Coordination chemistry of N-heterocyclic stannylenes: A combined synthetic and Moessbauer spectroscopy study

Article abstract of DOI:10.1021/ic101920x

The N-heterocyclic stannylenes (NHSns), [(Dipp)-N(CH₂) _nN(Dipp)Sn] (Dipp = 2,6-ⁱPr₂C₆H ₃; n = 2, 1; n = 3, 5) and [(^tBu)N(CHMe) ₂N(^tBu)Sn] (10) are competent ligands toward a variety of transition metal centers, as seen in the complexes [W(CO)₅ 3 1] (2), [(OC)₄Fe(μ-1)₂Fe(CO)₄] (3), [(OC) ₄Fe(μ-1)Fe(CO)₄] (4), [Fe(CO)₄ 3 5] _n (6, n = 1 or 2), [(OC)₄Fe(μ-5)Fe(CO₅) ₄] (7), [Ph₃PPt(μ-1)₂PtPPh₃] (8), [Fe(CO)₄ 3 10] (11), and [(η⁵-C₅H ₅)-(OC)₂Mn 3 10] (12). X-ray crystallographic studies show that the NHSns are structurally largely unperturbed binding to the metal, but in contrast to the parent NHCs, NHSns often adopt a bridging position across dinuclear metal units. The balance between terminal and bridging positions for the stannylene is evidently closely balanced as shown by the observation of both monomers and dimers for 6 in the solid state and in solution. ¹¹⁹Sn and ⁵⁷Fe Moessbauer spectroscopy of the complexes shows the tin atoms in such complexes to be consistent with electron deficient Sn(II) centers.

Full text of DOI:10.1021/ic101920x

ARTICLE
pubs.acs.org/IC
Coordination Chemistry of N-Heterocyclic Stannylenes: A Combined
€
Synthetic and Mossbauer Spectroscopy Study
Stephen M. Mansell,† Rolfe H. Herber,*,‡ Israel Nowik,‡ Douglas H. Ross,† Christopher A. Russell,*,† and
Duncan F. Wass†
†School of Chemistry, University of Bristol, Cantock’s Close, Bristol, #BS8 1TS, U.K.
‡Racah Institute of Physics, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel
S Supporting Information
b
ABSTRACT: The N-heterocyclic stannylenes (NHSns), [(Dipp)-
N(CH2)nN(Dipp)Sn] (Dipp = 2,6- iPr2C6H3; n = 2, 1; n = 3, 5) and
[(tBu)N(CHMe)2N(tBu)Sn] (10) are competent ligands toward a
variety of transition metal centers, as seen in the complexes [W(CO)5 1]
3
(2), [(OC)4Fe(μ-1)2Fe(CO)4] (3), [(OC)4Fe(μ-1)Fe(CO)4] (4),
[Fe(CO)4 5]n (6, n = 1 or 2), [(OC)4Fe(μ-5)Fe(CO)4] (7),
3
[Ph3PPt(μ-1)2PtPPh3] (8), [Fe(CO)4 10] (11), and [(η5-C5H5)-
3
(OC)2Mn 10] (12). X-ray crystallographic studies show that the
3
NHSns are structurally largely unperturbed binding to the metal, but
in contrast to the parent NHCs, NHSns often adopt a bridging position
across dinuclear metal units. The balance between terminal and bridg-
ing positions for the stannylene is evidently closely balanced as shown by the observation of both monomers and dimers for 6 in the
solid state and in solution. 119Sn and 57Fe M€ossbauer spectroscopy of the complexes shows the tin atoms in such complexes to be
consistent with electron deficient Sn(II) centers.
’ INTRODUCTION
silylenes16 and N-heterocyclic germylenes,16e,17 but recent reports
have extended this to the two heaviest elements in the group.17d,17e,18
These were prepared by the salt metathesis of the appropriate
dilithiated diamide with MCl2 (M = Sn, Pb) and showed a high
degree of thermal stability compared to the corresponding
species with unsaturated backbones.18b,18d The degree of aggre-
gation displayed by these species was closely related to the steric
bulk of the substituents on the N centers.
Alongside reports of the preparation of these low-valent, low-
coordination number species has been an exploration of their
coordination chemistry, in particular toward the transition ele-
ments. The parent NHCs are classed as being extremely strong
σ-donors, with π-acceptance being of much less importance.19
Almost all structurally characterized examples of NHCs bonded
to metal atoms display terminal coordination through the carbene
lone pair although a few examples of abnormal bonding through
another carbon atom in the heterocyclic ring have been ob-
served.20 Examples of NHCs which bridge metal centers are very
rare with a search of the Cambridge Structural Database revealing
only 12 examples of bridging coordination.21 In each case, the
formation of bridging carbenes is observed where the NHC is part
of a multidentate ligand, with the bridging position being adopted
N-heterocyclic carbenes (NHCs) are one of the most in-
tensely studied classes of compounds in the past two decades.1 A
vast body of work has been published on their fascinating chem-
istry since the first report of their isolation as stable species,2
including, notably, their ability to act as ligands in catalytically
active systems.3 More recently, they have attracted attention
because of their ability to form and stabilize highly unusual main
group species.4 In seeking to modify the NHC unit, chemists
have adopted a series of strategies such as changing the substit-
uents on the nitrogen atoms,1c,5 varying the backbone of the
ligand,6 or through use of a chiral scaffold.7 An alternative strat-
egy involves exchange of one or both of the N-centers for other
units, leading, for example, to cyclic alkyl-amino carbenes8 or
P-heterocyclic carbenes,9 which themselves have demonstrated
rather different but exciting new chemistry.8b Our own interest
and those of several other groups10 has focused on exchanging
the carbene carbon atom for other isovalent units. Representative
examples have been reported of ionic species containing donor
atoms from group 13,11 group 15,12 and even group 16.13 In addi-
tion, there is a rich chemistry reported for the neutral group 14
homologues of carbenes, a topic where initial reports14 predate the
discovery of NHCs by a considerable number of years.15 The
chemistry of these group 14 N-heterocyclic carbene analogues has
focused largely on the lighter congeners which are N-heterocyclic
Received: September 20, 2010
Published: February 14, 2011
r
2011 American Chemical Society
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Scheme 1. Coordination of N-Heterocyclic Stannylenes to
Table 1. 119Sn Chemical Shifts of Complexes 2, 3, 6, 8, 11,
and 12 and Comparison to Those of the Respective Free
Stannylene
Transition Metal Fragments (R = 2,6-iPr2C6H3)
119Sn chemical
compound 119Sn chemical shift (multiplicity)a shift of free stannylene
2
426.2 (s with 183W satellites
1J(119Sn-183W) = 1150 Hz)
525.3 (s)
366
3
366
291
366
454
454
6
498.0 (s)
8
500 (br. s)
11
12
534.0 (s)
625 (br. s)
a 112 MHz, 25 ꢀC, C6D6
as a consequence of the geometric constraints of the multidentate
ligand rather than an express preference for a bridging position.22
The heavier homologues of NHCs have a much less well-
developed coordination chemistry than N-heterocyclic carbenes.23
N-heterocyclic silylenes are usually observed as a terminal ligand,
although the bridging mode has been identified with Pd(0)
precursors.24 The coordination chemistry of other (non NHC
type) silylene complexes formed at a transition metal center are
reviewed elsewhere.25 Similarly, N-heterocyclic germylenes have
been shown to coordinate terminally to a range of transition metal
centers, all in a terminal bonding mode.17a,17b,26 The co-
ordination chemistry of N-heterocyclic stannylenes, the subject
of this report, is also relatively under explored. Homoleptic
nickel(0) complexes of a N-heterocyclic stannylene were reported
in 1990,27 but the wider field of coordination complexes of other
stannylene ligands dates back still further. In this regard we note
the pioneering work of the groups of Lappert and Veith probing
the chemistry of a variety of stannylenes, for example,
[Sn{E(SiMe3)2}2] (E = N, CH)23b and [Sn{N(tBu)}2SiMe2],28
showing them to be versatile ligands to a range of transition metal
centers. Hahn et al. have recently prepared and explored the
coordination chemistry of benzannulated N-heterocyclic stanny-
lenes, preparing chelating bis(stannylene) complexes of Ni(0),29
Pt(0),30 and Mo(0) and using computational chemistry to
investigate the σ/π-donor and π-acceptor effects.31
Figure 1. Thermal ellipsoid plot of 2. All hydrogen atoms have been
removed for clarity, and thermal ellipsoids are set at 30% probability.
