Synthesis, characterization, and reactivity of an N-heterocyclic germanium(II) hydride: Reversible hydrogermylation of a phosphaalkyne

Article abstract of DOI:10.1021/om200823x

The β-diketiminato germanium(II) chloride complex [( ^ButMesNacnac)GeCl] (^ButMesNacnac = [{N(Mes)C(Bu ^t)}₂CH]^-, Mes = mesityl) has been prepared and spectroscopically characterized. Treating this compound and a less bulky system, [(^MeMesNacnac)GeCl] (^MeMesNacnac = [{N(Mes)C(Me)} ₂CH]^-), with K[HBEt₃] led to differing outcomes, namely, the formation of the novel diamido germylene [( ^ButMesNacnacH)Ge:] (^ButMesNacnacH = [N(Mes)C(H)(Bu ^t)C(H)C(Bu^t)N(Mes)]^2-) and the germanium(II) hydride [(^MeMesNacnac)GeH], both of which were crystallographically characterized. The former product forms via a 1,3-hydrogen migration reaction involving [(^ButMesNacnac)GeH] as an unstable reaction intermediate. Reactions of [(^MeMesNacnac)GeH] with CO₂ and P≡CBu^t proceed at or below ambient temperature, in the absence of a catalyst, to give the crystallographically characterized hydrogermylation products [(^MeMesNacnac)GeOC(=O)H] and [(^MeMesNacnac) GeC(Bu^t)=PH], with complete regioselectivity. When a solution of the latter was gently heated, the hydrogermylation reaction was partially reversed, regenerating [(^MeMesNacnac)GeH] and P≡CBu^t. Heating to higher temperatures led to the irreversible isomerization of [( ^MeMesNacnac)GeC(Bu^t)=PH] to (E)-[(^MeMesNacnac) GeP=C(H)(Bu^t)], theoretical studies of which indicate is the thermodynamic product of the original hydrogermylation reaction.

