Iminoborylene complexes: evaluation of synthetic routes towards BN-allenylidenes and unexpected reactivity towards carbodiimides

Article abstract of DOI:10.1039/c5dt00131e

The synthetic and reaction chemistries of cationic iminoborylene complexes [L_nM=B=N=CR₂]⁺, which feature a unique heterocumulene structure, have been systematically investigated. Precursors of the type CpFe(CO)₂B(Cl)NCAr₂ (Ar = p-Tol/Mes, 5c/d) have been generated by B-centred substitution chemistry using CpFe(CO)₂BCl₂ and suitable lithiated ketimines - a reaction which is found to be highly sensitive to the steric bulk at both the metal fragment and the ketimino group. Carbonyl/phosphine exchange (using PCy₃ or PPh₃), followed by halide abstraction allows for the generation of the cationic iminoborylenes [CpFe(PR₃)(CO)(BNCAr₂)]⁺[BAr^X ₄]^- (R = Cy, Ar = p-Tol/Mes, 12c/d; R = Ph, Ar = Mes, 13d; Ar^X = 3,5-X₂C₆H₃ where X = Cl, CF₃) which have been characterized spectroscopically and by X-ray crystallography. The reactivity of these iminoborylene systems towards a range of nucleophiles and unsaturated substrates has been investigated. The latter includes the first examples of M=B metathesis reactivity with a carbodiimide, and results in Fe=B cleavage and formation of the isonitrile complexes [CpFe(PCy₃)(CO)(CNR)]⁺[BAr^Cl ₄]^- (R = ⁱPr/Cy, 16/17).

Full text of DOI:10.1039/c5dt00131e

Volume 44 Number 25 7 July 2015 Pages 11243–11672
Dalton
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An international journal of inorganic chemistry
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PAPER
J. Niemeyer, S. Aldridge et al.
Iminoborylene complexes: evaluation of synthetic routes towards
BN-allenylidenes and unexpected reactivity towards carbodiimides

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PAPER
Iminoborylene complexes: evaluation of synthetic
routes towards BN-allenylidenes and unexpected
reactivity towards carbodiimides†‡
Cite this: Dalton Trans., 2015, 44,
11294
J. Niemeyer,*a M. J. Kelly,b I. M. Riddlestone,b D. Vidovic§b and S. Aldridge*b
The synthetic and reaction chemistries of cationic iminoborylene complexes [LnMvBvNvCR2]+, which
feature a unique heterocumulene structure, have been systematically investigated. Precursors of the type
CpFe(CO)2B(Cl)NCAr2 (Ar = p-Tol/Mes, 5c/d) have been generated by B-centred substitution chemistry
using CpFe(CO)2BCl2 and suitable lithiated ketimines – a reaction which is found to be highly sensitive to
the steric bulk at both the metal fragment and the ketimino group. Carbonyl/phosphine exchange (using
PCy3 or PPh3), followed by halide abstraction allows for the generation of the cationic iminoborylenes
−
[CpFe(PR3)(CO)(BNCAr2)]+[BArX
]
(R = Cy, Ar = p-Tol/Mes, 12c/d; R = Ph, Ar = Mes, 13d; ArX = 3,5-
4
X2C6H3 where X = Cl, CF3) which have been characterized spectroscopically and by X-ray crystallography.
The reactivity of these iminoborylene systems towards a range of nucleophiles and unsaturated substrates
has been investigated. The latter includes the first examples of MvB metathesis reactivity with a carbodi-
imide, and results in FevB cleavage and formation of the isonitrile complexes [CpFe(PCy3)(CO)(CNR)]+-
Received 12th January 2015,
Accepted 10th February 2015
DOI: 10.1039/c5dt00131e
www.rsc.org/dalton
[BArCl − (R = iPr/Cy, 16/17).
]
4
developed.4 These species feature a mono-substituted boron
fragment, and are of particular interest due to their close
Introduction
The investigation of boron-transition metal complexes has relationship with archetypal organometallic complexes.5 Along
attracted widespread attention in recent years. A number of these lines, fluoroborylene (LnMBF) and aminoborylene
novel classes of compounds featuring conventional 2-centre (LnMBNR2) species have been have been synthesized, rep-
2-electron metal–boron bonds have been studied, not only resenting isolobal analogues of classical carbonyl (LnMCO)6
with respect to their structural and bonding properties, but and vinylidene (LnMCCR2) complexes.7,8
also with a view to targeting new modes of reaction chemistry.1
Reactivity-wise the chemistry of many borylene complexes
Within this area, boryl complexes, LnM(BX2), featuring a di- is dominated by the electrophilicity of the boron centre, which
substituted boron-fragment coordinated at M were the first to underpins their use in C–H activation9 or cycloaddition reac-
be discovered,2 and have subsequently been implicated in a tions.7a,10 One possibility, with precedent in organometallic
number of unprecedented transformations, such as the bory- systems, to further broaden the scope of reactivity of tran-
lation of unactivated hydrocarbon substrates.3
sition-metal boron complexes is by the introduction of further
More recently, reliable synthetic routes to subvalent tran- elements of unsaturation into the boron ligand. Thus, for
sition metal borylene complexes, (LnM)x(BX), have also been example, Braunschweig and co-workers have achieved this by
use of boryl ligands containing B–X double or triple bonds
(X = NR, O or CR2, Scheme 1).11 Taking this idea further, we have
recently communicated12 the first examples of cationic imino-
borylene complexes [LnMvBvNvCR2]+ featuring an extended
array of unsaturated bonds (Scheme 1).13 Such complexes can be
viewed as hetero-analogues of well-known allenylidene com-
plexes,14 which show a highly versatile reaction chemistry result-
aInstitute of Organic Chemistry, Department of Chemistry, University of Duisburg-
Essen, Universitätsstrasse 7, 45141 Essen, Germany.
E-mail: jochen.niemeyer@uni-due.de
bInorganic Chemistry Laboratory, Department of Chemistry, University of Oxford,
South Parks Road, Oxford, OX1 3QR, UK. E-mail: SimonAldridge@chem.ox.ac.uk
†Dedicated to the memory of Professor Ken Wade FRS.
‡Electronic Supplementary Information (ESI) available: Synthetic details and ing from their dual α, γ-electrophilicity and β-nucleophilicity.
analytical data for all compounds. CCDC 1037786 and 1037787 (crystallographic
With this in mind, we set out to uncover new patterns of reacti-
data for 6 and 16). For ESI and crystallographic data in CIF or other electronic
vity for iminoborylene complexes which are otherwise inaccessible
format see DOI: 10.1039/c5dt00131e
to known alkyl- or aminoborylene systems.4,7
§Current address: Division of Chemistry and Biological Chemistry, School of
Herein, we now report in full on synthetic approaches
towards iminoborylene systems, and their reaction chemistry
Physical and Mathematical Sciences, Nanyang Technological, University, 21
Nanyang Link, 637371 Singapore.
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Scheme 3 Synthesis of iminoboryl complexes 5 via dichloro-iminobor-
anes (top) or dichloroboryl iron precursors (bottom).
Scheme 1 Highly unsaturated metal–boron complexes featuring
iminoboryl, oxoboryl, alkylideneboryl (top) and iminoborylene ligands
(bottom, also showing the isolobal relationship with allenylidenes).
upper). While Cl2B(NvCPh2), 2a, was readily synthesized
according to Wade’s original procedure,18 it showed no reactiv-
ity towards ferrate 1. Assuming that the dimeric nature of 2a
(indicated by its 11B NMR shift of δB = −7 ppm) is responsible
for its low reactivity, we attempted to generate monomeric
dichloro-(imino)boranes by the use of bulkier ketimino substi-
tuents (e.g. R = Mes or Trip). These syntheses were initially fru-
strated by a ligand redistribution reaction which apparently
occurs on exposure to continuous vacuum [yielding ClB-
(NvCR2)2], and which prevents isolation of the pure dichloro
(ketimino)-boranes.19 This problem could be circumvented by
in situ generation (see ESI‡), which generates the corres-
ponding monomeric compounds Cl2B(NvCR2) (R = Mes/Trip
(2d/e), δB = 26/27 ppm). However, these systems do not show
clean reactivity towards 1, with the starting borane being the
predominant species in the reaction mixtures even under
forcing conditions.
both with respect to anionic nucleophiles and unsaturated
substrates. A key finding is the discovery of novel metathesis-
type reactivity towards carbodiimides, RNCNR.
