Planar-chiral ferrocenylphosphine-borane complexes featuring agostic-type B-H?E (E = Hg, Sn) interactions

Article abstract of DOI:10.1039/c6dt04791b

The synthesis of ferrocenylphosphine-borane adducts 1,2-fc(E)(PPh₂·BH₃) (E = SnR₂R′, HgX; 1,2-fc = 1,2-ferrocenediyl) that are substituted with organotin or organomercury Lewis acid moieties in ortho-position is presented. Several compounds that feature two ferrocenylphosphine-borane moieties bridged by Sn or Hg are also introduced. The products are fully characterized by multinuclear NMR spectroscopy, high-resolution MALDI-TOF mass spectrometry and elemental analysis. The attachment of the Lewis acid substituent to the same Cp ring of the ferrocene results in planar-chirality and the close proximity between the boron hydride group and the Lewis acid is expected to allow for agostic-type B-H?E (E = Sn, Hg) interactions. Structural investigations by X-ray diffraction reveal a short B-H?Sn contact of 2.755(4) ? for 1,2-fc(SnMe₂Cl)(PPh₂·BH₃), which is only the second reported example of such a short agostic-type contact involving a coordinatively saturated tin(iv) center. In contrast, for the tetraorganotin derivatives 1,2-fc(SnMe₃)(PPh₂·BH₃) and [1,2-fc(PPh₂)](μ-SnMe₂)[1,2-fc(PPh₂·BH₃)], in which the Lewis acidity of the tin atom is weaker than in 1,2-fc(SnMe₂Cl)(PPh₂·BH₃), the B-H?Sn distances are much longer but still within the sum of the van der Waals radii of Sn and H (∑_vdW = 3.27 ?). The chloromercury-substituted ferrocenylphosphine-borane 1,2-fc(HgCl)(PPh₂·BH₃) shows a similarly short B-H?Hg contact of 2.615(5) ? (∑_vdW = 3.15 ?). Inspection of the extended structure of 1,2-fc(HgCl)(PPh₂·BH₃) reveals that the Lewis acidic mercury atom is also involved in intermolecular Hg?Cl interactions with a neighboring molecule. An analysis of ³¹P and ¹¹B NMR data reveals a correlation between the chemical shifts and the Lewis acidity of the adjacent organotin/mercury substituent. Structure optimization of 1,2-fc(SnMe₃)(PPh₂·BH₃) and 1,2-fc(SnMe₂Cl)(PPh₂·BH₃) by density functional theory (DFT) indicates B-H?Sn contacts respectively of 3.129 and 2.631 ? that are close to the experimental values. Natural bond orbitals (NBO) and atom in molecules (AIM) analyses reveals a B-H→Sn donor-acceptor interaction energy of 5.46 kcal mol^-1 and a B-H?Sn bond path for the chlorodimethyltin-substituted derivative with a modest electron density ρ(r) of 0.0082 a.u. and a positive Laplacian at the bond critical point.

Full text of DOI:10.1039/c6dt04791b

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Planar-chiral ferrocenylphosphine-borane
complexes featuring agostic-type
Cite this: DOI: 10.1039/c6dt04791b
B–H⋯E (E = Hg, Sn) interactions†
Alain C. Tagne Kuate,a,b Roger A. Lalancette*a and F. Jäkle
a
*
The synthesis of ferrocenylphosphine-borane adducts 1,2-fc(E)(PPh2·BH3) (E = SnR2R’, HgX; 1,2-fc = 1,2-
ferrocenediyl) that are substituted with organotin or organomercury Lewis acid moieties in ortho-position
is presented. Several compounds that feature two ferrocenylphosphine-borane moieties bridged by Sn or
Hg are also introduced. The products are fully characterized by multinuclear NMR spectroscopy, high-
resolution MALDI-TOF mass spectrometry and elemental analysis. The attachment of the Lewis acid sub-
stituent to the same Cp ring of the ferrocene results in planar-chirality and the close proximity between
the boron hydride group and the Lewis acid is expected to allow for agostic-type B–H⋯E (E = Sn, Hg)
interactions. Structural investigations by X-ray diffraction reveal a short B–H⋯Sn contact of 2.755(4) Å for
1,2-fc(SnMe2Cl)(PPh2·BH3), which is only the second reported example of such a short agostic-type
contact involving a coordinatively saturated tin(IV) center. In contrast, for the tetraorganotin derivatives
1,2-fc(SnMe3)(PPh2·BH3) and [1,2-fc(PPh2)](μ-SnMe2)[1,2-fc(PPh2·BH3)], in which the Lewis acidity of the
tin atom is weaker than in 1,2-fc(SnMe2Cl)(PPh2·BH3), the B–H⋯Sn distances are much longer but still
within the sum of the van der Waals radii of Sn and H (∑vdW = 3.27 Å). The chloromercury-substituted ferro-
cenylphosphine-borane 1,2-fc(HgCl)(PPh2·BH3) shows a similarly short B–H⋯Hg contact of 2.615(5) Å
(∑vdW = 3.15 Å). Inspection of the extended structure of 1,2-fc(HgCl)(PPh2·BH3) reveals that the Lewis
acidic mercury atom is also involved in intermolecular Hg⋯Cl interactions with a neighboring molecule.
An analysis of 31P and 11B NMR data reveals a correlation between the chemical shifts and the Lewis
acidity of the adjacent organotin/mercury substituent. Structure optimization of 1,2-fc(SnMe3)(PPh2·BH3)
and 1,2-fc(SnMe2Cl)(PPh2·BH3) by density functional theory (DFT) indicates B–H⋯Sn contacts respect-
ively of 3.129 and 2.631 Å that are close to the experimental values. Natural bond orbitals (NBO) and atom
in molecules (AIM) analyses reveals a B–H→Sn donor–acceptor interaction energy of 5.46 kcal mol−1
and a B–H⋯Sn bond path for the chlorodimethyltin-substituted derivative with a modest electron density
ρ(r) of 0.0082 a.u. and a positive Laplacian at the bond critical point.
Received 20th December 2016,
Accepted 11th April 2017
DOI: 10.1039/c6dt04791b
rsc.li/dalton
actions is relevant to bond activation processes, can help
describe various reaction mechanism, and is critical to the
Introduction
Investigations into bonding situations where an X–H σ bond development of new synthetic pathways. In the case of intra-
coordinates to a metal center are of fundamental importance molecular bonding situations, these σ-interactions have been
for understanding catalytic processes that involve organo- termed “agostic”.1–4 This description originally referred
metallic complexes. Indeed, the formation of 3c–2e inter- specifically to coordination of C–H σ bonds to low valent
transition metals. Over time, the term has been applied more
generally and any X–H σ-bonded intramolecularly to low-valent
aDepartment of Chemistry, Rutgers University-Newark, 73 Warren Street, Newark,
or electron-deficient elements is nowadays frequently referred
NJ 07102, USA. E-mail: fjaekle@andromeda.rutgers.edu,
to as “agostic”.
rogerlal@andromeda.rutgers.edu
bDepartment of Chemistry, Faculty of Sciences, University of Dschang, P.O. Box 67,
Dschang, Cameroon
The coordination chemistry of phosphine-borane adducts
of type BH3·L (L = PMe3, PPh3) or B2H4·2PMe3 has been exten-
sively studied in regards to the nature of the B–H bonding to
transition metals and the ensuing complex stabilization.5 The
formation of a σ complex has been postulated as an intermedi-
ate in the C–H bond activation of the corresponding isoelectro-
†Electronic supplementary information (ESI) available: Details for the synthesis
and characterization of FcPPh2·BH3, NMR and mass spectra of all new
compounds (PDF), FT-IR spectral data, computational details. CCDC
1523540–1523543. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c6dt04791b
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nic alkanes.6 In view of the importance of reactions involving able interest. Wrackmeyer et al. proposed the first intra-
alkanes to the petroleum industry, phosphine-borane adducts molecular Si–H⋯B interaction for compound III (Fig. 1)19 and
serve as easy-to-handle alternatives for investigating this Chen et al. recently confirmed an intermolecular inter-
σ-bonding character.7 The first observation of a B–H⋯E inter- action for Et3Si–H⋯Al(C6F5)3 by single crystal X-ray diffrac-
action in a complex between a metal and a phosphine-borane, tion.20 Müller et al. reported multiple Si–H⋯Al interactions,
B2H4·2PMe3 and ZnCl2, has been reported three decades ago resulting in a formally hexacoordinate Al species.21 These
by Snow et al.8 This important discovery has provided the interactions have frequently been referred to as agostic or
basis for further exploration of the coordination of transition three-center interactions, based on the significant covalent
metals with mono- or bidentate phosphine-borane adducts. character derived from theoretical calculations.18,21,22 The
Several structurally characterized neutral complexes have since term “charge-inverted hydrogen bond” has been proposed by
been described, including [CpMo(CO)2{BH3·PPh[N(SiMe3)2]}],9 Jablonski to be more appropriate considering the Xδ+–Hδ−
[M(CO)n(B2H4·2PMe2)] (M
=
Cr, Mo, W;
n
=
4, 5),10 donation to a lone pair vacancy.23
The coordination chemistry of ferrocenyl-substituted phos-
[M(CO)5(BH3·L)] (M = Cr, Mo, W; L = PMe3, PPh3),11 [CpMn
(CO)2(BH3·PMe3)],12 [Cp*M(CO)2(BH3·PMe3)] (M = Fe, Ru);13 in phine-borane adducts remains scarcely developed despite
these complexes the B–H bond acts as a two-electron donor to potential advantages, including the robustness and unique
stabilize the central metal atom in an η1- or η2-binding mode. steric demand of ferrocene, and the opportunity for further
Bisphosphine-borane adducts that feature both a BH3-pro- studies on the electronic and redox properties of such
tected phosphine and a free phosphine moiety have emerged complexes.
