Spontaneous dehydrocoupling in peri-substituted phosphine-borane adducts

Article abstract of DOI:10.1039/c5dt02539g

Bis(borane) adducts Acenap(PiPr₂·BH₃)(PRH·BH₃) (Acenap = acenaphthene-5,6-diyl; 4a, R = Ph; 4b, R = ferrocenyl, Fc; 4c, R = H) were synthesised by the reaction of excess H₃B·SMe₂ with either phosphino

Full text of DOI:10.1039/c5dt02539g

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Text for ToC
Borane adducts of peri-substituted bis(phosphines) eliminate hydrogen at room temperature,
without the presence of catalysts. These reactions are the first examples of “spontaneous”
phosphine-borane dehydrocoupling.

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Spontaneous Dehydrocoupling in Peri-
Substituted Phosphine-Borane Adducts
Laurence J. Taylor, Brian A. Surgenor, Piotr Wawrzyniak, Matthew J. Ray, David
B. Cordes, Alexandra M. Z. Slawin and Petr Kilian
School of Chemistry, EastCHEM, University of St Andrews, St Andrews, Fife, UK KY16 9ST
Abstract
Bis(borane) adducts Acenap(PiPr2·BH3)(PRH·BH3) (Acenap = acenaphthene-5,6-diyl; 4a, R = Ph; 4b, R
= ferrocenyl, Fc; 4c, R = H) were synthesised by the reaction of excess H3B·SMe2 with either
phosphino-phosphonium salts [Acenap(PiPr2)(PR)]+Cl− (1a, R = Ph; 1b, R = Fc), or bis(phosphine)
Acenap(PiPr2)(PH2) (3). Bis(borane) adducts 4a–c were found to undergo dihydrogen elimination at
room temperature, this spontaneous catalyst-free phosphine-borane dehydrocoupling yields BH2
bridged species Acenap(PiPr2)(µ-BH2)(PR·BH3) (5a, R = Ph; 5b, R = Fc; 5c, R = H). Thermolysis of 5c
results in loss of the terminal borane moiety to afford Acenap(PiPr2)(µ-BH2)(PH) (14). Single crystal X-
ray structures of 3, 4b and 5a–c are reported.
Introduction
Dehydrocoupling reactions (E–H + E’–H → E–E’ + H2) are an interesting and effective way of
generating bonds between main-group elements, with concomitant evolution of H2. Reactions of this
type show applications not only in inorganic synthesis, but also in hydrogen storage, transfer
hydrogenation and polymer synthesis.1–6
Although dehydrocoupling reactions that occur by thermal or autocatalytic routes are
known,7,8 the vast majority of recent work has focused on catalysis, particularly with transition
metals.6,9–11 In particular, amine-borane adducts have attracted considerable interest as potential
hydrogen storage molecules.12,13 However, dehydrocoupling reactions in the chemically related
phosphine-boranes have received far less attention.11,14
Dehydrocoupling of phosphine-boranes to form poly(phosphinoboranes) was first reported
in the 1950s. Early work in this area is limited, with polymerisations yielding low molecular weight
polymers which were often poorly characterised.15,16 In 1999 the Manners’ group pioneered the use
of transition metal catalysts in the synthesis of poly(phosphinoboranes)17–20 and more recently
B(C6F5)3 has been used as a metal-free dehydrocoupling catalyst.21 Thus formed inorganic polymers
have interesting and unusual physical properties, which set them apart from the more traditional
carbon-based polymers.17,18
While catalysts are incredibly useful, they are often expensive, especially when they contain
precious transition metals such as Rh or Ir. As such, it would be helpful to develop systems which
undergo dehydrocoupling without the addition of an external catalyst, but while still under mild
conditions. The work of our group has focused on peri-substitution, which is useful in
thermodynamically stabilising bonding motifs which are typically unstable at room temperature.22,23
However, lately we have been intrigued by the possibility of using peri-substitution to promote
reactivity that would typically require the addition of a catalyst. Due to the unique constraints of the
1

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peri-geometry, atoms in the peri-position (E) are forced into close proximity. Strain from the overlap
of occupied orbitals can be relieved by, either, the formation of a direct E–E bonding interaction or a
bridging motif between the two peri-atoms (E–X–E). As such, it was postulated that if two potentially
reactive groups were placed in the peri-positions, the rigid scaffold could lower the kinetic barrier of
the coupling reaction, promoting the formation of a direct bond or a bridging motif and hence
emulating the role of an external catalyst.
This was indeed found to be the case, as a series of peri-substituted phosphine-borane
adducts were synthesised and observed to undergo spontaneous intramolecular dehydrocoupling in
solution at room temperature. The results of these investigations are detailed below.
Results and Discussion
Bis(borane) Adducts 4a–c
Compounds 1a–b and 2 were used as the starting points for all of the reactions presented in this
work. Compound 2 was synthesised according to a previously published procedure,24 while
compounds 1a–b were synthesised via a modified version of the literature procedure.25
The synthesis and characterisation of the bis(borane) adduct 4a were recently reported by
our group.26 In its preparation, treatment of the phosphino-phosphonium salt 1a with excess
H3B·SMe2 resulted in borane mediated reduction to afford 4a as a yellow oil in quantitative yield
(Scheme 1). An analogous procedure was employed to obtain adduct 4b from the corresponding
phosphino-phosphonium salt 1b. The adduct 4b was isolated as an orange solid, which was
1
contaminated with the bridged compound 5b (≈ 20% as judged by H and 31P NMR). Pure 4b was
obtained by recrystallisation from acetonitrile.
Scheme 1: Synthesis of bis(borane) adducts 4a–c and BH2 bridged compounds 5a–c.
The 31P{1H} NMR spectrum of 4b exhibits broad singlets at δP 36.3 (iPr2P) and −7.7 (PFcH), and in the
31P NMR spectrum the signal at δP −7.7 is split into a broad doublet (1JPH = 395 Hz). Crystals of 4b
suitable for X-ray diffraction were grown from acetonitrile, the structure is shown in Figure 2 and
Tables 1–3. The structure of 4b is similar to the previously reported structure of 4a,26 with a P···P
distance of 3.521(1) Å and a large positive splay angle of +21.2(7)° (see Figure 1 for a definition),
indicating significant repulsion between the two peri-groups. Additionally, both phosphorus atoms
2

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show significant displacement from the mean plane of the acenaphthene ring (0.706 Å for P1, 0.546
Å for P9).
Figure 1: Definition of a splay angle.
The bis(borane) adduct 4c was synthesised from the novel primary phosphine 3 (Scheme 1),
which was obtained by clean reduction of the phosphonium-phosphoranide 2 with LiAlH4. The
31P{1H} NMR spectrum of compound 3 displays two doublets at δP −11.3 (iPr2P) and −101.2 (PH2),
4
with a substantial through-space coupling of JPP = 205 Hz. In the 31P NMR spectrum, the signal for
the PH2 group is split into a pseudo-quartet due to 1JPH = 204 Hz being very similar to that of 4JPP. The
1H NMR spectrum of 3 displays a doublet of doublets for the PH2 protons (δH 4.98, 1JHP = 204 Hz, 5JHP
5
4
= 48 Hz). This long range JHP interaction, in addition to the large JPP coupling, indicates a significant
through space contribution to coupling operates in this compound.27 Crystals of compound 3
suitable for single crystal X-ray diffraction were grown from THF, the structure is shown in Figure 2
and Tables 1–3. The structure indicates a clear repulsive interaction between the two phosphorus
moieties, with a P···P distance of 3.143(1) Å and a positive splay angle of 16.4(7)°. The purity of 3 as
1
obtained from the reaction was established by 31P, H and 13C NMR and was found to be sufficient
for further syntheses.
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Figure 2: Structures of 3 (top) and 4b (bottom) in the solid state. Carbon-bound hydrogen atoms omitted for clarity.