Results and discussion
The synthesis of terminal borylene complexes has been
achieved using a variety of different approaches, including
double salt elimination,4f,g metal-to-metal borylene transfer,15
and dehydrogenation of σ-borane complexes.16 Moreover,
halide abstraction from haloboryl complexes has been shown
to give access to cationic borylenes in a reliable fashion.17
Based on this approach, we envisaged the use of suitable
imino-functionalized haloboryl complexes as precursors,
which upon halide abstraction with sodium tetra-arylborates
would give the desired cationic iminoborylenes (Scheme 2).
For this reason, we shifted our synthetic strategy towards a
reversed order of bond formation reactions at boron, employ-
ing the known reaction of 1 with BCl3 to generate the iron
dichloroboryl complex 3 in situ (δB = 91 ppm).20 Complex 3 was
Synthesis of iminoboryl complexes
In order to put our synthetic efforts towards imino-substituted
systems on a comparable basis to known complexes, we
initially decided to target the [CpFe(CO)2] unit as the metal
fragment, given its successful use for the generation of related
cationic aminoborylenes.7a,b For the construction of precursors
featuring the necessary array of consecutive Fe–B–N–C bonds,
we evaluated two synthetic approaches, differing in the order
of formation of the relevant bonds to the boron centre
(Scheme 3).
then treated with
a series of ketiminolithium reagents
t
LiNvCR2 [R = Bu/Ph/p-Tol/Mes/Trip (4a–e)],21 to install the
B–N linkage (Scheme 3, lower). Accordingly, the reactions with
less bulky lithium salts (e.g. 4a–d) lead to clean formation of
1
the desired iminoboryl complexes 5a–d (as judged by H and
11B NMR spectroscopy), which could be purified by precipi-
tation from hexane in case of the p-tolyl- and mesityl-substi-
tuted complexes (5c/d, 38–52%); the high solubility of
complexes 5a/b, on the other hand, prevented their isolation
as pure compounds. By contrast, the reaction of the lithium
salt LiNvCTrip2 (4e) with 3 gives a different type of boron-con-
Mirroring existing synthetic routes to [CpFe(CO)2] boryl
complexes,17c we initially attempted the generation of com-
plexes of type 5 by reaction of the anionic [CpFe(CO)2]−
taining product, with the high field 11B chemical shift (δB
=
reagent
1 (as the sodium salt) with the corresponding
27 ppm) arguing against Fe–B bond formation. The product is
tentatively assigned as borane 2e, resulting from the nucleo-
philic displacement of the [CpFe(CO)2]− anion (rather than
chloride) from precursor 3. Such a transformation has recent
precedent,22 and is presumably induced by the large steric
bulk of the bis(triisopropylphenyl)ketimino group.
dichloro-(imino)boranes 2, thus establishing the B–N connec-
tivity prior to the formation of the Fe–B bond (Scheme 3,
Complexes 5c/d have been characterized spectroscopically,
showing the expected NMR resonances for the [CpFe(CO2)]
fragment (Cp: δH = 4.26/4.30 ppm, δC = 84.4/84.6 ppm, CO:
δC = 215.3/215.3 ppm) and [B–NvC] fragments (δC = 150.7/
153.1 ppm, δB = 50/47 ppm). Additionally, in the case of 5c
structural authentication could be achieved by X-ray crystallo-
Scheme 2 Target synthesis of iminoborylene complexes by halide
abstraction from halo(imino)boryl complexes.
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graphy (Fig. 2). In the solid state, 5c exhibits a near-linear
arrangement of the B–C–N unit [∠ B(11)–N(13)–C(14)
=
175.6(3)°], consistent with a significant degree of N → B
π-donation, a finding also reflected in the short B–N [1.349(4)
Å] and relatively long Fe–B bond lengths [2.016(4) Å].
Having established a viable synthetic route for the gen-
eration of complexes of type 5 by boron-centred substitution
chemistry, and with the steric constraints of the ketimino
nucleophile now apparent, we set out to investigate the scope
of this approach by variation of the metal fragment. Thus, we
generated the previously described tungsten dichloroboryl
complex 6 (δB = 91 ppm) alongside its bromo analogue 7 (δB =
84 ppm) by reaction of the tungstate Na[CpW(CO3)] with the
respective trihaloboranes.23
Although 6 has previously been reported by Schmid and
Nöth, it has not been structurally characterized, and given the
dearth of structural data available for dihaloboryl systems we
sought to investigate it crystallographically. Accordingly, the
solid-state structure of 6 (Fig. 1) features a W–B bond [2.22(2)
Å] which is considerably longer than in the corresponding
CpFe(CO)2BCl2 complex 3 [1.942(3) Å] (even taking into
account the larger van der Waals radius of tungsten vs. iron:
2.10 vs. 2.05 Å),20a,24 while the B–Cl bonds are in the expected
range [e.g. 1.78(1) and 1.79(1) Å for 6, cf. 1.781(6) and 1.783(4)
Å for 3].20a Due to the presence of three carbonyl co-ligands,
complex 6 is sterically rather congested when compared to 3,
as can be seen from the close B–CO contacts [B(13)–C(11)
2.37(2), B(13)–C(2) 2.53(2) Å, cf. B–C(1) 2.574(5), B–C(2)
2.638(6) Å for 3], a factor which presumably also leads to the
(near parallel) orientation of the BCl2 unit with respect to the
Cp(centroid)-W-B plane [∠ Cp(centroid)–W(1)–B(13)–Cl(14) =
9.5(9)°, cf. ∠ Cp(centroid)–Fe–B–Cl(1) = 100.7(2)° for 3].
Scheme 4 Attempted synthesis of tungsten iminoboryl complexes.
These results further suggest that the boron-centred substi-
tution reaction using a metal dihaloboryl complex is very sen-
sitive to the steric bulk of the substituents both on the metal
fragment and on the incoming nucleophile, with the partner-
ship of the less sterically demanding iron boryl complex 3 and
the less bulky iminolithium salts 4a–d uniquely bringing
about substitution at boron without breakage of the metal–
boron bond.
Synthesis of iminoborylene complexes
With the iminoboryl-complexes 5c/d in hand, we next
attempted the synthesis of the corresponding borylene com-
plexes by halide abstraction. Reaction of 5d with Na[BAr f
]
4
[Ar f = 3,5-(CF3)2C6H3] leads to the formation of the corres-
ponding cationic borylene [CpFe(CO2)(BNCMes2)]+, as indi-
cated by a downfield shift in the 11B NMR signal (δB = 75 ppm,
cf. 47 ppm for 5d). While this borylene complex could be
shown to be stable at −30 °C in solution over a period of
several days, it decomposes rapidly at room temperature. This
led us to investigate the use of more electron-rich metal frag-
ments in order to generate borylene species stabilized by more
efficient M → B π-backbonding. Thus, we attempted the photo-
lytic displacement of the π-acidic carbonyl-ligands in 5c/d by
strong σ-donor phosphine ligands. While attempts to substi-
tute both carbonyl ligands by reaction with chelating bis-phos-
phines (dppe/dmpe for example) failed to yield the desired
products,25 reaction of 5c/d with monodentate donors cleanly
gave the corresponding mixed phosphine/carbonyl complexes
(Scheme 5).26 Assuming that bulky trialkylphosphines would
lead to an additional kinetic stabilization of the corresponding
borylene complexes, we first used PCy3 in this substitution
chemistry, leading to the formation of the desired complexes
10c/d in moderate yields (48–60%). In order to further investi-
While 6 could be structurally characterized, its reactivity –
in terms of boron-centred substitution processes – proves to be
much less facile than the corresponding chemistry for 3. Thus,
in contrast to the clean reactivity observed in the iron case, no
M-B containing products could be observed upon reaction of
the representative ketiminolithium salts 4a/d with either of the
dihaloboryl-tungsten complexes 6 or 7. As judged by 11B NMR
spectroscopy, breakage of the W–B bond and extrusion of the
[CpW(CO)3]− unit generates instead the corresponding dihalo
(ketimino)boranes 2a/d (δB = 21/26 ppm) (Scheme 4).
