL). After removal of the solvent under re-
duced pressure, crystals suitable for X-ray crystallography were ob-
tained from solution in THF by layering with heptane (154 mg,
29% yield). 1H NMR (500 MHz, CDCl3, 258C): d=1.71, 1.87, 1.93,
2.08, 2.09 and 2.17 (b s, each 3H, Me of Mes), 2.22 (d, 2J(H,P)=
11.2 Hz, 3H, PMe), 2.30 (s, 3H, Me of C7), 3.74 (b s, 1H, C2H), 3.77
(s, 5H, Cp), 3.92 (s, 1H, C3H), 4.72 (s, 1H, C1H), 6.62, 6.66, 6.70 and
6.73 (b s, each 1H, meta-CH of Mes), 7.20 (s, 1H, C6H), 7.23 (b s,
Electrochemical measurements
Electrochemical measurements were performed within a Saffron
Omega Scientific glovebox under anhydrous nitrogen on a PARA-
METEK VersaSTAT3 potentiostat. Cyclic voltammetry (CV) measure-
ments were carried out using a silver quasi-reference electrode,
a glassy-carbon working electrode and a platinum auxiliary elec-
trode. The supporting electrolyte was a 0.1m solution of [nBu4N]
[PF6] in THF. The quasi-reference electrode was immersed in the
electrolyte solution for 48 h prior to the experiment and the NiCp2/
+
NiCp2 couple was used as an internal reference. Redox potentials
were subsequently re-calibrated to Fc/Fc+ (DE=À379 mV).[29] CV
experiments were typically performed at three different scan rates
3
2H, ortho-CH of Ph), 7.36 (m, 2H, meta-CH of Ph), 7.43 (t, J(H,H)=
of 0.05, 0.1 and 1.0 VsÀ1
.
8.1 Hz, 1H, para-CH of Ph), 7.63 (td, 3J(H,H)=7.9 Hz, 5J(H,P)=
1.8 Hz, 1H, para-CH of Ph) 7.78 ppm (overlapping m, 4H, ortho
and meta-CH of Ph); 13C{1H} NMR (126 MHz, CDCl3, 258C): d=18.6
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/or from the author (complete syn-
thetic/characterisation data, details of DFT calculations and CIFs for
all crystal structures; CCDC 1059115–1059133 contain the supple-
mentary crystallographic data for this paper. These data are provid-
ed free of charge by The Cambridge Crystallographic Data Centre.
1
(d, J(C,P)=55.0 Hz, PMe), 19.7 (Me), 20.6, 20.8, 23.3, 25.1, 26.6, and
27.5 (Me of Mes), 58.5 (C1), 64.7 (C3), 68.5 (Cp), 69.4 (C2), 85.5 (C9),
95.7 (C8), 109.2 (C4), 119.3 and 121.8 (ipso-quaternary C of Ph),
126.9 and 128.3 (meta-CH of Mes), 128.5 (meta-CH of Ph), 129.2
4
(meta-CH of Mes), 129.4 (ortho-CH of Ph), 130.8 (d, J(C,P)=9.5 Hz,
para-CH of Ph), 132.2 (CH of Ph), 133.6 (para-CH of Ph), 135.2 (d,
3J(C,P)=20.0 Hz, C6), 137.7 and 138.8 (para-quaternary C of Mes),
141.7 and 142.5 (ortho-quaternary C of Mes), 144.3 (C7), 145.9 and
150.0 (b s, ipso-quaternary C of Mes), 179.4 ppm (C5), C of the cya-
nide bound to the boron atom was not found; 11B{1H} NMR
(128 MHz, CDCl3, 258C): d=À13; 31P{1H} NMR (162 MHz, CDCl3,
258C): d=21.5 ppm; elemental analysis (%) calcd for
C47H47BFeNP·THF: C 76.99, H 6.97; found: C 77.17, H 6.94 (solvent
content verified by NMR); UV/Vis (THF): lmax (e, molÀ1 LÀ1 cmÀ1)=
532 nm (1650); crystallographic data: C47H47BFeNP; Mr =723.53;
monoclinic; space group C2/c; a=18.4347(2), b=12.65370(10), c=
36.2152(3) Š; b=97.0569(9)8; V=8383.82(13) Š3; Z=8; T=150 K;
l=1.54180 Š; 8735 reflections collected; 8735 independent
[R(int)=0.053] used in all calculations, with 460 refined parameters;
GOF on F2 =1.0162. R1 =0.0427; wR2 =0.1061 for observed unique
reflections [I>2s(I)] and R1 =0.0466; wR2 =0.1086 for all unique re-
flections; max. and min. residual electron densities 0.39 and
À0.60 eŠÀ3. CCDC reference: 1059119.
Keywords: anions · boranes · fluorides · iron · Lewis acids
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Received: April 21, 2015
Published online on July 14, 2015
Chem. Eur. J. 2015, 21, 11813 – 11824
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim