Calcium stannyl formation by organostannane dehydrogenation

Article abstract of DOI:10.1039/c9cc07289f

Reaction of the dimeric calcium hydride, [(BDI)CaH]₂ (1), with Ph₃SnH ensues with elimination of H₂ to provide [(BDI)Ca-μ₂-H-(SnPh₃)Ca(BDI)] (3) and [(BDI)Ca(SnPh₃)]₂ (4) alongside dismutation to Ph₄Sn, H₂ and Sn(0). DFT analysis indicates that stannyl anion formation occurs through deprotonation of Ph₃SnH and with retention of dinuclear species throughout the reactions.

Full text of DOI:10.1039/c9cc07289f

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Calcium stannyl formation by organostannane
dehydrogenation†
Cite this: Chem. Commun., 2019,
5, 12964
5
a
a
b
a
Louis J. Morris, Michael S. Hill, * Ian Manners, Claire L. McMullin,
*
a
a
Received 17th September 2019,
Accepted 2nd October 2019
Mary F. Mahon and Nasir A. Rajabi
DOI: 10.1039/c9cc07289f
rsc.li/chemcomm
Reaction of the dimeric calcium hydride, [(BDI)CaH]
ensues with elimination of H to provide [(BDI)Ca-l -H-(SnPh
3) and [(BDI)Ca(SnPh )] (4) alongside dismutation to Ph Sn, H
Sn(0). DFT analysis indicates that stannyl anion formation occurs
through deprotonation of Ph SnH and with retention of dinuclear
2
(1), with Ph
)Ca(BDI)]
and
3
SnH
2
2
3
(
3
2
4
2
3
species throughout the reactions.
The stoichiometric and catalytic reaction chemistry of heavier
alkaline earth species, particularly those of calcium, continues to
1
attract increasing attention. Central to these advances have been a
2,3
suite of highly nucleophilic molecular hydride derivatives, which
may be applied to an array of reductive multiple bond heterofunc-
4
tionalisation and cross-metathetical processes. Our own recent
research has concentrated on the coordinatively unsaturated
Scheme 1 Reaction of compound 1 with alkenes and the calcium-
mediated nucleophilic alkylation of benzene.
2
b-diketiminato derivative [(BDI)CaH] (1; BDI = HC{(Me)CN-2,6-i-
Pr : Scheme 1), which reacts directly with a wide range of
otherwise unactivated alkenyl CQC bonds. The resultant calcium the group 1 metals have received significant attention, calcium
s-organometallics have proven to be incredibly potent reagents in compounds containing bonds to heavier p-block elements are, in
2 6 3 2
C H }
5,6
10
3
,11
their own right and are capable of effecting the direct alkylation of general, very rare,
while the sole example of a direct calcium-
benzene via an unprecedented displacement of hydride (to reform to-tin bond is provided by Westerhausen’s bis(trimethylstannyl),
1
2
1
), and in which the polarising qualities of the calcium centre itself [(Me
3
Sn)
2 4
Ca(THF) ] (2). Although the route to this compound,
5,7
perform a key role (Scheme 1).
by treatment of calcium metal with hexamethyldistannane,
As an extension of this work, we have turned our attention to provided a reasonable yield of the stannyl product its hetero-
the potential of calcium derivatives of the heavier tetrels to geneous nature necessitated a long reaction time. For phenyl–tin
effect similarly challenging arene activation. The realisation of derivatives, the intermolecular migration of phenyl substituents
such processes would enable the direct nucleophilic access to can also provide a significant complication. Of most relevance to
triorganosilylated and triorganostannylated aryl derivatives the current work, Uhlig and co-workers have reported that reaction
which would themselves be valuable substrates for onward of metallic calcium with hexaphenyldistannane provided the
8
13
2 3 3 2
elaboration in, for example, catalytic Hiyama- or Stille-type charge separated species, [Ca(18-crown-6)(HMPA) ][Sn(SnPh ) ] .