Fig. 1 Molecular structure of 6 in the solid state, hydrogen atoms
omitted for clarity and thermal ellipsoids set at the 40% probability level.
Key bond lengths [Å] and angles [°]: W(1)–B(13) 2.22(2), B(13)–Cl(14)
1.78(1), B(13)–Cl(15) 1.79(1), Cl(14)–B(13)–Cl(15) 110.5(7).
Scheme 5 Synthesis of phosphine-substituted iminoboryl complexes
10 and 11 and halide abstraction to give borylenes 12 and 13.
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gate the influence of the steric/electronic properties of the formation can be followed by the downfield shifts in the
phosphine ligands, we also employed PPh3 in the reaction respective 11B signals (δB = 82/85/85 ppm for 12c/12d/13d),
with 5d, giving the triphenylphosphine-substituted boryl while the shifts of the 31P resonances are less informative (δP =
1
complex 11d (43%).
84.9/75.0/69.3 ppm). In the H and 13C NMR spectra, the two
Spectroscopic characterization of 10c/d and 11d clearly sets of distinct signals for the ketimino aryl substituents
signals the successful introduction of the phosphine co- merge to give a single set of resonances, indicating fast
ligands via 31P NMR spectroscopy (δP = 77.1/75.0/78.8 ppm for rotation of the BNCAr2 unit (e.g. δH = 2.48/2.31/2.34 ppm for
10c/10d/11d), while little change is observed in the respective the p-CH3 signal in 12c/12d/13c), which is not frozen out even
11B spectra (δB = 47/50/51 ppm). In addition, diastereotopic at low temperatures (down to −75 °C). The IR spectra of these
splitting is observed for the aryl substituents of the axially pro- new compounds are also informative. These feature not only a
chiral ketimino fragments, brought about by the formation of BNC stretch consistent with the analogous mode observed for
a chiral metal centre (e.g. p-CH3 groups in 10c/10d/11c: δH
=
allenylidenes [ν(BNC) = 1763/1753/1779 cm−1], but also blue-
2.09, 2.06/2.11, 2.09/2.12, 2.11 ppm). In addition, the shifted carbonyl stretching frequencies in comparison with
formation of a more electron-rich metal centre leads to the their chloroboryl precursors [ν(CO) = 1962/1969/1984 cm−1 for
expected red-shift of the CvO stretches in the respective 12c/12d/13c] consistent with weaker Fe → CO π-backbonding
IR-spectra [ν(CO) = 1902/1905/1909 cm−1 for 10c/10d/11d, in the cationic systems.
cf. ν(CO) = 2002, 1922/2005, 1937 cm−1 for 5c/d].
Attempts to obtain crystals of complexes 12c and 13d
Crystallographically, complexes 10c/d (Fig. 2) feature a revealed instead the tendency of each complex to slowly
−
piano-stool geometry around the central metal atom in the decompose over several days to [CpFe(PR3)(CO)2]+[BArCl
] (R =
4
solid state, with the M–C(O) distances reflecting a higher Cy, Ph, respectively); the combined steric bulk of the mesityl
degree of π-backbonding compared to 5c [1.716(2)/1.716(3) Å and tricyclohexyl substituents, however, render complex 12d
for 10c/d, cf. 1.758(3), 1.753(3) Å for 5c]. Interestingly (and in stable enough to be characterized by both X-ray crystallography
contrast to dicarbonyl-ligated 5c), complex 10c features a non- and by positive-ion ESI-MS, the latter being consistent with the
linear arrangement of the B–N–C unit [∠ B–N–C = 144.8(2)°], presence of the [CpFe(PCy3)(CO)2(BNCMes2)]+ cation (ESI‡).
implying a reduced degree of N → B donation. Consistently, Moreover, the solid state structure (Fig. 2) reveals two crystallo-
10c features a relatively long B–N [1.396(4) Å], with the accom- graphically independent species with almost identical struc-
panying shortening of the Fe–B bond [1.980(4) Å] presumably tural features. The cationic borylene component features a
reflecting augmented Fe → B donation. This balance of com- cumulene-type linear arrangement of the Fe–B–N–C unit [∠ Fe(1)–
peting π-donation to boron is clearly a fine one, however, as B(28)–N(29) = 170.9(5)°, ∠ B(28)–N(36)–C(37) = 175.3(2)°]. In
the closely related dimesityl system 10d features a linear B–N–C the solid state at least, the ketimino-group is orientated near-
arrangement [∠ B–N–C = 174.7(2)°] and bond lengths consist- parallel to the Cp(centroid)–Fe–B plane [∠ Cp(centroid)–Fe(1)–
ent with dominant N → B donation [B–N 1.368(3) Å, Fe–B C(30)–C(40) = 7.1°], and the Fe–B bond [1.835(6) Å] is notice-
2.015(2) Å].
Utilising the monophosphine boryl complexes 10c/d and comparable to that observed in monophosphine-substituted
11d as precursors, halide abstraction with Na[BArCl4] (ArCl
aminoborylene-complexes (e.g. 1.821(4) Å in [CpFe(CO)(PMe3)-
ably shorter than in the precursor 10d [2.015(2) Å], being
=
3,5-Cl2C6H3) leads to the clean formation of the desired bory- (BNCy2)]+).26 However, the observed B–N distance is rather
lene complexes 12c/d and 13 in yields of 55–77% (Scheme 5). short and the N–C distance is long [B–N 1.314(6) Å, N–C
In comparison with their dicarbonyl-supported analogues, 1.292(6) Å, cf. 1.368(3) Å and 1.263(3) Å for 10d], consistent
these complexes are more stable at room temperature, at least with a significant contribution from a resonance form contain-
+
when handled under inert atmosphere conditions. Borylene ing a Fe–BuN–CMes2 unit. Such a contribution is also con-
Fig. 2 Molecular structures of 5c, 10c, 10d and 12d in the solid state. Hydrogen atoms and counter-ion omitted for clarity and thermal ellipsoids
set at the 40% probability level (20% for 12d). Key bond lengths [Å] and angles [°]: (for 5c) Fe(1)–B(11) 2.016(4), B(11)–N(13) 1.349(4), N(13)–C(14)
1.277(4), Fe(1)–B(11)–N(13) 126.0(2), B(11)–N(13)–C(14) 175.6(3); (for 10c/d) Fe(1)–B(28) 1.980(4)/2.015(2), B(28)–N(30) 1.397(4)/1.368(3), N(30)–C(31)
1.269(3)/1.263(3), B(28)–N(30)–C(31) 144.8(2)/174.7(2); (for 12d) Fe(1)–B(28) 1.835(6), B(28)–N(29) 1.302(8), N(29)–C(30) 1.287(7), Fe(1)–B(28)–N(29)
170.9(5), B(28)–N(36)–C(37) 175.3(2).
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sistent with the low-field shift of the ketimino-carbon 13C reso- with 12c/d generated in situ by the reaction of 10c/d with Na-
nance, a feature also characteristic of allenylidene complexes [BArCl4], re-formation of the chloroboryls 10c/d is observed.
(δC = 187.0 ppm, cf. δC = 150.2 ppm for 10d).
This suggests that in the presence of NaCl (from the initial salt
metathesis) and KCN, in conjunction with 18-crown-6 as a
solubilizing agent, the addition of chloride is preferred over
the addition of cyanide. Presumably such an observation
reflects thermodynamic control due the more favourable B–Cl
bond enthalpy (ca. 128 vs. 107 kcal mol−1).28 The reaction of
the pure complex 12d with KCN does, however, lead to
addition of cyanide to the borylene. Once again, α-selectivity is
observed, yielding the corresponding cyano-substituted boryl-
complex 15d. Unfortunately, reaction of KCN with the less
sterically encumbered borylene 12c gives only decomposition
products, so that the influence of the aryl substituents on the
regio-selectivity could not be fully investigated in this case.