as bidentate ligands that are able to further stabilize the on the parent ferrocenyldiphenylphosphine-borane adduct
B–H⋯E interaction. An example of such
ligand is (FcPPh2·BH3, Fc = ferrocenyl),24 while related derivatives
A literature search shows only two reports
a
BH3·PPh2CH2PPh2 (BH3·dppm), which was first introduced by have been used as intermediates for ligands applied in
Martin et al.14 and used for the synthesis of Co, Cr, Ru and Rh asymmetric catalysis.25,26 Notably, ferrocenylphosphine-boranes
compounds. In these complexes the metal is chelated by the FcCH2PH2·BH3, FcPH2·BH3, FcP(tBu)H·BH3, and corres-
BH3 (η1) and free PPh2 group.15 Recently, Weller et al. ponding planar-chiral derivatives have been explored by Hey-
described an ionic variant, [Ph2HP–BH2–PHPh2]X (X = counter Hawkins et al. as precursors to redox-active phosphinoborane
anion), with BH2 as a linker between two phosphines; they oligomers and polymers via metal-catalyzed dehydrocoupling
used this ligand in the formation of a Rh complex that displays and cross-dehydrocoupling processes.27 Structural studies
a β B–H⋯Rh agostic interaction.16
on ferrocenyldiphenylphosphine-borane adducts with Lewis
In main group chemistry, agostic-type B–H⋯E interactions acidic metal centers in ortho position are exceedingly rare.
are comparatively much less studied. Short B–H⋯Sn(II)/Pb(II) Recently we reported the structures of BH3-adducts of ferro-
contacts in the monomeric dialkyltetrylene species I (Fig. 1) cene-fused phosphastannin and phosphaborin species formed
are thought to prevent dimerization into the corresponding bis en-route to ambiphilic P,Sn and P,B ligands; however, the
(tetrylene).17 Kawachi et al. postulated a weak interaction complex geometry was such that B–H⋯E (E = Sn, B) agostic-
between the borate hydrogen and an adjacent electron- type interactions are not feasible.28 In earlier work, we also
deficient silicon center in II (Fig. 1).18 On the other hand, examined intramolecular B–Cl⋯Sn contacts in ferrocene
interactions of the type R3Si–H⋯ER3 and R3Sn–H⋯ER3 (E = based heteronuclear bidentate Lewis acids 1,2-fc(BRCl)
electron-deficient Group 13 element) have attracted consider- (SnMe2Cl) (R = Me, Ph; fc = ferrocenediyl).29 In continuation
of our studies on ferrocene-based Lewis pair systems, we
report here a new series of planar-chiral organostannyl-
and chloromercurio-substituted ferrocenylphosphine-borane
adducts (IV, V) and discuss the possibility of agostic-type inter-
actions through coordination of B–H bonds to the Lewis acidic
tin and mercury atoms. We postulate that these interactions
are similar in nature to those involving coordination of silanes
or stannanes to electron-deficient Group 13 elements dis-
cussed above.
Results and discussion
The planar-chiral ferrocenylsulfinate-substituted phosphine-
borane (pS,Ss)-2, first prepared by Kagan from (Ss)-ferrocenyl
p-tolyl sulfoxide (1) and LDA followed by quenching with
PPh2Cl/BH3·THF,25a was chosen as starting material owing to
the possibility to easily replace the tolylsulfinate moiety and
Fig. 1 Examples of species with B–H⋯E and B⋯H–E (E = Si, Sn, Hg)
contacts.
attach Lewis acid groups in ortho-position to the phosphine-
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borane through lithiation and reaction with a metal halide as (pS)-3, which is indicative of an enhanced s-character of the
electrophile. Thus, slow addition of tBuLi to a solution of equatorial Sn–C(Me) bonds as typically observed for hypercoor-
(pS,Ss)-2 in THF at −78 °C and subsequent treatment dinate organotin species.32 The presence of the chloride sub-
with trimethyltin chloride afforded after purification by stituent induces a strong low field shift of the Cp proton close
column chromatography and recrystallization from hexanes to the tin atom with a signal appearing at δ = 5.34 ppm,
the trimethylstannyl-substituted ferrocenylphosphine-borane whereas the corresponding Cp proton in (pS)-3 appears at δ =
(pS)-3 as an orange crystalline solid in 58% yield (Scheme 1).
4.56 ppm. A single broad resonance is observed at higher field
1
The H NMR spectrum of (pS)-3 displayed the typical reso- for the BH3 protons, which is consistent with fast rotation
nances for a 1,2-substituted Cp ring, as well as a singlet with tin about the P–B bond on the NMR time scale. It is not surprising
satellites and a broad resonance at low frequency consistent that a separate signal for a bridging H atom involved in an
respectively with the attachment of the SnMe3 group at the agostic-type interaction cannot be detected considering that
ferrocene moiety and retention of BH3 at the phosphorus dynamic exchange persists even at very low temperature
atom. The formation of (pS)-3 is also reflected in a broad, non- according to previous studies. For instance, an activation
resolved resonance at δ = 18.1 ppm in the 31P NMR, a sharp energy of only ΔG# = 30 kJ mol−1 was estimated for the BH3
signal at δ = −37.5 ppm in the 11B NMR which is indicative of hydrogen exchange in the transition metal complex
the Ph2P·BH3 adduct, and a single resonance at δ = −5.6 ppm [M(CO)5(η1-H3BPMe3)] (M = Cr, Mn, W).17a,b,11 Importantly, the
in the 119Sn NMR spectrum that is slightly low field compared 119Sn NMR signal of (pS)-4 at δ = 67.0 ppm is split into a
to the chemical shift measured for ortho-(diphenylphosphino) doublet with a coupling constant of J = 12.3 Hz; the signal
ferrocenyl stannane (δ = −7.2 ppm).30 Relative to the sulfinate splitting is tentatively assigned to coupling of 119Sn with 31P as
precursor both the 31P ((pS,Ss)-2, δ = 16.7 ppm (ref. 31)) and a similar coupling has been reported for the hypercoordinated
the 11B NMR signal ((pS,Ss)-2, δ = −36.5 ppm) experienced ferrocenylstannane Fc(PPh2O)(SnMe2Cl) with a bridging phos-
slight low field shifts. The high resolution MALDI-TOF mass phine oxide moiety (29 Hz).33 This is in contrast to the precur-
spectrum of (pS)-3 showed a major peak at m/z = 534.0228 a.u. sor (pS)-3, for which only a singlet could be resolved. The
that corresponds to [(pS)-3 − BH3]+ and demonstrates the labi- 31P (δ = 15.9 ppm) and 11B (δ = −36.7 ppm) NMR chemical
lity of the BH3 group.
shifts are slightly upfield relative to those of (pS)-3, possibly as
Enhancement of the Lewis acidity at tin was achieved by a result of a weak B–H⋯Sn agostic-type interaction (vide infra).
methyl/chloride substituent exchange reaction. Treatment of A high resolution MALDI-TOF mass spectrum of (pS)-4
(pS)-3 with PhBCl2 in benzene at 50 °C overnight resulted in displayed peaks at m/z = 568.0015 a.u. and 553.9666 a.u.
quantitative conversion to a new product. Solvent evaporation respectively assigned to [(pS)-4]+ and [(pS)-4 − BH3]+.
and recrystallization of the crude material from hexanes at
The molecular structures of (pS)-3 and (pS)-4 in the solid
−30 °C gave a yellow-orange crystalline solid that was identi- state were determined by X-ray diffraction analysis and are
fied as the chlorodimethylstannyl-substituted ferrocenyl- depicted in Fig. 2. The H1A⋯Sn1 distance of 3.104(3) Å in
phosphine-borane (pS)-4 (Scheme 1) by multinuclear NMR (pS)-3 is only slightly shorter than the sum of the van der
spectroscopy and high resolution MALDI-TOF MS. The methyl/ Waals radii (∑vdW = 3.27 Å (ref. 34)) of H and Sn, indicative of
chloride exchange reaction occurs selectively and in high yield the absence of an agostic-type interaction or a very weak inter-
(96%) without any sign of borodestannylation or decomplexa- action that does not significantly impact the geometry about
1
tion of BH3. The H NMR spectrum of (pS)-4 showed two dis- the tin atom. The Sn atom adopts a monocapped tetrahedral
tinct resonances with tin satellites at δ = 1.27 ppm (2JSn,H
=
coordination geometry with H1A approaching via the tetra-
67/70 Hz) and δ = 0.49 ppm (2JSn,H = 62/64 Hz) assigned to the hedral face C1, C24 and C25. In contrast, the enhanced Lewis
diastereotopic methyl groups generated upon halogenation. acidity of the tin center in (pS)-4 results in an H1A⋯Sn1 dis-
2
The JSn,H coupling constants are much larger than that for tance of 2.755(4) Å, which is considerably shorter than that
Scheme 1 Synthesis of ferrocenylphosphine-borane adducts with organotin and mercury substituents in ortho position (only one enantiomer shown for rac-5).