Selected bond lengths (Å) and angles (deg): Sn1-W1 2.6889(3), Sn1-
N1 1.994(2), Sn1-N1A 1.993(2); N1-Sn1-N1A 83.57(13);
P
(angles at N1) 358.7.
reaction of the free N-heterocyclic stannylene with transition
metal species bearing labile ligands (Scheme 1). Coordination of
the stannylene results in a downfield shift of the 119Sn chemical
shift (these are collated in Table 1; NMR data collected for other
nuclei are detailed within the Experimental Section for the spe-
cific compound). The products are extremely air- and moisture-
sensitive but are generally quite thermally stable in the solid state,
with some samples showing no signs of decomposition until above
200 ꢀC. In this report, we will discuss each compound in turn be-
fore analyzing the M€ossbauer spectroscopy results separately.
Reaction (thf, r.t., 16 h) of [Sn{N(Dipp)CH2}2] (1) with the
distinctive bright yellow colored [W(CO)5(thf)] (Scheme 1)
gave a deep red solution. 119Sn NMR spectroscopy showed a sin-
glet at high frequency (Table 1) with 183W satellites. Recrystalliza-
tion of 2from n-hexane gave X-ray quality crystals which showed the
stannylene binding through the Sn center to the W(CO)5 fragment
(Figure 1).
In this study, we sought to investigate the coordination
chemistry of a class of N-heterocyclic stannylenes which we re-
cently reported containing saturated organic backbones and
probe the metal-ligand interaction using X-ray crystallography,
NMR spectroscopy, and M€ossbauer spectroscopy. In addition,
we describe a new and rare example of a chiral NHSn and
investigate its coordination chemistry.
In complex 2, the stannylene ring, which as a free species adopts a
non-planar conformation in the solid state, becomes planar on
coordination, a feature which has also been observed for a number of
saturated NHCs.32 Other dimensions of the stannylene remain
largely unaltered upon complexation.
’ RESULTS AND DISCUSSION
Bridging and Terminal Coordination of N-Heterocyclic
Stannylenes. To probe the coordination chemistry of N-hetero-
cyclic stannylenes, a number of complexes were prepared by
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Figure 3. Thermal ellipsoid plot of one of the two crystallographically
independent molecules in the asymmetric unit of 6. All hydrogen atoms
have been omitted for clarity, and thermal ellipsoids are set at 50%
probability. Selected bond lengths (Å) and angles (deg) for both molecules:
Sn1-N2 1.990(4), Sn1-N1 1.998(5), Sn1-Fe1 2.4299(10), Sn2-N3
2.008(4), Sn2-N4 2.018(5), Sn2-Fe2 2.4293(10); N2-Sn1-N1
Figure 2. Thermal ellipsoid plot of 3. All hydrogen atoms have been
omitted for clarity, and thermal ellipsoids are set at 30% probability.
Selected bond lengths (Å) and angles (deg): Sn1-N1 2.041(3), Sn1-
N2 2.045(3), Sn1-Fe1A 2.6688(7), Sn1-Fe1 2.6689(7), Fe1-Sn1A
2.6686(7); N1-Sn1-N2 84.08(14), Fe1A-Sn1-Fe1 98.640(17),
P
P
P
P
97.12(19), N3-Sn2-N4 94.9(2);
(angles at N1) 359.9,
(angles
Sn1A-Fe1-Sn1 81.358(17);
(angles at N1) 359.3,
(angles at N2)
P
P
at N2) 359.6,
(angles at N3) 360.0,
(angles at N4) 359.7.
359.8.
In C6D6 solution, the 1H NMR spectrum of 2 showed inequi-
valent methyl groups within each iso-propyl substituent; further-
more the backbone CH2 protons were observed as singlets
together with poorly resolved satellites with about 22 Hz coupl-
toluene had a yellow coloration, and a single crystal X-ray
diffraction study showed a rather different solid-state structure
(Figure 3).
The stannylene was observed to coordinate in a terminal
manner to a Fe(CO)4 fragment, adopting an equatorial position
with the stannylene parallel to the axial carbonyl ligands. The Sn-
Fe distances {2.430(1) and 2.429(1) Å for each of the independent
molecules in the unit cell} are short compared to literature values
{av. Fe-Sn distance is 2.56(7) Å},35 although we note a series of
examples where similarly short Sn-Fe contacts have been ob-
served, for example, in a bis(aryloxide)stannylene-Fe(CO)4 com-
pound (2.408(1) Å);36 [(toluene)Fe(SnAr2)2] {2.432(1) and
2.434(1) Å};37 and 2.436(1) Å in [(toluene)Fe(C2H4)SnAr2].38
The Sn-N bond lengths {1.990(4) to 2.018(5) Å} are similar to
those in the free stannylene,18d andtheNcentersareplanar(sumof
angles at all N atoms above 359.6ꢀ). However, the difference in
color between the observed solid-state crystals and in solution led
us to probe the solution behavior in more detail. NMR spectros-
copy data (þ25 ꢀC, C6D6) for both 3 and 6 are similar with sharp
peaks seen in the 119Sn NMR spectra at δ = 525.3 ppm (3) and
498.0 ppm (6). These are both shifted to higher frequency (andare
much sharper) than those seen in the respective free stannylenes
(366 ppm for 1 and 291 ppm for 5).18d 1H NMR spectroscopy
revealed two doublets and one septet for the iso-propyl groups and
unresolved coupling to 119Sn/117Sn was seen for the CH2 group in 3.
13C NMR spectroscopy revealed peaks diagnostic of the co-
ordinated stannylene ligands as well as a single peak for the CO
ligands at δ = 212.6 ppm in 3 and 212.5 ppm in 6. To determine
whether the bridging or terminal structure is adopted in solution for
either compound, DOSY experiments were performed on C6D6
solutions of both complexes (Table 2). Diffusion constants of
5.036 ꢁ 10-10 m2 s-1 for 3 and 4.951 ꢁ 10-10 m2 s-1 for 6 were
obtained and through the use of the Stokes-Einstein equation
3
ing arising from J coupling to the 119Sn and 117Sn nuclei; this
feature was not resolved in the free stannylene. 13C NMR spec-
troscopy revealed CO resonances at 197.7 and 194.4 ppm (cf.
W(CO)6 at 192 ppm33) with the much less intense signal at 197.7
ppm tentatively assigned to the trans carbonyl. IR spectroscopy in n-
hexane solution showed three CO stretches at 2076.2, 1960.4, and
1928.7 cm-1. These stretches are similar to the a1 bands of the
corresponding W(CO)5 complexes of [Sn{N(SiMe3)2}2] (2073,
1931 cm-1), PPh3 (2075, 1938 cm-1), and an N-heterocyclic
carbene (2060, 1927 cm-1),34 suggesting that the NHSn has similar
net donor/acceptor properties to these other ligands.
An alternative coordination mode for 1 was revealed from
the equimolar reaction (16 h, r.t., toluene) with [Fe2(CO)9]
(Scheme 1). Bright red crystals of the product, 3, were grown
from n-hexane, and X-ray crystallography showed them to
contain a diamond-shaped Fe2Sn2 core (Figure 2), with non-
planar SnN2C2 rings perpendicular to the Fe2Sn2 plane. The
stannylene units are relatively unfettered by complexation, with
comparatively short Sn-N bonds {2.041(3) and 2.045(3) Å}
which are only marginally longer than in both the free stannylene
and in complex 2, and planar nitrogen centers. The Fe-Sn distances
{which are symmetry imposed to be equal at 2.6689(7) Å} are
slightly longer than for non-bridging iron-stannylene complexes
(vide infra), while both the Fe-C and CO bonds are all identical
within error.