Complexes 14c/d and 15d were fully characterized by spectro-
scopic, mass spectrometric and, in case of 14d, by crystallo-
graphic methods.12 The 11B and 31P resonances (δB = 56/52/
41 ppm, δP = 76.3/74.1/73.3 ppm for 14c/14d/15d) are similar
to those of the corresponding chloroboryl complexes (δB = 47/
50 ppm, δP = 77.1/75.0 ppm for 10c/10d), which together with
the CvN ketimino-resonances (δC = 147.7/149.9/150.5 ppm)
verify the postulated structures resulting from α-attack at
boron. The observed high α-selectivity is presumably brought
about by the high electrophilicity of the boron centre in each
case, bearing in mind the fact that γ-selectivity has been
observed in the addition of a variety of nucleophiles (including
thiolate and cyanide) to cationic allenylidene complexes.14,29
Hoping to uncover more diverse patterns of reactivity, we
targeted a study of the reactivity of the iminoborylenes towards
unsaturated substrates. It has been shown that neutral bory-
lene complexes undergo borylene transfer reactions with
alkynes,10 insertion reactions with isonitriles and carbodi-
imides,30 and metathesis-type reactions with ketones,30 while
cationic borylenes oftentimes show contrasting reactivity, dis-
playing hydride transfer reactivity towards ketones,31 insertion
reactions with carbodiimides,32 and metathesis-type reactivity
with isocyanates and phosphine sulfides.7b
Reactivity of the iminoborylene complexes
With the crystallographic and spectroscopic analysis of imino-
borylene complex 12d hinting at a partial contribution from a
carbo-cationic resonance form, we set out to determine experi-
mentally whether selectivity for nucleophilic addition at either
the α- or γ-position would be observed. With this in mind, we
further sought to compare the addition chemistry of both the
mesityl- and p-tolyl substituted systems (12c/d, Scheme 6) in
order to investigate the influence of the steric loading at the
ketimino group.
In the first instance, we investigated whether reactions with
a chloride source (e.g. [PPh4]Cl) could be used to generate pro-
ducts of the type CpFe(CO)(PCy3){BNC(Cl)Mes2}, thus allowing
a formal α, γ-isomerization of the precursors 10c/d via imino-
borylene intermediates (i.e. a formal reversal of the conversion
of [LnMvCvC–CR2OH]+ to [LnMvC(OH)CvCR2]+ via the
corresponding allenylidene27). However, exclusive α-attack led
to the re-formation of the precursors 10c/d. In similar fashion,
the reaction of 12c/d with sodium thiophenolate leads to the
products of boron-centred nucleophilic attack, exclusively
giving the B(SPh) complexes 14, independent of the steric bulk
at the ketimino group. The syntheses of the thiolate-functiona-
lized boryl complexes 14c/d could also be achieved directly by
reaction of 10c/d with NaSPh in a boron-centred substitution
reaction, thus providing independent verification of com-
pound identity.
The situation is slightly different, however, when using
cyanide (KCN, 18-crown-6) as a nucleophile. In this case, boryl
precursors 10c/d are completely resistant towards substitution
at boron, so we investigated the reactivity of the corresponding
borylenes 12c/d towards CN−. On mixing KCN and 18-crown-6
In order to investigate the reactivity of our iminoborylene
complexes towards unsaturated substrates, we used the
mesityl-substituted complex 12d which shows the highest
resistance towards undesired hydrolysis and decomposition
reactions. Mixing of 12d with non-polar substrates such as 2,3-
dimethyl-butadiene and trimethylsilyl-acetylene in dichloro-
methane leads to no conversion, even at 40 °C, and over pro-
longed periods of time. While this result is consistent with the
fact that other cationic borylenes show little affinity for alkenes
or alkynes, we were surprised to find that mixing of 12d with
isopropyl-isocyanate also did not lead to any conversion (as
judged from in situ 1H and 11B NMR measurements). This con-
trasts with the chemistry of cationic aminoborylenes, which
react with isocyanates, RNCO, cleanly and under mild con-
ditions to give the corresponding isonitrile complexes
[CpFe(CO)2(NCR)]+ via a metathesis-type reaction.26
By contrast, the reaction of 12d with an excess of either
diisopropyl- or dicyclohexylcarbodiimide (RNvCvNR,
Scheme 6 Reactivity of the borylene-complexes 12c/d towards anionic
nucleophiles.
11298 | Dalton Trans., 2015, 44, 11294–11305
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This chemistry represents, to our knowledge, the first
example of metathesis-type reactivity of a borylene complex
towards a carbodiimide, and we therefore performed further
investigations in order to better understand the reaction
mechanism and to probe the fate of the boron-containing
[BvNvCMes2] heterocumulene fragment.
Upon mixing of the 12d with the respective carbodiimide
RNvCvNR at −60 °C in CD2Cl2, we observe the immediate
formation of an intermediate (18/19 for R = Pr/Cy), which is
Scheme 7 Reaction of iminoborylene complex 12d with carbodiimides
to give isonitrile complexes 16/17.
i
stable at temperatures below 0 °C. Accordingly, we were able to
characterize these species by multinuclear NMR spectroscopy.
1
In the H and 13C NMR spectra we observe a splitting of the
resonances for the ketimino aryl substituents (e.g. mesityl
p-CH3 in 18/19: δH = 2.30, 2.25/2.31, 2.25 ppm), as is also seen
for the boryl precursor 10d. In addition, we also observe two
i
sets of resonances for the carbodiimide Pr/Cy substituents
(e.g. for the N–CHR2 protons in 18/19: δH = 3.73, 3.01/3.24,
2.54 ppm), consistent with desymmetrization of the RNCNR
unit. These spectroscopic features are consistent with the for-
mation of either a Lewis acid–base adduct between the electro-
philic boron and one of the carbodiimide nitrogens,32a,b,33b or
with the formation of a [2 + 2]-cycloaddition product,30a both
of which have been observed as intermediates in the reactions
of carbodiimides with borylene complexes.
Fig. 3 Molecular structure of 16 in the solid state, hydrogen atoms and
counter-ion omitted for clarity and thermal ellipsoids set at the 40%
probability level. Key bond lengths [Å] and angles [°]: Fe(1)–C(26)
1.850(2), C(26)–N(27) 1.163(3), N(27)–C(28) 1.461(3), Fe(1)–C(26)–N(27)
176.7(2), C(26)–N(27)–C(28) 175.0(2).
Somewhat unexpectedly, the 11B and 31P NMR resonances
for 18/19 are shifted downfield in comparison to those
observed for the free borylene (δB = 91/91 ppm, δP = 80.8/
80.4 ppm for 18/19, cf. δB = 85 ppm, δP = 75.0 ppm for 12d).
While these shifts imply retention of the Fe–B linkage at this
stage in the reaction, they appear counter-intuitive for the for-
mation of either a B-bound Lewis acid–base adduct or a
[2 + 2]-cycloaddition product, both of which would be expected
to lead to an upfield shift in the 11B NMR resonance. Thus,
adducts of [CpFe(CO)2(BNR2)]+ (δB = 94 ppm) with carbodi-
imides or imines (adducts: δB = 71/54 ppm)7b,32b and the [2 + 2]
cyclo-addition product of CpMn(CO)2(BtBu) (δB = 144 ppm)
with carbodiimide (product: δB = 62 ppm) show upfield shifts
in the 11B signal, consistent with an increased coordination
number at boron.30c To an even greater extent, the 11B reso-
nances measured for 18 and 19 contrast with those observed
for the FevB insertion products formed in the reaction of the
same carbodiimides with cationic iron aminoborylene com-
plexes (e.g. δB = 25 ppm for [CpFe(CO)2{C(NCy)2BNCy2}]+).32b
In the 13C spectra the downfield shifts observed for the car-
bodiimide quaternary carbons (δC = 168.8/168.3 ppm for 18/19,
cf. δC = 140.2/139.9 ppm for free RNvCvNR with R = iPr/Cy)34
are consistent with the formation of a direct metal-carbon
interaction [cf. δC = 151.0, 162.0 for CpMn(CO)2{κ2-B(tBu)N(Cy)-
i
R = Pr/Cy) gives clean conversion within hours at room tem-
perature, to a single 31P containing species (δP = 76.4/76.5 ppm,
respectively) and a compound giving rise to a 11B signal at δB =
29/30 ppm. Rather than the carbodiimide insertion products
found for related aminoborylene complexes30,32 and organic
boranes,33 in situ spectroscopic analysis of the reaction
mixture in this case supports an alternative pathway. Thus, as
opposed to a characteristic low-field carbene 13C resonance
seen for either a mono- or a bis-carbodiimide insertion
product (e.g. [CpFe(CO)2vC(NCy)2BNCy2]+, δC = 251.5 ppm or
[CpFe(CO)2 = C(NCy)2B(NCy)2CNCy2]+, δC = 224.0 ppm),32b we
observe the corresponding quaternary carbon resonance at
δC = 153.6/153.8 ppm (for R = iPr/Cy). This observation
suggests the formation of the isonitrile complexes [CpFe(CO)-
−
i
(PCy3)(NCR)]+[BArCl
]
(16/17, R = Pr/Cy, Scheme 7), which
4
could also be detected by positive-ion ESI MS (SI).