9
cross coupling protocols. Although organostannyl anions of Herein, we report facile access to triphenylstannyl-calcium deriva-
tives through the reaction of compound 1 and commercially
a
available Ph
3
SnH.
Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, UK.
E-mail: msh27@bath.ac.uk
Addition of 2–3 equivalents of Ph SnH to a C D solution
of compound 1 induced a rapid bubbling and the gradual
development of a pale orange colouration. Analysis by Sn{ H}
NMR spectroscopy after 60 minutes revealed quantitative con-
sumption of the tin hydride and the appearance of a major new
resonance at d ꢀ139.8 ppm, which appeared alongside a lower
3
6 6
b
Department of Chemistry, University of Victoria, Victoria, BC V8P 5C2, Canada
Electronic supplementary information (ESI) available: General synthetic experi-
†
119
1
mental details, NMR spectra, X-ray analysis of compounds 3 and 4, details of the
computational analysis and atomic coordinates of the DFT computed structures.
CCDC 1951246 and 1951247. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c9cc07289f
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Scheme 2 Synthesis of compounds 3 and 4 and the calcium-centred dismutation of Ph SnH.
1
5
intensity signal at d ꢀ126 ppm and was readily identified as stabilisation of alkaline-earth complexes is well established.
1
4
6
Ph Sn by comparison to literature values. The corresponding Although unsupported Z -arene interactions between benzene
4
1
H NMR spectrum evidenced the formation of a predominant and toluene and calcium-centred cations have recently been
1
6
17
BDI-containing compound (3, Scheme 2) that was most readily observed by our own group and by Harder and co-workers,
characterised by the emergence of a new BDI g-methine signal at these structures display significantly longer Ca–arene centroid
d 4.75 ppm and a singlet, resonating with half the relative distances (ca. 2.9 Å) than that observed in 3 [2.5999(9) Å]. This
intensity, at d 3.83 ppm. This latter signal also displayed latter distance is, thus, more typical of those observed in a
characteristic satellites separated by 94 Hz, within the typical variety of N-mesityl- and N-di-isopropylphenyl calcium amidinate
2
119/117
1
14
range for a J(
Sn– H) coupling.
and triazenide derivatives in which the arene substituent is a
18
While no significant changes to the NMR spectra were tethered component of the coordinated anion. Although the
observed on standing overnight, the origin of these observations Ca2–Sn1 bond length [3.2137(4) Å] is similar to that observed in
was resolved by an X-ray diffraction analysis, which was performed Westerhausen’s calcium trimethylstannyl [2, 3.2721(3) Å], the
on a colourless single crystal grown from a hexane/toluene solution strength of this Ca–arene interaction ensures that the coordination
at ꢀ30 1C. The results of this analysis (Fig. 1) confirmed that environment of the tin centre is significantly perturbed toward a
compound 3 is an unsymmetrical dinuclear calcium species in trigonal pyramidal geometry. The C70–Sn1–Ca2 bond angle
12
which two {(BDI)Ca} units are connected by m
and triphenylstannyl ligands.
2
-bridging hydride subtended by the apical C70–C75- containing phenyl substituent
is just 96.16(5)1, whereas the sum of the C58–Sn1–Ca2, C58–Sn1–
The tin-centred ligand is bonded to Ca2 through Sn1 and C64 and C64–Sn1–Ca2 angles amounts to 354.51, with Sn1 lying
6
interacts with Ca1 via an Z -interaction of its (C70–C75) phenyl only 0.331(1) Å out of the basal plane of the pyramid defined by
substituent. The significance of secondary interactions in the Ca2, C58 and C64.
8
Repetition of the reaction in d -toluene provided similar
observations at 298 K. Cooling of this sample to 246 K, however,
resulted in significant broadening of the BDI g-methine reso-
nance at d 4.71 ppm, which was observed to split into two well
resolved signals at 4.60 and 4.83 ppm below 211 K. Although a
single set of phenyl–tin resonances was also observed at 298 K,
below 220 K a series of resonances at d 6.05, 6.72, and 8.10 ppm,
and integrating in a 2 : 1 : 2 ratio relative to the two 1H BDI
g-methine signals, could be discriminated. Although the Ca–Sn
bond apparently rotates freely at room temperature, these latter
6
signals may be attributed to the persistence of the Ca-Z -phenyl
‡
ꢀ1
interaction observed in the solid state structure (DG = 48 kJ mol ).