The related reaction (toluene, r.t., 16 h) of a N-heterocyclic
stannylene with a three-carbon backbone (5) with [Fe2(CO)9]
(Scheme 1) proceeded in an apparently similar manner, giving a
dark orange/red solution. However, crystals of 6 grown from
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Table 2. Structures of the Iron-Tin Complexes 3 and 6 in Solution and the Solid State
crystal structure
volume/Å3
DOSY experiments
compound
radius/Å
diffusion coefficient/m2 s-1
calculated hydrodynamic radius/Å
solution structure
3 (dimer)
1289.0
694.5
6.75
5.5
5.036 ꢁ 10-10
4.951 ꢁ 10-10
6.7
6.8
dimer
dimer
6 (monomer)
(D = kT/6πηRH, k = Boltzmann constant, η = viscosity of C6D6 at
25 ꢀC: 6.46 ꢁ 10-4 kg m-1 s-1,39 RH = hydrodynamic radius), the
hydrodynamic radii of the species were calculated; 6.7 Å for 3 and
6.8 Å for 6. It is evident from the results that the molecules are of a
very similar size in solution and through calculating the crystal-
lographic radius from the crystal structure volume, (5.5 Å for 6 and
6.75 Å for 3), it is seen that both compounds are dimeric in solution.
IR spectroscopy of 3 in n-hexane showed CO stretches at
2091.8, 2039.9, 2014.6, and 1962.2 cm-1 and very similar
absorptions in the solid state. In contrast, n-hexane solutions of
6 gave stretches at 2061.5, 2052.1, 1990.2, and 1970.0 cm-1 and a
rather different set of absorptions in the solid state (2089.8,
2059.7, 2042.9, 2021.4, 1966.9, and 1940.7 cm-1), thus corro-
borating the other evidence of different structures in solution and
the solid state.
In light of this observation that alternative structures are
accessible for 6 in the solid state and in solution, a detailed re-
examination of a crystalline sample from the reaction of 5 with
[Fe2(CO)9] was undertaken. This revealed that indeed, a very
small number of red crystals were present among the bulk of
yellow crystals of 6. X-ray diffraction experiments revealed the
structure of this minor product, 7, to contain a bridging stannylene
ligand, but this time only one bridging an Fe-Fe bond between
Fe(CO)4 units (Figure 4).
Figure 4. Thermal ellipsoid plot of one of the four crystallographically
independent molecules in the asymmetric unit of 7. All hydrogen atoms
have been removed for clarity, and thermal ellipsoids are set at 50%
probability. Selected bond lengths (Å) for all four molecules and selected
angles for a representative molecule (deg): Sn1-N1 2.026(2), Sn1-N2
2.037(2), Sn1-Fe2 2.5734(4), Sn1-Fe1 2.5764(4), Fe1-Fe2 2.8572(5),
Sn2-N4 2.034(2), Sn2-N3 2.032(2), Sn2-Fe4 2.5664(4), Sn2-Fe3
2.5769(4), Fe3-Fe4 2.8557(5), Sn3-N6 2.022(2), Sn3-N5 2.036(2),
Sn3-Fe5 2.5464(4), Sn3-Fe6 2.5901(4), Fe5-Fe6 2.8600(5), Sn4-N8
2.030(2), Sn4-N7 2.036(2), Sn4-Fe7 2.5697(4), Sn4-Fe8 2.5798(4),
Fe7-Fe8 2.8571(5); N1-Sn1-N2 96.64(8), Fe2-Sn1-Fe1 67.40(1);
The analysis of its solid-state structure is complicated by the
presence of four crystallographically independent molecules in
the asymmetric unit. However, the majority of bond dimensions
are comparable to those seen in the other Fe-stannylene com-
plexes 3, 4, and 6.
A detailed re-examination of the 1H NMR spectrum from the
reaction mixture showed 6 as the major product as well as another
set of very minor peaks that are consistent with the structure of 7.
Peaks at 3.87 (septet), 3.02 (multiplet), and 1.50 ppm (doublet)
were assigned to 7, with the remaining resonances not being
identified as they presumably overlap with the dominant signals
from the major product, 6.
P
P
(angles at N1) 358.8,
(angles at N2) 359.2.
appear to be linked with an Fe-Fe bond. The Sn-N bonds are
similar to those previously seen in 1-3, and the nitrogen atoms
are planar. In contrast to 7, the bridging stannylene leans signi-
ficantly more toward one iron center than the other, although the
reason for this is not readily apparent.
The different bonding modes apparent for stannylene 5 in
complexes 6 and 7 led us to speculate whether the same vari-
ability may be observed for other stannylenes. Hence we reviewed
the 1H NMR spectroscopic data from the crude reaction mixture
used to isolate 3 which revealed a minor set of peaks with signals at
3.96 (septet), 3.50 (singlet), and 1.48 ppm (doublet) which may
correspond to a minor product 4. Crystallization of the mother
liquor after initial crystallizationof3 led to materialthat was mostly
free diamine, Dipp(H)NCH2CH2N(H)Dipp. However, a few red
crystals of 4 were observed in the mixture and were suitable for
analysis by X-ray crystallography, and the molecular structure of
this product was obtained (Figure 5). Unfortunately, this is the
only unequivocal data on this pure compound as contamination
with the free diamine, the very small yields, and the high air- and
moisture-sensitivity of all the compounds involved prevented us
from acquiring complete analytical data for 4.
The bridging stannylene motif across Fe-Fe vectors in 3, 4,
and 7 was also seen using other metals. Reaction of 1 with
[(Ph3P)2Pt(C2H4)] gave a red solution, and crystals suitable for
X-ray crystallography were grown from benzene. This showed 8
as containing a butterfly shaped Sn2Pt2 core with two stannylene
units bridging the Pt-Pt vector of a (Ph3P)Pt-Pt(PPh3) frag-
ment (Figure 6). The SnN2C2 rings are twisted, with the SnN2
unit lying at an angle of 73.2ꢀ to the Sn2Pt plane, in contrast to
the orthogonal arrangement seen in 3. The Sn-N bonds are
short and slightly unsymmetrical {2.043(2) and 2.028(2) Å} and
the nitrogen atoms are planar. The Sn-Pt distances are short
and similar {the unique distances are 2.6096(2), 2.5995(2), and
2.5994(2) Å} and the angle between the Pt1-Sn1-Pt1A and
Pt1-Sn1A-Pt1A planes is 153.4ꢀ, in contrast to the planar arrange-
ment (i.e., 180ꢀ) seen in 3. Unusually, the PPh3 ligands pendant
to the Pt centers adopt a markedly cisoid orientation about the
Pt-Pt vector, the distance of which is 2.6393(2) Å, and is
consistent with a single bond.40 We note that other tin species
The molecular structure was very similar to that of 7, with a
single stannylene ligand bridging two Fe(CO)4 fragments which
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Scheme 2. Synthesis and Coordination Chemistry of a Chiral
N-Heterocyclic Stannylenea
a All compounds shown were formed as racemic mixtures.
(cf. 0.07 ppm in both 1 and 2). 31P NMR spectroscopy revealed a
broad resonance at 52.0 ppm with broad 195Pt satellites (5173
Hz) but with no discernible coupling to tin; heating the sample to
60 ꢀC did not cause any further sharpening of the signal. 119Sn
NMR spectroscopy of this complex (Table 1) only revealed an
extremely broad signal at about 500 ppm, possibly on account of
coupling to the multiple spin-active nuclei in the complex.
In summary, these results show that NHSns have a propensity
to adopt bridging coordination modes. Relative to NHCs, the heavier
analogues generally possess a more inert lone pair of electrons in
a nominal sp2-hybrid orbital and a reactive, Lewis acidic empty
p-orbital.10 The Lewis-acidity of the empty p-orbital often leads
to dimerization or oligomerization of NHSns18d,42 via interac-
tions of the divalent atom with ring-nitrogen donors of adjacent
molecules. Presumably the observation of bridging modes for
NHSns across transition metal vectors is a manifestation of the
same effect.