In case of 16, we were also able to isolate the metal-contain-
ing species by crystallization and unambiguously confirm its
structure by X-ray crystallography (Fig. 3). In the solid state,
complex 16 shows a piano-stool geometry, with the isonitrile
unit featuring a linear geometry [∠ C(26)–N(27)–C(28) =
175.0(2)°], brought about by the presence of the C–N
triple bond [C(26)–N(27) 1.163(3) Å]. In solution, complexes 16
and 17 show very similar spectroscopic features, e.g. reso-
CNCy}
and
CpMn(CO)2{κ2-B(tBu)OCPh2},
respectively],
although not with complete insertion into the FevB bond (cf.
δC = 251.0 ppm for [CpFe(CO)2{C(NCy)2BNCy2}]+, which fea-
tures partial FevC carbenoid character). Moreover, the obser-
vation of resonances at δc ≈ 180 ppm for the γ-carbons of the
FeBNC units (along with the downfield 11B shifts for 18/19),
suggests retention of a substantial degree of delocalization
along the hetero-cumulene framework. With this in mind, we
nances in the 1H and 13C NMR spectra for the Cp (δH
=
4.92/4.92 ppm, δC = 84.2/84.3 ppm for 16/17) and CuN–CHR2
units (CH: δH = 4.08/3.85 ppm, δC = 51.3/57.3 ppm for
16/17).
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Scheme 8 Pathway of the reaction of borylene complex 12d with carbodiimides. Formation of the intermediates 18/19 and subsequent reaction to
give the final products 16/17 and 20/21 (tentative structures).
suggest the formation of an unsymmetrical [2 + 2]-cyclo-
addition product featuring a strong interaction between the
Conclusions
metal and the central carbodiimide carbon and a relatively Our investigation of the possible synthetic routes to iminobory-
weak N → B interaction (Scheme 8).
lene complexes (12/13) has given insight into the scope of
Repeated attempts to obtain structural information on 18/ metal-fragments, ketimino substituents and ancillary ligands
19 by crystallization at low temperatures failed to give crystals which allow for successful formation of the desired cationic
suitable for X-ray analysis, and in contrast to the [2 + 2] heterocumulenes. For the synthesis of the iminoboryl-precur-
cycloaddition products of carbodiimides with CpMn- sors, it is found that an optimal level of overall steric bulk, in
(CO)2(BtBu),30c solutions of 18/19 are labile, yielding isonitrile combination with the correct order of bond formation (Fe–B
complexes 16/17 within a few hours at room temperature. The prior to B–N bond formation), is required for the generation of
activation barriers for this step could be determined in each the boryl complexes CpFe(CO)2{B(Cl)NCAr2} (Ar = p-Tol/Mes,
case by following of the intensity of the cyclopentadienyl 1H 5c/d). The use of reagents with increased steric bulk on either
signals as a function of time. Values of 21.8 0.1 kcal mol−1 the metal [CpW(CO)3 vs. CpFe(CO)2] or the ketimino side (Trip
and 22.1 0.1 kcal mol−1 (at T = 25 °C) are thus obtained for vs. Mes) leads primarily to products resulting from M–B bond
18 and 19, respectively (Scheme 8).
breakage, illustrating the sensitivity of the boron-centred sub-
Finally, we sought to establish the fate of the boron-con- stitution reaction to steric factors.
taining fragment in the final reaction mixture. When the reac-
While direct halide abstraction from complexes 5c/d leads to
tion is performed with a stoichiometric amount of either thermally unstable borylene species, the substitution of one car-
carbodiimide, the 1H NMR spectra show the isonitrile com- bonyl ligand for a tertiary phosphine drastically increases
plexes 16/17, together with a number of products containing complex stability, leading to the isolation of the cationic hetero-
mesityl- or isopropyl/cyclohexyl groups, respectively. Only in allenylidenes [CpFe(PR3)(CO)(BNCAr2)]+ as borate salts (12c/d,
the presence of an excess of carbodiimide, could well-defined 13d). The reactivity of these complexes towards nucleophilic
boron-containing products be isolated. The 11B resonances substrates is dominated by the high electrophilicity of the
i
(δB = 29/30 ppm for R = Pr/Cy) indicate a three-coordinate boron centre, leading exclusively to α-attack, while the reactivity
boron centre without any metal–boron interaction, while the towards unsaturated substrates leads to unprecedented trans-
i
1H NMR spectra show the presence of three inequivalent Pr- formations. While no reactivity is observed towards isocyanates,
i
or Cy-groups [e.g. CHR2-units: δH = 3.90/3.39/3.61 for Pr-(I/II/ we observe clean metathesis-type reactivity with carbodiimides.
III), 3.04/2.96/3.45 for Cy-(I/II/III), (for numbering see This contrasts with the insertion-type reactivity of closely related
Scheme 8)]. The 13C-NMR and GHMBC-data indicate that all amino- and alkylborylene complexes towards the same sub-
three alkylamino-substituents are bound to a central quater- strates. Spectroscopic analysis of the reaction mixtures leads to
nary carbon (δC = 152.4/ 155.9 ppm), with two of the alkyl- identification of the boron-containing reaction products as the
groups (I and III, respectively) being in close proximity as seen coordinatively trapped heteroallenes (20/21), with the metal-con-
from NOE difference spectra. Taken together, these obser- taining products being unambiguously identified as the iso-
vations suggest that in the presence of excess carbodiimide, nitrile complexes (16/17). This reactivity is unprecedented and
RIINvCvNRIII, coordinative trapping of the initial metathesis represents the first example of a productive metathesis-type
product [RINvBvNvCMes2] leads to the formation of a reaction of a borylene compound with a carbodiimide.