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1
nances at δ = 14.6 and δ = −36.4 ppm while the H NMR spec-
trum exhibits the expected pattern for a 1,2-disubstituted ferro-
cene with the most downfield shifted Cp-proton observed at
δ = 4.74 ppm. A MALDI-TOF mass spectrum confirmed the iden-
tity of 5 with a major peak at m/z = 605.9874 a.u. for [5 − BH3]+.
X-ray diffraction analysis of a single crystal of rac-5 (Fig. 3)
was performed in order to identify any short B–H⋯Hg contacts
in the solid state. The H1C⋯Hg1 distance in the structure of
rac-5 is 2.615(5) Å (∑vdW = 3.15 Å (ref. 34)), indicative of an
agostic-type interaction similar to the one observed for (pS)-4.
In addition, inspection of the extended structure revealed the
formation of dimers via short intermolecular Hg1⋯Cl1 con-
tacts of 3.191(1) Å (∑vdW = 3.80 Å (ref. 34)). This dimerization
brings about significant steric strain, because the Cl1 atom
comes into close proximity of the free Cp ring of the second
molecule as reflected in a short intermolecular Cl1–CpH6 dis-
tance of 2.666(9) Å (Fig. 3). Consequently the Hg1–Cl1 moiety
is bending away from the ferrocene framework, resulting
in a distortion of the linear geometry at the mercury atom
(C1–Hg1–Cl1 = 174.34(11)°).
We next pursued the synthesis of corresponding organotin
and mercury-bridged diferrocenes. Lithiation of (pS,Ss)-2 and
subsequent reaction with 0.5 equiv. of HgCl2 at low tempera-
ture gave the mercury-substituted bis(ferrocenylphosphine-
borane) (pSpS)-6 in 27% yield (Scheme 2). Retention of the
BH3 groups in (pSpS)-6 is evidenced by a single, broad reso-
nance at δ = 16.8 ppm in the 31P NMR spectrum, which is
slightly low field shifted relative to that of the chloromercury
derivative rac-5, whereas the boron signal at δ = −36.0 ppm
appears at an almost identical chemical shift. The 1H NMR
spectrum shows three signals for the substituted and one for
the free Cp rings, which is consistent with the formation of a
single enantiomer with C2-symmetry. As in the case of the
monoferrocenes, the Ph groups are chemically inequivalent
Fig. 2 (a) Molecular structure of enantiomerically pure (pS)-3 (50%
ellipsoids, H atoms omitted for clarity except for B–H). Selected bond
lengths, interatomic distances (Å) and angles (°): Sn1–C1 2.141(3),
Sn1–C23 2.155(3), Sn1–C24 2.154(3), Sn1–C25 2.137(3), P1–C2 1.791(3),
P1–B1 1.924(4), B1⋯Sn1 3.912(4), H1A⋯Sn1 3.104(3), C1–Sn1–C24
116.80(13), C1–Sn1–C25 109.88(12), C24–Sn1–C25 109.13(13),
H1A⋯Sn1–C23 174.51(5), P1–C2–C1–Sn1 −2.00(5); the B–H distances
were fixed at 1.19 Å. Flack parameter 0.038(3). (b) Molecular structure of
enantiomerically pure (pS)-4 (50% ellipsoids, H atoms omitted for clarity
except for B–H). Selected bond lengths, interatomic distances (Å) and
angles (°): Sn1–Cl1 2.4022(9), Sn1–C1 2.133(3), Sn1–C23 2.143(3), Sn1–
C24 2.127(4), P1–C2 1.793(3), P1–B1 1.925(4), B1–H1A 1.13(4), B1–H1B
1.13(4), B1–H1C 1.11(4), B1⋯Sn1 3.585(5), H1A⋯Sn1 2.755(4), C1–Sn1–
C23 121.17(14), C1–Sn1–C24 112.94(13), C23–Sn1–C24 115.20(14),
H1A⋯Sn1–Cl1 177.22(7), B1–H1A⋯Sn1 129.32(7), P1–C2–C1–Sn1 −2.42(5);
the B–H distances were fully refined. Flack parameter: 0.020(3).
found for (pS)-3. As a consequence, the tin atom in (pS)-4 is
pyramidalized (∑ equatorial angles = 349.3°) and adopts a dis-
torted trigonal bipyramidal coordination geometry with H1A
and Cl1 in axial positions. The P1–B1 bond in (pS)-4 (1.925(4)
Å) is similar in length to the corresponding bond in (pS)-3
(P1–B1 = 1.924(4) Å). The hydrogen atoms on boron were
found on the difference map and refined, yielding B–H dis-
tances of 1.11(4), 1.13(4) and 1.13(4) Å for (pS)-4. To the best
of our knowledge, compound (pS)-4 represents only the
second reported example of such a short B–H⋯Sn agostic-type
interaction involving a tin(IV) center. In a previous report, an
even shorter B–H⋯Sn contact of 2.349 Å was found for the
anion [10-endo-(SnPh3)-10-í-H-7,8-nido-C2B9H10]−, indicative of
an α-agostic interaction, but in this case the tin atom is
directly bound to a boron atom of the carborane.35 Other
known examples involve (cyclic) dialkylstannylenes (I, Fig. 1)
having low-valent tin(II) centers stabilized by short B–H⋯Sn
contacts.17
We next turned our attention to the replacement of the tri-
methylstannyl group in (pS)-3 by a Lewis acidic mercury chlo-
ride with the aim of structurally investigating a possible
agostic-type B–H⋯Hg interaction. The synthesis was achieved
by reacting (pS)-3 with a slight excess of HgCl2 in acetone for
1 h. Precipitation in water and recrystallization from CH2Cl2/
hexanes afforded the chloromercurio-substituted ferrocenyl-
phosphine-borane rac-5 as a yellow crystalline solid in 62%
yield (Scheme 1). The 31P and 11B NMR spectra display reso-
Fig. 3 Molecular structure of rac-5 (50% ellipsoids, H atoms omitted
for clarity except for B–H); a pair of enantiomers is displayed. Selected
bond lengths, interatomic distances (Å) and angles (°): Hg1–C1 2.038(4),
Hg1–Cl1 2.3274(9), P1–C2 1.781(4), P1–B1 1.933(5), B1–H1A 1.00(5), B1–
H1B 1.18(5), B1–H1C 1.16(5), H1C⋯Hg1 2.615(5), B1⋯Hg1 3.149(5),
H1B⋯Hg1 2.929(5), Hg1⋯Cl1# 3.191(1) Å and Cp–H6⋯Cl1# 2.666(9) Å.
C1–Hg1–Cl1 174.34(11), P1–C2–C1–Hg1 0.21(4); the B–H distances
were fully refined.
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The structure was finally confirmed by single crystal
X-diffraction analysis (Fig. 4). The ferrocene units in (pS,pS)-7
are positioned almost perpendicular to each other with an
interplanar angle between the substituted Cp-rings of 82.7(1)°.
The molecular structure is marked by an B1–H1A⋯Sn1 contact
that is slightly shorter than for the tetraorganostannane (pS)-
3, but still much longer than for the chlorostannane (pS)-4.
Consistent with a rather weak B–H⋯Sn interaction is the small
sum of the equatorial angles at Sn of 337.5° (involving C1,
C23, C24), which is closer to the value expected for a tetra-
hedral than a trigonal bipyramidal configuration. A relatively
short P2⋯Sn1 contact of 3.882(9) Å that is in the range of the
sum of the van der Waals radii of P and Sn (∑vdW = 3.97 Å (ref.
34)) appears to also contribute to distortions of the tetrahedral
geometry at Sn.
An earlier band from the chromatographic separation gave
after recrystallization from hexanes a yellow crystalline solid in
low yield (6%) that was identified as the hydridodimethyl-
stannyl-substituted
ferrocenylphosphine-borane
(pS)-8
(Scheme 2). High resolution MALDI-TOF MS analysis con-
firmed the formation of (pS)-8 with a dominant peak at m/z =
520.0041 a.u. assigned to [{(pS)-8} − BH3]+. (pS)-8 most likely
originates from initial formation of (pS)-4 as an intermediate
by 1 : 1 reaction between the lithio derivative of (pS,Ss)-2 and
dimethyltin chloride. The latter may then be reduced to the
hydride either by tert-butyllithium or FcPPh2·BH3, which was
identified as a major product of the reaction. Literature
precedents describe the synthesis of 1,1′-fc(SnMe2H)2 by
reduction of the chloro precursor with LiAlH4.36 The 119Sn
NMR spectrum of (pS)-8 in CDCl3 exhibits a single resonance
at δ = −96.3 ppm close to that reported for 1,1′-fc(SnMe2H)2
Scheme 2 Synthesis of bis(ferrocenylphosphine-borane) adducts with
organotin and mercury bridges.
and thus give rise to two sets of resonances. The formation of
(pSpS)-6 was further confirmed by peaks in the MALDI-TOF
mass spectrum for [(pSpS)-6 − BH3]+ at m/z = 954.1053 a.u.
and [(pSpS)-6 − 2BH3]+ at m/z = 940.0734 a.u. In order to inves-
tigate the presence of an agostic-type B–H⋯Hg interaction in
(pSpS)-6 in the solid state, X-ray diffraction data were collected
on a single crystal obtained by recrystallization from hexanes,
but all attempts to solve the structure proved unsuccessful.
The successful synthesis of (pSpS)-6 by salt elimination
reaction starting from (pS,Ss)-2 prompted us to apply a similar
method for the preparation of the corresponding dimethyl-
stannyl-bridged bis(ferrocenylphosphine-borane). Lithiation of
(pS,Ss)-2 and subsequent reaction with 0.5 equiv. of Me2SnCl2
gave after aqueous workup an oily red residue. The latter was
subjected to column chromatography on silica gel using
hexanes : Et2O : Et3N as the eluent. One of the later fractions
was concentrated and repeated recrystallization of the residue
in hexanes at low temperature allowed deposition of a first
crop of orange solid that appeared to be the parent ferrocenyl-
1
phosphine-borane, FcPPh2·BH3, according to an H NMR ana-
lysis (see ESI† for full characterization). A second crop contain-
ing very few orange crystals was collected after some days and
its analysis revealed the formation of the expected dimethyl-
stannyl-substituted bis(ferrocenylphosphine-borane), but with
only one remaining BH3 group ((pSpS)-7, Scheme 2). The
1H NMR spectrum exhibits eight proton resonances for the
Fig. 4 Molecular structure of enantiomerically pure (pSpS)-7 (50%
ellipsoids, H atoms omitted for clarity except for B–H). Selected bond
lengths, interatomic distances (Å) and angles (°): Sn1–C1 2.158(4), Sn1–
inequivalent ferrocene fragments and two independent signals C23 2.150(3), Sn1–C24 2.147(3), Sn1–C25 2.151(4), P1–C2 1.801(4), P2–
C26 1.815(4), P1–B1 1.914(4), B1⋯Sn1 3.732(5), H1A⋯Sn1 3.006, P2⋯Sn1
at δ = 1.20 ppm (2JSn,H = 62 Hz) and δ = 0.02 ppm (2JSn,H
=
3.882(9), C1–Sn1–C23 107.14(14), C1–Sn1–C24 118.73(13), C1–Sn1–C25
103.22(13), C23–Sn1–C24 111.62(15), C23–Sn1–C25 104.62(14), C24–
Sn1–C25 110.31(14), C25–Sn1⋯H1A 173.97(5), P2–Sn1⋯H1A 136.72(7),
P1–C2–C1–Sn1 3.44(5), P2–C26–C25–Sn1 −8.67(5); the B–H distances
were fixed at 1.19 Å. Flack parameter: 0.027(2).
56 Hz) for the diastereotopic methyl groups. Peaks at m/z =
889.0534 a.u. and m/z = 905.0495 a.u. corresponding to
[{(pSpS)-7} − BH3 + H]+ and the oxidized species [{(pSpS)-7} −
BH3 + O]+ were identified by MALDI-TOF MS.
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(δ = −102.4 ppm, C6D6).36 Due to coupling to the methyl Refinement of the Flack parameter gave values of 0.038(3),
protons, the Sn–H proton is split into a septet resonance at δ = 0.020(3), and 0.027(2) respectively for (pS)-3, (pS)-4, and
5.27 ppm in the 1H NMR spectrum. Similar to (pS)-4, the (pSpS)-7 (Fig. 2 and 4), demonstrating their planar-chirality
methyl groups in (pS)-8 are diastereotopic and give rise to two with the expected pS configurations according to the CIP
close but independent doublet resonances at δ = 0.18 ppm (Cahn–Ingold–Prelog) protocol. Although no proof of enantio-
2
(3JHH = 1.8 Hz, JSn,H = 55/58 Hz) and 0.17 ppm (3JHH = 1.8 Hz, meric purity, the chirality of the bulk samples was further
2
2JSn,H = 58/60 Hz). The JSn,H coupling constants, however, are established by measurement of the specific rotations in
much smaller than those for (pS)-4, indicative of less s-charac- CHCl3, which is respectively [α]2D0 = +167 (c 0.25), +74 (c 0.16),
ter of the corresponding Sn–C bonds. The 31P and 11B NMR +50 (c 0.22) and −47 (c 0.18) for (pS)-3, (pS)-4, (pS)-8 and
spectra offer evidence that the BH3 is still attached to the phos- (pRpR)-10. The reasons for racemization in the conversion of
phine moiety, given that broad resonances are observed in the (pS)-3 to rac-5 have not been established. One possibility is
expected range at δ = 18.5 ppm and δ = −38.5 ppm. They are that the BH3 group directs attack of the HgCl2 electrophile into
more similar to those of (pS)-3 than (pS)-4, suggesting that the less hindered C–H ortho-position, opening up a competing
agostic-type B–H⋯Sn contacts are not present. We were unable pathway to the direct mercuriodestannylation at the sterically
to further assess the molecular structure of (pS)-8, because the more hindered C–Sn position.37
refinement of a dataset for crystals obtained from hexanes
failed.
The structural studies in this manuscript reveal a relatively
long B–H⋯Sn contact in the structures of (pS)-3 and (pSpS)-7
The desired dimethylstannyl-bridged bis(ferrocenylphos- containing a tetraorganotin moiety, which is known to be a
phine-borane) is more readily obtained by first introducing poor Lewis acid. The selective conversion of (pS)-3 into (pS)-4
the dimethyltin bridge and then adding the PPh2·BH3 moi- with PhBCl2 induces a shorter B–H⋯Sn contact as a result of
eties, thereby avoiding undesirable side reactions such as the the enhanced Lewis acidity of the tin atom. Similarly, rac-5
reduction processes discussed above. We recently reported a exhibits a short B–H⋯Hg contact, but the Lewis acidic
high yield synthesis of the enantiomerically pure dimethyl- mercury center is also involved in intermolecular Hg⋯Cl inter-
stannyl-substituted bis(ferrocenylsulfinate) (pRSSpRSs)-9.28a actions that result in formation of discrete dimers in the solid
Treatment of (pRSSpRSs)-9 with two equiv. of tBuLi at −78 °C state. Looking at the heteronuclear solution NMR data we find
followed by addition of Ph2PCl and BH3·THF furnished the di- that within the series of stannyl- and mercury-substituted com-
methylstannyl-bridged
bis(ferrocenylphosphine-borane) pounds, the 31P NMR signals are shifted upfield and the 11B
(pRpR)-10 in 38% yield (Scheme 2). The formation of (pRpR)- NMR signals shifted downfield in the order (pS)-8 < (pS)-3 ≈
10 was further confirmed by high resolution MALDI-TOF MS (pRpR)-10 < (pSpS)-6 < (pS)-4 ≈ rac-5 (Table 1). This trend
with a dominant peak at m/z = 889.0544 a.u. for [(pRpR)-10 − tracks well with the expected Lewis acidity of the stannyl/
1
2BH3 + H]+. The H NMR spectrum of (pRpR)-10 displays only mercury group (highest for (pS)-4 and rac-5).38 It is also in
four signals for the ferrocene fragments and one signal for the good agreement with the magnitude of the agostic-type
methyl groups, suggesting that a single isomer is gener- B–H⋯E interactions as determined by X-ray crystallography.
ated. Signals at δ = 17.9 ppm in the 31P NMR and at δ = Interestingly, in the 1H NMR spectra we observe distinct down-
−37.7 ppm in the 11B NMR spectrum are close to those of (pS)- field shifts for the BH3 protons of (pS)-4, rac-5, and (pSpS)-6
3. The 119Sn NMR spectrum of (pRpR)-10 shows a singlet at δ = (1.7–2.0 ppm) relative to the parent FcPPh2·BH3 (1.25 ppm).