Figure 5. Thermal ellipsoid plot of 4. All hydrogen atoms have been
removed for clarity, and thermal ellipsoids are set at 30% probability.
The carbon atoms forming the backbone of the stannylene ligand (C13)
are disordered over two sites and only one set is displayed. Selected bond
lengths(Å) andangles(deg):Sn1-N1 2.0205(18), Sn1-N1A 2.0205(18),
Sn1-Fe1 2.5739(5), Sn1-Fe2 2.5296(5), Fe1-Fe2 2.8965(7); N1A-
Sn1-N1 84.23(10).
Synthesis and Coordination Chemistry of a Chiral N-
Heterocyclic Stannylene. Given the importance of chiral
ligands in enantioselective reactions, we were keen to explore
the synthesis of chiral N-heterocyclic stannylenes.43 This was
achieved by dilithiation of chiral diamine 9 and reaction with
SnCl2 at low temperature in diethyl ether (Scheme 2).
The analogous silylene has been reported by reduction of a
dihalodiaminosilane, and interesting chemistry of this species has
been reported.44 Crystals of 10 suitable for X-ray crystallography
revealed the monomeric nature of the stannylene (Figure 7) with
both enantiomers present in the asymmetric unit. The two mole-
cules in other respects are very similar, with symmetric Sn-Nbonds
varying between 2.019(4) and 2.039(4) Å and planar nitrogen
atoms. No close intermolecular contacts were found akin to those in
1, which is unsurprising given the absence of aryl groups in 10.18d In
other respects the stannylenes show similar physical attributes, with
compound 10 subliming at 0.5 Torr and 100 ꢀC (compared to
160 ꢀCat7.5ꢁ 10-5 Torr for 1and 200 ꢀCat1ꢁ 10-2 Torr for 5),
and displays thermochromic behavior as solutions of the stanny-
lene frozen in n-hexane at -196 ꢀC are bright red, whereas at
room temperature the stannylene is yellow.18d 1H NMR spec-
troscopy showed a sharp singlet for the tert-butyl group, a doublet
for the methyl group, and a quartet for the CH protons. 119Sn
NMR spectroscopy showed a singlet resonance at 454 ppm,
consistent with a two-coordinate tin center.
Figure 6. Thermal ellipsoid plot of 8. All hydrogen atoms and the three
benzene solvate molecules have been removed for clarity. The thermal
ellipsoids for the Sn, Pt, N, and P atoms are set at 50% probability, while
the C atoms are represented as spheres of 0.1 Å radius. Selected bond
lengths (Å) and angles (deg): Pt1A-Sn1 2.5994(2), Pt1-Sn1 2.6096(2),
Pt1A-Pt1 2.6392(2), Sn1-N2 2.027(2), Sn1-N1 2.043(2), Sn1-Pt1A
2.5995(2); Sn1A-Pt1-Sn1 114.078(6), P1-Pt1-Pt1A 165.30(2), N2-
P
Sn1-N1 83.10(9), Pt1A-Sn1-Pt1 60.882(6);
(angles at N1) 358.3,
P
(angles at N2) 358.0.
have been reported to bridge both homonuclear Pd and Pt
bonds,41,26b although the precise arrangement identified in 8 is
unique.
1H NMR spectroscopy revealed sharp signals for the stanny-
lene units with a notably large difference in chemical shift
between the nonequivalent CHMe2 environments of 0.22 ppm
The coordination chemistry of this chiral N-heterocyclic stan-
nylene was then probed by reaction with [Fe2(CO)9] in toluene
to afford a direct comparison with the coordination behavior of
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Figure 7. Thermal ellipsoid plot of one of the two molecules (the S,S
enantiomer is shown; the other molecule in the asymmetric unit is the R,
R enantiomer) in the asymmetric unit of 10. All hydrogen atoms have
been removed for clarity and thermal ellipsoids are set at 50% prob-
ability. Selected bond lengths (Å) and angles (deg): Sn1-N2 2.019(4),
Sn1-N1 2.038(4), Sn2-N3 2.033(5), Sn2-N4 2.039(4), N2-Sn1-
P
N1 82.05(17), N3-Sn2-N4 81.97(18);
(angles at N1) 360.0,
(angles at N2) 359.7, (angles at N3) 359.6, (angles at N4) 360.0.
P
P
P
Figure 9. Thermal ellipsoid plot of 12. All hydrogen atoms have been
removed for clarity, and thermal ellipsoids are set at 30% probability.
Only one position for each of the carbon atoms in the disordered
stannylene backbone (C16, C17, C18 and C19) is shown. Selected bond
lengths (Å) and angles (deg): Cpcentroid-Mn1 1.774, Mn1-Sn1 2.4705(4),
P
Sn1-N1 2.013(2), Sn1-N2 2.013(2), N1-Sn1-N2 83.52(10);
P
(angles at N1 for component shown) 353.2,
nent shown) 354.5.
(angles at N2 for compo-
free stannylene 10, but with unresolved 119Sn and 117Sn satellites
3
with J coupling of about 36 Hz now observed, as seen for the
other stannylene complexes.
Coordination of the chiral stannylene was also observed by
reaction with [CpMn(CO)2(thf)], generated by photolytic cleav-
age of [CpMn(CO)3] in thf. The dark red color of [CpMn-
(CO)2(thf)] faded and the orange product (12) was recrystal-
lized from n-hexane. A structural study was carried out using
X-ray crystallography and revealed terminal coordination of the
N-heterocyclic stannylene to the manganese center (Figure 9).
The stannylene remains puckered and appears relatively unper-
turbed by coordination to the Mn center. 119Sn NMR spectros-
copy (Table 1) showed a very broad resonance at 625 ppm, with
the broadness probably being a consequence of coupling to the
quadrupolar manganese atom (55Mn, 100% I = 5/2).
Figure 8. Thermal ellipsoid plot of 11. All hydrogen atoms have been
removed for clarity, and thermal ellipsoids are set at 50% probability.
Selected bond lengths (Å) and angles (deg): Sn1-N1 2.0127(16), Sn1-
P
Fe1 2.4683(4); N1-Sn1-N1A 84.65(9);
(angles at N1) 359.7.
€
’ MoSSBAUER SPECTROSCOPY STUDIES
non-chiral NHSns. Crystallization of the product from n-hexane
afforded 11 as red crystals which X-ray crystallographic experi-
ments showed to contain terminal coordination of the stanny-
lene to the Fe(CO)4 fragment in the equatorial position with a
2-fold rotation axis running through the Fe-Sn bond (Figure 8).
In contrast to 6, the N-Sn-N plane does not lie in the plane of
the axial carbonyl ligands, being significantly twisted (by 40.3ꢀ)
away from this plane. The Fe-Sn bond is marginally longer
{2.4683(4) Å} than the bond lengths seen in 6 {2.430(1) and
2.429(1) Å}. However, the Sn-N bond lengths {2.0127(16) Å}
remain very short and the N-Sn-N angle {84.65(9)ꢀ} is only
slightly more open than that seen in the free stannylene {82.05(17)
and (81.97(18)ꢀ}.