Treatment of primary bis(phosphine) 3 with excess H3B·SMe2 afforded bis(borane) adduct 4c
as the major product (δP 38.0 (br s, iPr2P), −40.8 (br s, PH2)), although the reaction was not clean.
Even with a large excess (12 equivalents) of H3B·SMe2, traces of starting material were found in the
31P{1H} NMR spectra of the crude mixture after the reaction. In addition, a broad singlet at δP 44.0
along with a sharp singlet at δP −101.4 were observed in this spectrum. Rather revealingly, the signal
at δP −101.4 splits into a triplet (1JPH = 207 Hz) in the 31P NMR spectrum which, together with the
chemical shift values, allowed these signals to be assigned to the monoborane adduct 6 (Scheme 2).
In the crude mixture, 4c and 6 were present in a ratio of approximately 5:1. A number of minor,
unidentified P containing side products were also formed.
H3B
BH3
PH2
BH3
Pr P
PH2
iPr2P
i
2
DCM
H2O
(excess)
4c
6
Scheme 2: Synthesis of monoborane adduct 6.
4

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In sharp contrast to compounds 4a and 4b, which are stable towards both air and moisture,
compound 4c is rather moisture sensitive. On a preparative scale, treatment of a dichloromethane
solution of 4c with degassed water afforded 6 as a yellow solid in near quantitative yield (Scheme 2).
The new compound was characterised by 1H, 31P, 31P{1H}, 13C{1H}, 11B, and 11B{1H} NMR spectroscopy.
Primary phosphine-borane adducts have been less extensively studied than secondary or
tertiary phosphine-boranes, and are known to be generally less stable.28 In addition, steric hindrance
arising from the peri-geometry is likely to further destabilise the bis(borane) adduct 4c with respect
to the monoborane adduct 6. This corresponds well with our observations of the instability of 4c
towards moisture, as well as the difficulty in getting complete conversion to the bis(borane) adduct.
Compound 4c could not be isolated in analytically pure form due to its crystallisation being
extremely difficult, whilst its sensitivity to air and moisture prevented chromatographic purification.
Compound 4c exhibits two broad singlets in the 31P{1H} NMR spectrum at δP 38.0 (iPr2P) and
δP −40.8 (PH2). In the 31P NMR spectrum, the signal at δP −40.8 splits into a broad triplet (1JPH = 379
Hz). One particularly distinctive signal is observed for the PH2 group in the 1H NMR spectrum, which
is split into a doublet of quartets (δH 6.14, 1JHP = 377 Hz, 3JHH = 7.1 Hz) due to coupling to the adjacent
BH3 hydrogen atoms. This, therefore, provides strong evidence that BH3 is bound to PH2 in this
1
molecule. In contrast, the PH2 signal in the H NMR spectrum of compound 6 appears as a sharp
doublet (δH 4.48, 1JHP = 207 Hz), indicating the absence of a co-ordinated borane.
It should be noted that, unlike compounds 1a–b, treatment of the phosphonium-
phosphoranide 2 with H3B·SMe2 does not result in borane mediated reduction to give 4c, but instead
yields the “push-double pull” bis(borane) adduct 7 (Scheme 3).29
Scheme 3: Synthesis of the “push-double pull” bis(borane) adduct 7.
Spontaneous Intramolecular Dehydrocoupling of 4a–c to give 5a–c
When compound 4a was allowed to stand in solution in DCM, the signals corresponding to the
bis(borane) adduct (δP 39.4 (br s, iPr2P) and −6.6 (br s, PhPH))26 were gradually replaced by a broad
doublet (δP 13.9, PiPr2, 2JPP ≈ 84.0 Hz) and a very broad signal in which coupling could not be resolved
(δP −26.3, PPh),30 corresponding with the formation of 5a (Figure 3). Complete conversion to 5a was
achieved after 8 days at room temperature (Scheme 1). 1H and 31P NMR spectroscopy confirmed that
the H atom directly bonded to phosphorus had been lost. Additionally, 11B{1H} NMR spectroscopy
1
1
revealed a broad pseudo-triplet (δB −39.4, JBP ≈ 69 Hz) and a broad doublet (δB −33.6, JBP ≈ 46 Hz),
consistent with the presence of one bridging P–B–P motif and one terminal B–P motif (Figure 4).
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Figure 3: Stacked 31P{1H} NMR spectra showing the gradual formation of compound 5a from 4a over 8 days.
Figure 4: 11B{1H} NMR of compound 5a
Crystals of 5a suitable for single crystal X-ray diffraction were grown from d6-DMSO. The
structure confirmed 5a to contain one bridging BH2 and one terminal BH3 motif (Figure 5, Tables 1–
3). A significant reduction of strain is observed in 5a in comparison to 4a, with a reduced P···P
distance of 3.1295(8) Å and smaller splay angle of +15.1(4)° (c.f. 3.61 Å and +24.4(4)° in 4a),26 as well
as decreased displacements of the P atoms from the mean plane of the acenaphthene ring (0.338 Å
for P1, 0.327 Å for P9; c.f. 0.478 and 0.816 Å in 4a).26
Based on the identity of compound 5a, it seemed likely that the bis(borane) adduct 4a had
undergone a phosphine-borane dehydrocoupling reaction. In order to confirm the evolution of
hydrogen, a solution of 4a in C6D6 was prepared and left to stand in a sealed NMR tube. After 1 day,
some conversion to compound 5a was observed by 31P{1H} NMR, and a sharp singlet of dissolved H2
was observed in the 1H NMR (δH 4.47).31 In another experiment, the conversion of 4a to 5a (in CDCl3)
1
was followed over several days at room temperature by H NMR spectroscopy.32 The reaction was
found to follow simple first order kinetics, with an approximate rate constant of 0.04 h−1. It is likely
that the driving force for this reaction is the reduction in strain on going from 4a to 5a, coupled with
the entropic gain from hydrogen evolution.
Spontaneous dehydrocoupling reactions occurring at room temperature are rather rare,
with a few examples involving very reactive precursors such as primary/secondary stibines or
bismuthines.7 In recent work by the Manners’ group, a series of primary arylamine-borane adducts
were found to undergo spontaneous dehydrocoupling at room temperature, with the rate of
dehydrocoupling increasing with decreasing electron density on the aryl substituent.8 This reactivity
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was attributed to weak B–N bonding and the increased acidity of the N–H bonds in arylamine-
boranes. By contrast, while dehydrocoupling of phosphine-boranes has been observed in the
presence of catalysts17–19,33 or at very high temperatures,15,16 spontaneous, room temperature
dehydrocoupling of a phosphine-borane adduct is without precedent in the literature.
The compound 5a bears some similarities to two cyclic boronium salts, 10 and 13, reported
by Mikołajczyk et al.34 and Costa and Schmidbaur35 (Scheme 4). Compounds 5a, 10 and 13 are all
formed by the treatment of peri-substituted precursors with borane, and all consist of two peri-
phosphorus atoms bridged by a BH2 unit. However, compounds 10 and 13 are ionic species; 10 is
thought to form via the mono(borane) adduct 9, which then reduces the halogenated solvent to
form 10.34 Compound 13 exists in equilibrium with the bis(borane) adduct 12 and forms via hydride
−
transfer to give a BH2 bridge and a BH4 counterion.35 Although these reactions are significantly
different from the dehydrocoupling observed in 4a, in all cases the driving force for the formation of
the BH2 bridge is most likely the same – reduction of strain resulting from the peri-substitution
geometry. In compound 4a this is achieved via hydrogen evolution, while for 9 and 12 (which contain
no P–H bonds) the formation of the boronium salts is preferred.
Scheme 4: Formation of the cyclic boronium salts 10 and 13.