−13.1 ppm, a resonance that is shifted to high field when com- This is in sharp contrast to the upfield shifts typically observed
pared with that of the SnMe2-bridged diferrocenylphosphine- upon complexation of phosphine-borane ligands to transition
borane fc2(μ-SnMe2)(μ-PPh·BH3) (δ = −29.6).28a Unfortunately, metal complexes.10 Looking at prior NMR spectroscopic
despite several attempts, single crystals suitable for analysis of studies on main group species, in several cases upfield shifts
the structure of (pRpR)-10 in the solid state by X-ray diffraction have been reported (e.g., Et3SiH⋯Al(C6F5)3; III in Fig. 1);19,20
could not be obtained.
however, there is also precedent for downfield shifts for com-
The stereoselectivity of the transformations described in plexes with proposed Si–H⋯Al contacts.21 It is possible that in
here deserves a further comment. Except for rac-5 all X-ray our case spin–orbit effects due to the heavy metals play an
diffraction studies were performed on single enantiomers. important role.
Table 1 Comparison of heteronuclear NMR data for compounds 1,2-fc(PPh2·BH3)(R) with different substituents R in ortho position to phosphorous
Compound (R)
δ (119Sn) (ppm)
J(117/119Sn,1H) Sn–Me (Hz)
δ (31P) (ppm)
δ (11B) (ppm)
δ (1H) BH3 (ppm)
FcPPh2·BH3
16.0
18.5
18.1
17.9
15.9
16.8
14.6
−38.4
−38.5
−37.5
−37.7
−36.7
−36.0
−36.4
1.25
1.37
1.32
1.34
1.70
1.91
1.99
(pS)-8 (SnMe2H)
(pS)-3 (SnMe3)
(pRpR)-10 (SnMe2Fc′)
(pS)-4 (SnMe2Cl)
(pSpS)-6 (HgFc′)
rac-5 (HgCl)
−96.3
−5.6
−13.1
+67.0
55/58, 58/60
55
60
62/64, 67/70
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An unexpected result is seen for (pS)-8, for which the NMR 3.069 kcal mol−1 22c
,
and the interaction energies determined
data would seem to indicate very weak agostic-type inter- by NBO analysis of strongly hydrogen-bonded organic com-
actions. This suggests that the structure of the Sn–H derivative pounds were found to be within 4.7–12.8 kcal mol−1 39
On the
.
(pS)-8 resembles more that of (pS)-3 than the Sn–Cl derivative other hand, the B–H⋯Sn donor–acceptor interaction energies
(pS)-4 with the small H atom positioned close to the BH3 in tin(II) compound I (Fig. 1, X = CH2CH2) were calculated to
moiety rather than in trans-position. The smaller steric be 20 and 30 kcal mol−1 for the rac- and meso-isomers, indicat-
demand of the hydrogen in comparison to a Me group may ing much stronger interactions.17a
result in even less distortions at the Sn atom ultimately
placing this compound at the top of the list.
In good agreement with the NBO calculations and X-ray
diffraction analyses, atoms in molecules (AIM) analysis of the
The nature of the B–H⋯Sn interactions was further investi- optimized structure of (pS)-4 indicated a B–H⋯Sn bond path
gated for compounds (pS)-3 and (pS)-4 using computational (Fig. 5), which is characterized by an electron density ρ(r) of
3
tools. The optimized geometries obtained by density func- 0.0082e/a0 at the bond critical point (BCP) and a positive
5
tional theory (DFT) calculations at the B3LYP/dgdzvp level of Laplacian of ∇2ρ(r) = +0.040694e/a0 . In contrast, no BCP was
theory, indicated interatomic B–H⋯Sn distances of 3.129 and found during AIM analysis of the optimized structure of (pS)-
2.631 Å for (pS)-3 and (pS)-4 respectively, which are close to 3, consistent with no or unremarkable secondary interactions.
those determined by X-ray diffraction analysis (3.104(3) and The calculated electron density value at the BCP in (pS)-4 is
2.755(4) Å). Thus, as observed experimentally, a much stronger slightly lower than that reported by Kawachi for the B–H⋯Si
B–H⋯Sn interaction is predicted for (pS)-4, in accordance with interaction in compound II (0.0156e/a0 , Fig. 1)18 and the
3
an increased Lewis acidity and pyramidalization (∑ equatorial R3C–H⋯SiR2FR′ interaction reported by Gabbaï22c (0.0168e/a0 ).
3
angles = 350.8°) of the tin atom. Only a small difference in the
P–B bond lengths (1.939 Å for (pS)-3; 1.944 Å for (pS)-4) is
found. However, the B–H bond that is pointing toward Sn in
compound (pS)-4 is elongated by ca. 1% relative to the other
Conclusions
B–H bonds (1.217 vs. 1.208 Å), giving rise to a bent B–H⋯Sn We have prepared a series of ferrocene-based planar-chiral
linkage (131.86°; experimental value: 129.32°). Examination of organostannyl and mercury-substituted ferrocenylphosphine-
the B1–H1→Sn1 second order perturbation in the optimized borane adducts, in which the BH3 group at the phosphorus
structures of (pS)-3 and (pS)-4 by NBO analysis indicated inter- atom is involved in B–H⋯E interactions with Lewis acidic tin
action energies (E(2)) of 0.20 and 5.46 kcal mol−1, respectively. or mercury atoms in close proximity. The nature of the inter-
The latter suggests a significant donor–acceptor interaction action was investigated in the solid state by X-ray structural
between the B1–H1A moiety and the Sn center only in (pS)-4. analyses and in solution by multinuclear NMR spectroscopy.
For comparison, a deletion calculation for a R3C–H⋯SiR2FR′ DFT calculations on (pS)-3 and (pS)-4, and corresponding
interaction increased the total energy of the molecule by NBO and AIM analyses further support the notion that an
increase in the Lewis acidity of the tin atom results in a
B–H→Sn donor–acceptor interaction with
a perturbation
energy typical of compounds with moderately strong hydrogen
bonding.
Thus, our investigations provide new insights into the
nature of agostic-type interactions involving phosphine-
boranes and organotin(IV) and chloromercury moieties, and
attempts to synthesize the corresponding organoboryl-
substituted ferrocenylphosphine-boranes, which may feature
B–H⋯B agostic-type interactions, are currently in progress. As
planar-chiral bidentate ligands with a Lewis acid moiety in the
bridge, the diferrocenes (pSpS)-6 and (pRpR)-10 are also prom-
ising for use in coordination chemistry with transition metals
and asymmetric synthesis.
Experimental section
General methods
All reactions were carried out under an atmosphere of dry
nitrogen using high vacuum Schlenk-line techniques or in an
inert-atmosphere glove box (Innovative Technologies).
Commercial-grade solvents (toluene, hexanes) were purified by
a solvent purification system from Innovative Technologies,
Fig. 5 AIM contour plots of (a) the electron density and (b) the calcu-
lated Laplacian for 4. The bond path between Sn1 and the bridging
hydrogen (H6) is indicated by a dashed black line and the corresponding
BCP as a green circle.
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degassed and stored over sodium-potassium (NaK) alloy prior solved by direct methods and refined by full-matrix least
to use. Tetrahydrofuran was distilled from sodium/benzo- squares based on F2 with all reflections using the SHELXTL
phenone and dichloromethane from CaH2, degassed and V6.14 program package.49,50 Non-hydrogen atoms were refined
1
stored under nitrogen atmosphere. 499.9/599.7 MHz H NMR, with anisotropic displacement coefficients. All H atoms were
125.7/150.8 MHz 13C NMR, 192.4 MHz 11B NMR, 202.5 MHz found in electron-density difference maps and carbon-bound
31P NMR, and 223.6 MHz 119Sn NMR spectra were recorded on H atoms were treated as idealized contribution unless noted
a Bruker Avance III HD NMR spectrometer (Bruker BioSpin, differently. For (pS)-3 and (pSpS)-7 the boron-bound H atoms
Billerica, MA) equipped with a 5 mm broadband gradient were allowed to refine positionally, but were constrained to be
SmartProbe (Bruker, Billerica, MA) or a 600 INOVA NMR at a fixed distance to the boron atom; for (pS)-4, and rac-5 they
spectrometer (Varian Inc., Palo Alto, CA) equipped with a were allowed to refine freely. Structural data have been de-
boron-free 5 mm dual broadband gradient probe (Nalorac, posited with the Cambridge Structure Database as supplemen-
Varian Inc., Martinez, CA). Chemicals shifts (δ) are given in tary publications CCDC 1523540–1523543.
