Enabling and Probing Oxidative Addition and Reductive Elimination at a Group 14 Metal Center: Cleavage and Functionalization of E-H Bonds by a Bis(boryl)stannylene

Article abstract of DOI:10.1021/jacs.6b00710

By employing strongly σ-donating boryl ancillary ligands, the oxidative addition of H₂ to a single site Sn^II system has been achieved for the first time, generating (boryl)₂SnH₂. Similar chemistry can also be achieved for protic and hydridic E-H bonds (N-H/O-H, Si-H/B-H, respectively). In the case of ammonia (and water, albeit more slowly), E-H oxidative addition can be shown to be followed by reductive elimination to give an N- (or O-)borylated product. Thus, in stoichiometric fashion, redox-based bond cleavage/formation is demonstrated for a single main group metal center at room temperature. From a mechanistic viewpoint, a two-step coordination/proton transfer process for N-H activation is shown to be viable through the isolation of species of the types Sn(boryl)₂·NH₃ and [Sn(boryl)₂(NH₂)]^- and their onward conversion to the formal oxidative addition product Sn(boryl)₂(H)(NH₂).

Full text of DOI:10.1021/jacs.6b00710

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Article
Enabling and probing oxidative addition and reductive
elimination at a Group 14 metal center: cleavage and
functionalization of E-H bonds by a bis(boryl)stannylene
Andrey V. Protchenko, Joshua I. Bates, Liban M. A. Saleh, Matthew P. Blake, Andrew D. Schwarz,
Eugene L. Kolychev, Amber L. Thompson, Cameron Jones, Philip Mountford, and Simon Aldridge
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b00710 • Publication Date (Web): 16 Mar 2016
Downloaded from http://pubs.acs.org on March 21, 2016
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Journal of the American Chemical Society
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Enabling and probing oxidative addition and reductive elimination at
a Group 14 metal center: cleavage and functionalization of E-H
bonds by a bis(boryl)stannylene
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Andrey V. Protchenko, Joshua I. Bates, Liban M. A. Saleh, Matthew P. Blake, Andrew D. Schwarz, Eugene L. Kolychev,
Amber L. Thompson, Cameron Jones, Philip Mountford and Simon Aldridge*
Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK
and School of Chemistry, PO Box 23, Monash University, Melbourne, VIC, 3800, Australia.
ABSTRACT: By employing strongly σꢀdonating boryl ancillary ligands the oxidative addition of H2 to a single site SnII system has
been achieved for the first time, generating (boryl)2SnH2. Similar chemistry can also be achieved for protic and hydridic EꢀH bonds
(NꢀH/OꢀH, SiꢀH/BꢀH, respectively). In the case of ammonia (and water, albeit more slowly), EꢀH oxidative addition can be shown
to be followed by reductive elimination to give an Nꢀ (or Oꢀ) borylated product. Thus, in stoichiometric fashion, redoxꢀbased bond
cleavage/formation is demonstrated for a single Main Group metal center at room temperature. From a mechanistic viewpoint, a
twoꢀstep coordination/proton transfer process for NꢀH activation is shown to be viable through the isolation of species of the types
ꢀ
.
Sn(boryl)2 NH3 and [Sn(boryl)2(NH2)] and their onward conversion to the formal oxidative addition product Sn(boryl)2(H)(NH2).
Introduction
the metallylene singlet ground state and triplet excited state
(ꢀEst) is inversely correlated with reactivity.5a,10 The very
strong σꢀdonor capabilities of boryl substituents are hypotheꢀ
sized to raise the energy of the metallylene HOMO, and so
reduce the HOMO/LUMO separation (and thus the related
singletꢀtriplet gap).9a As such, from a kinetic as well as a therꢀ
modynamic perspective, (boryl)SnII systems were perceived as
being potential candidates for the activation of EꢀH bonds.
Oxidative addition and reductive elimination represent funꢀ
damental chemical steps familiar from undergraduate textꢀ
books and key to numerous societally important catalytic proꢀ
cesses.1 The associated Mn+/M(n+2)+ redox shuttle, while facile
for noble metals, is not well established for Main Group sysꢀ
tems.2 This reflects the fact that although oxidative bond actiꢀ
vation by subꢀvalent Main Group systems is well known (e.g.
for 6ꢀvalenceꢀelectron carbenes),3 subsequent regeneration of
the reduced state via reductive elimination is typically not
thermodynamically viable.4
In the current study we demonstrate that by targeted choice
of ancillary substituents, SnII systems can be synthesized that
are capable of a range of oxidative addition processes, relevant
not only to H2, but also to protic and hydridic EꢀH bonds.
Mechanistically, through the isolation of potential intermediꢀ
ate species we establish experimentally the viability of a twoꢀ
step activation process for ammonia, involving initial coordiꢀ
nation at the metal centre, followed by NꢀtoꢀSn proton transꢀ
fer. In addition, in the cases of NH3 and H2O, this net oxidaꢀ
tive addition process can be shown to be followed by reductive
elimination to give an Nꢀ (or Oꢀ) borylated product, thus
providing demonstration (in stoichiometric fashion) of redoxꢀ
based bond cleavage/formation at a Main Group metal.
With reductive elimination in mind, one potential strategy is
to target metallylenes featuring the heavier Group 14 eleꢀ
ments, i.e. to exploit metals with intrinsically weaker MꢀE
bonds (and a more favorable MII/MIV potential). The thermoꢀ
dynamic balance between facilitating reductive elimination
and maintaining the capability for oxidative addition is a fine
one, however: diarylgermylene systems (Ar2Ge:) have been
reported to react with EꢀH bonds via oxidative addition to give
GeIV products, but the corresponding stannylenes react via a
concerted exchange process, releasing arene and leading to the
formation of SnII systems containing SnꢀE linkages (E = H,
NH2, OR etc.).5ꢀ7 To date – while addition to distannynes
(RSnSnR) has been reported – no experimental evidence exists
for the oxidative addition of the archetypal EꢀH bond (i.e. that
in H2) at a mononuclear SnII system to give the corresponding
SnIV dihydride.6,7
We hypothesized that the SnII/SnIV redox couple might be
manipulated to promote oxidative addition by the use of more
strongly σꢀdonating ancillary ligands. Thus, given the fact that
boryl ligands (ꢀBX2) are known to be extremely strong σꢀ
donors (to greater extent even than hydride, alkyl or aryl doꢀ
nors),8 we targeted oxidative bond activation by boryltin(II)
systems.9 In addition, with the kinetics of EꢀH oxidative addiꢀ
tion in mind, it has been reported that the energy gap between
Experimental
General methods and instrumentation. All manipulations
were carried out using standard Schlenk line or dryꢀbox techꢀ
niques under an atmosphere of argon or dinitrogen. Solvents
were degassed by sparging with dinitrogen and dried by passꢀ
ing through a column of the appropriate drying agent. THF
was refluxed over potassiumꢀsodium alloy and distilled prior
to use. NMR spectra were measured in C6D6 which was dried
over potassium, distilled under reduced pressure and stored
under dinitrogen in Teflon valve ampoules. NMR samples
were prepared under dinitrogen in 5 mm Wilmad 507ꢀPP tubes
1
fitted with J. Young Teflon valves. H and 13C{1H} NMR
spectra were recorded on Varian MercuryꢀVX 300 MHz,
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Journal of the American Chemical Society
Page 2 of 12
Bruker Avance III HD nanobay 400 MHz or Bruker Avance Sn{B(NDippCH)2}2(H)(SiH3), 4a: A degassed solution of 1
1
2
3
4
5
6
7
8
III 500 MHz spectrometer at ambient temperature unless statꢀ
ed otherwise and referenced internally to residual protioꢀ
solvent (1H) or solvent (13C) resonances and are reported relaꢀ
tive to tetramethylsilane (δ = 0 ppm). 11B{1H} NMR spectra
were referenced to external Et2O·BF3. Assignments were conꢀ
firmed using two dimensional 1Hꢀ1H and 13Cꢀ1H NMR correlaꢀ
tion experiments. Chemical shifts are quoted in δ (ppm) and
coupling constants in Hz. Coupling to 117/119Sn nuclei was conꢀ
firmed by measuring the spectrum of the same sample at a
different spectrometer frequency. Elemental analyses were
(95 mg, 0.106 mmol) in C6H6 (2 mL) was exposed to an atꢀ
mosphere of silane. After 5 min of stirring at 20 °C the yelꢀ
lowꢀgreen color disappeared, all volatiles were removed in
vacuo and the residue was extracted with hexane (3 × 5 mL).
The extract was evaporated to dryness, and the resulting lightꢀ
brown solid washed with a small amount of cold hexane and
dried in vacuo yielding colorless crystalline 4a (76 mg, 0.082
mmol, 77.5%). Anal. found (calcd. for C52H76B2N4SiSn): C,
1
67.65 (67.48); H, 8.38 (8.28); N, 6.21 (6.05) %. H NMR
9
(C6D6):
δ
7.19 (4 H, t, 3J = 7.7 Hz, pꢀH of Ar), 7.09 (8 H, d, 3J
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= 7.7 Hz, mꢀH of Ar), 6.16 (4 H, s, with 119/117Sn satellites
carried out by London Metropolitan University. Starting mateꢀ
i
4J(SnꢀH) = 9.5 Hz, NCH), 3.21 (4 H, septet, J = 6.83 Hz,
3
rials Sn{B(NDippCH)2}2 (Dipp
=
2,6ꢀC6H3 Pr2) (1),
3
Sn{NDipp(SiMe3)}{B(NDippCH)2} (2), and Si{NDippꢀ
(SiMe3)}{Si(SiMe3)3} were synthesised according to pubꢀ
lished procedures.9a,11 Silane was prepared in a similar manner
to that described in ref 12 from SiCl4 (2.78 g, 16.3 mmol) and
LiAlH4 (0.63 g, 16.6 mmol) in Et2O (20 mL) in a degassed
400 mL ampoule and used without fractionation (CAUTION:
SiH4 reacts violently with air!). Ammonia was dried by conꢀ
densing onto a Na mirror and distilling the required amount
into a Young’s tap ampoule of such volume so that one atꢀ
mosphere pressure of NH3 was achieved at RT. Phenylsilane
and BH3⋅NMe3 (Aldrich) were carefully degassed and stored
under a protective atmosphere.