Compound 1. In common with the other samples investigated
in this study (with the exception of 6) the 119Sn ME spectra consist
of doublets, and evidence more than one Sn atom site. In the case of
1, thesetwositesconsistofamajorandminor spectralcontribution,
the latter presumably due to a byproduct or the result of minor
sample degradation prior to examination. In any event, the major
site in 1 evidence an isomer shift (I.S.) of 2.27 ( 0.01 mm sec-1
and a quadrupole splitting (Q.S.) of 2.25 ( 0.04 mm sec-1 at 90 K,
indicative of the expected Sn(II) nature of the metal center, with a
major contribution to the Q.S. arising from the lone pair. A
representative spectrum is shown in Supporting Information,
Figure S1, and the hyperfine and derived parameters are summar-
ized in Table 3. The I.S. of the minor constituent is nearly the same
as that of the major one, but the Q.S. parameter is smaller by ∼38%,
strongly suggesting a different ligation mode of the tin atom in the
minor constituent, supportive of our hypothesis of the dimeric form
As expected, coordination of the stannylene results in a down-
1
field shift in the 119Sn NMR chemical shift (Table 1), and H
NMR spectroscopy of 11 showed similar signals compared to the
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Table 3. 119Sn and 57Fe Mo€ssbauer Data for Compounds 1-3, 5, and 6a
compound/param
1
2
3
5
6
units
IS(90) (Sn)
2.271(2)
1.652(12)
2.812
1.926(48)
2.801
2.173(6)
2.279(6)
1.53(2)
0.986(52)
-0.012(4)
0.49(4)
mm s-1
mm s-1
mm s-1
mm s-1
QS(90) (Sn)
IS(90) (Fe)
2.460(20)
-0.024(5)
1.866(5)
2.76(2)
QS (90) (Fe)
k2Æxave2æM (Sn)
k2Æxave2æX (Sn)
k2Æxave2æM (Fe)
k2Æxave2æX (Fe)
2.60
3.87
2.07
3.56
3.74(1)
4.48(1)
1.65(11)
2.56(3)
2.41(3)
1.040(13)
1.17(10)
1.09(3)
1.048(18)
a The values in parentheses indicate the experimental error in the last digit(s).
of 1. The area ratio of the two sites at 90 K is ∼3:1, and this ratio is
not a sensitive parameter, consistent with the organometallic
natures of the two sites.
ranges 90 < T < 190 and 90 < T < 248 K, respectively. As noted
above the 119Sn spectra consist of a major and minor component
with an area ratio of ∼2.1:1. The tin hyperfine parameters are
included in Table 3 and are not otherwise remarkable, being
consistent with a Sn(II) oxidation state of the metal atom. The
examination of the F parameters at 100 K again shows that FX >
FM but the difference is only ∼13%; that is, much smaller than
the corresponding differences noted for 1 and 2. The most
obvious rationalization of this fact is that in 3, the tin atom, being
coordinated to two iron centers as well as by the two nitrogen
atoms of the 5-membered ring, is much more securely “tethered”
and is less sensitive to the effects of the libration and torsional
modes referred to above. The hyperfine parameters for the major
component extracted from the 57Fe ME spectra of 3 are included
in Table 3 and a representative spectrum is shown in Supporting
Information, Figure S2.
As noted earlier,45 the temperature-dependence of the recoil-
free fraction, f, for an optically thin absorber, as derived from the
temperature-dependence of the area under the resonance curve,
can be expressed by the parameter FM.T = k2Æxave2æ, where k is the
wave vector of the appropriate ME radiation, and Æxave2æ is the
mean-square-amplitude-of-vibration (msav) of the metal atom at
temperature T. The F parameter can also be extracted from the Uij
values of the single crystal X-ray diffraction data, FX,T, and
compared to the corresponding ME value, FM.T. This comparison
is effected in Table 3, from which it is seen that for 1 at 100 K,
FM,100 = 2.56 whereas FX,100 = 3.87. This large difference has been
noted previously46 for other organotin complexes, in contrast to
iron organometallics, and may be related to the presence of
relatively low frequency librational or torsional modes involving
the tin atom in the covalent structure, or to the presence of static
crystal imperfections to which the X-ray data are sensitive, but the
ME data are not. In the present instance, the Sn atom is two-
coordinate and ligated to the N atoms in the monomeric structure
by two σ bonds of length 1.989 Å and bond angle 84.15(8)ꢀ. This
configuration permits a large Uij value in the X-ray data compared
to the corresponding ME value. The relatively small resonance
effect magnitude, as well as the rapid decrease of f with increasing
temperature prevents the extension of the temperature-dependent
ME data to higher values. The 147.9 K data show an effect
magnitude of 0.37% and required accumulating 14.7 ꢁ 106
counts/channel for reasonable statistical accuracy.
Here, again, there is evidence of a multiplicity of 57Fe resonances,
roughly in the ratio of 1:0.19:0.09 at 90 K. The three IS parameters
are very similar, but the observed QS values range from 1.68 for the
major resonance to 0.54 for the smallest. Because of the lower
energy of the 57Fe resonance compared to that of 119mSn, it is
possible in this case to extend the ME measurements over a
significantly longer range, and hence to determine the temperature
dependencies of the IS and f parameters to better accuracy. The
Meff value48 for the major constituent calculated from these data is
∼150 ( 15 Da, a value indicative of the strong covalent ligation of
the Fe atom. In the present case it is interesting to note that FM,100
and FX,100 are nearly identical, as has been previously reported45,49
for other organometallics in which the metal-ligand bonding
involves largely π-orbitals of the transition metal, and where this
group is well “tethered” to the remainder of the molecule.
Compound 5. Not surprisingly, the major Sn ME resonance
hyperfine parameters observed for 5 are very similar to those
noted for 1, since the only significant difference between the two
arises from the 5- versus 6-membered ring incorporating the
metal atom (Figure 10). In this case, the minor resonance (∼5%
of the total spectral area) has an IS close to 0 mm sec-1 and is
most probably due to a minor contamination by Sn(IV) oxide.
The major constituent hyperfine parameters are included in
Table 3 and are not otherwise remarkable. The F values are
FX,173 = 4.48 and FM,173 = 3.74, consistent with the earlier
observations noted above. A comparison of the FM values at
various temperatures and their extrapolation to Tf0, to the single
crystal X-ray FX value determined at 173 K is summarized
graphically in Figure 10.
Compound 2. Here, again, the ME spectra consist of a major
and minor constituent, the area ratio at 90 K being ∼3.0:1. The
hyperfine parameters are included in Table 3, from which it is
noted that the IS in 2 is very much smaller than in 1. In fact, the
value of this parameter is close to that normally associated with
Sn in the þ4 oxidation state. However, given the determined
stoichiometry of this compound, it is clear that the unusually
small value of the IS must be associated with the ligation of the tin
atom to the pentacarbonyl tungsten ligand, which effectively acts
as an electron-withdrawing group.47 The magnitude of the QS
parameter in 2 is similar to that observed in other complexes in
which the tin atom is ligated to a transition metal center (e.g.,
vide infra), and again a major contribution arises from the lone
pair associated with the Sn center. The comparison of the FX and
FM parameters at 100 K follows the same behavior as that noted
for 1, above. The origin of the minor tin resonance in the ME
spectra of 2 has not been further elucidated.
One further comment is appropriate with respect to 5: because
of the relatively “clean” nature of the ME spectra, it is observed
that the area ratio of the two components of the QS doublet is
Compound 3. For this compound, both the 119mSn and 57Fe
ME resonances have been determined over the temperature
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determination using a conventional apparatus. Elemental compositions
(C, H, and N) were determined by sealing samples in airtight aluminum
boats in a glovebox and were recorded on a Carlo Erba EA1108 CHN
elemental analyzer. Solution NMR spectroscopy samples were pre-
pared using dry and degassed deuterated solvent in airtight NMR tubes
sealed with a Young’s tap. 119Sn spectra were run on Jeol Eclipse and
Lambda 300 MHz spectrometers and referenced to an external sample
of SnMe4. 1H and 13C NMR spectra were run on Jeol Eclipse 300 and
Lambda 300 MHz spectrometers and were referenced to the internal
solvent peaks. DOSY experiments were performed using a Varian
500 MHz spectrometer at 25.0 ꢀC using peak integrations to give
diffusion coefficients.
Temperature-dependent M€ossbauer spectra were acquired in trans-
mission geometry by using both the 57Fe and the 119mSn resonances.
The former was collected using a ∼50 mCi source of 57Co in a Rh
matrix, and all iron isomer shifts are referred to the center of a room
temperature R-Fe absorption spectrum which was also used for spectro-
meter calibration. The latter were collected using a 2 mCi 119 mSn source
in a BaSnO3 matrix, and all tin isomer shifts are referenced to a room
temperature BaSnO3 absorber spectrum. Temperatures were monitored
using the Daswin program, and are considered stable to (0.2 K over the
data acquisition intervals.
Figure 10. Parameter k2Æxave2æ for the 119Sn M€ossbauer resonance in 5.