When the reaction was left to stand for a further three days
at room temperature the reaction mixture became opaque with
the deposition of a brown-grey solid, which was tentatively
4
assigned as a precipitate of Sn metal. Crystals of Ph Sn (confirmed
by a unit cell check), were also observed to form, coinciding with
119
1
the emergence of a new signal at d ꢀ160.6 ppm in the Sn{ H}
NMR spectrum. Removal of solvent from the reaction mixture,
extraction of the solid residue into hexane/toluene and crystal-
Fig. 1 ORTEP representation of compound 3 (25% probability ellipsoids).
Hydrogen atoms, except H1, iso-propyl methyl groups and occluded
solvent molecules are removed for clarity. Selected bond lengths (Å) and angles
(
1): Sn1–Ca2 3.2137(4), Sn1–C58 2.174(2), Sn1–C64 2.190(2), Sn1– lisation at ꢀ30 1C yielded single crystals of the hydride-free dimeric
C70 2.2258(19), Ca1–N1 2.3443(16), Ca1–N2 2.3318(15), Ca1–C70 3.0649(18),
Ca1–C71 2.9837(19), Ca1–C72 2.904(2), Ca1–C73 2.864(2), Ca1–C74 2.898(2),
Ca1–C75 2.9815(19), Ca2–N3 2.3275(18), Ca2–N4 2.3306(17), C58–Sn1–Ca2
calcium triphenylstannyl, compound 4. Compound 4 (Fig. 2) is a
centrosymmetric dimer, in which each pseudo-four-coordinate
calcium centre is bound by a BDI ligand and a direct Ca–Sn
s bond to a triphenylstannyl anion. This Ca–Sn distance
is closely comparable to that of compound 3 [3.3221(6) Å].
1
27.45(6), C58–Sn1–C64 98.00(8), C58–Sn1–C70 98.48(8), C64–Sn1–Ca2
29.08(6), C64–Sn1–C70 98.61(7), C70–Sn1–Ca2 96.16(5), N1–Ca1–N2
9.59(5), N3–Ca2–N4 81.18(6).
1
7
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Fig. 3 Computed HOMOs (BP86/BS1) for (a) 3 and (b) 4.
a
b
of the reaction at room temperature, the Sn–H bond and Ca–H
bonds are broken via a dinuclear transition state TSAB at only
ꢀ
1
+
8.3 kcal mol . Natural Bond Orbital (NBO) analysis of TSAB
a
b
indicated partial charges of ꢀ0.03 and ꢀ0.25 on H and H ,
respectively (Fig. 4b), emphasising that H elimination results from
what is essentially an acid–base reaction where Ph SnH is depro-
tonated by the calcium hydride. The newly formed H–H bond in
2
3
Fig. 2 ORTEP representation of compound 4 (25% probability ellipsoids).
Hydrogen atoms and iso-propyl methyl groups are removed for clarity.
0
Selected bond lengths (Å) and angles (1): Sn1–Ca1 3.3221(5), Sn1– the subsequent intermediate B remains substantially polarised,
a
b
C34 2.222(3), Sn1–C36 2.218(3), Sn1–C42 2.196(3), Ca1–N1 2.319(2), with partial charges of +0.01 (H ) and ꢀ0.27 (H ). Isomerisation via
0
0
Ca1–N2 2.330(2), Ca1–C34 2.987(3), Ca1–C35 2.917(3), C34–Sn1–Ca1
ꢀ1
C and dihydrogen loss provides species D at ꢀ11.9 kcal mol
,
0
130.80(8), C36–Sn1–Ca1 122.05(9), C36–Sn1–C34 94.66(12), C42–Sn1–Ca1
ꢀ1
from which 3 is formed (DG = ꢀ26.7 kcal mol ) through realisa-
1
11.37(9), C42–Sn1–C34 92.91(11), C42–Sn1–C36 97.30(12), N1–Ca1–N2
tion of the Ca–p arene interaction. Assessment of the reaction of 3
8
2.69(8). Symmetry operations to generate primed atoms; 1 ꢀ x, 1 ꢀ y, 1 ꢀ z.