CHMe2), 2.92 (4 H, septet, J = 6.83 Hz, CHMe2), 2.27 (3 H,
d, 3J = 3.83 Hz, with 119/117Sn satellites 2J(SnꢀH) = 39.9 Hz and
29Si satellites 1J(SiꢀH) = 191.2 Hz, SiH3), 1.66 (1 H, quartet, 3J
= 3.83 Hz, with 119/117Sn satellites J(SnꢀH) = 1223 and 1281
1
Hz, SnH), 1.06ꢀ1.13 (48 H, four overlapping doublets,
CHMe2). 13C{1H} NMR (C6D6):
δ 146.17 (oꢀC of Ar), 145.74
(oꢀC of Ar), 140.61 (ipsoꢀC of Ar), 127.93 (pꢀCH of Ar),
124.28 (mꢀCH of Ar), 123.73 (mꢀCH of Ar), 123.28 (119/117Sn
3
satellites J(SnꢀC) = 32.9 Hz, NCH), 28.92 (CHMe2), 28.47
(CHMe2), 25.96 (CHMe2), 25.89 (CHMe2), 24.31 (CHMe2),
22.80 (CHMe2). 11B{1H} (C6D6):
108.8.
δ
27.9. 29Si{1H} (C6D6): δ –
Syntheses of novel compounds. Included here are synthetic
Sn{B(NDippCH)2}2(H)(SiH2Ph), 4b: Phenylsilane (10.8
mg, 12.3 µL, 0.10 mmol) was added to a solution of 1 (75 mg,
0.084 mmol) in C6D6 (0.7 mL). Almost immediately the yelꢀ
t
and characterizing data for 3ꢀ9, 1.NH3 and 1.NH2 Bu. The corꢀ
responding data for H2NB(NDippCH)2, Sn{B(NDippꢀ
CH)2}2Cl2, {(MeCCH2)2}Sn{NDipp(SiMe3)}{B(NDippCH)2}
and Si{Si(SiMe3)3}{N(SiMe3)Dipp}(H)(NH2) are included in
the supporting information. An alternative synthesis of
H2NB(NDippCH)2 has recently been reported.9c Reliable miꢀ
croanalytical data for intermediate compounds 1.NH3, 7 and 9
proved impossible to obtain due to their ready onward reacꢀ
tivity yielding H2NB(NDippCH)2 and HB(NDippCH)2.
1
lowꢀgreen color disappeared and H NMR spectrum showed
clean formation of a single product (excess PhSiH3 was also
present). The mixture was transferred into a twoꢀsection crysꢀ
tallization tube and volatiles removed in vacuo. The residue
was dissolved in hexane (1 mL) and the tube sealed under
vacuum. The solution was slowly concentrated at room temꢀ
perature while large colorless blocks formed. The resulting
crystals were washed with a small amount of cold hexane and
dried in vacuo yielding 4b (59 mg, 0.059 mmol, 70.1%). Anal.
found (calcd. for C58H80B2N4SiSn): C, 69.79 (69.54); H, 8.18
Sn{B(NDippCH)2}2H2, 3: A solution/suspension of 1 (137
mg, 0.153 mmol) in hexane (3 mL) was degassed by two
freezeꢀpumpꢀthaw cycles and exposed to 1 atm of H2 at 20 °C.
The mixture was stirred at this temperature until the color had
changed to pale brown (ca. 3 h). The mixture was filtered and
concentrated to a small volume; crystallization was initiated
by partial freezing with liquid N2 and continued at room temꢀ
perature. The resulting colourless crystals were washed with a
small amount of cold hexane and dried in vacuo yielding 3
(110 mg, 0.123 mmol, 80.3%). Anal. found (calcd. for
C52H74B2N4Sn): C, 69.64 (69.74); H, 8.26 (8.33); N, 6.23
(8.05); N, 5.47 (5.59) %. 1H NMR (C6D6):
δ 7.36 (2 H, m, oꢀH
of Ph), 7.21 (4 H, t, 3J = 7.7 Hz, pꢀH of Ar), 7.05ꢀ7.11 (8+3 H,
m, mꢀH of Ar and mꢀ and pꢀH of Ph), 6.15 (4 H, s, with
119/117Sn satellites 4J(SnꢀH) = 9.4 Hz, NCH), 3.86 (2 H, d, J =
3
2
3.69 Hz, with 119/117Sn satellites J(SnꢀH) = 64.5 Hz and 29Si
satellites 1J(SiꢀH) = 190.3 Hz, SiH2Ph), 3.17 (4 H, septet, J =
3
3
6.88 Hz, CHMe2), 2.96 (4 H, septet, J = 6.88 Hz, CHMe2),
3
1
1.84 (1 H, t, J = 3.69 Hz, with 119/117Sn satellites J(SnꢀH) =
1197 and 1252 Hz, SnH), 1.12 (12 H, d, 3J = 6.88 Hz,
1
3
(6.26) %. H NMR (C6D6):
δ 7.20 (4 H, t, J = 7.7 Hz, pꢀH of
3
3
CHMe2), 1.08 (12 H, d, J = 6.88 Hz, CHMe2), 1.03 (12 H, d,
Ar), 7.07 (8 H, d, J = 7.7 Hz, mꢀH of Ar), 6.20 (4 H, s, with
3
3J = 6.88 Hz, CHMe2), 0.94 (12 H, d, J = 6.88 Hz, CHMe2).
119/117Sn satellites J(SnꢀH) = 10 Hz, NCH), 3.03 (8 H, septet,
4
13C{1H} NMR (C6D6):
δ 146.30 (oꢀC of Ar), 145.91 (oꢀC of
3J = 6.9 Hz, CHMe2), 2.22 (2 H, s, with 119/117Sn satellites
Ar), 140.86 (ipsoꢀC of Ar), 137.19 (119/117Sn satellites 3J(SnꢀC)
1J(119Snꢀ1H) = 1337 and J(117Snꢀ1H) = 1277 Hz, SnH2), 1.09
1
= 12.9 Hz, oꢀCH of Ph), 133.18 (119/117Sn satellites J(SnꢀC) =
2
3
3
(24 H, d, J = 6.9 Hz, CHMe2), 1.03 (24 H, d, J = 6.9 Hz,
CHMe2). 13C{1H} NMR (C6D6):
δ
145.91 (oꢀC of Ar), 140.34
8.6 Hz, ipsoꢀC of Ph), 128.75 (pꢀCH of Ph), 127.94 (pꢀCH of
Ar), 127.61 (mꢀCH of Ph), 124.25 (mꢀCH of Ar), 123.75 (mꢀ
(ipsoꢀC of Ar), 127.76 (pꢀCH of Ar), 123.64 (mꢀCH of Ar),
3
CH of Ar), 123.57 (119/117Sn satellites J(SnꢀC) = 32.6 Hz,
122.56 (s with 119/117Sn satellites J(SnꢀH) = 34 Hz, NCH),
3
28.71 (CHMe2), 25.37 (CHMe2), 23.73 (CHMe2). 11B{1H}
NCH), 28.91 (CHMe2), 28.44 (CHMe2), 26.11 (CHMe2),
25.96 (CHMe2), 24.07 (CHMe2), 23.00 (CHMe2). 11B{1H}
(C6D6):
δ 28.3. Crystallographic data (for 3): C52H74B2N4Sn,
(C6D6):
δ
28.1. 29Si{1H} (C6D6): δ –63.1 (with 119/117Sn satelꢀ
Mr = 895.50, a = 12.6324(2) Å, b = 20.8368(3) Å, c =
19.7169(3) Å, β = 95.4397(14)°, monoclinic, P 21/c, V =
1
lites J(SnꢀSi) = 409.4 and 428.5 Hz). Crystallographic data
(for 4b): C58H80B2N4SiSn, Mr = 1001.79, a = 12.5270(4) Å, b
= 12.6173(5) Å, c = 21.1768(8) Å, α = 79.505(3)°, β =
5166.51(14) Å3, Z = 4, R1 for 9810 [data intensity I > 2
σ(I)]
unique reflections = 0.0390, wR2 (all 10686 unique reflecꢀ
tions) = 0.0395. CCDC ref: 1055214.
89.404(3)°, γ = 60.709(4)°, triclinic, P ͞
1, V = 2858.2(2) Å3, Z
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Journal of the American Chemical Society
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s, with 119/117Sn satellites J(SnꢀH) = 10.8 Hz, NCH), 6.13 (br
s, with 119/117Sn satellites 1J(SnꢀH) = 1385 and 1450 Hz, SnH),
3.17 (4 H, septet, 3J = 6.85 Hz, CHMe2), 3.05 (4 H, septet, 3J =
= 2, R1 for 10834 [data intensity I > 2σ(I)] unique reflections
1
2
3
4
5
6
7
8
= 0.0614, wR2 (all 11774 unique reflections) = 0.0767. CCDC
ref: 1055215.
3
.