The open data points refer to the ME data, the filled data point to that
extracted from the single crystal X-ray data at 173 K, and the five-pointed
star data point refers to the extrapolated value as Tf0.
The M€ossbauer samples were transferred from sealed Pyrex tubes to
O-ring equipped Perspex sample holders in a glovebox, and immediately
cooled to liquid nitrogen temperature prior to spectroscopic examina-
tion. To ascertain that there was no sample loss during the extended data
collection intervals (crucial for the data interpretation related to the m.
sav values) the transmission rate through the sample was monitored
both before and after each temperature run.
essentially temperature independent over the range 100 < T <
200 K. Hence it is clear that there is no significant Gol’danskii-
Karyagin effect50 in the tin atom motion, and that the vibrational
amplitudes are essentially isotropic over this temperature range,
in agreement with the Uij values reported at 173 K.
Compound 6. The 119Sn ME spectra of this compound show
only a single doublet, facilitating the subsequent data analysis.
The IS value at 90 K (Table 3) is that associated with a tin atom
ligated to a transition element, as noted above, indicating that
there is a significant diminution in the s-electron density at Sn.
The QS value at 90 K is particularly small, and this parameter is
not at all sensitive to temperature. The FX,100 and FM,100 values
are included in Table 3, and their ratio (1.55:1) is consistent with
those reported earlier for these tin resonances, except that un-
usually the X-ray value is slightly smaller than the M€ossbauer value.
The area ratio of the two components of the QS doublet is tem-
perature insensitive (1.08 ( 0.05 over the range 92 < T < 202 K)
again indicating isotropic motion of the tin atom in this solid. The Fe
resonance in 6, which could be acquired over only a limited
temperature range (90 < T < 150 K), consists of the usual doublet,
and the IS parameter is similar to that observed for 3. On the other
hand, the QS for this resonance is only about 25% of that observed
for 3 reflecting the fact that the Fe atom in 6 is five-coordinate (and
six-coordinate in 3) reflecting the major crystallographic difference
in the two iron sites. The agreement between the Fx or M values at
100 K is satisfactory, given the limited temperature range over which
data could be accumulated, and suggest that there are no major low-
frequency librational or torsional modes influencing the iron atom
motion in this compound.
Stannylenes 1 and 5 were synthesized as reported previously.18d
A
racemic mixture of the chiral diamine 1044a and [(Ph3P)2Pt(C2H4)]51
were synthesized as previously described. Despite repeated attempts,
satisfactory elemental analyses for a number of the products could not be
obtained. This is presumably due to the range of different products
isolated from some of the reactions and the high air- and moisture-
sensitivity of the compounds.
[(CO)5W-Sn{N(Dipp)CH2}2] (2). A solution of W(CO)6 (0.257 g,
0.73 mmol) dissolved in thf (20 cm3) was irradiated with a UV lamp for
2 h forming a clear yellow solution indicative of [W(CO)5(thf)]. A
solution of [Sn{N(Dipp)CH2}2] (1) (0.363 g, 0.73 mmol) in thf
(10 cm3) was then added, and the reaction mixture was stirred overnight.
All of the solvent was removed from the red/orange solution, and the
solid was extracted into n-hexane (30 cm3) and filtered (porosity 3 sinter
with Celite). The red solution was reduced in volume in vacuo and placed
at -20 ꢀC for 1 week upon which red/purple crystals were observed
(0.220 g, 0.268 mmol, 37%).
1H NMR (300 MHz, 25 ꢀC, C6D6): δ (ppm) 7.13-7.11 (m, Ar H),
3.81 (s with unresolved 119Sn and 117Sn satellites 3J(1H-Sn) ≈ 22 Hz,
CH2), 3.62(septet3J(1H-1H) =6.97Hz, DippCH), 1.35(d3J(1H-1H) =
6.97 Hz, Dipp Me), 1.28 (d 3J(1H-1H) = 6.97 Hz, Dipp Me). 13C NMR
(75.5 MHz, 25 ꢀC, C6D6): δ (ppm) 197.7 (trans W-CO), 194.4 (cis
W-CO), 146.8 (ortho Ar C), 144.1 (ipso Ar C), 127.3 (para Ar C), 124.4
(meta Ar C), 60.6 (CH2), 29.2, 26.0, 24.8 (iPr carbons).119Sn NMR (112
1
MHz, 25 ꢀC, C6D6): δ (ppm) 426.2 (s with 183W satellites J(119Sn-
183W) = 1150 Hz). IR (n-hexane): ν(cm-1) 2076.2 (CO stretch), 1960.4
(CO stretch), 1928.7 (shoulder, CO stretch). Elemental Analysis: Calcu-
lated (%): C 45.34, H 4.66, N 3.41. Found (%): C 45.60, H 4.99, N 3.54.
[(CO)4Fe(μ-Sn{N(Dipp)CH2}2)]2 (3). To a suspension of [Fe2(CO)9]
(144 mg, 0.40 mmol) in toluene (10 mL) was added a solution of
[Sn{N(Dipp)CH2}2] (1) (197 mg, 0.40 mmol) in toluene (10 mL),
and the mixture was stirred overnight. A clear, dark red solution was
observed, and all the solvent and any [Fe(CO)5] produced was removed
under vacuum. The solid was extracted with toluene (30 mL) and filtered
(porosity 3 sinter with Celite) giving a clear dark red solution which was
’ EXPERIMENTAL SECTION
General Experimental Procedures. All air- and moisture-sen-
sitive materials were weighed out, isolated, and stored in an argon-filled
Saffron Beta glovebox. The solvents used were distilled HPLC grade and
further dried and degassed using a commercially available solvent puri-
fication system (Anhydrous Engineering). Deuterated benzene was
dried over potassium then vacuum transferred and kept over molecular
sieves in the glovebox. Melting points were determined by sealing
the sample in melting point tubes under argon in a glovebox prior to
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Table 4. Selected Crystallographic and Data Collection Parameters for Compounds 2-4, 6-8, and 10-12
2
3
4
6
7
color, habit
red prism
red prism
red block
yellow plate
0.20 ꢁ 0.12 ꢁ 0.03
C34.5H44FeN2O4Sn
725.26
red block
size/mm
0.11 ꢁ 0.10 ꢁ 0.02
C31H38N2O5SnW
821.19
0.13 ꢁ 0.08 ꢁ 0.05
C60H76Fe2N4O8Sn2
1330.32
0.20 ꢁ 0.13 ꢁ 0.10
C34H38Fe2N2O8Sn
833.05
0.21 ꢁ 0.14 ꢁ 0.04
C35H40Fe2N2O8Sn
847.08