ppm and were referenced internally to deuterated solvent
Synthesis of trimethylstannyl-substituted
ferrocenylphosphine-borane (pS)-3
signals (CDCl3 7.26 (1H), 77.36 (13C); C6D6 7.15 (1H), 128.62
(13C)). Coupling constants (J) are reported in Hertz (Hz) and
splitting patterns are indicated as s (singlet), d (doublet), pt To a solution of (pS,Ss)-2 (1.023 g, 1.96 mmol) in THF (40 mL)
(pseudo triplet), t (triplet), brs (broad singlet), nr (not that was cooled to −78 °C, was added slowly a solution of
resolved), brm (broad multiplet) and m (multiplet), and the tBuLi in pentane (1.7 M, 1.3 mL, 2.20 mmol, 1.1 equiv.). The
following abbreviations are used for signal assignments: Ph = mixture was stirred for 10 min and a solution of Me3SnCl
phenyl, Fc = ferrocenyl, Cp = cyclopentadienyl, Me = methyl. (0.43 g, 1.1 mmol, 1.1 equiv.) in THF (10 mL) was slowly
IR spectra were recorded on a Thermo Scientific Nicolet 6700 added. After stirring for 1 h at this temperature, the reaction
FT-IR device. Optical rotation analyses were performed on an mixture was allowed to warm up to room temperature over-
Autopol III polarimeter, Rudolph Research Analytical, using a night. The THF solvent was removed in vacuum and the
tungsten-halogen light source operating at λ = 589 nm. High- residue dissolved in Et2O (50 mL) and quenched by addition
resolution matrix-assisted laser desorption ionization-time of of aqueous sodium bicarbonate. The combined organic layer
flight mass spectrometry (MALDI-TOF MS) data were acquired was then separated, washed with water, brine, dried with
on a Bruker Ultraflextreme. Elemental analyses were per- sodium sulfate, and the solvent was removed in vacuum to
formed by Quantitative Technologies Inc., Whitehouse, NJ. leave an orange viscous oil. The latter was purified by column
Structural optimization with full NBO analysis and frequency chromatography on silica gel with hexanes/Et3N (100 : 1) and
calculations of the optimized structures were performed using recrystallized in hexanes at −27 °C to give orange X-ray quality
1
the Gaussian 09 suite of programs40,41 with the B3LYP func- crystals. Yield 0.62 g (58%). [α]D20 (c = 0.25, CHCl3) = +167°. H
tional42 and dgdzvp basis set43 for all elements. The dgdzvp NMR (499.9 MHz, CDCl3, 25 °C): δ = 7.69 (t, 3JH,H = 8.8 Hz, 2H,
3
3
basis set has been employed for DFT studies on tin containing Ph), 7.51 (t, JH,H = 7.3 Hz, 1H, Ph), 7.45 (t, JHH = 8.3 Hz, 2H,
compounds.44 The AIM analysis was performed with the Ph), 7.45–7.30 (m, 5H, Ph), 4.56 (nr, 1H, Cp), 4.48 (nr, 1H, Cp),
AIMAll professional program applying wavefunctions obtained 4.23 (s, 5H, free Cp), 3.99 (nr, 1H, Cp), 1.32 (brm, 3H, BH3),
2
from DFT calculations at the B3LYP/dgdzvp level of theory.45
0.16 (s, JSn,H = 55 Hz, 9H, SnMe3). 13C{1H} NMR (158.8 MHz,
CDCl3, 25 °C): δ = 133.7 (d, 2JP,C = 10 Hz, o-Ph), 133.0 (d, 2JP,C
10 Hz, o-Ph), 132.3 (d, 1JC,P = 61 Hz, i-Ph), 131.2 (s, p-Ph), 131.1
Me3SnCl, PhBCl2, Me2SnCl2, HgCl2, BH3·THF (1.0 M in THF), (d, JP,C = 62 Hz, i-Ph), 130.9 (s, p-Ph), 128.6 (d, JP,C = 10 Hz,
n-butyllithium (1.6 M in hexanes), and t-butyllithium (1.7 M in m-Ph), 80.4 (d, JP,C = 13 Hz, JSn,C = 49 Hz, Cp), 76.5 (d, JP,C = 7.1
=
Materials
1
3
2
1
pentane) were purchased from commercial sources and used Hz, Cp), 75.9 (d, JP,C = 25 Hz, i-Cp–Sn), 75.6 (d, JP,C = 77 Hz,
without further purification. Ph2PCl was purchased from i-Cp–P), 73.3 (d, JP,C = 5.7 Hz, JSn,C = 39 Hz, Cp), 69.9 (s, free
Aldrich and distilled prior to use. Lithium diisopropylamide Cp), −5.8 (s, JSn,C = 370/377 Hz, SnMe). 31P{1H} NMR
1
(LDA) was freshly prepared by addition of n-butyllithium (202.5 MHz, CDCl3, 25 °C): δ = 18.1 (nr). 119Sn{1H} NMR
(1.6 M in hexanes) to a THF solution of diisopropylamine (dis- (223.6 MHz, CDCl3, 25 °C): δ = −5.6 (s). 11B{1H} NMR
tilled from sodium/benzophenone prior to use) at 0 °C. (160.4 MHz, CDCl3, 25 °C): −37.5 (nr). High-resolution
Ferrocenylphosphine-borane sulfinate ((pS,Ss)-2) and trimethyl- MALDI-TOF MS (positive mode, anthracene): m/z 534.0228
1H2731P56Fe120Sn
12
stannyl-substituted bis(ferrocenylphosphine-borane) disulfi- ([(pS)-3
−
BH3]+, 100%, calcd for
C
25
nate ((pRSspRSs)-9) have been previously reported.25a,28
534.0222). Elem. anal. for C25H30BFePSn: calcd C 54.91,
H 5.53; found C 55.09, H 5.45.
X-ray diffraction analysis
Synthesis of chlorodimethylstannyl-substituted
ferrocenylphosphine-borane (pS)-4
Reflections were collected on a Bruker SMART APEX II CCD
Diffractometer using CuKα (1.54178 Å) radiation at 100 K. Data
processing, Lorentz-polarization, and face-indexed numerical To a solution of PhBCl2 (0.095 g, 0.060 mmol, 1.1 equiv.) in
absorption corrections were performed using SAINT, APEX, C6D6 (1 mL) was added (pS)-3 (0.030 g, 0.055 mmol) in an
and SADABS computer programs.46–48 The structures were NMR tube. The mixture was left standing for two hours at
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room temperature and heated to 50 °C overnight. Analysis by resolution MALDI-TOF MS (positive mode, anthracene): m/z
119Sn and 11B NMR showed full conversion to a new ferrocene 605.9874
([rac-5
−
BH3]+,
100%,
calcd
for
12
species and PhMeBCl as a byproduct (δ(11B) = 65.3 ppm, δ(1H)
C
22
1H1835Cl56Fe200Hg31P 605.9880); m/z 940.0681 ([rac-6 −
= 1.08 ppm for Me). The solvent was evaporated under vacuum 2BH3]+, 50%, calcd for
C
44
1H3656Fe2200Hg31P2 940.0703).
12
to leave an orange residue that was dissolved in hexanes and Elem. anal. for C22H21BClFeHgP: calcd C 42.68, H 3.42; found
stored at −30 °C. X-ray quality crystals of (pS)-4 were obtained C 43.20, H 3.12.
after 24 h. Yield 0.030 g (96%). [α]2D0 (c = 0.16, CHCl3) = +74°.
Synthesis of mercury-bridged bis(ferrocenylphosphine-borane)
(pSpS)-6
1H NMR (599.7 MHz, C6D6, 25 °C): δ = 7.67 (m, 2H, Ph), 7.26
(m, 2H, Ph), 7.04 (m, 1H, Ph), 6.98 (m, 2H, Ph), 6.91 (m, 1H,
3
Ph), 6.83 (m, 2H, Ph), 5.34 (m, 1H, Cp), 4.30 (pt, JH,H
=
To a solution of (pS,Ss)-2 (0.52 g, 1.0 mmol) in THF (40 mL)
2.4 Hz, 1H, Cp), 4.04 (s, 5H, free Cp), 3.96 (m, 1H, Cp), 1.70 that was cooled to −78 °C, was added slowly a solution of
2
(brm, 3H, BH3), 1.27 (s, JSn,H = 67/70 Hz, 3H, SnMe), 0.49 (s, tBuLi in pentane (1.7 M, 0.65 mL, 1.1 mmol, 1.1 equiv.). The
2JSn,H = 62/64 Hz, 3H, SnMe). 13C{1H} NMR (158.8 MHz, C6D6, mixture was stirred for 10 min and a solution of HgCl2 (0.12 g,
25 °C): δ = 134.3 (d, 2JP,C = 10 Hz, o-Ph), 132.9 (d, 2JP,C = 10 Hz, 0.45 mmol, 0.45 equiv.) in THF (10 mL) was slowly added.