6.85 Hz, CHMe2), 1.10 (24 H, d, J = 6.85 Hz, CHMe2), 1.09
Sn{B(NDippCH)2}2(H)(BH2 NMe3), 5: Trimethylamine boꢀ
3
3
(12 H, d, J = 6.85 Hz, CHMe2), 1.06 (12 H, d, J = 6.85 Hz,
rane (7.3 mg, 0.10 mmol) was added to a solution of 1 (102
mg, 0.114 mmol) in C6H6 (2 mL). A preliminary NMRꢀ
monitored experiment showed that no apparent reaction ocꢀ
curred at room temperature, but the starting stannylene was
consumed after 11 h at 50 °C. Accordingly, on a preparative
scale, the reaction mixture was heated overnight at 50 °C proꢀ
ducing a light brown solution. The mixture was then transꢀ
ferred into a twoꢀsection crystallization tube and all volatiles
removed in vacuo. Hexane (2 mL) was added to the residue
and the tube was sealed under vacuum. Crystallisation by slow
evaporation at room temperature gave colorless crystals conꢀ
taminated with a brown powder. The product was recrystalꢀ
lized again by extraction with warm hexane and storing the
concentrated extract at 4 °C. This yielded clear blocks, which
were washed with a small amount of cold hexane and dried in
vacuo turning into a white powder (26 mg, 0.027 mmol,
3
CHMe2), −2.09 (1 H, d, J = 1.43 Hz, with 119/117Sn satellites
2J(SnꢀH) = 24.6 Hz, SnOH). 13C{1H} NMR (C6D6):
δ 146.30
(oꢀC of Ar), 145.94 (oꢀC of Ar), 139.77 (ipsoꢀC of Ar), 128.53
(C6H6), 128.00 (pꢀCH of Ar), 123.80 (mꢀCH of Ar), 123.69
(mꢀCH of Ar), 122.89 (119/117Sn satellites J(SnꢀC) = 38.1 Hz,
3
9
NCH), 28.77 (CHMe2), 28.66 (CHMe2), 25.56 (CHMe2),
10
11
12
13
14
15
16
17
18
19
20
21
22
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24
25
26
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29
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42
43
44
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48
49
50
51
52
53
54
55
56
57
58
59
60
25.45 (CHMe2), 23.95 (CHMe2), 23.61 (CHMe2). 11B{1H}
(C6D6):
δ 29.8. Crystallographic data (for 6): C52H74B2N4OSn,
Mr = 911.48, a = 21.22087(16) Å, b = 12.54026(11) Å, c =
19.63737(17) Å, β = 97.8858(8)°, monoclinic, P 21/c, V =
5176.39(7) Å3, Z = 4, R1 for 10463 [data intensity I > 2
σ(I)]
unique reflections = 0.0491, wR2 (all 10735 unique reflecꢀ
tions) = 0.0464. CCDC ref: 1055217.
Sn{B(NDippCH)2}2(NH3), 1.NH3: (i) Bulk material: Comꢀ
pound 1 (265 mg, 0.297 mmol) was dissolved/suspended in
hexane (5 mL), degassed by three freezeꢀpumpꢀthaw cycles
and cooled to 0 °C. When exposed to an atmosphere of dry
ammonia, the yellowꢀgreen solution turned orange and undisꢀ
solved crystals of 1 disappeared over a period of ca. 1 min,
while small orange crystals of the product began to appear on
the walls of the ampoule. After stirring for a further 10 min the
ampoule was carefully opened to vacuum and the solvent and
excess ammonia removed under reduced pressure, yielding
23.5%). Anal. found (calcd. for C55H84B3N5Sn): C, 68.51
1
(68.35); H, 8.92 (8.76); N, 7.16 (7.25) %. H NMR (C6D6):
δ
7.21 (4 H, t, 3J = 7.4 Hz, pꢀH of Ar), 7.16 (8 H, d, 3J = 7.4 Hz,
mꢀH of Ar + C6D5H), 6.22 (4 H, s, with 119/117Sn satellites
4J(SnꢀH) = 6.5 Hz, NCH), 3.30 (4 H, septet, J = 6.83 Hz,
3
3
CHMe2), 3.17 (4 H, septet, J = 6.83 Hz, CHMe2), 2.14 (2 H,
br s, BH2), 1.73 (9 H, s, NMe3), 1.12ꢀ1.21 (48 H, four overlapꢀ
ping doublets, CHMe2 + CH2 of hexane), 1.11 (1 H, br s, with
1
119/117Sn satellites J(SnꢀH) = 893.9 and 936.2 Hz, SnH), 0.88
1
light orange microneedles. H NMR (C6D6) showed almost
(6 H, t, Me of hexane). 13C{1H} NMR (C6D6):
δ
146.71 (oꢀC
quantitative conversion into 1.NH3 at this point, and as recrysꢀ
tallization (from either hexane, pentane or cyclohexane) on a
preparative scale requires very careful handling (prolonged
manipulation in solution results in decomposition) this product
was typically used in further reactions as is.
of Ar), 146.54 (oꢀC of Ar), 142.22 (ipsoꢀC of Ar), 127.19 (pꢀ
CH of Ar), 123.86 (mꢀCH of Ar), 123.78 (mꢀCH of Ar),
3
123.05 (119/117Sn satellites J(SnꢀC) = 24.2 Hz, NCH), 54.42
3
(
119/117Sn satellites J(SnꢀC) = 36.3 Hz, NMe3), 31.92 (CH2 of
hexane), 28.57 (CHMe2), 28.53 (CHMe2), 26.82 (CHMe2),
26.35 (CHMe2), 23.76 (CHMe2), 23.48 (CHMe2), 23.01 (CH2
(ii) Single crystals: A degassed solution of 1 (50 mg, 0.056
mmol) in C6H6 (1 mL) was exposed to an atmosphere of amꢀ
monia and shaken by hand until the color changed from yelꢀ
lowꢀgreen to bright orange (ca. 1 min). Volatiles were reꢀ
moved in vacuo and the residue extracted with nꢀhexane. The
extract was then concentrated until the onset of crystallisation
and stored at 4 °C to produce large redꢀorange blocks of 1.NH3
suitable for Xꢀray diffraction. Further concentration and storꢀ
age of the mother liquor produced a second crop of slightly
of hexane), 14.31 (Me of hexane). 11B{1H} (C6D6):
δ 31.9 (3ꢀ
coordinate boryl), –8.26 (BH2NMe3). Crystallographic data
(for 5): C55H84B3N5Sn, Mr = 966.43, a = 14.1360(2) Å, b =
16.8914(2) Å, c = 25.9133(4) Å, β = 99.1667(12)°, monoclinꢀ
ic, P 21/c, V = 6108.47(15) Å3, Z = 4, R1 for 12064 [data intenꢀ
sity I > 2σ(I)] unique reflections = 0.0412, wR2 (all 12712
unique reflections) = 0.0505. CCDC ref: 1055216.
less pure compound (total: 33 mg, 0.036 mmol, 65%). 1H
Sn{B(NDippCH)2}2(H)(OH), 6: To a solution of 1 (108
mg, 0.121 mmol) in C6H6 (2 mL) was added dropwise wet
benzene (prepared by stirring dry, deoxygenated benzene (20
mL) with excess water (1 mL)) until the color change from
yellowꢀgreen to light brown was complete (ca. 9 mL). All
volatiles were removed in vacuo and the residue was dried for
2 h at 20 °C. The solid was redissolved in dry benzene and the
solution was filtered to remove cloudiness. The resulting filꢀ
trate was slowly evaporated almost to dryness, while large
colorless blocks were growing, and stored overnight at 4 °C to
complete crystallisation. Washing with a small amount of cold
benzene and drying in vacuo yielded 6 (93.7 mg, 0.102 mmol,
85%). Anal. found (calcd. for C52H74B2N4OSn): C, 68.66
(68.52); H, 8.28 (8.18); N, 6.16 (6.15) %. Compound 6 is only
sparingly soluble in hexane and, when heated to reflux, reactꢀ
ed to give HB(NDippCH)2, HOB(NDippCH)2 and tin metal;
thus Xꢀray quality crystals were obtained from methylcycloꢀ
3
NMR (C6D5CD3, ref.: internal SiMe4):
δ
7.06 (4 H, t, J = 7.6
3
Hz, pꢀH of Ar), 6.97 (8 H, d, J = 7.6 Hz, mꢀH of Ar), 6.25 (4
H, s, NCH), 3.62 (4 H, br, CHMe2), 3.25 (4 H, br, CHMe2),
1.11 (48 H, br, CHMe2), 0.27 (3 H, br s, NH3). 13C{1H} NMR
(C6D5CD3, ref.: internal SiMe4):
δ 146.47 (br, oꢀC of Ar),
142.02 (ipsoꢀC of Ar), 127.29 (pꢀCH of Ar), 124.07 (br, mꢀCH
of Ar), 122.82 (NCH), 28.77 (CHMe2), 25.79 (br, CHMe2),
23.95 (v br, CHMe2). 11B{1H} (C6D5CD3):
δ 45.1. Crystalloꢀ
graphic data (for 1.NH3): C52H74B2N5Sn, Mr = 909.50, a =
19.4712(2) Å, b = 14.5386(2) Å, c = 18.7150(2) Å, β =
105.3864(5)°, monoclinic, P 21/c, V = 5108.04(5) Å3, Z = 4, R1
for 9836 [data intensity I > 2σ(I)] unique reflections = 0.0475,
wR2 (all 11614 unique reflections) = 0.1302. CCDC ref:
1055211.