The ferrocenyl substituted bis(borane) adduct (4b) was also found to undergo spontaneous
dehydrocoupling in solution, albeit at a slower rate than 4a. A solution of 4b left standing in CDCl3
achieved approximately 80% conversion to 5b after 2 weeks. Owing to the slow rate of reaction, 5b
was more conveniently synthesised by refluxing 4b in THF for 4 days. The observed trend in
dehydrocoupling rates (4a > 4b) correlates with the acidity of the P–H hydrogen, which is higher in
4a due to the more electron withdrawing nature of the phenyl substituent as compared to the
ferrocenyl substituent.
Compound 5b demonstrates a similar 31P{1H} NMR spectrum to 5a, displaying a broad
1
doublet (δP 16.1, JPP = 92.6 Hz, iPr2P) and a very broad unresolved signal (δP −32.0, PFc) located
upfield of the corresponding signals for 4b (ΔδP ≈ 20–25). Once again, 1H and 31P NMR spectroscopy
confirmed the loss of H directly bonded to phosphorus, and the 11B NMR spectrum displayed two
distinct boron environments. Crystals of 5b suitable for single crystal X-ray diffraction were grown
from acetonitrile. Obtained data is of somewhat poor quality, but is sufficient to demonstrate the
connectivity of the molecule. The crystal structure is shown in Figure 5 with data in Tables 1–3 and is
broadly similar to that seen for 5a.
The dehydrocoupled product of the primary bis(borane) adduct 4c was obtained by treating
3 with excess H3B·SMe2 in DCM and then, without isolating 4c, allowing the reaction mixture to stir
at room temperature for 11 days. After this time, no peaks for 4c could be observed in the 31P{1H}
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NMR spectrum of the reaction mixture. The resultant bridged compound 5c is significantly more
inert than the corresponding bis(borane) adduct, and was stable enough to be purified by flash
column chromatography. As with the previous compounds, 5c displays peaks in the 31P{1H} NMR
spectrum with ΔδP ≈ 25–30 upfield of the corresponding resonances for the parent bis(borane)
adduct 4c. Additionally, in the 31P NMR spectrum of 5c, the signal for the PH group (δP −69.5)
appears as a doublet (1JPH = 339 Hz) as opposed to the triplet seen for 4c.
Figure 5: Structures of 5a (left), 5b (centre), and 5c (right) in the solid state. Carbon-bound hydrogen atoms and second
molecule in asymmetric unit (for 5b and 5c) omitted for clarity.
Crystals of 5c suitable for single crystal X-ray diffraction were grown from slow diffusion of
hexane into its concentrated solution in DCM. The structure is presented in Figure 5, with data in
Tables 1–3. One interesting point of note is that, in contrast to 5a–b, compound 5c displays almost
no out-of-plane displacement of the peri-phosphorus atoms (0.050 Å [0.042 Å] for P1, 0.068 Å [0.101
Å] for P8, values in square brackets are for the second molecule in the asymmetric unit). This can be
attributed to the significantly reduced steric demands of the hydrogen substituent.
Given the presence of vicinal P–H and B–H bonds in compound 5c, the thermal
decomposition of this compound was investigated to verify whether a further molecule of
dihydrogen could be eliminated. After refluxing 5c in xylenes for 3 days, partial conversion (≈ 26 %
by 31P NMR) to a new compound, compound 14, was observed. Compound 14 shows two
resonances in its 31P{1H} NMR spectrum, a broad multiplet (δP 11.0, PiPr2) and a sharp singlet (δP
−136.9, PH). The low frequency chemical shift of the singlet suggests that 14 forms by loss of BH3
1
from 5c (Scheme 5). Furthermore, the 31P NMR spectrum shows a significant reduction in the JPH
coupling constant (5c, 1JPH = 339 Hz; 14, 1JPH = 185 Hz), consistent with an increase in electron density
on phosphorus due to loss of the Lewis acidic BH3.36 Due to the slow rate of the reaction, complete
conversion to 14 was not achieved and this compound was not isolated pure. Attempts to drive the
reaction to completion by prolonged heating resulted in decomposition.
H2
B
H2
B
Pr
H
i
iPr
H
P
P
P
P
iPr
BH3
iPr
Reflux, Xylenes
- BH3
5c
14
Scheme 5: Proposed initial product of thermal decomposition of 5c.
8

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Conclusion
Bis(borane) adducts 4a–c were formed by either borane mediated reduction of phosphino-
phosphonium salts 1a–b, or by treatment of the bis(phosphine) 3 with excess H3B·SMe2. All three
adducts were found to undergo spontaneous intramolecular dehydrocoupling in solution, resulting
in the formation of a P–B bond to afford the novel BH2 bridged compounds 5a–c. This reaction is
surprisingly facile, occurring at room temperature and in the absence of a catalyst (albeit in some
cases at a slow rate). The ease with which the reaction proceeds can be attributed to the unique
constraints of the peri-geometry; the two reactive moieties are held in close proximity and the
repulsive interaction between them introduces considerable strain into the system, which is reduced
on formation of a bridging P–B–P motif.
This interesting reaction serves as a demonstration of the utility of peri-substitution for
promoting unusual or unexpected reactivity. Furthermore, it highlights how manipulation of the
steric properties of a molecule can eliminate the need for a catalyst, which could be a potentially
interesting alternative approach to developing compounds for hydrogen storage.
Table 1: Selected bond lengths (Å) and angles (deg) for 3, 4b, 5a–c.
3
C1–P1
1.849(4)
99(1)
C9–P9
1.850(4)
94(1)
C1–P1–H1a
H1a–P1–H1b
4b
C1–P1–H1b
90(2)
C1–P1
1.817(3)
1.929(4)
109.7(2)
105(2)
C9–P9
1.842(3)
1.980(4)
117(2)
B1–P1
B9–P9
C1–P1–B1
C1–P1–H1
5a
B1–P1–H1
C9–P9–B9
113.6(2)
C1–P1
1.818(2)
1.932(2)
1.928(2)
112.21(8)
106.33(8)
117.28(9)
C9–P9
B9–P9
1.812(2)
1.927(2)
B1–P1
B9–P1
C1–P1–B1
C1–P1–B9
B9–P1–B1
5b†
C9–P9–B9
P9–B9–P1
110.95(8)
108.55(9)
C1–P1
1.82(1) [1.85(1)]
1.94(2) [1.94(2)]
1.94(2) [1.94(2)]
111.6(7) [108.6(7)]
111.3(7) [107.4(7)]
114.0(9) [118.2(8)]
C9–P9
B9–P9
1.82(1) [1.80(2)]
1.92(2) [1.90(2)]
B1–P1
B9–P1
C1–P1–B1
C1–P1–B9
B9–P1–B1
C9–P9–B9
P9–B9–P1
114.1(7) [108.3(7)]
110.9(9) [106.6(8)]
5c†
C1–P1
1.820(2) [1.821(2)]
1.937(2) [1.930(2)]
1.922(2) [1.914(2)]
111.90(9) [112.48(9)]
107.56(8) [107.54(8)]
118.16(9) [117.60(9)]
C9–P9
B9–P9
1.808(2) [1.807(2)]
1.910(2) [1.911(2)]
B1–P1
B9–P1
C1–P1–B1
C1–P1–B9
B9–P1–B1
C9–P9–B9
P9–B9–P1
110.15(8) [109.62(8)]
109.1(1) [109.0(1)]
† Measurements for second molecule in asymmetric unit shown in square brackets
Table 2: Peri-distances (Å), splay angles (deg) and out-of-plane displacements for 3, 4b, 5a–c.
3
4b
5a
5b†
5c†
P1···P9
3.143(1)
+16.4(7)
0.148
3.521(1)
+21.2(7)
0.706
3.1295(8)
+15.1(4)
0.338
3.181(5) [3.081(5)]
+16(3) [+15(3)]
0.332 [0.213]
3.1214(6) [3.1145(6)]
+16.5(3) [+16.3(3)]
0.050 [0.042]
splay angle
out-of-plane displacement (P1)
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out-of-plane displacement (P9)
0.068
0.546
0.327
0.450 [0.296]
0.068 [0.101]
† Measurements for second molecule in asymmetric unit shown in square brackets
Table 3: Crystallographic data for 3, 4b, 5a–c.