with a second molecule of Ph
3
SnH (Fig. S23, ESI†) indicated that
6
the formation of 4 occurs via a similarly polarised and kinetically
Dimer propagation occurs in a similar manner, through twofold Z -
phenyl-to-calcium interactions, albeit the Ca-centroid distance in 4 is
slightly shorter at 2.5376(19) Å. The presence of the more sterically
demanding triphenylstannyl ligand in the place of the bridging
hydride of 3 also induces some significant geometric adjustments at
the calcium centre, which projects some 1.339(3) Å out of the mean
plane defined by N1/N2/C1–C5 of the BDI ligand. In addition,
although the geometry at tin is somewhat distorted from a perfect
tetrahedron, the three C–Sn–C and Ca–Sn–C angles fall within a
much closer range (92.9–97.31 and 111.4–130.81, respectively) than
those imposed by the trigonal pyramidal geometry of compound 3.
The electronic structures of compounds 3 and 4 were assessed
by Density Functional Theory (DFT, see ESI† for full details of
methodology). The HOMOs of both compounds are of a similar
energy (ca. ꢀ4.45 eV, see Fig. S20, ESI†) and, emphasising the
highly ionic nature of the Ca–Sn bonding, effectively comprise
donation of the filled tin 5p orbitals to the cationic calcium
centre(s) (Fig. 3). The Ca–arene interactions (3: HOMOꢀ27,
HOMOꢀ28; 4: HOMOꢀ21, HOMOꢀ22) are also largely electro-
static in nature and originate from interactions between the Lewis
acidic tin centres and the almost degenerate pair of highest
energy filled p-orbitals associated with the coordinated C
6 5
H
rings (Fig. S20, ESI†).
3
The reaction between compound 1 and Ph SnH was also
investigated by DFT. Although a pathway (Fig. S22, ESI†)
invoking fragmentation of the dimeric hydride (1) was considered,
the initial monomerisation process was found to be significantly
endergonic (DG = +33.7 kcal mol ). A more viable pathway was
computed to occur without any necessary dimer fragmentation
1
9
ꢀ
1
Fig. 4 (a) DFT calculated (BP86-D3(BJ)-benzene/BS2//BP86/BS1) free
energy (kcal mol ) profile for the reaction of 1 with HSnPh
ꢀ1
3
to form 3,
relative to 1 + HSnPh ; (b) transition state TSAB showing calculated NBO
charges of H and H . BDI ligand frameworks shown as wireframe for
3
b
a
(
Fig. 4a). In this case, although formation of encounter complex A is
ꢀ
1
endergonic by +7.6 kcal mol , consistent with the observed facility clarity.
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‡
accessible transition state leading to H
ꢀ
2
elimination (DG
=
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G. Ballmann, J. Pahl, H. Zijlstra, C. Farber and S. Harder, Organome-
tallics, 2016, 35, 3350–3360.
ꢀ1
12.1 kcal mol ).
Further experiments to scale up and optimise the synthesis
of 3 and 4 were hampered by the co-crystallisation of both
compounds and the facility of the system toward redistribution
3
V. Leich, T. P. Spaniol, L. Maron and J. Okuda, Angew. Chem., Int.
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tentatively suggest that the production of elemental tin may be
ascribed to the dismutation of the Ph SnH reagent to Ph Sn
and unstable SnH , which further eliminates H . Under this regime,
4
Sn and metallic tin. We
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4
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(
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This journal is ©The Royal Society of Chemistry 2019
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