Onward reaction of 1.NH3: A sample of 1.NH3 was moniꢀ
tored by in situ multinuclear NMR spectroscopy while mainꢀ
hexane at 4 °C. 1H NMR (C6D6):
δ
7.18 (4 H, t, 3J = 7.7 Hz, pꢀ
H of Ar), 7.15 (s, C6H6), 7.07 (8 H, m, mꢀH of Ar), 6.16 (4 H,
o
tained at 20 C for 4ꢀ5 days, revealing the formation of two
metalꢀfree products: the known compound HB(NDippCH)2,13
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and H2NB(NDippCH)2 (identified by comparison of its 1H
Page 4 of 12
zene/hexane (1:3) at 4 °C produced almost colourless blocks
1
2
3
4
5
6
7
8
NMR spectral signals with those of independently prepared
sample (see ESI)). Although tin metal was also observed to be
formed, we were also able to isolate small (but reproducible)
quantities of a tin cluster species identified solely on the basis
of Xꢀray crystallography as nidoꢀSn11{B(NDippCH)2}4 and
nidoꢀSn10{B(NDippCH)2}4. NMR monitoring further shows
that the onward reaction of 1.NH3 proceeds via an intermediate
(which achieves maximum concentration after ca. 2 d), identiꢀ
fied as Sn{B(NDippCH)2}2(H)(NH2) (7) through comparison
of its SnꢀH and SnꢀNH2 1H NMR signals with an authentic
sample (prepared as outlined below). After 5 d no residual
of benzeneꢀsolvated Sn{B(NArCH)2}2(NH2)(H) (46 mg, 0.051
mmol, 29%), which lose crystallisation solvent when taken
from the mother solution and were unsuitable for Xꢀray difꢀ
fraction. Single crystals were obtained by crystallisation from
methylcyclohexane at 4 °C (during crystallisation solutions of
Sn{B(NArCH)2}2(NH2)(H) gradually turned dark brown or
black resulting in low yields of pure crystals). 1H NMR
(C6D6):
δ 7.16 (4 H, m, pꢀH of Ar + C6D5H), 7.05 (8 H, m, mꢀ
H of Ar), 6.16 (4 H, s, with 119/117Sn satellites J(SnꢀH) = 9.9
4
9
Hz, NCH), 4.80 (br t, with 119/117Sn satellites J(SnꢀH) = 1365
1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3
and 1430 Hz, SnH), 3.25 (4 H, septet, J = 6.87 Hz, CHMe2),
1
signals of 1.NH3 were observed in H NMR spectrum, and
3
3
3.00 (4 H, septet, J = 6.87 Hz, CHMe2), 1.13 (12 H, d, J =
6.87 Hz, CHMe2), 1.09 (36 H, overlapping doublets, CHMe2),
volatiles were removed in vacuo; the residue was extracted
with hexane (1 mL) into a crystallisation tube to remove the
black precipitate of tin metal. The resulting solution was conꢀ
centrated and stored at 4 °C for 2 d producing a very small
quantity of black (brown in thin layer) blockꢀlike crystals of
distorted hexagonal shape (1:4 mixture of Sn10/Sn11 clusters).
Similar (by Xꢀray diffraction) crystals were isolated reproducꢀ
ibly from three different experiments in very low yield. Furꢀ
ther crystallisation at −30 °C gave needle crystals containing
both HB(NDippCH)2 and H2NB(NDippCH)2.
−1.91 (2 H, br d, with 119/117Sn satellites J(SnꢀH) = 23 Hz,
2
SnNH2). Tin satellites for the SnH peak at
identified at 6.58 and 6.50 (the other pair of satellites overꢀ
laps with the septet at
3.00 ppm). 13C{1H} NMR (C6D6):
δ 4.80 ppm were
δ
δ
δ
146.15 (oꢀC of Ar), 145.80 (oꢀC of Ar), 140.12 (ipsoꢀC of Ar),
127.85 (pꢀCH of Ar), 124.00 (mꢀCH of Ar), 123.65 (mꢀCH of
Ar), 122.89 (119/117Sn satellites J(SnꢀC) = 36.0 Hz, NCH),
3
28.80 (CHMe2), 28.66 (CHMe2), 25.78 (CHMe2), 25.55
(CHMe2), 24.04 (CHMe2), 23.34 (CHMe2). 11B{1H} (C6D6):
29.4. Crystallographic data (for 7): C52H75B2N5Sn, Mr
δ
=
Sn{B(NDippCH)2}2(tBuNH2), 1.tBuNH2: To a solution of 1
(27 mg, 0.030 mmol) in C6H6 (1 mL) was added excess
tBuNH2 (10 µL, 7.0 mg, 0.096 mmol) resulting in immediate
color change from yellowꢀgreen to bright orange. Volatiles
were removed in vacuo, the residue was extracted with nꢀ
hexane, the extract was concentrated to an oily drop which
was stored in a fridge (4 °C) for several days producing large
orange blocks of 1.tBuNH2 suitable for Xꢀray diffraction (after
drying in vacuo became nonꢀtransparent; 14 mg, 0.014 mmol,
48%). Anal. found (calcd. for C56H83B2N5Sn): C, 69.26
(69.58); H, 8.20 (8.66); N, 7.31 (7.25) %. The product is exꢀ
tremely sensitive towards hydrolysis by traces of water abꢀ
sorbed on glass surfaces (much more so than the starting mateꢀ
rial 1) leading to contamination by 6 and HOB(NDippCH)2.
910.51, a = 21.2882(3) Å, b = 12.5521(2) Å, c = 19.6851(2)
Å, β = 98.1681(13)°, monoclinic, P 21/c, V = 5206.7(1) Å3, Z
= 4, R1 for 10221 [data intensity I > 2σ(I)] unique reflections
= 0.0515, wR2 (all 10798 unique reflections) = 0.1147. CCDC
ref: 1446877.
[Sn{B(NDippCH)2}(ꢁ-NH2)]2, 8: A degassed solution of 2
(60 mg, 0.080 mmol) in C6H6 (2 mL) was exposed to an atꢀ
mosphere of ammonia and shaken by hand until the color
changed from purple to light brown (~0.5 min). Volatiles were
removed in vacuo, and the residue extracted into cyclohexane;
the solution was concentrated and stored at 4 °C for 2 d yieldꢀ
ing pale yellow block crystals of 8 (36 mg, 0.068 mmol of the
monomer, 86%). Single crystals suitable for the Xꢀray diffracꢀ
tion were obtained from nꢀhexane, but the presence of residual
solvent in the NMR sample prevented unambiguous assignꢀ
ment of the NH2 group resonance; thus, cyclohexane was the
solvent of choice for bulk preparation. Anal. found (calcd. for
1H NMR (C6D6):
δ 7.15 (4 H, m, pꢀH of Ar + C6D5H), 7.08 (8
3
H, d, J = 7.7 Hz, mꢀH of Ar), 6.32 (4 H, s, NCH), 3.28ꢀ3.67
(8 H, br, CHMe2), 1.79 (2 H, br s, NH2), 1.15 (48 H, br,
CHMe2), 0.56 (9 H, s, CMe3). 13C{1H} NMR (C6D6):
δ 146.52
(oꢀC of Ar), 142.17 (ipsoꢀC of Ar), 127.41 (pꢀCH of Ar),
124.33 (mꢀCH of Ar), 123.50 (NCH), 49.09 (CMe3), 28.77
(CHMe2), 28.73 (CMe3), 26.49 (CHMe2), 23.76 (v br,
C26H38BN3Sn): C, 59.74 (59.81); H, 7.21 (7.34); N, 8.16 (8.05)
%. 1H NMR (C6D6):
of Ar), 7.04 (4 H, d, J = 7.7 Hz, mꢀH of Ar), 6.40 (2 H, s,
δ
7.10 (2 H, dd, J = 6.2 and 8.8 Hz, pꢀH
3
3
CHMe2). 11B{1H} (C6D6):
δ 45.7. Crystallographic data (for
3
NCH), 3.44 (4 H, septet, J = 6.9 Hz, CHMe2), 1.23 (12 H, d,
.
1.tBuNH2 (0.25 C6H14)): C57.5H86.5B2N5Sn, Mr = 988.16, a =
20.3402(2) Å, b = 12.83950(10) Å, c = 22.8290(3) Å, β =
102.8998(11)°, monoclinic, P 21/c, V = 5811.51(11) Å3, Z = 4,
3J = 6.9 Hz, CHMe2), 1.15 (12 H, d, 3J = 6.9 Hz, CHMe2), 0.88
(2 H, br s, NH2). 13C{1H} NMR (C6D6):
δ 146.41 (oꢀC of Ar),
140.45 (ipsoꢀC of Ar), 127.58 (pꢀCH of Ar), 123.55 (mꢀCH of
Ar), 121.50 (NCH), 28.64 (CHMe2), 26.11 (CHMe2), 23.79
R1 for 10758 [data intensity I > 2σ(I)] unique reflections =
0.0368, wR2 (all 11964 unique reflections) = 0.0292. CCDC
ref: 1055212.
(CHMe2). 11B{1H} (C6D6):
δ 45.5. Crystallographic data (for
7): C52H75B2N6Sn2, Mr = 1043.21, a = 14.0818(3) Å, b =
12.3113(2) Å, c = 15.8745(2) Å, β = 99.2666(15)°, monoclinꢀ
ic, P 21/c, V = 2716.17(8) Å3, Z = 2, R1 for 5398 [data intensity
Reaction of 1.NH3 with K[B(C6F5)4]: isolation of
Sn{B(NDippCH)2}2(H)(NH2), 7: To a solution of 1.NH3 (160
mg, 0.175 mmol) in benzene (2 mL) was added solid
K[B(C6F5)4] (162 mg, 0.225 mmol). Upon stirring at 20 °C the
colour of the mixture gradually changed from orange to light
brown. After 1 h the mixture was filtered and the filtrate was
evaporated in vacuo. A sample of the resulting crystalline maꢀ
terial (21 mg) contained ca. 70% Sn{B(NDippCH)2}2(NH2)ꢀ
(H), together with unreacted 1.NH3 (ca. 10%) and small quanꢀ
tities of 3, HB(NDippCH)2 and H2NB(NArCH)2 (as deterꢀ
mined by 1H NMR spectroscopy). Recrystallization from benꢀ
I > 2σ(I)] unique reflections = 0.0223, wR2 (all 5596 unique
reflections) = 0.0236. CCDC ref: 1055219.