3
4b
5a
5b
5c
chemical formula
C18H24P2
302.34
C28H38B2FeP2
514.02
C24H32B2P2
404.08
C28H36B2FeP2
C18H28B2P2
327.99
formula weight
512.01
crystal dimensions (mm)
0.12 x 0.10 x 0.03
triclinic
P -1
0.10 × 0.10 × 0.01
monoclinic
P 21/c
0.10 × 0.06 × 0.06
triclinic
0.20 × 0.03 × 0.01
monoclinic
P 21
0.18 × 0.12 × 0.08
monoclinic
P 21/c
crystal system
space group
a (Å)
P -1
7.4543(19)
8.4923(15)
14.361(5)
79.88(3)
82.10(3)
66.47(2)
818.3(4)
2
15.806(5)
13.022(4)
12.835(4)
90.0000
94.825(6)
90.0000
2632.4(14)
4
8.3375(11)
9.4724(14)
16.108(2)
101.8880(17)
93.165(3)
112.772(3)
1134.9(3)
2
15.409(5)
11.059(3)
16.712(6)
90.0000
114.651(5)
90.0000
2588.3(14)
4
14.0458(16)
18.841(2)
14.4671(14)
90.0000
101.176(3)
90.0000
3768.3(7)
8
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
Dcalc (g cm−3
)
1.227
1.297
1.182
1.314
1.156
μ (cm−1
)
2.546
7.088
1.989
7.207
2.244
no. rflns measured (unique)
5176 (2878)
0.0643
27526 (4765)
0.0513
14052 (4112)
0.0362
34241 (9417)
0.0948
45236 (6914)
0.0360
R1†
wR2‡
0.1599
0.1533
0.1041
0.2647
0.1045
† I > 2σ(I), R1 = Σ(||Fo| - |Fc||)/Σ|Fo|. ‡ wR2 = [Σ[w(Fo2 - Fc2)2]/Σ[w(Fo2)2]]1/2, w = 1/[σ2(Fo2) + [(ap)2 +bp], where p =[ (Fo2) + 2Fc2]/3.
Experimental
General Procedures
All experiments were carried out using standard Schlenk technique or glove box unless otherwise
stated. Solvents were dried on an MBraun solvent purification system and stored over molecular
sieves prior to use. 5-Bromo-6-diisopropylphosphinoacenaphthene and phosphonium-
phosphoranide 2 were synthesised according to literature procedures.24 Where possible, new
1
compounds were fully characterized by 31P, 31P{1H}, H and 13C{1H} NMR, including measurement of
1H{31P}, H−H DQF COSY, H−P HMQC, H−C HSQC, and H−C HMBC experiments. The NMR numbering
scheme for all compounds discussed is shown in Scheme 6.
Instrumentation
All NMR spectra were recorded using a JEOL GSX Delta 270, a Bruker Avance 300, Bruker Avance
400, Bruker Avance 500 or Bruker Avance III 500 spectrometer. 85% H3PO4 was used as an external
standard in 31P, BF3·OEt2 in CDCl3 was used as an external standard in 11B, and TMS was used as an
1
internal standard in H and 13C NMR. Measurements were performed at 25 °C unless otherwise
indicated. All IR and Raman spectra were obtained in the range 4000–300 cm−1 on a Perkin-Elmer
System 2000 NIR Fourier transform spectrometer. Mass spectra were acquired by Mrs Caroline
Horseburgh at the University of St Andrews on a Micromass LCT. Elemental analysis (C, H and N) was
performed by Mr Stephen Boyer at London Metropolitan University.
X-ray experimental
Table 3 lists details of data collections and refinements. Data for compound 3 were collected at
−180(1) °C by using a Rigaku Mercury70 diffractometer. Data for compounds 4b and 5b were
collected at −180(1) °C by using a Rigaku XtaLAB P200 diffractometer. Data for compounds 5a and 5c
were collected at −100(1) °C by using a Rigaku XtaLAB P200 diffractometer. All instruments use Mo
10

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Kα radiation (λ = 0.71075 Å). Intensities were corrected for Lorentz polarization and for absorption.
The structures were solved by direct methods. Refinements were done by full-matrix least-squares
based on F2 using SHELXTL.37 CCDC 1410480-1410484 contain the supplementary crystallographic
data for this article. These data can be obtained free of charge via www.
ccdc.cam.ac.uk/data_request/cif, by emailing data_request@ccdc.cam. ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336033. Ellipsoids in all ORTEPs are drawn with 50% probability.
Scheme 6: NMR numbering scheme for all compounds discussed.
[Acenap(PiPr2)(PPh)][Cl] Phosphino-phosphonium 1a
Synthesis adapted from method published by Kilian et al.25
5-Bromo-6-diisopropylphosphinoacenaphthene (4.00 g, 11.45 mmol) was dissolved in diethyl ether
(100 mL) and cooled to −78 °C. nBuLi (4.58 mL of a 2.5 M solution in hexanes, 11.45 mmol) was
added dropwise with stirring. The solution was stirred for 2 hours at −78 °C. A solution of
dichlorophenylphosphine (1.55 mL, 2.05 g, 11.45 mmol) in diethyl ether (10 mL) was added dropwise
over 30 minutes at −78 °C and the solution left to warm to room temperature overnight. The white
precipitate was collected by filtration, washed with diethyl ether (3 × 10 mL) and dried in vacuo to
yield 1a as a fine white powder (4.109 g). Accurate yield could not be determined due to
contamination with LiCl, which however poses no problems for further syntheses. The 1H and 31P{1H}
NMR of the product were in good agreement with previously published data.25
1H NMR δH (270 MHz; CDCl3) 8.80 (1H, dd, 3JHP = 9.2 Hz, 3JHH = 7.3 Hz, 2-H), 7.84 (1H, ≈ t 3JHH = 6.8 Hz
3JHP = 6.8 Hz, 8-H), 7.72 (1H, dd, 3JHH = 7.3 Hz, 4JHP = 2.9 Hz, 3-H), 7.58 (1H, dd, 3JHH = 7.2 Hz, 4JHP = 2.6
Hz, 7-H), 7.49–7.22 (5H, m, 5 × Ph CH), 3.94–3.75 (1H, m, iPr CH), 3.75–3.61 (1H, m, iPr CH), 3.58 (4H,
br s, 11-H, 12-H), 1.39 (3H, dd, 3JHP = 19.3 Hz, 3JHH = 6.9 Hz, iPr CH3), 1.14 (3H, dd, 3JHP = 18.9 Hz, 3JHH
=
7.0 Hz, iPr CH3), 1.03 (3H, dd, 3JHH = 7.1 Hz, 3JHP = 3.8 Hz, iPr CH3), 0.95 (3H, dd, 3JHH = 7.1 Hz, 3JHP = 3.8
Hz, iPr CH3).
31P{1H} NMR δP (109 MHz; CDCl3) 61.3 (d, iPr2P), −35.3 (d, PPh), 1JPP = 304 Hz.