empirical formula
M
crystal system
orthorhombic
Pnma
monoclinic
P21/n
monoclinic
P21/m
monoclinic
P21
triclinic
space group
P1
a/Å
16.6243(3)
18.9240(7)
10.4192(2)
90
13.4655(16)
11.7855(13)
19.163(2)
90
9.7677(3)
18.7855(6)
10.3823(3)
90
9.7070(4)
36.8631(15)
9.7894(4)
90
17.6709(5)
21.4182(6)
22.3233(6)
61.7630(10)
89.7080(10)
81.8280(10)
7349.1(4)
8
b/Å
c/Å
R/deg
β/deg
90
102.069(2)
90
112.1130(10)
90
101.050(2)
90
γ/deg
V/Å3
90
3277.86(15)
4
2973.9(6)
2
1764.93(9)
2
3438.0(2)
4
Z
μ/mm-1
4.307
1.364
1.564
1.186
1.504
T/K
100
100
100
100
100
θmin,max
2.55,32.87
0.941
1.69,27.50
0.997
2.12, 27.48
1.000
3.47,33.13
0.995
1.04,27.48
0.997
completeness to θmax
reflections: total/independent
Rint
36025/5899
0.0507
25856/6822
0.0595
27016/4176
0.0388
38795/19511
0.0692
152337/33588
0.0419
final R1 (I > 2σ) and wR2 (all data)
largest peak, hole/e Å-3
0.0285, 0.0542
0.859, -1.543
1.664
0.0404, 0.1015
1.226, -0.630
1.486
0.0300, 0.0837
1.416, -1.072
1.568
0.0576, 0.1102
1.420, -1.764
1.401
0.0319, 0.0788
0.907, -0.603
1.531
F
calc/g cm-3
Flack parameter
n/a
n/a
n/a
0.36(2)
n/a
8
10
yellow rod
11
12
color, habit
red prism
yellow block
orange rod
0.40 ꢁ 0.11 ꢁ 0.11
C19H31MnN2O2Sn
493.09
size/mm
0.40 ꢁ 0.20 ꢁ 0.05
0.30 ꢁ 0.08 ꢁ 0.08
C12H26N2Sn
317.06
0.19 ꢁ 0.06 ꢁ 0.04
C16H26FeN2O4Sn
484.93
empirical formula
C112H130N4P2Pt2Sn2
2221.70
monoclinic
C2/c
M
crystal system
monoclinic
P21/c
monoclinic
C2/c
monoclinic
P21/c
space group
a/Å
19.8884(6)
26.9543(8)
19.0243(6)
90
23.3287(6)
6.4072(2)
21.6934(5)
90
17.9372(6)
9.8950(3)
12.1219(5)
90
6.6047(8)
17.932(2)
17.704(2)
90
b/Å
c/Å
R/deg
β/deg
92.934(2)
90
117.6900(10)
90
111.787(2)
90
94.222(2)
90
γ/deg
V/Å3
10185.1(5)
4
2871.19(13)
8
1997.82(12)
4
2091.2(4)
4
Z
μ/mm-1
3.302
1.756
1.996
1.812
T/K
100
100
100
100
θmin,max
1.27,28.35
0.996
1.75,28.27
0.997
3.26,27.53
0.995
2.31,27.51
0.998
completeness to θmax
reflections: total/independent
Rint
75254/12690
0.0527
65157/7120
0.0457
9783/2291
0.0237
18763/4801
0.0276
final R1 (I > 2σ) and wR2 (all data)
largest peak, hole/e Å-3
0.0243, 0.0721
0.731, -0.756
1.449
0.0367, 0.0927
4.861, -1.070
1.467
0.0195, 0.0518
1.128, -0.518
1.612
0.0264, 0.0874
0.616, -1.251
1.566
F
calc/g cm-3
Flack parameter
n/a
n/a
n/a
n/a
reduced in vacuo and placed at -20 ꢀC for 3 days upon which red crystals
were observed (89 mg, 0.067 mmol, 33% yield).
CH2), 3.63 (septet 3J(1H-1H) = 6.97 Hz, Dipp CH), 1.30 (d
3J(1H-1H) = 6.97 Hz, Dipp Me), 1.27 (d 3J(1H-1H) = 6.97 Hz, Dipp
Me). 13C NMR (75.5 MHz, 25 ꢀC, C6D6): δ (ppm) 212.6 (Fe-CO),
146.4 (ortho Ar C), 143.9 (ipso Ar C), 126.8 (para Ar C), 124.0 (meta Ar C),
1H NMR (300 MHz, 25 ꢀC, C6D6): δ (ppm) 7.13-7.00 (m, Ar H),
3.75 (s with unresolved 119Sn and 117Sn satellites 3J(1H-Sn) ≈ 27 Hz,
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59.6 (CH2), 28.7, 24.8, 24.8 (iPr carbons). 119Sn NMR (112 MHz,
25 ꢀC, C6D6): δ (ppm) 525.3 (s). IR (n-hexane): ν(cm-1) 2091.8 (m,
CO stretch), 2039.9 (s, CO stretch), 2014.6 (s, CO stretch), 1962.2 (m,
CO stretch). IR (nujol mull): ν(cm-1) 2092.7 (w), 2046.1 (s), 2008.9
(s), 1995.2 (s). m.p.: Decomposed above 205 ꢀC.
solution was observed, and all the solvent and any [Fe(CO)5] produced
was removedunder vacuum. Thesolidwas extractedwithhexane(20mL)
and filtered(porosity 3 sinter with Celite) giving a clear red solution which
was reducedinvacuoandplacedat-20ꢀC for3 daysuponwhichorange/
red crystals were observed (98 mg, 0.20 mmol, 43% yield).
[(CO)4Fe-Sn{N(Dipp)CH2}2CH2] (6). To a suspension of [Fe2(CO)9]
(151 mg, 0.41 mmol) in toluene (10 mL) was added a solution of [Sn-
{N(Dipp)CH2}2CH2] (5) (212 mg, 0.41 mmol) in toluene (10 mL), and
the mixture was stirred overnight. A dark orange/red solution was observed,
and all the solvent and any [Fe(CO)5] produced was removed under
vacuum. The solid was extracted with hexane (30 mL) and filtered (porosity
3 sinter with Celite) giving a clear orange/red solution which was reduced in
vacuo and placed at -20 ꢀC overnight upon which yellow crystals were
observed (123 mg, 0.18 mmol, 44% yield).
1H NMR (300 MHz, 25 ꢀC, C6D6): δ (ppm) 3.03 (q 3J(1H-1H) =
6.04 Hz with unresolved 119Sn and 117Sn satellites 3J(1H-Sn) ≈ 36 Hz,
CH), 1.15 (s, tert-butyl), 0.96 (d, 3J(1H-1H) = 6.04 Hz, Me). 13C NMR
(75.5 MHz, 25 ꢀC, C6D6): δ (ppm) 214.2 (Fe-CO), 61.6 (NCH), 56.8
(NC(CH3)3), 33.4 (C(CH3)3), 27.3 (CHCH3). 119Sn NMR (112
MHz, 25 ꢀC, C6D6): δ (ppm) 534.0. IR (n-hexane): ν(cm-1
)
2034.0 (s), 1963.9 (m), 1934.4 (s), 1907.2 (s). m.p.: 70-73 ꢀC. Decom-
posed above 119 ꢀC.
rac-[Cp(CO)2Mn-Sn{N(tBu)CHMe}2] (12). A solution of [CpMn-
(CO)3] (100 mg, 0.49 mmol) dissolved in thf (20 cm3) was irradiated
with a UV lamp for 2 h forming a deep red/purple solution. A solution of
rac-[Sn{N(tBu)CHMe}2] (10) (155 mg, 0.49 mmol) in thf (10 cm3)
was then added, and the reaction mixture was stirred overnight. All of the
solvent was removed from the clear orange solution, and the solid was
extracted into n-hexane (20 cm3) and filtered (porosity 3 sinter with
Celite). The bright orange solution was reduced in volume in vacuo and
placed at -20 ꢀC, and the product was formed as yellow crystals in two
crops (56 mg, 0.114 mmol, 23%).
1H NMR (300 MHz, 25 ꢀC, C6D6): δ (ppm) 7.14-7.09 (m, Ar H),
3
3
3.61 (septet J(1H-1H) = 6.96 Hz, Dipp CH), 3.38 (t J(1H-1H) =
4.95 Hz, N-CH2), 2.10 (m, CH2) 1.35 (d 3J(1H-1H) = 6.96 Hz, Dipp
Me), 1.27 (d 3J(1H-1H) = 6.96 Hz, Dipp Me). 13C NMR (75.5 MHz,
25 ꢀC, C6D6): δ (ppm) 212.5 (Fe-CO), 146.9 (ortho Ar C), 144.8 (ipso
Ar C), 127.4 (para Ar C), 124.3 (meta Ar C), 58.4 (N-CH2), 35.2
(CH2), 28.4, 25.4, 24.9 (iPr carbons). 119Sn NMR (112 MHz, 25 ꢀC,
C6D6): δ (ppm) 498.0 (s). IR (n-hexane): ν(cm-1) 2061.5 (m), 2052.1
(m), 1990.2 (s), 1970.0 (s). IR (nujol mull): ν(cm-1) 2089.8 (w),
2059.7 (w sh.), 2042.9 (s), 2021.4 (w sh.), 1966.9 (s sh.), 1940.7 (s).
m.p.: 164-168 ꢀC. Decomposed above 178 ꢀC.