1
o-Ph), 132.4 (d, JP,C = 64 Hz, i-Ph), 132.2 (s, p-Ph), 131.5 (s, After stirring for 1 h at this temperature, the reaction mixture
1
3
p-Ph), 130.2 (d, JP,C = 64 Hz, i-Ph), 129.2 (d, JP,C = 12 Hz, was allowed to slowly warm up to 25 °C overnight. The THF
m-Ph), 128.8 (d, one peak overlapping with C6D6, m-Ph), 81.6 solvent was removed in vacuum, the residue dissolved in
2
(d, JP,C = 12 Hz, Cp), 79.9 (d, JP,C = 26 Hz, i-Cp–Sn), 76.0 (d, toluene (20 mL) and filtered to remove insoluble solid. The
JP,C = 4.3 Hz, JSn,C = 43 Hz, Cp), 75.2 (d, JP,C = 5.9 Hz, Cp), 74.4 toluene solvent was evaporated and the crude product was
1
1
(d, JP,C = 77 Hz, i-Cp–P), 71.0 (s, free Cp), 6.1 (s, JSn,C = 509/ extracted with hexanes (20 mL) at 50 °C for two hours. After fil-
535 Hz, SnMe), 3.5 (s, JSn,C = 450/468 Hz, SnMe). 31P{1H} NMR tration and solvent evaporation, the remaining residue was
(202.5 MHz, CDCl3, 25 °C): δ = 15.9 (nr). 119Sn{1H} NMR subjected to column chromatography on silica gel with Et2O/
2
(223.6 MHz, CDCl3, 25 °C): δ = 67.0 (d, JSn,P = 12.3 Hz). hexanes (1 : 2) as eluent. The first band was concentrated,
11B{1H} NMR (160.4 MHz, CDCl3, 25 °C): −36.7 (nr). High- washed with hexanes and dried in vacuum. Recrystallization
resolution MALDI-TOF MS (positive mode, anthracene): m/z from CH2Cl2/hexanes by slow evaporation at room temperature
12
568.0015 ([(pS)-4]+, 5%, calcd for
C
24
1H2711B35Cl31P56Fe120Sn afforded a yellow crystalline solid consisting of a mixture of
568.0001); m/z 553.9666 ([M
−
BH3]+, 100%, calcd for (pSpS)-6 (ca. 90%) and a small amount of byproducts, which
12
C
24
1H2435Cl31P56Fe120Sn 553.9669); m/z 534.9882 ([M − BH3 proved to be impossible to remove despite many efforts to
− Cl + O]+, 45%, calcd for
m/z 518.9921 ([M
C
1H2416O31P56Fe120Sn 534.9936); obtain (pSpS)-6 in analytically pure form. Yield: 0.26 g (27%).
12
24
−
BH3
−
Cl]+, 30%, calcd for 1H NMR (499.9 MHz, CDCl3, 25 °C): δ = 7.82 (m, 4H, Ph), 7.55
12
C
24
1H2431P56Fe120Sn
518.9987).
Elem.
anal.
for (m, 2H, Ph), 7.51 (m, 4H, Ph), 7.36 (m, 6H, Ph), 7.30 (m, 4H,
Ph), 4.67 (nr, 2H, Cp), 4.65 (pt, 3JH,H = 2.3 Hz, 2H, Cp), 4.27 (s,
10H, free Cp), 4.25 (nr, 2H, Cp), 1.91 (brm, 6H, BH3). 13C{1H}
C24H27BClFePSn: calcd C 50.82, H 4.80; found C 51.00, H 4.64.
Synthesis of chloromercury-substituted ferrocenylphosphine-
2
NMR (158.8 MHz, CDCl3, 25 °C): δ = 133.7 (d, JP,C = 10 Hz,
borane rac-5
1
2
o-Ph), 133.3 (d, JP,C = 62 Hz, i-Ph), 132.4 (d, JP,C = 10 Hz,
1
A solution of (pS)-3 (0.11 g, 0.20 mmol) in acetone (2 mL) was o-Ph), 131.4 (s, p-Ph), 131.1 (d, JP,C = 65 Hz, i-Ph), 130.6 (s,
added to a solution of HgCl2 (0.053 g, 0.20 mmol) in acetone p-Ph), 128.7 (nr, m-Ph), 128.6 (nr, m-Ph), 105.9 (d, 2JP,C = 29 Hz,
1
(2 mL). The mixture was stirred for one hour at room tempera- i-Cp–Hg), 81.8 (d, JP,C = 13 Hz, Cp), 75.6 (d, JP,C = 80 Hz,
ture and then added to water (40 mL). Upon addition, a yellow i-Cp–P), 74.7 (nr, Cp), 74.6 (nr, Cp), 69.8 (s, free Cp). 31P{1H}
precipitate formed that was collected by filtration, washed with NMR (202.5 MHz, CDCl3, 25 °C): δ = 16.8 (nr). 11B{1H} NMR
hexanes and dried under high vacuum. Recrystallization from (160.4 MHz, CDCl3, 25 °C): −36.0 (nr). High-resolution
CH2Cl2/hexanes by slow solvent evaporation at room tempera- MALDI-TOF MS (positive mode, anthracene): m/z 954.1053
12
ture gave X-ray quality yellow crystals of rac-5. Yield: 0.072 g ([(pSpS)-6 − BH3]+), 20%, calcd for
C
44
1H3911B56Fe2200Hg31P2
(62%). [α]2D0 (c = 0.16, CHCl3) = +14°. 1H NMR (499.9 MHz, 954.1037; m/z 940.0734 ([(pSpS)-6 − 2BH3]+), 100%, calcd for
12
CDCl3, 25 °C): δ = 7.76 (m, 2H, Ph), 7.57 (m, 1H, Ph), 7.51 (m,
2H, Ph), 7.43 (m, 1H, Ph), 7.35 (m, 4H, Ph), 4.74 (nr, 1H, Cp),
4.47 (nr, 1H, Cp), 4.45 (nr, 1H, Cp), 4.11 (s, 5H, free Cp), 1.99
(brm, 3H, BH3). 13C{1H} NMR (158.8 MHz, CDCl3, 25 °C): δ =
C
1H3656Fe2200Hg31P2 940.0703.
44
Attempted synthesis of dimethylstannyl-bridged
bis(ferrocenylphosphine-borane) from (pS,Ss)-2:
isolation of (pSpS)-7 and (pS)-8
2
2
133.6 (d, JP,C = 10 Hz, o-Ph), 132.3 (d, JP,C = 10 Hz, m-Ph),
1
132.0 (s, p-Ph), 131.8 (d, JC,P = 64 Hz, i-Ph), 131.3 (s, p-Ph), To a solution of (pSSs)-2 (1.01 g, 1.94 mmol) in THF (60 mL)
129.8 (d, JP,C = 67 Hz, i-Ph), 129.1 (d, JP,C = 10 Hz, m-Ph), that was cooled to −78 °C, was added slowly a solution of
129.0 (d, JP,C = 10 Hz, m-Ph), 88.7 (d, JP,C = 25 Hz, i-Cp–Hg), tBuLi in pentane (1.7 M, 1.26 mL, 2.13 mmol, 1.1 equiv.). The
1
3
3
2
78.2 (d, JP,C = 11 Hz, Cp), 74.9 (d, JP,C = 5.9 Hz, Cp), 74.3 (d, mixture was stirred for 10 min and a solution of Me2SnCl2
1
JP,C = 5.7 Hz, Cp), 74.3 (d, JP,C = 77 Hz, i-Cp–P), 70.6 (s, free (0.26 g, 1.16 mmol, 0.6 equiv.) in THF (10 mL) was slowly
Cp). 31P{1H} NMR (202.5 MHz, CDCl3, 25 °C): δ = 14.6 (nr). added. After stirring for 1 h at this temperature, the reaction
11B{1H} NMR (160.4 MHz, CDCl3, 25 °C): −36.4 (nr). High- mixture was allowed to slowly warm up to 25 °C overnight. The
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THF solvent was removed in vacuum and the residue dissolved Synthesis of dimethylstannyl-bridged bis
in Et2O (50 mL) and quenched by addition of aqueous sodium (ferrocenylphosphine-borane) (pRpR)-10
bicarbonate. The combined organic layer was then separated,
To a solution of (pRSspRSs)-9 (0.18 g, 0.23 mmol) in THF
(40 mL) that was cooled to −78 °C, was added slowly a solution
of tBuLi in pentane (1.7 M, 0.27 mL, 0.46 mmol, 2.1 equiv.).