[KSn{B(NDippCH)2}2(NH2)]2, 9: Solid KN(SiMe3)2 (55
mg, 0.276 mmol) was added to a solution of 1.NH3 (252 mg,
0.276 mmol) in benzene (5 mL) at 20 °C. After stirring for 5
min, a deep yellow solution formed. Volatiles were removed
in vacuo leading to the solution undergoing a color change to
brown, and yellow crystalline material to precipitate. A small
amount of the concentrated supernatant solution was sealed in
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Journal of the American Chemical Society
groups reduce the HOMOꢀLUMO gap through destabilization
a crystallisation tube and stored at 4 °C, producing yellow
of the HOMO,16 and the fact that ligands based around relaꢀ
tively electropositive atoms such as boron or silicon are
among the strongest σꢀdonors currently available to the synꢀ
thetic chemist.8 Moreover, in contrast to the destabilization of
the LUMO (and widening of the HOMOꢀLUMO gap) effected
by πꢀdonor substituents such as amido groups, it would also be
expected that the moderate πꢀacceptor properties of the boryl
ligand would, if anything, reduce the HOMOꢀLUMO gap, and
hence also ꢀEst.
1
2
3
4
5
6
7
8
blocks suitable for Xꢀray crystallography. Washing of the
crystalline product with cold hexane resulted in rapid darkenꢀ
ing, therefore a small sample for spectroscopic characterisaꢀ
tion was prepared by crystallization from cyclohexane at 4 °C.
The sample showed very broad resonances for Dipp substituꢀ
ents at 20 °C, which started to resolve at −10 °C, however
1
further cooling resulted in precipitation. H NMR (C6D5CD3,
ref.: internal SiMe4):
δ 7.01ꢀ7.19 (12 H, br s, pꢀ and mꢀH of
Ar), 6.08 (4 H, s, NCH), 3.05 (8 H, v br s, CHMe2), 1.20 (48
9
H, br, CHMe2), −2.09 (2 H, br s, NH2). 11B{1H} (C6D6):
δ
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The effect of net charge was also considered, given the
isolobal relationship between formally anionic boryl and
chargeꢀneutral Nꢀheterocyclic carbene ligands, with the findꢀ
ing that preferential stabilization of the HOMO in the NHCꢀ
ligated cation leads to an enhanced magnitude of ꢀEst.
Bis(boryl)germylene and –silylene systems were also evaluatꢀ
ed (and shown to possess relatively lowꢀlying triplet states),
although in our hands such systems have yet to be accessed
synthetically.17 As such, borylꢀstannylenes were considered to
be a logical starting point for experimental studies, offering a
workable compromise between accessibility and reactivity.
41.4. Attempts to obtain reliable 13C NMR data for 9 were
frustrated by its low solubility in compatible solvents. Crystalꢀ
lographic data (for 9.(C6H6)): C55H77B2KN5Sn, Mr = 987.66, a
= 17.8585(1) Å, b = 43.5512(2) Å, c = 19.1937(1) Å, β =
117.1098(3)°, monoclinic, P 21/a, V = 13288.00(12) Å3, Z = 8,
R1 for 19623 [data intensity I > 2σ(I)] unique reflections =
0.0576, wR2 (all 30117 unique reflections) = 0.1812. CCDC
ref: 1446876.
X-ray crystallography. Xꢀray diffraction data were collected
at 150 K using either a Nonius Kappa CCD or Oxford Diffracꢀ
tion (Agilent) SuperNova A diffractometer.14a Structures were
solved with SuperFlip,14b and refined by fullꢀmatrix least using
CRYSTALS.14c Any disorder was modelled as per the CIF and
hydrogen atoms were treated in the usual manner.14d Where
the structure contained large solvent accessible voids comprisꢀ
ing weak, diffuse electron density, the discrete Fourier transꢀ
forms of the void regions were treated as contributions to the
A and B parts of the calculated structure factors using
PLATON/SQUEEZE integrated within the CRYSTALS softꢀ
ware.14e
Table 1: DFT calculated singlet-triplet separation energies,
ꢀEst, for model Group 14 metallylenes :MXY and
[:MX(L)]+.a
ꢀEst (kcal molꢀ1)
M
Sn
Sn
Sn
Sn
Sn
Sn
Ge
Ge
Si
charge
0
0
0
+1
0
0
0
0
0
X
Y
B(NMeCH)2
B(NMeCH)2
B(NMeCH)2
B(NMeCH)2
Si(SiH3)3
B(NMeCH)2
N(SiMe3)Me
PMe2
C(NMeCH)2
Si(SiH3)3
N(SiMe3)Me
B(NMeCH)2
N(SiMe3)Me
B(NMeCH)2
N(SiMe3)Me
12.8
23.5
14.8
22.3
14.5
24.5
10.5
24.2
7.8
Si(SiH3)3
B(NMeCH)2
B(NMeCH)2
B(NMeCH)2
B(NMeCH)2
Computational details. DFT calculations were performed
using the Amsterdam Density Functional (ADF) Package
Software 2012.15aꢀc Unrestricted calculations were performed
using the VoskoꢀWilkꢀNusair local density approximation
with exchange from Becke,15d and correlation corrections from
Perdew (BP).15e Slaterꢀtype orbitals (STOs)15f were used for
the triple zeta basis set with an additional set of polarization
functions (TZP). The large frozen core basis set approximation
was applied with no molecular symmetry. The general numerꢀ
ical integration was 6. Relativistic effects were addressed usꢀ
ing the scalar Zeroth Order Regular Approximation
(ZORA).15gꢀi No significant imaginary frequencies were obꢀ
served for the optimized geometry of the model complexes
:MXY and [:MX(L)]+. See ESI for the run files for frequency
calculations, which contain coordinates for the optimized geꢀ
ometries of model complexes
Si
0
21.4
a
The corresponding value of ꢀEst for the amido(silyl)silylene,
Si{Si(SiMe3)3}{N(SiMe3)Dipp}, is calculated to be 24.8 kcal molꢀ1.11
Reactivity towards E-H bonds (E = H, B, Si). Diamidoꢀ
stannylenes have previously been shown to be unreactive toꢀ
wards H2.18 Given the predictions of enhanced reactivity assoꢀ
ciated with the incorporation of boryl substituents, the reacꢀ
tions of dihydrogen with (amido)borylstannylene 2 and homoꢀ
leptic bis(boryl) stannylene 1 were therefore investigated
(Scheme 1).9a Consistent with DFT predictions, we find that
oxidative addition of H2 at the tin center in 1 is facile, occurꢀ
ring steadily at room temperature to give the corresponding
bis(boryl)tin(IV) dihydride 3. By contrast, under analogous
conditions 2 is unreactive towards dihydrogen, consistent with
a significantly higher barrier to HꢀH bond activation.19,20
Results and Discussion
Characterization of 3 has been achieved by standard specꢀ
troscopic, analytical and crystallographic techniques; particuꢀ
larly diagnostic is the new NMR signal at δH = 2.22 ppm due
Computational screening. The singletꢀtriplet energy separaꢀ
tion (ꢀEst) has previously been shown to be of key importance
in determining activation barriers in the oxidative addition of
EꢀH bonds at metallylene centers.5a,10 With this in mind, we set
out to probe (via DFT methods) the scope for tuning this gap
in stannylene and related systems by variation in the peripherꢀ
al substituents. These studies reveal that the incorporation of
silyl, phosphido, or better still boryl substituents should lead to
values for ꢀEst of <15 kcal molꢀ1 (Table 1), while the use of
amido substituents, in particular, leads to significant stabilizaꢀ
tion of the singlet state (ꢀEst > 21 kcal molꢀ1). Such findings
are in line with the notion that strongly σ electronꢀdonating
to the SnH2 unit, for which both /
117 119Sn satellites can be reꢀ
solved [1J(117Snꢀ1H) = 1277 Hz; 1J(119Snꢀ1H) = 1337 Hz]. Conꢀ
sistently, the molecular structure determined by single crystal
Xꢀray diffraction (Figure 1) features two tinꢀbound hydrogen
atoms in the difference Fourier map, and a significantly widꢀ
ened BꢀSnꢀB angle compared to bis(boryl)stannylene precurꢀ
sor 1 [135.6(1) vs. 118.8(3)o]. Similar phenomena have previꢀ
ously been observed in the oxidative addition of dihydrogen to
lighter group 14 analogues [e.g. 109.7(1) and 120.1(1)o, reꢀ
spectively, for a (boryl)amidosilylene and the corresponding
Si(IV) dihydride].9a,21 In similar fashion, the BꢀSnꢀB angle
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Page 6 of 12
Scheme 1: Contrasting reactivity of bis(boryl)- and (ami-
do)boryl stannylene complexes 1 and 2 towards H2.
elimination is observed from samples of 3 upon storing for
several days at room temperature.
1
2
3
4
5
6
7
8
9
The presence of a relatively small singletꢀtriplet gap also faꢀ
cilitates the activation of a range of other EꢀH bonds by 1,
including hydridic systems such as BꢀH and SiꢀH bonds. Thus,
silanes such as PhSiH3 or SiH4 itself, and boranes such as
Me3N.BH3, undergo EꢀH oxidative addition to generate the
corresponding silylꢀ or boryltin(IV) hydrides in moderate to
good yield (4a, 4b, 5; Scheme 2).26,27 To our knowledge, the
oxidative addition of SiꢀH and BꢀH bonds of this sort to give
SnIV products has no precedent.26
Dipp
Dipp
N
N
H2
N
N
N
N
B
B
B
B
(1 atm.)