1H NMR (300 MHz, 25 ꢀC, C6D6): δ (ppm) 4.04 (s with broad
satellites ≈12 Hz, Cp), 3.21 (q 3J(1H-1H) = 6.23 Hz, CH), 1.29 (s, tert-
3
[(Ph3P)Pt(μ-Sn{N(Dipp)CH2}2)]2 (8). [(Ph3P)2Pt(C2H4)] (150 mg,
0.20 mmol) and [Sn{N(Dipp)CH2}2] (1) (100 mg, 0.20 mmol) were
dissolved in C6D6 (1.0 cm3) giving a deep red solution which was transferred
to an NMR tube equipped with a Young’s tap. Upon standing for a week, the
sample crystallized as bright red crystals suitable for X-ray analysis.
1H NMR (300 MHz, 25 ꢀC, C6D6): δ (ppm) 7.42 (d J = 7.14 Hz,
8 H, Ar H), 7.23 (m, Ar H), 7.02-6.90 (m 12 H, Ar H), 3.97 (br. s, 4 H,
butyl), 1.10 (d, J(1H-1H) = 6.23 Hz, Me). 13C NMR (75.5 MHz,
25 ꢀC, C6D6): δ (ppm) 231.1 (s, weak, Mn-CO), 230.6 (s, weak, Mn-
CO), 78.2 (Cp), 62.3 (NCH), 56.2 (NC(CH3)3), 34.0 (C(CH3)3), 28.9
(CHCH3). 119Sn NMR (112 MHz, 25 ꢀC, C6D6): δ (ppm) 625
(broad). IR (n-hexane): ν(cm-1) 1921.6 (s), 1855.4 (s). m.p.: 120-
126 ꢀC. Decomposed above 144 ꢀC.
X-ray Crystallography. Crystals suitable for X-ray diffraction
analysis of 2-4, 6-8, 10-12 were grown as described above, mounted
in an inert oil and then transferred to the cold gas stream of the dif-
fractometer. Experiments were made with a Bruker-AXS Kappa-APEX-II
four circle diffractometer52 employing Mo-KR radiation (λ = 0.71073 Å),
except 2 where a Oxford diffraction Gemini four-circle diffractometer emp-
loying Mo-KR radiation (λ = 0.71073 Å) was used. Intensities were inte-
grated from several series of exposures, and absorption corrections were
applied based on multiple- and symmetry-equivalent measurements.53
Structures were solved by direct methods and refined by least-squares on
weighted F2 values for all reflections.54 All non-hydrogen atoms were
assigned anisotropic displacement parameters and refined without positional
constraints. Hydrogen atoms were constrained to ideal geometries and
refined with fixed isotropic displacement parameters. Refinement proceeded
smoothly to give the residuals shown in Table 4. Complex neutral-atom
scattering factors were used.55 Crystals of 10 were twinned, and the twin law
[-1 0 -1 0 -1 0 0 0 1] was used with the BASF refining to 0.49.
CCDC reference numbers 807655 (2), 807656 (3), 807657 (4),
807658 (6), 807659 (7), 807660 (8), 807661 (10), 807662 (11), and
807663 (12).
3
CH2), 3.81 (septet J(1H-1H) = 6.96 Hz, 4 H, Dipp CH), 1.35 (d
3J(1H-1H) = 6.96 Hz, 12 H, Dipp Me), 1.13 (d 3J(1H-1H) = 6.96 Hz,
12 H, Dipp Me). 1H NMR (300 MHz, 60 ꢀC, C6D6): δ (ppm) spec-
trum identical to that at room temperature. 31P{1H} NMR (121.4 MHz,
1
25 ꢀC, C6D6): δ(ppm) 52.0 (br. s with broad 195Pt satellites J(31P-
195Pt) = 5173 Hz). 31P{1H} NMR (121.4 MHz, 60 ꢀC, C6D6):δ(ppm) 52.8
(br. s with broad 195Pt satellites 1J(31P-195Pt) = 5215 Hz). 119Sn NMR
(112 MHz, 25 ꢀC, C6D6): δ (ppm) extremely broad signal at about 500.
rac-[Sn{N(tBu)CHMe}2] (10). nBuLi(5.2mL1.6Msolutioninhexane,
8.42 mmol) was added to a solution of {HN(tBu)CHMe}2 (9) (0.838 g,
4.2 mmol) in Et2O (40 mL), and the clear colorless solution was stirred for
2 h. This solution was then transferred slowly by cannula to a -78 ꢀC
suspension of SnCl2 (0.793 g, 4.2 mmol) in Et2O (20 mL), and the cloudy
orange solution was allowed to warm to room temperature and stir
overnight. The solvent was removed under reduced pressure, and the
solid was extracted with n-hexane (60 mL) and filtered (porosity 3 sinter
with Celite). The bright yellow solution was reduced in volume in vacuo
and then placed at -20 ꢀC overnight upon which yellow crystals were
observed. (0.818 g, 2.58 mmol, 61% yield). The product sublimes at 5 ꢁ
10-1 Torr and 100 ꢀC, and displays thermochromic behavior as frozen
solutions of the stannylene in n-hexane at -196 ꢀC are bright red.
1H NMR (300 MHz, 25 ꢀC, C6D6): δ (ppm) 3.32 (q 3J(1H-1H) =
6.23 Hz, CH), 1.25 (s, tert-butyl), 1.20 (d, 3J(1H-1H) = 6.23 Hz, Me).
13C NMR (75.5 MHz, 25 ꢀC, C6D6): δ (ppm) 64.3 (NCH), 55.9
(NC(CH3)3), 34.0 (C(CH3)3), 28.5 (CHCH3). 119Sn NMR (112
MHz, 25 ꢀC, C6D6): δ (ppm) 454. Elemental Analysis: Calculated
(%): C 45.46, H 8.27, N 8.84. Found (%): C 45.33, H 8.77, N 8.50. m.p.:
172-177 ꢀC. Decomposed above 210 ꢀC.
’ CONCLUSIONS
In summary we have shown that N-heterocyclic stannylenes
are competent ligands toward a range of different transition
metals (W, Fe, Pt, and Mn). The binding of NHSns to metals
structurally has only minimal effect on the NHSn. Interestingly,
whereas NHSns will bind in a terminal manner to transition
metal centers, they also show a distinct propensity to adopt a
bridging position across dinuclear metal units. The balance
between terminal and bridging positions for the stannylene is
evidently closely balanced as shown by the observations of both
monomers and dimers for the same molecular unit in both solid
rac-[(CO)4Fe-Sn{N(tBu)CHMe}2] (11). To a suspension of [Fe2-
(CO)9] (172 mg, 0.47 mmol) in toluene (10 mL) was added a solu-
tion of rac-[Sn{N(tBu)CHMe}2] (10) (150 mg, 0.47 mmol) in toluene
(10 mL), and the mixture was stirred overnight. A dark orange/red
2261
dx.doi.org/10.1021/ic101920x |Inorg. Chem. 2011, 50, 2252–2263

Inorganic Chemistry
ARTICLE
state and in solution. 119Sn and 57Fe M€ossbauer spectroscopy of
the complexes showed significant changes to the environment
around the Sn centers upon coordination to transition metals.
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’ ASSOCIATED CONTENT
S
Supporting Information. M€ossbauer spectra of 1 and 3.
b
Crystallographic data in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: herber@vms.huji.ac.il (R.H.H.), chris.russell@bris.ac.uk
(C.A.R.). Fax: þ972 (0)2 658 6347 (R.H.H.), þ44 (0)117 925
1295 (C.A.R.). Phone: þ972 (0)2 658 4244 (R.H.H.), þ44
(0)117 928 7599 (C.A.R.).
’ ACKNOWLEDGMENT
We acknowledge Dr. Craig Butts (Bristol) for help with DOSY
experiments and Dr. Natalie Fey (Bristol) for advice on this
manuscript. S.M.M. would like to thank the University of Bristol
for funding. We are also indebted to David Mocatta for the care-
ful glovebox transfer of the many air/moisture sensitive samples
to the ME sample holders prior to spectroscopic examination.
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