The mixture was stirred for 10 min and a solution of PPh2Cl
(0.087 g, 0.49 mmol, 2.2 equiv.) in THF (5 mL) was slowly
added, followed by addition of a solution of BH3·THF (1.0 M in
THF, 0.7 mL, 0.66 mmol, 3.0 equiv.). After stirring for 1 h at
this temperature, the reaction mixture was slowly allowed to
warm up to 25 °C overnight. The THF solvent was removed in
vacuum and the residue dissolved in Et2O (30 mL) and
quenched by addition of aqueous sodium bicarbonate. The
combined organic layer was then separated, washed with
water, brine, dried with sodium sulfate, and the solvent was
removed in vacuum to leave a yellow oily residue. Purification
by column chromatography on silica gel with hexanes/THF/
Et3N (80 : 20 : 1) afforded (pRpR)-10 as a yellow solid that was
obtained in analytically pure form after washing several times
with hot hexanes. Yield 0.070 g (38%). [α]2D0 (c = 0.18, CHCl3) =
−47°. 1H NMR (499.9 MHz, CDCl3, 25 °C): δ = 7.69 (m, 4H,
Ph), 7.50 (m, 6H, Ph), 7.45 (m, 6H, Ph), 7.38 (m, 4H, Ph), 4.29
washed with water, brine, dried with sodium sulfate, and the
solvent was removed in vacuum to leave a red oily residue. The
latter was purified by column chromatography on silica gel
with hexanes/Et2O/Et3N (80 : 20 : 1).
The first orange band gave an oily residue after evaporation
that did not show evidence for the expected product by 1H
NMR. A second band was concentrated and the residue recrys-
tallized in hexanes at −27 °C to give an orange crystalline
solid, which was identified to be the hydridodimethylstannyl-
substituted ferrocenylphosphine-borane (pS)-8 by multinuc-
lear NMR and MALDI-TOF MS analyses. Yield 0.060 g (6%).
[α]2D0 (c = 0.22, CHCl3) = +50°. 1H NMR (599.7 MHz, CDCl3,
25 °C): δ = 7.67 (m, 2H, Ph), 7.51 (m, 1H, Ph), 7.45 (m, 2H, Ph),
3
7.44–7.38 (m, 5H, Ph), 7.34 (m, 2H, Ph), 5.27 (sept, JH,H
=
3
2.4 Hz, 1H, SnH), 4.59 (pt, JH,H = 2.4 Hz, 1H, Cp), 4.48 (m,
1H, Cp), 4.24 (s, 5H, free Cp), 4.08 (m, 1H, Cp), 1.37 (brm, 3H,
3
2
BH3), 0.18 (d, JH,H = 1.8 Hz, JSn,H = 55/58 Hz, 3H, SnMe),
0.17 (d, JH,H = 1.8 Hz, JSn,H = 58/60 Hz, 3H, SnMe). 13C{1H}
3
2
2
NMR (158.8 MHz, CDCl3, 25 °C): δ = 133.6 (d, JP,C = 10 Hz,
o-Ph), 133.1 (d, 2JP,C = 9 Hz, o-Ph), 132.0 (d, 1JP,C = 61 Hz, i-Ph),
(pt, 3JH,H = 2.3 Hz, 2H, Cp), 4.11 (s, 10H, free Cp), 3.94 (nr, 2H,
1
131.4 (s, p-Ph), 131.1 (s, p-Ph), 130.9 (d, JP,C = 65 Hz, i-Ph),
2
Cp), 3.88 (nr, 2H, Cp), 1.34 (brm, 6H, BH3), 0.46 (s, JSn,H
=
128.7 (m, m-Ph), 81.1 (d, JP,C = 12 Hz, JSn,C = 51 Hz, Cp),
60 Hz, 6H, SnMe2). 13C{1H} NMR (125.7 MHz, CDCl3, 25 °C):
δ = 133.7 (d, JP,C = 9 Hz, o-Ph), 133.2 (d, JP,C = 9 Hz, o-Ph),
132.4 (d, 1JP,C = 56 Hz, i-Ph), 131.3 (d, 1JP,C = 60 Hz, i-Ph), 131.3
(s, p-Ph), 131.1 (s, p-Ph), 128.6 (m, m-Ph), 82.2 (d, JP,C = 12 Hz,
Cp), 78.0 (d, JP,C = 25 Hz, i-Cp–Sn), 76.2 (d, JP,C = 6.4 Hz, Cp),
74.9 (d, JP,C = 72 Hz, i-Cp–P), 73.2 (d, JP,C = 4.5 Hz, Cp), 70.3
1
76.7 (d, JP,C = 8.7 Hz, Cp), 75.8 (d, JP,C = 77 Hz, i-Cp–P), 73.7
2
2
2
(d, JC,P = 5.9 Hz, JSn,C = 40 Hz, Cp), 72.9 (d, JP,C = 23 Hz, i-Cp–
Sn), 70.4 (s, free Cp), −7.4 (s, SnMe), −8.9 (s, SnMe). 31P{1H}
NMR (202.5 MHz, CDCl3, 25 °C): δ = 18.5 (nr). 119Sn{1H} NMR
(223.6 MHz, CDCl3, 25 °C): δ = −96.3 (s). 11B{1H} NMR
(160.4 MHz, CDCl3, 25 °C): −38.5 (nr). High-resolution
MALDI-TOF MS (positive mode, anthracene): m/z 534.9943
2
1
(s, free Cp), −0.86 (s, SnMe2). 31P{1H} NMR (202.5 MHz, CDCl3,
25 °C): δ = 17.9 (nr). 119Sn{1H} NMR (223.6 MHz, CDCl3,
25 °C): δ = −13.1 (s). 11B{1H} NMR (160.4 MHz, CDCl3, 25 °C):
−37.7 (nr). High-resolution MALDI-TOF MS (positive mode,
([(pS)-8
−
BH3
−
H
+
O]+), 50%, calcd for
12
C
24
1H2416O56Fe31P120Sn 534.9936; m/z 520.0041 ([(pS)-8 −
BH3]+), 100%, calcd for
C
24
1H2556Fe31P120Sn 520.0065. Elem.
12
anthracene): m/z 905.0513 ([(pRpR)-10 − 2BH3 + O]+, 30%,
anal. for C24H28BFePSn: calcd C 54.10, H 5.80; found C 54.50,
H 5.10.
12
calcd for
C
46
1H4316O56Fe231P2120Sn 905.0517); m/z 889.0544
([(pRpR)-10
−
2BH3
+
H]+,
889.0568).
100%,
Elem.
calcd
anal.
for
for
The third band was collected from the column and concen- 12
trated. Repeated recrystallization in hexanes at −27 °C gave
first a crop of crystals corresponding to FcPPh2·BH2 (0.20 g,
27% yield). A second crop of just a few orange crystals was
obtained from the hexanes solution after a few more days and
C
46
1H4356Fe231P2120Sn
C46H48B2Fe2P2Sn: calcd C 60.39, H 5.29; found C 60.07, H 4.95.
identified to be the dimethyl(ferrocenylphosphino)stannyl- Acknowledgements
substituted ferrocenylphosphine-borane adduct (pSpS)-7 by
X-ray, MALDI-TOF MS and 1H NMR analysis. Yield: 0.005 g
(3%). 1H NMR (499.9 MHz, CDCl3, 25 °C): δ = 7.68 (m, 2H, Ph),
7.63 (m, 2H, Ph), 7.20–7.28 (m, 14H, Ph), 7.24 (m, 2H, Ph),
4.54 (nr, 1H, Cp), 4.23 (nr, 1H, Cp), 4.19 (nr, 1H, Cp), 4.17 (nr,
1H, Cp), 3.98 (nr, 1H, Cp), 3.95 (s, 5H, free Cp), 3.83 (s, 5H,
A. C. T. K. thanks the Deutsche Forschungsgemeinschaft
(DFG) for a postdoctoral fellowship. A 500 MHz NMR spectro-
meter used in these studies was purchased with support from
the NSF-MRI program (1229030). The X-ray diffractometer was
purchased with support from the NSF-CRIF program (0443538)
and Rutgers University (Academic Excellence Fund).
free Cp), 3.81 (nr, 1H, Cp), 1.38 (brm, 3H, BH3), 1.20 (s, 2JSn,H
62 Hz, 3H, SnMe3), 0.02 (s, JSn,H = 56 Hz, 3H, SnMe3).
=
2
High-resolution MALDI-TOF MS (positive mode, anthracene):
m/z 905.0495 ([(pSpS)-7
− BH3 +
O]+), 100%, calcd
References
12
for
C
46
1H4316O56Fe231P2120Sn 905.0517; m/z 889.0534
([(pSpS)-7 − BH3 + H]+, 45%, calcd for
C
46
1H4356Fe231P2120Sn
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889.0568).
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