Dipp
Dipp
Dipp
Dipp
H
H
Sn
Sn
20oC
N
N
Dipp
Dipp
1
3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Dipp
N
Here too, the appearance of the respective SnꢀH resonance
in the 1H NMR spectrum is diagnostic of the addition process.
Thus, for example, in the case of 4a, formed by the activation
of SiH4, the signal in question is a quartet [δH = 1.66 ppm,
3J(1Hꢀ1H) = 3.8 Hz] with tin satellites [1J(117Snꢀ1H) = 1223 Hz;
1J(119Snꢀ1H) = 1281 Hz], while the corresponding doublet SiH3
resonance shows coupling to both silicon and tin [δH = 2.27
H2
N
B
N
(1 atm.)
Dipp
Dipp =
no reaction
Sn
20oC
Me3Si
Dipp
2
1
2
ppm, J(29Siꢀ1H) = 191.2 Hz; J(117/119Snꢀ1H) = 39.9 Hz]. In
similar fashion, the analogous signals for the tinꢀ and siliconꢀ
bound hydrogens in 4b are a triplet [3J(1Hꢀ1H) = 3.7 Hz, with
117/119Sn satellites: 1J(117Snꢀ1H) = 1197 Hz; 1J(119Snꢀ1H) = 1252
Hz] and a lower field doublet [with 29Si and 117/119Sn satellites
1J(29Siꢀ1H) = 190.3 Hz; 2J(117/119Snꢀ1H) = 64.5 Hz]. In the case
1
of 5, Hꢀ1H couplings cannot be resolved, presumably due to
11
.
broadening by the quadrupolar B nucleus of the ꢀBH2 NMe3
group; the presence of two distinct 11B environments is clearly
signaled, however, by resonances at δB = 31.9 (for the threeꢀ
coordinate boryl substituent) and ꢀ8.3 ppm (for the fourꢀ
coordinate baseꢀstabilised boryl ligand). In addition, for both
4b and 5, structural authentication has proved possible by sinꢀ
gle crystal Xꢀray diffraction, which yields the heavy atom
skeleton together with likely hydrogen atom positions from the
difference Fourier map (Figure 1).
Scheme 2: Oxidative addition of hydridic E-H bonds to 1
to give silyl- and boryl SnIV species 4 and 5.
Figure 1: Molecular structures of 3, Sn{B(NDippCH)2}2Cl2,
4b and 5 as determined by X-ray crystallography. Most H
i
atoms omitted and Pr groups shown in wireframe format
for clarity; thermal ellipsoids set at the 50% probability
level. Key bond lengths (Å) and angles (o): (for 3) Sn(1)-B
2.242(2), 2.246(2), Sn(1)-H 1.72(2), 1.72(2), B(1)-Sn(1)-B(2)
135.6(1); (for Sn{B(NDippCH)2}2Cl2) Sn(1)-B 2.250(3),
2.252(3), Sn(1)-Cl 2.353(1), 2.358(1), B(1)-Sn(1)-B(2)
136.3(1); (for 4b) Sn(1)-B 2.248(5), 2.260(3), Sn(1)-H(1)
1.67(6), Sn(1)-Si(1) 2.593(1), B(1)-Sn(1)-B(2) 137.0(1); (for
5) Sn(1)-B(1/2) 2.274(2), 2.284(2), Sn(1)-H(1) 1.68(2), Sn(1)-
B(3) 2.272(3), B(3)-N(5) 1.621(4), B(1)-Sn(1)-B(2) 114.4(1).
Interestingly, while the structure of 4b features the wide Bꢀ
SnꢀB angle [137.0(1)o] characteristic of other SnIV systems
(such as 3 and Sn{B(NDippCH)2}2Cl2), 5 possesses a much
narrower angle at the metal center [114.4(1)o] and somewhat
longer SnꢀB(heterocycle) bonds [2.274(2), 2.284(2) Å cf.
measured for Sn{B(NDippCH)2}2Cl2 [which can be syntheꢀ
sized through the reaction of 1 with Ph3CCl or UCl4 (ESI)]
shows a comparable widening [136.3(1)o]. Presumably, these
observations reflect the shortening of the SnꢀB bonds on oxiꢀ
dation from SnII to SnIV [e.g. 2.242(2), 2.246(2) Å for 3, cf.
2.290(8) (mean) for 1] and the consequent need to widen the
BꢀSnꢀB angle to minimize the repulsive interactions between
the sterically very demanding boryl substituents.
.
2.242(2), 2.246(2) Å for 3]. The bond to the –BH2 NMe3 ligꢀ
and is of similar length [2.272(3) Å] despite the higher coordiꢀ
nation number at B(3) and is therefore clearly best described
as a simple 2ꢀcentre, 2ꢀelecton covalent bond (cf. 2.23 Å for
the sum of the respective covalent radii).28 We therefore atꢀ
tribute the narrower geometry of the bis(boryl)tin backbone in
While oxidative cleavage of dihydrogen at isolated carbene,
silylene and germylene compounds has been reported previꢀ
ously,5,22ꢀ25 to our knowledge the transformation of 1 to 3 repꢀ
resents the first example of simple oxidative addition of H2 to
a monoꢀmetallic SnII system to generate a SnIV product. This
process appears to be irreversible, and no hint of HꢀH or BꢀH
5
to the higher steric demands of the relatively
.
bulky ꢀBH2 NMe3 ligand (cf. H in 3) in association with the
short SnꢀE distance (cf. SiH2Ph in 4b). An alternative descripꢀ
tion of 5 as a donor/acceptor adduct formed between a
stannylene Lewis acid and Me3N.BH3 (acting as an essentially
unactivated σ(BH) donor) appears unlikely on the basis of the
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Journal of the American Chemical Society
short SnꢀB separation and the large SnꢀH coupling constant
dearth of transition metal systems capable of effecting such a
(ca. 900 Hz).29
transformation,30 and the relevance of NꢀH activation to a
number of potentially important industrial processes.31 Thus,
the reactivity of 1 towards NH3 has also been explored
(Scheme 4).32
1
2
3
4
5
6
7
8
Reactivity towards protic E-H bonds: water. The activation
of protic EꢀH linkages by 1 has also been investigated, with
both OꢀH and NꢀH bonds being shown to be amenable to oxiꢀ
dative addition. In contrast to compounds 3ꢀ5, however, the
resulting SnIV products are prone to further reaction leading to
BꢀE reductive elimination (E = O, N).
Addition of excess ammonia to a benzene solution of 1 rapꢀ
idly yields a compound of composition 1.NH3, which is charꢀ
acterized by a marked downfield shift in the NH3 resonance,
from δH = −0.17 (free NH3) to 0.32 ppm (isolated 1.NH3; samꢀ
ples containing excess NH3 show intermediate shift), and by
slow onward reaction in C6D6 solution. Monitoring of the latꢀ
ter by in situ NMR measurements leads to two key observaꢀ
tions: (i) the formation of an intermediate species characterꢀ
ized by mutually coupled (1Hꢀ1H COSY spectrum) broad resꢀ
onances with relative intensities of 1:2, which, although labile,
persists for up to four days under certain conditions, and (ii)
the formation of the final products, namely a ~1:1 mixture of
H2NB(NDippCH)2 and HB(NDippCH)2, together with reduced
tin species. The amidoborane H2NB(NDippCH)2 can be synꢀ
thesized independently from KNH2 and BrB(NDippCH)2 for
proof of composition (see ESI).9c In addition, the metalꢀ
containing product is shown to be primarily elemental tin,
although we were also able (reproducibly) to isolate very
small quantities of two tin clusters, identified as nidoꢀ
Sn11{B(NDippCH)2}4 and nidoꢀSn10{B(NDippCH)2}4 on the
basis of Xꢀray crystallographic studies (see ESI), which also
speak to the reductive nature of BꢀN/BꢀH bond formation.
In the case of water, the hydroxytin(IV) hydride
Sn{B(NDippCH)2}2(H)(OH), 6, resulting from the oxidative
addition of one OꢀH bond can be isolated in very good yield
(85%, Scheme 3) and characterized by multinuclear NMR, Xꢀ
ray crystallography and elemental microanalysis. As in the
cases of HꢀH, BꢀH and SiꢀH oxidative addition products, the
resulting SnꢀH linkage is characterized in solution by large
satellite couplings to 117/119Sn [1J(117Snꢀ1H) = 1385 Hz;
1J(119Snꢀ1H) = 1456 Hz], while the (sharper) SnOH resonance
can be resolved into a doublet [3J(1Hꢀ1H) = 1.4 Hz] with longꢀ
er range couplings to tin [2J(117/119Snꢀ1H) = 24.6 Hz].
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
While the oxidative addition of water to a single Main
Group metal center is very rare, precedent does exist for SnII,
with Pörschke and coꢀworkers reporting the synthesis of
Sn{CH(SiMe3)2}2(H)(OH) from the corresponding stannylene
as long ago as 1998.6a,6d The molecular structures of these two
systems are similar, featuring SnꢀO distances of 1.977(2) (for
6; Scheme 3) and 1.984(1) Å; however, while
Sn{CH(SiMe3)2}2(H)(OH) aggregates into centrosymmetric
dimers via pairs of OH…O hydrogen bonds, 6 is mononuclear
in the solid state, presumably reflecting the greater steric bulk
of the ancillary boryl ligands. The wide BꢀSnꢀB angle measꢀ
ured for 6 [138.7(1)o] is consistent with those reported for 3,
4b and Sn{B(NDippCH)2}2Cl2.