trialkyl-guanidinate, which is bound to the BvNvCMes
heterocumulene fragment. The resulting triaminoboranes of
the type RNvC(NR)2BNCMes2 [with R = Pr (20) and R = Cy
Experimental
General considerations
i
(21), Scheme 8] thus resemble the metalla-amidinates [CpFe-
(CO)2{C(NCy)2BNCy2}]+ formed by mono-carbodiimide inser-
All reactions involving air- or moisture-sensitive compounds
were carried out under an inert atmosphere by using Schlenk-
tion in the case of aminoborylene systems.32
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type glassware or in a glovebox. UV photolysis experiments 0.564 mmol, 1.1 equiv.) were dissolved in toluene (20 mL) in a
were carried out using a Spectral Energy mercury arc lamp quartz Schlenk tube. The mixture was irradiated (UV-lamp)
(1 kW) with samples contained within quartz Schlenk vessels. and the reaction was monitored by 1H NMR. After complete
Solvents were dried using an MBraun SPS800 prior to use. consumption of the starting material (ca. 4 h), the mixture was
NMR-solvents were dried over molecular sieves and degassed filtered and the solvent was removed. The residue was dried in
before use when necessary. Solid starting materials were dried vacuo overnight (thorough drying important!) and then sus-
on high vacuum before use when necessary. Unless otherwise pended in pentane (20 mL). The mixture was stirred vigorously
noted, all starting materials were commercially available and for 30 min, the resulting solid isolated by filtration, washed
were used without further purification. The following com- with pentane (2 × 20 mL) and dried in vacuo. The product 11d
1
pounds were synthesized according to literature procedures was isolated as a beige solid (160 mg, 0.222 mmol, 43.3%). H
(for references see ESI‡): Na[B(3,5-Cl2C6H3)4, Na[B(3,5- NMR (500 MHz, [D6]benzene, 298 K): δ = 7.56 (m, 6 H, o-Ph),
(CF3)2C6H3)4], Na[CpW(CO)3], Na[CpFe(CO)2] (1), (Ph2CNBCl2)2 6.99 (m, 3 H, p-Ph), 6.90 (m, 6 H, m-Ph), 6.70 (s, 2 H, m-MesA),
3
(2b), CpFe(CO)2BCl2 (3), ketimino lithium salts 4a/b/c/d, boryl 6.66 (s, 2 H, m-MesB), 4.42 (d, J(P,H) = 1.0 Hz, 5 H, Cp), 2.40
complexes 5c/d, 10c/d, 14c/d and 15d, borylene complexes (s, 6 H, o-CH3A), 2.27 (s, 6 H, o-CH3B), 2.12 (s, 3 H, p-CH3B),
12c/d. For the synthesis of 4e see ESI.‡
2.11 (s, 3 H, p-CH3A); 13C NMR (126 MHz, [D6]benzene, 298 K):
The following instruments were used for physical characteri- δ = 221.8 (d, 2J(P,C) = 29.7 Hz, CO), 150.1 (CN), 138.3 (d, 1J(P,C)
zation of novel compounds: IR: Nicolet Magna-IR 560; NMR: = 41.9 Hz, i-Ph), 137.9 (o-MesB), 137.8 (p-MesA), 137.7 (i-MesB),
Bruker AVC500 (1H: 500 MHz; 13C: 125 MHz); Bruker DRX500 137.6 (i-MesA), 137.4 (p-MesB), 136.6 (o-MesA), 133.6 (d, 2J(P,C) =
(11B: 160 MHz), Varian Unity500 (1H: 500 MHz; 13C: 125 MHz, 9.9 Hz, o-Ph), 130.2, 130.1 (m-MesA/B), 129.4 (d, 4J(P,C) = 1.8 Hz,
11B: 160 MHz), Varian Mercury VX-300 (31P: 122 MHz, 19F: p-Ph), 127.8 (m-Ph), 84.5 (Cp), 21.9 (o-CH3B), 21.6 (o-CH3A),
282 MHz, 11B: 96 MHz). Mass spectra of compounds 12d, 16 20.92, 20.89 (p-CH3A/B); 11B NMR (160 MHz, [D6]benzene,
and 17 were recorded on a Bruker Microtof mass-spectrometer. 298 K): δ = 51 (ν1/2 = 1150 Hz);.31P NMR (122 MHz, [D6]benzene,
All other mass spectra were measured by the EPSRC National 298 K): δ = 78.8; IR (KBr): ν bar = 3059 (w), 2976 (w), 2923 (w),
Mass Spectrometry Service Centre, Swansea University. For all 2857 (w), 1909 (s, CO), 1769 (m), 1747 (m), 1609 (w), 1479 (w),
crystallographic studies, diffraction data were collected at 1438 (m), 1261 (w), 1161 (w), 1092 (w), 1073 (w), 1029 (w), 877
150 K using an Enraf Nonius Kappa CCD diffractometer or an (w), 851 (m), 824 (w) cm−1; HR-MS (EI): m/z: 692.2216, calcd for
Oxford Diffraction/Agilent Technologies SuperNova instru- (C42 H42 10B Cl Fe N P)+ = 692.2217 [(M − CO)+]; elemental
ment. For complete analytical data (including 2D-NMR data) microanalysis: (calcd for C43H42BClFeNOP) C 71.53, H 5.86,
and for details concerning the determination of the activation N 1.94; (measd) C 71.16, H 5.66, N 2.10.
energies, see the ESI.‡
[CpFe(PPh3)(CO)(BNCMes2)]+[B(3,5-(CF3)2C6H3)4]− (13d). 11d
(30.0 mg, 0.0416 mmol, 1 equiv.) and Na[B(3,5-(CF3)2C6H3)4]
(36.9 mg, 0.0416 mmol, 1 equiv.) were dissolved in fluoroben-
Syntheses
CpW(CO)3BCl2 (6).23 Na[CpW(CO)3] (250 mg, 0.702 mmol, zene (2 mL) and the mixture was stirred for five min. The solu-
1 equiv.) was suspended in hexanes (20 mL) and boron trichlo- tion was filtered (glovebox) and the solvent was removed to
ride (0.70 mL of a 1 M solution in hexanes, 0.702 mmol, give the product as a dark-red solid (49.4 mg, 0.0319 mmol,
1
1 equiv.) was added at −78 °C. The mixture was stirred at 76.6%). H NMR (500 MHz, [D2]dichloromethane, 298 K): δ =
−78 °C for 30 min, warmed to room temperature and stirred 7.74 (bs, 8 H, o-ArF), 7.56 (s, 4 H, p-ArF), 7.43 (m, 3 H, p-Ph),
for another 4 h. The mixture was filtered and the filtrate 7.29 (m, 6 H, m-Ph), 7.26 (m, 6 H, o-Ph), 6.96 (s, 4 H, m-Mes),
cooled to −30 °C. After storage for 24 h, the supernatant was 4.95 (s, 5 H, Cp), 2.34 (s, 6 H, p-CH3), 2.03 (s, 12 H, o-CH3); 13
C
removed by filtration to give to product as a white powder NMR (126 MHz, [D2]dichloromethane, 298 K): δ = 213.4 (d,
1
(35 mg, 0.084 mmol, 12%). Storage of the mother liquor at 2J(P,C) = 25.8 Hz, CO), 188.3 (CN), 162.1 (q, J(B,C) = 50.0 Hz,
−30 °C for another 24 h gave crystals suitable for X-ray crystal- i-ArF), 144.1 (p-Mes), 139.5 (i-Mes), 138.8 (o-Mes), 135.2 (b, o-ArF),
lography. 1H NMR (300 MHz, [D8]toluene, 248 K): δ = 4.38 (s, 5 134.4 (d, 1J(P,C) = 50.6 Hz, i-Ph), 132.8 (d, 2J(P,C) = 10.3 Hz,
H, Cp); 13C NMR (75 MHz, [D8]toluene, 248 K): δ = 218.1 (CO), o-Ph), 131.7 (d, 4J(P,C) = 2.4 Hz, p-Ph), 131.4 (m-Mes), 129.3 (d,
2
3
215.7 (CO), 94.1 (Cp); 11B NMR (96 MHz, [D8]toluene, 248 K): δ 3J(P,C) = 10.7 Hz, m-Ph), 129.2 (qq, J(F,C) = 31.5 Hz, J(B,C) =
= 91 (ν1/2 = 940 Hz); Crystallographic data: C8H5BCl2O3W, Mr 2.9 Hz, m-ArF), 124.9 (q, 1J(F,C) = 272.6 Hz, CF3), 117.8 (sept, 3J
414.69, monoclinic, P21/n, a = 7.8866(4), b = 11.0772(5), c = (F,C) = 3.8 Hz, p-ArF), 86.0 (Cp), 21.6 (o-CH3), 21.5 (p-CH3); 11B
12.3944(6) Å, β = 97.450(2)°, V = 1073.65(9) Å3, Z = 4 ρc = 2.565 NMR (96 MHz, [D2]dichloromethane, 298 K): δ = 85 (ν1/2 = 820
Mg m−3, T = 150 K, λ = 0.71073 Å. 11 681 reflections collected, Hz), −6 (ν1/2 = 6 Hz); 19F NMR (282 MHz, [D2]dichloromethane,
2427 independent [R(int) = 0.0069], which were used in all cal- 298 K): −62.8; 31P NMR (122 MHz, [D2]dichloromethane,
culations. R1 = 0.0578, wR2 = 0.1389 for observed unique reflec- 298 K): δ = 69.3 (b, (ν1/2 = 10 Hz); IR (KBr): ν bar = 2963 (w),
tions [F2 > 2σ(F2)] and R1 = 0.0834, wR2 = 0.1558 for all unique 1984 (s, CO), 1779 (m), 1608 (s), 1482 (w), 1435 (m), 1355 (s),
reflections. Max. and min. residual electron densities 3.87 and 1275 (s), 1141 (m), 1017 (w), 889 (w), 855 (s), 839 (m), 803 (m),
−4.01 e Å−3. CSD reference: 1037787.