[Acenap(PiPr2)(PFc)][Cl] Phosphino-phosphonium 1b
Synthesis adapted from method published by Kilian et al.25
5-Bromo-6-diisopropylphosphinoacenaphthene (1.00 g, 2.86 mmol) was dissolved in diethyl ether
(20 mL) and cooled to −78 °C. nBuLi (1.14 mL of a 2.5 M solution in hexanes, 2.86 mmol) was added
dropwise with stirring. The solution was stirred for 2 hours at −78 °C. A suspension of
dichloroferrocenylphosphine (0.82 g, 2.86 mmol) in diethyl ether (20 mL) was added dropwise over
30 minutes at −78 °C and the solution left to warm to room temperature overnight. The orange
precipitate was collected by filtration, washed with diethyl ether (3 × 5 mL) and dried in vacuo to
yield 1a as a fine orange powder (1.480 g). Accurate yield could not be determined due to
11

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contamination with LiCl, which however poses no problems for further syntheses. The 1H and 31P{1H}
NMR of the product were in good agreement with previously published data.25
3
3
1H NMR δH (270 MHz; CDCl3) 8.62 (1H, dd, JHP = 9.0 Hz, JHH = 7.3 Hz, 2-H), 8.10–8.01 (1H, m, 8-H),
7.67 (1H, dd, 3JHH = 7.2 Hz, 4JHP = 2.7 Hz, 3-H), 7.62 (1H, dd, 3JHH = 7.0 Hz, 4JHP = 2.2 Hz, 7-H), 4.75–4.71
(1H, m, CpH), 4.65–4.55 (1H, m, CpH), 4.34 (5H, s, CpH), 4.30–4.25 (2H, m, 2 × CpH), 3.56 (4H, s, 11-
H, 12-H), 3.21–3.19 (2H, m, 2 × iPr CH), 1.35–0.92 (12H, m, 4 × iPr CH3).
31P{1H} NMR δP (109 MHz; CDCl3) 56.1 (d, iPr2P), −37.3 (d, PFc), 1JPP = 313 Hz.
Acenap(PiPr2)(PH2) Bis(phosphine) 3
To a stirred suspension of LiAlH4 (0.334 g, 8.8 mmol) in THF (15 mL) cooled to -78 °C, a suspension of
2 (0.50 g, 1.35 mmol) in THF (20 mL) was added slowly via cannula. The resultant bright pink solution
was allowed to warm to room temperature, with stirring, overnight. The solution was cooled to 0 °C
and degassed water (2.5 mL) was added dropwise with stirring. The mixture was then filtered to
remove insoluble impurities. Volatiles were removed in vacuo to give 3 as a pink solid (0.298 g, 0.986
mmol, 73%). The compound is highly soluble in most organic solvents, a small amount of crystals of 3
suitable for single crystal X-ray diffraction were grown from THF.
mp 140–144 °C.
IR (Nujol mull) νmax/cm−1 2293w, 2240m (PH), 1604w, 840m, 790m.
Raman (glass capillary) νmax/cm-1 3058s (ArH), 2948s and 2929s and 2866s (CH), 2294m and 2241s
(PH), 1605m, 1567s, 1331vs, 585s.
1H NMR δH (400 MHz; C6D6) 7.79–7.72 (1H, m, 2-H), 7.60 (1H, dd, 3JHH = 7.1 Hz, 3JHP = 3.3 Hz, 8-H), 7.12
(1H, dt, 3JHH = 7.2 Hz, 4JHH = 1.3 Hz, 7-H), 6.93 (1H, d, 3JHH = 7.1 Hz, 3-H), 4.98 (2H, dd, 1JHP = 204 Hz, 5JHP
= 47.8 Hz, PH2), 3.04–2.83 (4H, m, 11-H, 12-H), 2.12–1.99 (2H, m, 2 × iPr CH), 1.17 (6H, dd, 3JHP = 14.3
Hz, 3JHH = 6.9 Hz, 2 × iPr CH3), 1.00 (6H, dd, 3JHP = 12.3 Hz, 3JHH = 7.0 Hz, 2 × iPr CH3).
13C{1H} NMR δC (101 MHz; C6D6) 148.8 (s, qC-6), 147.7 (d, 4JCP = 1.9 Hz, qC-4), 140.4 (m, qC-5, qC-10),
139.7 (s, C-2), 134.5 (d, 2JCP = 2.3 Hz, C-8), 131.0 (dd, 1JCP = 23.9 Hz, 3JCP = 7.4 Hz, qC-9), 125.9 (d, 1JCP
=
1
19.8 Hz, qC-1), 119.8 (s, C-3), 119.4 (s, C-7), 30.3 (s, C-11/C-12), 29.9 (s, C-11/C-12), 26.4 (d, JCP
=
15.9 Hz, iPr CH), 26.4 (d, 1JCP = 15.8 Hz, iPr CH), 20.4 (s, iPr CH3), 20.3 (s, iPr CH3), 20.2 (s, 2 × iPr CH3).
31P NMR δP (162 MHz; C6D6) −11.3 (dm, 1JPP = 205 Hz, iPr2P), −101.2 (≈ q, 1JPP = 205, 1JPH = 204 Hz, PH2).
31P{1H} NMR δP (162 MHz; C6D6) −11.3 (d, iPr2P), −101.2 (d, PH2), 1JPP = 205 Hz.
MS (ES+) m/z 301.1 (100%, M – H)
HRMS (ES+) Found: 301.1278. Calc. for C18H23P2 (M − H): 301.1275.
Acenap(PiPr2·BH3)(PFcH·BH3) Bis(borane) 4b
Borane dimethylsulfide (0.10 mL, 94%, 0.99 mmol) was added to a stirred suspension of 1b (120 mg,
0.23 mmol) in THF (30 mL) at −78 °C. The reacꢀon was sꢀrred for 2 hours at −78 °C, then allowed to
warm to RT and stirred overnight. Volatiles were removed in vacuo to afford 4b as an orange solid.
The crude product contained the bridged compound 5b as a minor byproduct (approximately 20%).
Analytically pure material, as well as crystals suitable for single crystal X-ray diffraction, was obtained
from acetonitrile at 5 °C (50 mg, 0.10 mmol, 42%).
mp 154–155 °C.
Found: C 65.56; H 7.56. Calc. for C28H38FeB2P2: C 65.43; H 7.45.
IR (KBr disk) νmax/cm−1 2966m and 2928m (CH), 2374vs (PH), 2345s (BH), 1638m, 1604m, 1460m,
1414m, 1387m, 1316m, 1256m, 1182m, 1071s, 1028s, 929m, 831s, 671m, 643m, 492m, 446m.
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3
1H NMR δH (400 MHz; CDCl3) 8.09 (1H, dd, 3JHP = 13.2 Hz, JHH = 7.4 Hz, 2-H), 7.81 (1H, dd, 3JHP = 16.4
3
1
3
3
Hz, JHH = 7.3 Hz, 8-H), 7.70 (1H, dq, JHP = 393 Hz, JHH = 6.1 Hz, P–H), 7.39 (1H, d, JHH = 7.4 Hz, 3-H),
7.31 (1H, d, 3JHH = 7.3 Hz, 7-H), 4.59–4.55 (2H, m, 2 × CpH), 4.48–4.43 (2H, m, 2 × CpH), 4.32 (5H, s, 5
× CpH), 3.38 (4H, s, 11-H, 12-H), 3.16–3.06 (1H, m, iPr CH), 3.06–2.95 (1H, m, iPr CH), 1.60–0.40 (6H,
br m, 2 × BH3), 1.48–1.37 (9H, m, 3 × iPr CH3), 0.99 (3H, dd, 3JHP = 15.2 Hz, 3JHH = 6.8 Hz, iPr CH3).
13C{1H} NMR δC (101 MHz; CDCl3) 152.3 (s, qC-6), 151.9 (s, qC-4), 140.8 (≈ t, 3JCP = 7.9 Hz, qC-5), 140.1
2
2
1
(d, JCP = 7.5 Hz, C-2), 136.9 (d, JCP = 14.3 Hz, C-8), 133.1–132.9 (m, qC-10), 123.5 (d, JCP = 53.0 Hz,
qC-9), 120.3 (d, 3JCP = 12.9 Hz, C-7), 119.5 (d, JCP = 43.4 Hz, qC-1), 119.5 (d, JCP = 10.6 Hz, C-3), 74.3
1
3
(d, JCP = 15.9 Hz, Cp CH), 72.7 (d, JCP = 6.6 Hz, Cp CH), 72.4 (d, JCP = 3.6 Hz, Cp CH), 71.4 (d, JCP = 9.0 Hz,
1
Cp CH), 70.2 (s, 5 × Cp CH), 66.2 (d, JCP = 68.3 Hz, Cp qC), 30.2 (s, C-11/C-12), 30.0 (s, C-11/C-12),
25.5 (dd, 1JCP = 29.1 Hz, 3JCP = 2.1 Hz, iPr CH), 23.7 (d, 1JCP = 33.3 Hz, iPr CH), 19.4 (s, iPr CH3), 18.3 (s,
iPr CH3), 17.9–17.3 (m, iPr CH3), 17.5 (s, iPr CH3).