In contrast to the SnIV compounds formed by the oxidative
addition of hydridic EꢀH bonds, however, 6 undergoes further
reaction at elevated temperatures. Thus, refluxing in hexane
leads to the formation of equimolar amounts of HB(NDippꢀ
CH)2 and HOB(NDippCH)2 together with tin metal. This
chemistry is thought to be initiated by the reductive eliminaꢀ
tion of the strong BꢀO bond from 6 and appears closely related
to the onward reactivity of the corresponding ammonia activaꢀ
tion product (vide infra).
Scheme 4: Reaction of 1 with ammonia: coordination, N-H
activation and subsequent B-N/B-H reductive elimination.
Dipp
Dipp
N
N
N
N
N
N
NH3
(excess)
(Boryl)H
+
(Boryl)NH2
B
B
B
Dipp
Dipp
1 d
Dipp
Dipp
4 d
H
1
Sn
Sn
+
NH2
1 min
reduced Sn
species
NH3
B
N
N
Dipp
Dipp
1.
7
NH3
Our interpretation of these data in terms of a likely mechaꢀ
nistic pathway involves initial formation of a simple doꢀ
nor/acceptor adduct 1.NH3 featuring the stannylene 1 acting as
a Lewis acid. Indeed, this intermediate species can be isolated
by removal of volatiles in vacuo at short reaction times and
characterized by multinuclear NMR spectroscopy and Xꢀray
crystallography (Scheme 4 and Figure 2). 1.NH3 is labile toꢀ
wards further onward reaction and our initial attempts to obꢀ
tain single crystals were unsuccessful. With this in mind we
Scheme 3: Oxidative addition of the O-H bond in water to
1 to give hydroxy SnIV hydride 6. Molecular structure of 6
as determined by X-ray crystallography. Most H atoms
i
omitted and Pr groups shown in wireframe format for
clarity; thermal ellipsoids set at the 50% probability level.
Key bond lengths (Å) and angles (o): Sn(1)-B(1/2) 2.251(2),
2.259(2), Sn(1)-H(1) 1.76, Sn(1)-O(1) 1.977(2), B(1)-Sn(1)-
B(2) 138.7(1).
t
also synthesized the related tertꢀbutylamine adduct 1.NH2 Bu
t
through the corresponding reaction of 1 with BuNH2, and
additionally characterized this system by standard spectroꢀ
scopic and crystallographic techniques (Figure 2).
From a structural perspective, both 1.NH3 and 1.NH2 Bu can
t
be shown to feature the Nꢀdonor coordinated essentially perꢀ
pendicular to the plane of a SnB2 unit, which is itself largely
unperturbed from that found in the free stannylene [e.g. for
1.NH3: ∠N(2)ꢀSn(1)ꢀB(3/33) = 92.4(1), 94.6(1)o; ∠B(3)ꢀ
Sn(1)ꢀB(33) = 119.3(1)o]. Such data are consistent with deꢀ
scriptions of these systems as Lewis acid/base adducts formed
by interaction of the amine lone pair with the formally vacant
pπ orbital at tin. The SnꢀN bond lengths for both systems
[2.355(3) and 2.350(2) Å] are in keeping with other threeꢀ
Reactivity towards protic E-H bonds: ammonia. The oxidaꢀ
tive addition of NꢀH bonds in ammonia represents a high proꢀ
file challenge in synthetic chemistry in part because of the
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Journal of the American Chemical Society
Page 8 of 12
coordinate Nꢀdonor stannylene adducts [e.g. 2.345(6) Å for an
Spectroscopically, such an (amido)hydride species is conꢀ
intramolecular pyridine stabilized (dialkyl)stannylene].33
sistent with the pattern of 1H NMR resonances observed in situ
[δH = 4.80 (1H, SnH), −1.90 ppm (2H, SnNH2)]. These can be
compared to shifts of δH = 6.13 (1H, SnH), −2.09 ppm (1H,
SnOH) for 6, and δH = 5.22 (1H, SiH), 0.99 ppm (2H, SiNH2)
for the thermally stable (and crystallographically characterꢀ
ized) silicon (amido)hydride Si{Si(SiMe3)3}{N(SiMe3)Dipp}ꢀ
(H)(NH2) synthesized via the reaction of the acyclic silylene
Si{Si(SiMe3)3}{N(SiMe3)Dipp} with ammonia (ESI).
1
2
3
4
5
6
7
8
9
The apparently comparable rates of formation of 7 (from
1.NH3), and its onward conversion (to H2NB(NDippCH)2 and
HB(NDippCH)2) render it impossible to obtain significant
amounts of this compound via this route. More rapid converꢀ
sion of 1.NH3 into 7 can be achieved by employing an alternaꢀ
tive acid/base synthetic methodology (vide infra). Single crysꢀ
tals of 7 suitable for Xꢀray crystallography could be obtained
via the Lewis acid (K[B(C6F5)4]) catalyzed isomerization of
1.NH3 (Figure 2). The crystals of 7 so obtained are isomorꢀ
phous with those of 6, but can be shown unequivocally to conꢀ
tain the (amido)tin hydride on the basis of multinuclear NMR
measurements made on single crystalline samples reꢀdissolved
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2: Molecular structures of 1.NH3 (upper left),
t
1.NH2 Bu (upper right) and 7 as determined by X-ray crys-
1
tallography. Most hydrogen atoms omitted, and iPr groups
shown in wireframe format for clarity; thermal ellipsoids
set at the 50% probability level. Key bond lengths (Å) and
angles (o): (for 1.NH3) Sn(1)-B 2.311(3), 2.306(3), Sn(1)-N(2)
2.355(3), B(3)-Sn(1)-B(33) 119.3(1), N(2)-Sn(1)-B 92.4(1),
in C6D6. Moreover, the H NMR signals associated with the
SnH and SnNH2 units are found at chemical shifts indistinꢀ
guishable from those measured in situ for the intermediate
species in the 1/NH3 reaction.
The onward lability of 7 is in line with (i) previous literature
reports describing facile coupling between adjacent metalꢀ
bound borane moieties and electronꢀrich coꢀligands;35 and (ii)
the observation that isolated (hydroxy)hydride 6 also underꢀ
goes elimination of equimolar quantities of hydroꢀ and hyꢀ
droxyborane (albeit more slowly). In the case of
Sn{B(NDippCH)2}2(H)(NH2), reductive elimination of the
strong BꢀN bond formed by coupling of πꢀdonor amido and πꢀ
acceptor boryl components would represent a thermodynamic
driving force, and account for the formation of
H2NB(NDippCH)2. Subsequent BꢀH extrusion then generates
the observed hydroborane HB(NDippCH)2 and a source of
Sn0, which is either precipitated as the metal itself, or trapped
by unreacted 1.NH3 to generate the observed small quantities
of Sn10 and Sn11ꢀclusters. We favor this ordering of the reducꢀ
tive elimination steps (i.e. BꢀN formation preceding BꢀH) on
the basis that the SnII amide [Sn{B(NDippCH)2}(NH2)]2 can
be shown to be stable with respect to BꢀN elimination, and
thus not a viable intermediate under the conditions employed.
t
94.6(1); (for 1.NH2 Bu) Sn(1)-B 2.323(2), 2.338(2), Sn(1)-
N(5) 2.350(2), B(1)-Sn(1)-B(2) 118.3(1), N(5)-Sn(1)-B
90.3(1), 95.4(1); (for 7): Sn(1)-B 2.251(4), 2.259(3), Sn(1)-
N(58) 1.969(5), B(2)-Sn(1)-B(27) 138.2(1).
The behavior of 1.NH3 in solution is found to be strongly
dependent on the concentration of free ammonia present.
Thus, while isolated single crystalline samples react slowly
(<50% conversion to a mixture of 7, H2NB(NDippCH)2,
HB(NDippCH)2 over 44 hours at 20oC), the presence of a twoꢀ
fold excess of ammonia leads to more rapid onward reaction
with little of the adduct remaining after 21 hours under otherꢀ
wise analogous conditions. The spectroscopic properties of
1.NH3 are also dependent on the presence of additional free
ammonia in solution. Thus, isolated crystalline samples of the
adduct display two broad resonances for the Dipp methine
o
protons at 20 C, which sharpen on cooling below 0oC, and
o
which form one broad signal at 50 C. This observation is asꢀ
cribed to slow rotation about the SnꢀB bonds in 1.NH3 on the
NMR timescale, which renders inequivalent the two Dipp
groups associated with each boryl ligand. Conceivably, the
fluxional process occurring in the higher temperature regime
may involve either rotation about the SnꢀB bonds, or loss of
the NH3 ligand/reꢀccordination at either of the two faces of the
SnB2 unit. With the latter mechanism in mind, it is notable that
exchange is more facile in the presence of added ammonia
(with a sharp septet being observed for the Dipp methine proꢀ
tons at 20oC).
Scheme 5: Protonolysis of amidostannylene 2 by ammonia.
Molecular structure of 8 as determined by X-ray crystal-
i
lography. Most hydrogen atoms omitted, and Pr groups
shown in wireframe format for clarity; thermal ellipsoids
set at the 50% probability level. Key bond lengths (Å) and
angles (o): Sn(1)-B(1) 2.309(2), Sn(1)-N(3) 2.215(1), Sn(1)-
N(3') 2.227(1), B(1)-Sn(1)-N(3) 90.4(1), B(1)-Sn(1)-N(1')
92.9(1), N(1)-Sn(1)-N(1') 80.7(1).