745 (m), 713 (m) cm−1. Attempts to obtain reproducible micro-
CpFe(PPh3)(CO){B(Cl)NCMes2}
(11d). 5d
(250
mg, analytical data for 11d were frustrated by its ready decompo-
0.512 mmol, 1 equiv.) and triphenylphosphine (148 mg, sition in solution during recrystallization.
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1
3
[CpFe(PCy3)(CO)(CNiPr)]+[B(3,5-Cl2-C6H3)4]−
(16). 10d (Cy-1), 38.8 (d, J(P,C) = 19.8 Hz, C-1), 30.9 (C-3A/B), 30.7 (d, J
(126 mg, 0.170 mmol, 1 equiv.) and Na[B(3,5-Cl2C6H3)4] (P,C) = 2.3 Hz, C-3A/B), 28.0 (d, J(P,C) = 10.8 Hz, C-2A/B), 27.9
(105 mg, 0.170 mmol, 1 equiv.) were dissolved in fluoroben- (d, 2J(P,C) = 9.8 Hz, C–2A/B), 26.4 (C-4); 11B NMR (96 MHz,
zene (5 mL). The mixture was stirred for five minutes and di- [D2]dichloromethane, 293 K): δ = −7 (ν1/2 = 16 Hz); 31P NMR
isopropylcarbodiimide (26.5 μl, 21.5 mg, 0.170 mmol, 1 equiv.) (122 MHz, [D2]dichloromethane, 293 K): δ = 76.5; HR-MS (EI):
was added. The mixture was stirred overnight, filtered into a m/z: 538.2876, calcd for (C31 H49 Fe N O P)+ = 538.2896
layering Schlenk and layered with hexanes. After 7 d, the super- [(M-B(C6H3Cl2)4)+].
2
natant was removed and the product was isolated as
Intermediate 18. 12d (41.7 mg, 0.0321 mmol, 1 equiv.) was
yellow crystals suitable for X-ray crystallography (55.0 mg, dissolved in [D2]dichloromethane in an NMR tube and the
0.0503 mmol, 29.6%). 1H NMR (500 MHz, [D2]dichloro- solution was cooled to −78 °C. Diisopropylcarbodiimide
methane, 298 K): δ = 7.03 (m, 8 H, o-ArCl), 7.00 (m, 4 H, p-ArCl), (5.0 μl, 4.05 mg, 0.0321 mmol, 1 equiv.) was added and the
4.92 (d, 3J(P,H) = 0.7 Hz, 5 H, Cp), 4.08 (sept, 3J(H,H) = 6.6 Hz, NMR tube transferred to the precooled NMR spectrometer.
1
1 H, CH), 1.95 (m, 3 H, H-1), 1.89 (m, 12 H, H-2A/B, H-3A/B), NMR spectra for intermediate 18 were measured at 263 K. H
3
1.79 (m, 3 H, H-4), 1.39 (d, J(H,H) = 6.6 Hz, 6 H, CH3, CH3′), NMR (500 MHz, [D2]dichloromethane, 263 K): δ = 7.03 (m, 8 H,
1.34 (m, 6 H, H-3A′/B′), 1.29 (m, 9 H, H-2A′/B′, H-4′); 13C NMR o-ArCl), 7.00 (m, 4 H, p-ArCl), 6.99 (bs, 2 H, m-MesA), 6.93 (bs,
2
(126 MHz, [D2]dichloromethane, 298 K): δ = 215.7 (d, J(P,C) = 2 H, m-MesB), 4.70 (s, 5 H, Cp), 3.73 (sept, 3J(H,H) = 6.6 Hz,
1
3
24.7 Hz, CO), 165.0 (q, J(B,C) = 49.4 Hz, i-ArCl), 153.6 (b, CN), 1 H, CH(I)), 3.01 (sept, J(H,H) = 6.8 Hz, 1 H, CH(II)), 2.30 (s,
133.4 (m-ArCl), 133.2 (q, 2J(B,C) = 4.2 Hz, o-ArCl), 123.4 (p-ArCl), 3 H, p-CH3A), 2.25 (s, 3 H, p-CH3B), 2.11 (bs, 6 H, o-CH3A), 2.05
84.2 (Cp), 51.3 (CH), 38.7 (d, J(P,C) = 20.3 Hz, C-1), 30.8 (d, J (bs, 6 H, o-CH3B), 1.82 (m, 9 H, H-1, H-3A/B), 1.79 (m, 6 H,
1
3
(P,C) = 1.2 Hz, C-3A/B), 30.7 (d, 3J(P,C) = 3.2 Hz, C-3A/B), 27.9 (d, H-2A/B), 1.71 (m, 3 H, H-4), 1.35 (d, 3J(H,H) = 6.6 Hz, 3 H,
2J(P,C) = 10.8 Hz, C-2A/B), 27.8 (d, 2J(P,C) = 9.9 Hz, C-2A/B), 26.4 CH3(I)), 1.32 (m, 6 H, H-3A′/B′), 1.31 (d, J(H,H) = 6.6 Hz, 3 H,
3
(C-4), 23.29, 23.27 (CH3, CH3′); 11B NMR (96 MHz, [D2]dichloro- CH3(I)′), 1.18 (m, 6 H, H-2A′/B′), 1.17 (m, 3 H, H-4′), 1.13 (d, J
3
methane, 298 K): δ = −7 (ν1/2 = 21 Hz); 31P NMR (122 MHz, (H,H) = 6.8 Hz, 3 H, CH3(II)), 0.52 (d, J(H,H) = 6.8 Hz, 3 H,
3
[D2]dichloromethane, 298 K): δ = 76.4; IR (KBr): ν bar = 2961 CH3(II)′); 13C NMR (126 MHz, [D2]dichloromethane, 263 K): δ =
(m), 2935 (m), 2854 (m), 2164 (s, CN), 1991 (s, CO), 1566 (m), 220.9 (d, 2J(P,C) = 27.5 Hz, CO), 180.9 (CN), 168.8 (NCN), 164.7
1544 (s), 1446 (m), 1421 (m), 1390 (m), 1369 (m), 1262 (s), 1139 (q, 1J(B,C) = 49.5 Hz, i-ArCl), 143.6 (p-MesA), 143.2 (p-MesB),
(m), 846 (m), 800 (s), 783 (s), 710 (m), 703 (m) cm−1; HR-MS 138.8 (o-MesA), 137.0 (o-MesB), 136.0 (i-MesB), 135.0 (i-MesA),
(EI): m/z: 498.2545, calcd for (C24 H45 Fe N O P)+ = 498.2583 133.1 (m-ArCl), 133.0 (q, 2J(B,C) = 4.0 Hz, o-ArCl), 131.4 (m-
[(M-B(C6H3Cl2)4)+]; elemental microanalysis: (calcd for MesA), 130.9 (m-MesB), 123.1 (p-ArCl), 81.2 (Cp), 46.7 (CH(I)),
1
C52H57BCl8FeNOP) C 57.12, H 5.25, N 1.28; (measd) C 56.88, 46.0 (CH(II)), 39.5 (d, J(P,C) = 19.6 Hz, C-1), 29.9 (2 × C-3A/B),
H 4.99, N 1.30.