31P NMR δP (162 MHz; CDCl3) 36.2 (br s, iPr2P), −7.7 (d, 1JPH = 395 Hz, PFcH).
31P{1H} NMR δP (162 MHz; CDCl3) 36.3 (br s, iPr2P), −7.7 (br s, PFcH).
11B NMR δB (160 MHz; CDCl3) −39.2 (br m, 2 × BH3).
11B{1H} NMR δB (160 MHz; CDCl3) −38.7 (br m, BH3), −39.4 (br m, BH3).
MS (ES−) m/z 485.1 (12%, M – 2BH3 − H), 499.2 (86, M – BH3 − H), 513.2 (100, M − H).
HRMS (ES−) Found: 513.1919. Calc. for C28H37P2FeB2 (M + H): 513.1906
Acenap(PiPr2·BH3)(PH2·BH3) Bis(borane) 4c
Borane dimethylsulfide (0.40 mL, 94%, 3.98 mmol) was added to a stirred solution of 3 (100 mg, 0.33
mmol) in DCM (5 mL) at −78 °C. The reacꢀon was allowed to warm to room temperature over 1 h,
then stirred for 30 minutes. Volatiles were removed in vacuo to afford crude 4c as an off-white sticky
solid (105 mg). Compound was not purified due to its high sensitivity towards moisture and oxygen,
which prevented chromatographic separation. NMR data was assigned from the crude product
mixture.
3
1H NMR δH (400 MHz; CDCl3) 8.26 (1H, dd, 3JHP = 14.1 Hz, JHH = 7.5 Hz, 2-H), 8.19 (1H, dd, 3JHP = 19.3
3
1
3
Hz, JHH = 7.3 Hz, 8-H), 7.49–7.42 (2H, m, 3-H, 7-H), 6.14 (2H, dq, JHP = 377 Hz, JHH = 7.1 Hz, PH2),
3.44 (4H, s, 11-H, 12-H), 2.90–2.77 (2H, m, 2 × iPr CH), 1.50–0.30 (6H, br m, 2 × BH3), 1.38 (6H, dd,
3JHP = 14.6 Hz, 3JHH = 6.9 Hz, 2 × iPr CH3), 1.06 (6H, dd, 3JHP = 15.6 Hz, 3JHH = 7.1 Hz, 2 × iPr CH3).
31P NMR δP (162 MHz; CDCl3) 38.0 (br s, iPr2P), −40.8 (br s, PH2).
31P{1H} NMR δP (162 MHz; CDCl3) 38.0 (br s, iPr2P), −40.8 (t, 1JPH = 379 Hz, PH2).
11B NMR δB (96 MHz; CDCl3) −40.1 (br m, 2 × BH3).
11B{1H} NMR δB (96 MHz; CDCl3) −40.1 (br m, 2 × BH3).
Acenap(PiPr2)(μ-BH2)(PPh·BH3) 5a
Borane dimethylsulfide (0.15 mL, 94%, 1.49 mmol) was added to a stirred solution of 1a (150 mg,
0.363 mmol) in THF (5 mL) at −78 °C. The reacꢀon was sꢀrred for 2 hours at −78 °C, then allowed to
warm to room temperature and stirred overnight. Volatiles were removed in vacuo to afford the
bis(borane) adduct 4a, which was re-dissolved in DCM (5 mL) and stirred at room temperature for 8
days. Volatiles were removed in vacuo to afford 5a as an off-white solid in near quantitative yield
(0.145 g, 0.359 mmol, 99%). Crystals suitable for single crystal X-ray diffraction were grown from d6-
DMSO at room temperature.
mp 230 °C (decomp).
Found: C 71.23; H 8.05. Calc. for C24H32B2P2: C 71.34; H 7.98.
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IR (KBr disk) νmax/cm−1 3030w (ArH), 2970m and 2934m and 2872m (CH), 2449m, 2364vs (BH),
2258m, 1597s, 1488m, 1453s, 1436s, 1388m, 1340m, 1248m, 1139m, 1111m, 1057vs, 882m, 847s,
829m, 739s, 699vs, 666m, 614m, 472m, 402m.
Raman (glass capillary) νmax/cm−1 3060 (s, Ar–H), 2942 (s, νC–H), 2895 (m), 2453 (m), 2388 (m), 2340
(m, νB–H), 1599 (s), 1578 (s), 1444 (s), 1419 (s), 1343 (vs), 1002 (s), 832 (m), 739 (m), 573 (s).
3
1H NMR δH (500 MHz; CDCl3) 8.01 (1H, dd, 3JHP = 12.2 Hz, JHH = 7.2 Hz, 2-H), 7.67 (1H, dd, 3JHP = 10.3
3
Hz, JHH = 7.2 Hz, 8-H), 7.59–7.51 (2H, m, o-Ph CH), 7.42 (1H, d, 3JHH = 7.5 Hz, 7-H), 7.40 (1H, d, 3JHH
=
7.3 Hz, 3-H), 7.30–7.23 (3H, m, m/p-Ph CH), 3.50–3.39 (4H, m, 11-H, 12-H), 2.76–2.64 (1H, m, iPr CH),
3
2.38–2.26 (1H, m, iPr CH), 2.10–0.70 (5H, br m, BH2 and BH3), 1.28 (3H, dd, JHP = 14.9 Hz, 3JHH = 7.1
Hz, iPr CH3), 1.25 (3H, dd, 3JHP = 16.0 Hz, 3JHH = 7.2 Hz, iPr CH3), 1.17 (3H, dd, 3JHP = 15.7 Hz, 3JHH = 7.0
Hz, iPr CH3), 0.93 (3H, dd, 3JHP = 15.9 Hz, 3JHH = 7.2 Hz, iPr CH3).
13C{1H} NMR δC (126 MHz; CDCl3) 153.0 (m, qC-6), 149.7 (s, qC-4), 139.8 (dd, JCP = 8.6 Hz, JCP = 6.5
Hz, qC-5), 138.5 (d, 2JCP = 8.1 Hz, C-2), 137.6 (dd, 1JCP = 42.1 Hz, 3JCP = 7.0 Hz, i-Ph qC), 135.2 (dd, 2JCP
9.1 Hz, 2JCP = 5.3 Hz, qC-10), 134.2 (s, C-8), 132.5 (d, 2JCP = 8.7 Hz, o-Ph CH), 128.9 (d, 4JCP = 2.1 Hz, p-
3
3
=
Ph CH), 128.1 (d, 3JCP = 9.2 Hz, m-Ph CH), 124.6 (dd, 1JCP = 41.4 Hz, 3JCP = 5.0 Hz, qC-1), 121.0 (d, 3JCP
=
9.6 Hz, C-3), 118.8 (d, 3JCP = 8.9 Hz, C-7), 114.5 (dd, JCP = 56.2 Hz, 3JCP = 3.5 Hz, C-9), 30.6 (s, C-11/C-
12), 30.2 (s, C-11/C-12), 24.3 (dd, 1JCP = 35.0 Hz, 3JCP = 1.8 Hz, iPr CH), 23.0 (dd, 1JCP = 35.3 Hz, 3JCP = 4.7
Hz, iPr CH), 18.4 (s, iPr CH3), 17.8 (s, iPr CH3), 16.7 (s, iPr CH3), 16.6 (s, iPr CH3).