Dipp
N
Following coordination of NH3, NꢀH bond cleavage then
occurs, generating the amidotin(IV) hydride Sn{B(NDippꢀ
CH)2}2(H)(NH2) (7), analogous to the (hydroxy)tin hydride 6
isolated from the reaction of 1 with water.34 The formation of
isolable phosphidotin(IV) hydrides by the oxidative addition
of PꢀH bonds to diaryl stannylenes has also been reported reꢀ
cently.14
N
H2
N
B
NH3
(excess)
Dipp
Sn
Sn
2
Dipp
N
H2
0.5 min
B
N
N
Dipp
8
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Journal of the American Chemical Society
a complex of composition
with the presence of significant electron density at the SnII
Independent synthesis of
1
[Sn{B(NDippCH)2}(NH2)] can be achieved by the reaction of
amido(boryl)stannylene 2 with ammonia. In this case converꢀ
sion proceeds rapidly (< 1 min) via a formal acid/base reacꢀ
tion. Thus, protonolysis generates the free aniline
HN(SiMe3)Dipp and the dinuclear primary amidostannylene
[Sn{B(NDippCH)2}(ꢁꢀNH2)]2 (8; Scheme 5), the identity of
which has been confirmed crystallographically. Such chemisꢀ
try, leading as it does to the isolation of a SnII amide, mirrors
the reactivity towards ammonia reported by Power and coꢀ
workers for diarylstannylene systems.5 Moreover, in line with
the mechanistic proposals outlined above, in our hands 8 is
found to be essentially inert to BꢀN reductive elimination.
centre, and accordingly, reaction of 9 with the dialkylaniliniꢀ
um salt [HNMe2Ph][B(C6F5)4], in C6D6 at 20oC generates a
~2:1 mixture of Sn{B(NDippCH)2}2(H)(NH2) (7) and 1.NH3.
This mixture presumably reflects the kinetics of protonation at
Sn and N, since the mixture transforms to yield solely the
thermodynamic product (7) after six hours at 20oC. As such,
the notion of NꢀH oxidative addition occurring via coordinaꢀ
tion of NH3, followed by Nꢀdonor mediated proton transfer,
can be shown to have some experimental validity. Finally, we
note that the transformation of 1.NH3 into 7 can also be
brought about, cleanly and in catalytic fashion by employing
ca. 10 mol% of the solventꢀfree potassium borate K[B(C6F5)4].
Mechanistically, this process is proposed to occur via Lewis
acid sequestration of NH3 from 1.NH3 by K+, resulting in the
presence in solution of a small quantity of species of the type
[K(NH3)x]+. The enhanced acidity of the NꢀH protons in such
species presumably then facilitates isomerization via protonaꢀ
tion at tin. Consistent with this hypothesis, the direct converꢀ
sion of 1.NH3 to 7 can also be brought about by the addition of
a catalytic amount (ca. 5 mol%) of [HNMe2Ph][B(C6F5)4] (see
ESI).
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Classical mechanisms of concerted EꢀH bond activation at a
single transition metal center involve donation of electron
density from the EꢀH σ bonding orbital and backꢀbonding into
the corresponding σ* orbital.1 The activation of H2 by acyclic
silylenes is also thought to involve initial interaction of the Hꢀ
H σꢀbonding MO with the pπ orbital of the electrophilic siliꢀ
con center,9a while alternative mechanisms exploiting the
strongly nucleophilic carbonꢀcentred lone pair have been adꢀ
vanced for EꢀH activation by related carbene compounds.24
With regard to the activation of ammonia by metallylene sysꢀ
tems, an alternative mechanistic pathway which has been proꢀ
posed computationally,5b,34,36 involves a second molecule of
NH3 acting as a proton shuttle, in effect transferring H+ from a
coordinated ammonia molecule to the Group 14 center. Thus,
computational studies from Power and Nagase, Pörschke and
Sicilia, for example, all propose that such a mechanism ought
to be viable for the activation of ammonia (and indeed water)
by low valent Group 14 metal centers.5b,6a,6d,34,36 In the case of
ammonia activation by 1, we wondered whether the signifiꢀ
cantly increased rate of onward reaction of 1.NH3 in the presꢀ
ence of excess ammonia might be explained on the basis of a
similar mechanism (Scheme 6). Therefore, with a view to
probing the plausibility of such deprotonation at
N/reprotonation at Sn acid/base chemistry we set out to isolate
the conjugate base of 1.NH3, and examine its behavior in the
presence of ammonium salts (i.e. sources of H+).
Figure 3: Molecular structure of dimeric 9 as determined
by X-ray crystallography. Most hydrogen atoms omitted,
and selected carbon atoms shown in wireframe format for
clarity; thermal ellipsoids set at the 50% probability level.
Key bond lengths (Å) and angles (o): Sn(1)-B 2.302(4),
2.314(5), Sn(1)-N(2) 2.088(8), Sn(1)…K(142) 3.745(2), B(11)-
Sn(1)-B(41) 102.7(2), N(2)-Sn(1)-B 102.8(2), 104.3(2).
Scheme 6: Potential proton shuttling mechanism for N-H
activation in 1.NH3 involving a second molecule of ammo-
nia.
Conclusions
In conclusion, we have demonstrated that a bis(boryl) ancilꢀ
lary ligand set can facilitate the oxidative addition of a range
of EꢀH bonds at SnII from both a thermodynamic and a kinetic
viewpoint. Moreover, in the case of OꢀH and NꢀH bonds, the
formation via oxidative addition of a tinꢀbound πꢀdonor ligand
offers a facile route to EꢀB bond formation (E = O, N) and reꢀ
reduction of the tin center. Thus, a singleꢀcenter redoxꢀbased
bond modification process is unequivocally established for a
Main Group metal. From a mechanistic viewpoint, a twoꢀstep
coordination/proton transfer process for NꢀH activation is
shown to be viable through the isolation of intermediate speꢀ
Dipp
Dipp
H
Dipp
H
N
N
N
H
N
N
N
N
N
N
B
B
B
B
B
B
H
N
NH3
Dipp
Dipp
Dipp
Dipp
Dipp
Dipp
-NH3
Sn
Sn
Sn
H
N
NH2
Dipp
NH3
Dipp
H2
N
N
N
Dipp
1.NH3
In the event, the reaction of K[N(SiMe3)2] with 1.NH3 does
indeed lead to deprotonation of the tinꢀbound ammonia moleꢀ
cule and to the formation of the potassium salt of the
[Sn{B(NDippCH)2}2(NH2)]ꢀ anion. Confirmation of the nature
of the product could be obtained crystallographically, with the
structure shown to feature centrosymmetric dimers in the solid
state (9; Figure 3).
ꢀ
.
cies of the types Sn(boryl)2 NH3 and [Sn(boryl)2(NH2)] , and
protonation of the latter to give Sn(boryl)2(H)(NH2).
ASSOCIATED CONTENT
Additional synthetic/characterizing data, copies of NMR spectra
for new compounds, all CIFs and complete details of DFT calcuꢀ
lations (including run files). This material is available free of
charge via the Internet at http://pubs.acs.org.
Deprotonation is accompanied by a marked shortening of
the SnꢀN distance [2.088(8) Å cf. 2.355(2) Å for 1.NH3], and
Sn(1) is also engaged in a contact with the potassium cation
[d(Sn…K) = 3.745(2) Å] which is well within the sum of the
respective Van der Waals radii. This observation is consistent
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(8) (a) Zhu, J.; Lin, Z.; Marder, T. B. Inorg. Chem. 2005, 44,
9384–9390; (b) Braunschweig, H.; Dewhurst, R. D.; Schneider, A.
Chem. Rev. 2010, 110, 3924ꢀ3957.
AUTHOR INFORMATION
Corresponding Author
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(9) Syntheses of boryltin systems: (a) Protchenko, A. V.; Birjkuꢀ
mar, K. H.; Dange, D.; Schwarz, A. D.; Vidovic, D.; Jones, C.;
Kaltsoyannis, N.; Mountford, P.; Aldridge, S. J. Am. Chem. Soc.
2012, 134, 6500–6503; (b) Protchenko, A. V.; Dange, D.; Schwarz,
A. D.; Tang, C. Y.; Phillips, N.; Mountford, P.; Jones, C.; Aldridge, S.
Chem Commun. 2014, 50, 3841ꢀ3844. For a related (boryl)amidotin
compound see: (c) Hadlington, T. J.; Abdalla, J. A. B.; Tirfoin, R.;
Aldridge, S.; Jones, C. Chem. Commun. 2016, 52, 1717ꢀ1720.
(10) Wang, Y.; Ma, J. J. Organomet. Chem. 2009, 694, 2567ꢀ2575.
(11) Protchenko, A. V.; Schwarz, A. D.; Blake, M. P.; Jones, C.;
Kaltsoyannis, N.; Mountford, P.; Aldridge, S. Angew. Chem., Int. Ed.
2013, 52, 568ꢀ571.
* Simon Aldridge, Oxford Chemistry, tel: +44 (01865) 285201,
fax: +44 (0)1865 272690, simon.aldridge@chem.ox.ac.uk
Author Contributions
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the manuꢀ
script.
Funding Sources
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We thank the Leverhulme Trust (F/08 699/E), the OUP John Fell
Fund, the ARC, the EU (Marie Curie grant PIEFꢀGAꢀ2013ꢀ
626441), NSERC and the EPSRC (EP/L025000/1 and
EP/K014714/1) for funding various aspects of this work.
(12) Finholt, A. E.; Bond, A. C., Jr.; Wilzbach, K. E.; Schlesinger,
H. I. J. Am. Chem. Soc, 1947, 69, 2692ꢀ2696.
(13) Segawa, Y.; Yamashita, M.; Nozaki, K. Science 2006, 314,
113ꢀ115.
ACKNOWLEDGMENT
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We acknowledge the NMSF, Swansea University.
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