27.9 (d, 2J(P,C) = 10.8 Hz, C-2A/B), 27.6 (d, 2J(P,C) = 8.6 Hz,
A
Crystallographic data. C52H57BCl8FeNOP, Mr 1093.28, mono- C-2A/B), 26.2 (C-4), 23.8 (CH3(I)′), 22.5 (CH3(II)), 21.8 (o-CH3
,
clinic, P21/n, a = 13.0423(1), b = 24.8523(2), c = 17.0945(1) Å, CH3(I)), 21.7 (o-CH3B), 21.3 (CH3(II)′), 21.2 (p-CH3A), 21.1
β = 108.2053(4)°, V = 5263.50(7) Å3, Z = 4 ρc = 1.380 Mg m−3, T = (p-CH3B); 11B NMR (160 MHz, [D2]dichloromethane, 263 K):
150 K, λ = 0.71073 Å. 23 500 reflections collected, 11 969 inde- δ = 91 (ν1/2 = 1470 Hz), −7 (ν1/2 = 11 Hz); 31P NMR (122 MHz,
pendent [R(int) = 0.000], which were used in all calculations. [D2]dichloromethane, 263 K): δ = 80.8 (ν1/2 = 5 Hz).
R1 = 0.0384, wR2 = 0.0905 for observed unique reflections [F2 >
Intermediate 19. 12d (20.0 mg, 0.0154 mmol, 1 equiv.), was
2σ(F2)] and R1 = 0.0590, wR2 = 0.0982 for all unique reflections. dissolved in [D2]dichloromethane in an NMR tube and the
Max. and min. residual electron densities 0.97 and −0.65 e Å−3
.
solution cooled to −78 °C. Dicyclohexylcarbodiimide (3.5 mg,
CSD reference: 1037786.
0.0170 mmol, 1.1 equiv.) was added and the NMR tube trans-
(17). 12d ferred to the precooled NMR spectrometer. The NMR-spectra
[CpFe(PCy3)(CO)(CNCy)]+[B(3,5-Cl2-C6H3)4]−
(20.0 mg, 0.0154 mmol, 1 equiv.) and dicyclohexylcarbodi- for intermediate 19 were measured at 263 K. 1H NMR
imide (7.0 mg, 0.0339 mmol, 2.2 equiv.) were dissolved in [D2]- (500 MHz, [D2]dichloromethane, 263 K): δ = 7.03 (m, 8 H,
dichloromethane in an NMR tube at −78 °C. The NMR tube o-ArCl), 7.00 (m, 4 H, p-ArCl), 6.99 (bs, 2 H, m-MesA), 6.93 (bs,
was warmed to room temperature and allowed to stand for 4 h. 2 H, m-MesB), 4.68 (s, 5 H, Cp), 3.24 (m, 1 H, Cy(I)-1), 2.54 (m,
The mixture, containing complex 17 and adduct 21 (reson- 1 H, Cy(II)-1), 2.31 (s, 3 H, p-CH3A), 2.25 (s, 3 H, p-CH3B), 2.10
ances not listed), was analyzed by NMR spectroscopy and mass (bs, 6 H, o-CH3A), 2.06 (bs, 6 H, o-CH3B), 1.90 (m, 1 H, Cy(I)-
spectrometry. 1H NMR (500 MHz, [D2]dichloromethane, 2B), 1.86 (m, 6 H, H-3A/B), 1.85 (m, 1 H, Cy(II)-2A), 1.84 (m, 4 H,
293 K): δ = 7.04 (m, 8 H, o-ArCl), 7.01 (m, 4 H, p-ArCl), 4.92 (d, Cy(I)-2A, H-1), 1.83 (m, 6 H, H-2A/B), 1.82 (m, 2 H, 2 × Cy-3A),
3J(P,H) = 0.9 Hz, 5 H, Cp), 3.85 (m, 1 H, Cy-1), 1.96, 1.69, 1.57, 1.72 (m, 3 H, H-4), 1.71 (m, 1 H, Cy(I)-2B′), 1.60 (m, 1 H, Cy(I)-
1.36 (each m, 10 H, Cy-2, Cy-3, Cy-4), 1.94 (m, 3 H, H-1), 1.88 2A′), 1.58 (m, 1 H, Cy-4), 1.55 (m, 1 H, Cy(II)-2B), 1.49 (m, 1 H,
(m, 12 H, H-2A/B, H-3A/B), 1.74 (m, 3 H, H-4), 1.32 (m, 6 H, Cy-4), 1.46 (m, 1 H, Cy-3B), 1.43 (m, 1 H, Cy-3B), 1.34 (m, 6 H,
H-3A′/B′), 1.27 (m, 9 H, H-2A′/B′, H-4′); 13C NMR (126 MHz, [D2]- H-3A′/B′), 1.31 (m, 1 H, Cy(II)-2A′), 1.23 (m, 8 H, 2 × Cy-3A′,
2
dichloromethane, 293 K): δ = 215.8 (d, J(P,C) = 25.7 Hz, CO), H-2A′/B′), 1.22 (m, 3 H, H-4′), 1.04 (m, 1 H, Cy-4′), 0.95 (m, 1 H,
1
165.0 (q, J(B,C) = 49.1 Hz, i-ArCl), 153.8 (CN), 133.5 (m-ArCl), Cy-3B′), 0.94 (m, 1 H, Cy-3B′), 0.70 (m, 1 H, Cy-4′), 0.12 (m, 1 H,
133.3 (q, 2J(B,C) = 4.2 Hz, o-ArCl), 123.4 (p-ArCl), 84.3 (Cp), 57.3 Cy(II)-2B′); 13C NMR (126 MHz, [D2]dichloromethane, 263 K):
11302 | Dalton Trans., 2015, 44, 11294–11305
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2
δ = 221.0 (d, J(P,C) = 27.6 Hz, CO), 180.8 (CN), 168.3 (NCN),
Acknowledgements
164.7 (q, 1J(B,C) = 49.2 Hz, i-ArCl), 143.6 (p-MesA), 143.1
(p-MesB), 138.8 (o-MesA), 136.8 (o-MesB), 136.0 (i-MesB), 135.0 We thank the EPSRC for funding and for access to the National
2
(i-MesA), 133.2 (m-ArCl), 133.0 (q, J(B,C) = 4.2 Hz, o-ArCl), 131.5 Mass Spectrometry Facility, Swansea University. J. Niemeyer
(m-MesA), 131.1 (m-MesB), 123.1 (p-ArCl), 81.2 (Cp), 55.5 (Cy(I)- thanks the DFG for a postdoctoral fellowship.
1), 53.5 (Cy(II)-1), 39.7 (d, 1J(P,C) = 17.0 Hz, C-1), 34.5 (Cy(I)-
2A), 32.7 (Cy(II)-2A), 32.5 (Cy(I)-2B), 31.6 (Cy(II)-2B), 30.0 (2 ×
2
2
C-3A/B), 28.0 (d, J(P,C) = 10.9 Hz, C-2A/B), 27.7 (d, J(P,C) = 8.8
Hz, C-2A/B), 26.62 (Cy-3A), 26.57 (Cy-3A), 26.3 (Cy-3B), 26.2 (C-4),
26.0 (Cy-4), 25.0 (Cy-4), 24.4 (Cy-3B), 22.2 (o-CH3B), 22.0
(o-CH3A), 21.3 (p-CH3A), 21.0 (p-CH3B); 11B NMR (160 MHz,
Notes and references
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80.4 (ν1/2 = 4 Hz).
Boron-containing product 20. 12d (10 mg, 0.0077 mmol,
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= 6.6 Hz, CH(III)), 3.39 (sept, J(H,H) = 6.4 Hz, CH(II)), 2.27 (s,
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6 H, p-CH3), 2.15 (s, 12 H, o-CH3), 1.10 (d, 6 H, J(H,H) = 6.2
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(p-Mes), 139.1 (i-Mes), 136.4 (o-Mes), 130.1 (m-Mes), 46.3 (CH-
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[D2]dichloromethane, 298 K): δ = 29 (ν1/2 = 390 Hz).
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equiv.) was dissolved in [D2]dichloromethane in an NMR tube
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1H NMR (500 MHz, [D2]dichloromethane, 293 K): δ = 6.85 (s,
4 H, m-Mes), 3.45 (m, Cy(III)-1), 3.04 (m, Cy(I)-1), 2.96 (m, Cy(II)-
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155.9 (NvC(NR2)), 139.7 (i-Mes), 139.1 (p-Mes), 136.6 (o-Mes),
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methane, 293 K): δ = 30 (ν1/2 = 350 Hz).
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11304 | Dalton Trans., 2015, 44, 11294–11305
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