31P NMR δP (202 MHz; CDCl3) 13.7 (br s, iPr2P), −26.4 (br s, PPh).
1
31P{1H} NMR δP (202 MHz; CDCl3) 13.9 (br d, 1JPP = 84.0 Hz, iPr2P), −26.3 (br m, PPh).
11B NMR δB (160 MHz; CDCl3) −33.6 (br m, BH3), −39.4 (br m, BH2).
11B{1H} NMR δB (160 MHz; CDCl3) −33.6 (br d, 1JBP = 46.2 Hz, BH3), −39.4 (br ≈t, 1JBP = 69.3 Hz, BH2).
MS (ES+) m/z 391.2 (28%, M – BH3 + H), 427.2 (100, M + Na)
HRMS (ES+) Found: 427.2052. Calc. for C24H32P2B2Na (M + Na): 427.2058.
Acenap(PiPr2)(μ-BH2)(PFc·BH3) 5b
Borane dimethylsulfide (0.10 mL, 94%, 0.99 mmol) was added to a stirred suspension of 1b (120 mg,
0.23 mmol) in THF (30 mL) at −78 °C. The reacꢀon was sꢀrred for 2 hours at −78 °C, then allowed to
warm to RT and stirred overnight to afford 4b, which was not isolated. The reaction mixture was
heated under reflux for 4 days. Volatiles were removed in vacuo to afford 5b as an orange oil.
Analytically pure material was obtained by filtering through silica gel, eluting with DCM (102 mg,
0.20 mmol, 87%). Crystals of 5b suitable for single crystal X-ray diffraction were grown by slow
evaporation from acetonitrile.
mp 180 °C (decomp)
Found: C 65.76; H 6.93. Calc. for C28H36FeB2P2: C 65.68; H 7.09.
IR (KBr disk) νmax/cm−1 2967m and 2932m (CH), 2437s, 2362vs (BH), 1598s, 1448m, 1387m, 1332m,
1255m, 1169s, 1105m, 1059s, 1025s, 828s, 670s , 492s, 453m.
Raman (glass capillary) νmax/cm−1 3110s (ArH), 2929s (CH), 2440w, 2381m (BH), 1601s, 1575s, 1447s,
1417s, 1335vs, 1173s, 1107vs, 1060m, 830m, 729m, 552m, 402m, 368m, 321s.
3
1H NMR δH (500 MHz; CDCl3) 7.71 (1H, dd, 3JHP = 11.5 Hz, JHH = 7.2 Hz, 2-H), 7.64 (1H, dd, 3JHP = 10.3
Hz, 3JHH = 7.2 Hz, 8-H), 7.35 (1H, d, 3JHH = 7.1 Hz, 7-H), 7.27–7.23 (1H, m, 3-H), 4.94 (1H, s, CpH), 4.43
(2H, s, 2 × CpH), 4.39 (5H, s, 5 × CpH), 4.32 (1H, s, CpH), 3.43–3.27 (4H, m, 11-H, 12-H), 2.90–2.80
(1H, m, iPr CH), 2.80–2.70 (1H, m, iPr CH), 2.20–0.60 (5H, br m, BH2 and BH3), 1.52 (3H, dd, 3JHP = 16.6
Hz, 3JHH = 7.3 Hz, iPr CH3), 1.42 (3H, dd, 3JHP = 16.3 Hz, 3JHH = 6.9 Hz, iPr CH3), 1.32 (3H, dd, 3JHP = 14.4
Hz, 3JHH = 7.1 Hz, iPr CH3), 1.11 (3H, dd, 3JHP = 15.5 Hz, 3JHH = 7.0 Hz, iPr CH3).
3
3
13C{1H} NMR δC (101 MHz; CDCl3) 152.7 (s, qC-6) , 148.1 (s, qC-4) , 139.6 (dd, JCP = 9.1 Hz, JCP = 6.4
Hz, qC-5), 135.9 (d, 2JCP = 5.7 Hz, C-2), 133.8 (m, qC-10), 133.5 (s, C-8), 130.2 (dd, 1JCP = 38.6 Hz, 3JCP
=
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1
3.2 Hz, qC-1), 120.8 (d, 3JCP = 8.5 Hz, C-3), 118.4 (d, 3JCP = 8.8 Hz, C-7), 114.6 (dd, JCP = 55.7 Hz, 3JCP
=
2.7 Hz, qC-9), 75.1 (dd, 1JCP = 53.3 Hz, 2JCP = 12.2 Hz, Cp qC), 74.2 (d, JCP = 10.4 Hz, Cp CH), 71.7 (d, JCP
= 5.9 Hz, Cp CH), 71.0 (d, JCP = 6.1 Hz, Cp CH), 70.6 (d, JCP = 6.8 Hz, Cp CH), 69.8 (s, 5 × Cp CH), 30.6 (s,
1
1
2
C-11/C-12), 30.0 (s, C-11/C-12), 25.9 (d, JCP = 34.6 Hz, iPr CH), 21.4 (dd, JCP = 36.0 Hz, JCP = 9.0 Hz,
iPr CH), 19.2 (s, iPr CH3), 17.8 (s, iPr CH3), 17.4 (d, 2JCP = 2.7 Hz, iPr CH3), 16.1 (d, 2JCP = 3.8 Hz, iPr CH3).
31P NMR δP (202 MHz; CDCl3) 16.1 (br s, iPr2P), −32.0 (br s, PFc).
31P{1H} NMR δP (202 MHz; CDCl3) 16.1 (br d, 1JPP = 92.6 Hz, iPr2P), −32.0 (br m, PFc).
11B NMR δB (160 MHz; CDCl3) −33.9 (br m, BH3), −41.0 (br m, BH2).
11B{1H} NMR δB (160 MHz; CDCl3) δ −33.7 (br m, BH3), −41.1 (br m, BH2).
MS (ES+) m/z 498.1 (100%, M − BH3), 512.2 (38, M).
HRMS (ES+) Found: 498.1483. Calc. for C28H33FeP2B (M – BH3): 498.1500.
Acenap(PiPr2)(μ-BH2)(PH·BH3) 5c
Borane dimethylsulfide (0.40 mL, 94%, 3.98 mmol) was added to a stirred solution of 3 (100 mg, 0.33
mmol) in DCM (5 mL) at −78 °C. The reacꢀon was allowed to warm to room temperature over 1 h,
then left to stir at room temperature for 11 days. Distilled water (10 mL) was added and the reaction
stirred for 1 h at room temperature. The product was extracted with DCM (3 × 10 mL) in air and the
combined washings were dried over MgSO4. Volatiles were removed in vacuo and the crude product
was purified by flash column chromatography on silica, eluting with DCM, to yield 5c as a white
crystalline solid (45 mg, 0.137 mmol, 41%). Crystals of 5c suitable for single crystal X-ray diffraction
were grown by diffusion of hexane into a concentrated solution of 5c in DCM.
mp 140 °C (decomp).
Found: C 65.89; H 8.70. Calc. for C18H28B2P2: C 65.92; H 8.61.
IR (KBr disk) νmax/cm−1 2975m and 2959s (CH), 2930m, 2872m, 2444s (BH), 2368vs (PH), 2259w,
1710w, 1597s, 1492m, 1461s, 1418m, 1387m, 1367w, 1333m, 1257m, 1217w, 1139m, 1103m,
1065vs, 1040m, 909s, 883m, 849s, 715s, 629m, 395w.
Raman (glass capillary) νmax/cm−1 3064m, 2962s, 2934vs (CH), 2901vs, 2462m (BH), 2392m, 2351vs
(PH), 1599s, 1575s, 1441s, 1420m, 1337vs, 1054m, 883m, 830m, 735m, 585m, 571s.
3
3
1H NMR δH (500 MHz; CDCl3) 8.37 (1H, dd, JHP = 13.4 Hz, JHH = 7.1 Hz, 2-H), 7.67 (1H, dd, 3JHP = 10.3
3
3
3
Hz, JHH = 7.2 Hz, 8-H), 7.46 (1H, d, JHH = 7.1 Hz, 3-H), 7.42 (1H, d, JHH = 7.2 Hz, 7-H), 4.89 (1H, br d,
1JHP = 328 Hz, PH), 3.45 (4H, s, 11-H, 12-H), 2.78–2.64 (1H, m, iPr CH), 2.55–2.41 (1H, m, iPr CH), 1.90–
0.60 (5H, br m, BH2 and BH3), 1.32 (3H, dd, 3JHP = 16.6 Hz, 3JHH = 6.9 Hz, iPr CH3), 1.29–1.23 (6H, m, 2
× iPr CH3), 1.07 (3H, dd, 3JHP = 16.0 Hz, 3JHH = 7.0 Hz, iPr CH3).
13C{1H} NMR δC (126 MHz; CDCl3) 153.1 (d, 4JCP = 2.0 Hz, qC-4), 150.1 (s, qC-6), 139.5 (dd, 3JCP = 8.2 Hz,
3JCP = 5.9 Hz, qC-5), 138.4 (d, 2JCP = 10.9 Hz, C-2), 135.8 (dd, 2JCP = 8.6, 2JCP = 2.2 Hz, qC-10), 134.4 (s, C-
8), 120.6 (d, 3JCP = 10.7 Hz, C-3), 119.1 (d, 3JCP = 8.9 Hz, C-7), 118.8 (dd, 1JCP = 41.0 Hz, 3JCP = 6.8 Hz, qC-
1), 112.6 (dd, 1JCP = 55.9 Hz, 3JCP = 4.1 Hz, qC-9), 30.5 (s, C-11/C-12), 30.0 (s, C-11/C-12), 25.1 (d, 1JCP
36.1 Hz, iPr CH), 22.4 (dd, JCP = 36.4, 3JCP = 4.5 Hz, iPr CH), 17.7 (d, 2JCP = 2.2 Hz, iPr CH3), 17.2 (s, iPr
CH3), 17.1 (s, iPr CH3), 16.6 (d, 2JCP = 1.8 Hz, iPr CH3).
=
1
31P NMR δP (202 MHz; CDCl3) 12.8 (br s, iPr2P), −69.5 (br d, 1JPH = 339 Hz, PH).
31P{1H} NMR δP (202 MHz; CDCl3) 12.8 (br d, 1JPP = 79.9 Hz, iPr2P), −69.5 (br m, PH).
11B NMR δB (160 MHz; CDCl3) −37.2 (br m, BH3), −41.9 (br m, BH2).
11B{1H} NMR δB (160 MHz; CDCl3) −37.1 (br m, BH3), −41.9 (br
≈
t, 1JBP = 63.0 Hz, BH2).
MS (ES+) m/z 351.2 (100%, M + Na)
HRMS (ES+) Found: 351.1753. Calc. for C18H28P2B2Na (M + Na): 351.1750
15

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DOI: 10.1039/C5DT02539G
Acenap(PiPr2·BH3)(PH2) 6
Borane dimethylsulfide (0.72 mL, 94%, 7.6 mmol) was added to a stirred solution of 3 (180 mg, 0.60
mmol) in DCM (10 mL) at 0 C. The reaction was stirred for 2 hours at room temperature, then
cooled to 0 C and degassed water (10 mL) was added cautiously. The reaction mixture was stirred
°
°
for a further 2 hours at room temperature, the organic layer was separated and volatiles removed in
vacuo to afford 6 as a yellow solid in quantitative yield (188 mg, 0.59 mmol, 99%).
1H NMR δH (400 MHz; CDCl3) 8.54 (1H, dd, 3JHP = 17.4 Hz, 3JHH = 7.4 Hz, 8-H), 8.04–8.00 (1H, m, 2-H),
7.33 (1H, d, 3JHH = 7.3 Hz, 7-H), 7.27 (1H, d, 3JHH = 7.2 Hz, 3-H), 4.48 (2H, d, 1JHP = 207 Hz, PH2), 3.63–
3.49 (2H, m, 2 × iPr CH), 3.38 (4H, s, 11-H, 12-H), 1.70–0.30 (3H, br m, BH3), 1.42 (6H, dd, 3JHP = 15.0
Hz, 3JHH = 7.0 Hz, 2 × iPr CH3), 0.88 (6H, dd, 3JHP = 16.2 Hz, 3JHH = 7.1 Hz, 2 × iPr CH3).
13C{1H} NMR δC (75 MHz; CDCl3) 152.3 (s, qC-6), 150.1 (s, qC-4), 143.8 (d, 2JCP = 4.6 Hz, C-8), 143.5 (br
2
3
3
2
d, JCP = 20.0 Hz, C-2), 140.9 (≈ t, JCP = 7.1 Hz, JCP = 7.1 Hz, qC-5), 137.0 (d, JCP = 26.6 Hz, qC-10),
120.5 (d, 1JCP = 43.7, qC-9), 119.8 (s, C-3), 119.2 (d, 3JCP = 14.7 Hz, C-7), 117.8 (d, 1JCP = 16.0 Hz, qC-1),
30.0 (s, C-11/C-12), 29.9 (s, C-11/C-12), 25.2 (d, 1JCP = 28.4 Hz, iPr CH), 24.8 (d, 1JCP = 28.3 Hz, iPr CH),
19.1 (s, 2 × iPr CH3), 18.9 (s, 2 × iPr CH3).
31P NMR δP (109 MHz; CDCl3) 44.5 (m, iPr2P), −101.3 (t, 1JPH = 207 Hz, PH2).
31P{1H} NMR δP (109 MHz; CDCl3) 44.0 (m, iPr2P), −101.4 (s, PH2).
11B NMR δB (96 MHz; CDCl3) −41.9 (br m, BH3).
11B{1H} NMR δB (96 MHz; CDCl3) −41.9 (br d, 1JBP = 66.4 Hz, BH3).
Acenap(PiPr2)(μ-BH2)(PH) 14
A suspension of 5c (0.100 g, 0.304 mmol) in xylenes (20 mL) was heated under reflux. At high
temperatures, all solid dissolved to give a yellow solution. After 3 days volatiles were removed in
vacuo to give a yellow solid (0.098 g) containing
≈ 74% 5c and ≈ 26% 14. Compound 14 was not
isolated pure due to its instability to air and moisture and NMR data was assigned from the mixture.
Attempts to bring the reaction to completion via prolonged heating under reflux resulted in
decomposition to a complex mixture of products.
1H NMR δH (400 MHz; CDCl3) 7.98 (1H, dd, 3JHP = 12.1 Hz, 3JHH = 7.0 Hz, 2-H), 7.61 (1H, dd, 3JHP = 9.9
Hz, 3JHH = 7.2 Hz, 8-H), 7.35 (1H, d, 3JHH = 7.2 Hz, 7-H), 7.25 (1H, d, 3JHH = 7.0 Hz, 3-H), 3.40–3.33 (4H,
m, 11-H, 12-H). Signals for H directly bound to phosphorus/boron and iPr groups were obscured by
signals from 5c or were too weak to be seen.
31P NMR δP (162 MHz; CDCl3) 11.1 (br s, iPr2P), −137.0 (br d, 1JPH = 185 Hz, PH).
31P{1H} NMR δP (162 MHz; CDCl3) 11.0 (m, iPr2P), −136.9 (s, PH).
Acknowledgements
This work was financially supported by the EPSRC and COST action CM1302 SIPs. The authors would
also like to thank the University of St Andrews NMR Service and to Mrs Caroline Horsburgh for
running the MS spectra.
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DOI: 10.1039/C5DT02539G
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17