Full text of DOI:10.1021/om200823x

ARTICLE
pubs.acs.org/Organometallics
Synthesis, Characterization, and Reactivity of an N-Heterocyclic
Germanium(II) Hydride: Reversible Hydrogermylation of a
Phosphaalkyne
Sam L. Choong, William D. Woodul, Christian Schenk, Andreas Stasch, Anne F. Richards, and
Cameron Jones*
School of Chemistry, Monash University, P.O. Box 23, Clayton, Victoria 3800, Australia
S Supporting Information
b
ABSTRACT: The β-diketiminato germanium(II) chloride
complex [(ButMesNacnac)GeCl] (ButMesNacnac = [{N(Mes)
C(But)}2CH]À, Mes = mesityl) has been prepared and spectro-
scopically characterized. Treating this compound and a less
bulky system, [(MeMesNacnac)GeCl] (MeMesNacnac = [{N-
(Mes)C(Me)}2CH]À), with K[HBEt3] led to differing out-
comes, namely, the formation of the novel diamido germylene
[(ButMesNacnacH)Ge:] (ButMesNacnacH = [N(Mes)C(H)
(But)C(H)C(But)N(Mes)]2‑) and the germanium(II) hydride [(MeMesNacnac)GeH], both of which were crystallographically
characterized. The former product forms via a 1,3-hydrogen migration reaction involving [(ButMesNacnac)GeH] as an unstable
reaction intermediate. Reactions of [(MeMesNacnac)GeH] with CO2 and PtCBut proceed at or below ambient temperature, in the
absence of a catalyst, to give the crystallographically characterized hydrogermylation products [(MeMesNacnac)GeOC(dO)H] and
[(MeMesNacnac)GeC(But)dPH], with complete regioselectivity. When a solution of the latter was gently heated, the hydro-
germylation reaction was partially reversed, regenerating [(MeMesNacnac)GeH] and PtCBut. Heating to higher temperatures led to
the irreversible isomerization of [(MeMesNacnac)GeC(But)dPH] to (E)-[(MeMesNacnac)GePdC(H)(But)], theoretical studies
of which indicate is the thermodynamic product of the original hydrogermylation reaction.
’ INTRODUCTION
2,6)8b has been the most studied as a reagent. It has been shown to
readily hydrogermylate alkynes, CO2, ketones, and diazoalkenes
under mild conditions and in the absence of a catalyst.9 Inaddition,
it has been used as a precursor to terminal germanium(II) azide
and hydroxide compounds,10 a monomeric germanium dithiocar-
boxylic acid analogue,9 and the diamido germylene 2.11 Given the
synthetic versatility of 1, it is surprising that related three-coordi-
nate germanium heterocycles incorporatingβ-diketiminate ligands
of varying steric bulk have not been reported. Very recently, we
attempted the preparation of a bulkier example, [(ButNacnac)GeH]
The chemistry of low-oxidation-state main-group complexes
has rapidly expanded since the turn of the millennium.1 Although
this is primarily a result of significant fundamental interest in the
unusual structural, bonding, and other properties of such com-
pounds, their generally high reactivity has led to increasing
explorations into their applications potential in recent times. In
this respect, it is emerging that low-oxidation-state p-block
compounds can participate in reactions (e.g., the activation of
saturated and unsaturated small molecules) that are normally
considered the realm of transition-metal complexes.2 Perhaps the
most poorly studied low-oxidation-state p-block systems are the
hydrides, which almost exclusively require sterically very de-
manding coligands to be capable of ambient existence.3 If a
greater range and understanding of these compounds could be
accessed, their reactivity could be further developed and com-
pared to that of d-block metal hydride complexes, which are of
undoubted importance to many areas of synthesis and catalysis.4
Most of the recent progress in this area has come from group 14
element(II) hydride complexes,3a a considerable number of which
have been kinetically stabilized by incorporation of bulky mono-
dentate (e.g., terphenyl5 or N-heterocyclic carbene6) or bidentate
(e.g., amidinate7 or β-diketiminate8) ligands. Of these, the mon-
omeric germanium(II) hydride complex [(DipNacnac)GeH] (1)
(Chart1;DipNacnac = [{N(Dip)C(Me)}2CH]À, Dip =C6H3Pri2-
(
ButNacnac = [{N(Dip)C(But)}2CH]À), either via the reaction of
the germanium(I) radical [(ButNacnac)Ge:]• (3) with Bun SnH
3
or by treating [(ButNacnac)GeCl] with the hydride source K-
[HBEt3].12 In both cases, the thermally unstable diamido germylene
4 was isolated instead. It was proposed that this was formed via the
intended product [(ButNacnac)GeH], which is unstable and under-
goes a 1,3-hydrogen migration to give 4. The instability of
[(ButNacnac)GeH] is thought to result from the unsaturated back-
bone of its very bulky β-diketiminate ligand being significantly more
localized than that of 1.12 This in turn leads to its imine C-center
being more susceptible to intramolecular nucleophilic attack.13
Herein, we report on the less bulky and more reactive analogues
Received: September 1, 2011
Published: September 29, 2011
r
2011 American Chemical Society
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Chart 1
crystalline 7 results from its high solubility in aliphatic organic
solvents. Despite its sterically less demanding ligand, compound
6 has thermal stability in the solid state (dec pt 160À162 °C)
similar to that of 1 (dec pt 170 °C)8b and is indefinitely stable in
solution at room temperature.19 In contrast, the diamido germy-
lene 7 is less stable in solution than its bulkier counterpart 4 and
decomposes over ca. 30 h at 20 °C to cleanly give the new
azabutadiene MesNdC(But)C(H)dC(H)But via a CÀN cleav-
age process. Compound 4 similarly decomposes, but over ca.
48 h, to give DipNdC(But)C(H)dC(H)But.13 It is likely that
the reasons for the differing formations of 6 and 7 from seemingly
very similar reactions are equivalent to those mentioned above
for the reactions that gave 1 and 4. However, in the present case,
following the reaction that ultimately gave 7 by 1H NMR
spectroscopy confirmed that [(ButMesNacnac)GeH] (8) is an
intermediate in the reaction and that this compound rapidly
isomerizes to 7 at 20 °C. What should be noted from these results
is that relatively small differences in the steric bulk of β-
diketiminate ligands can lead to major differences in their ability
to stabilize low-oxidation-state p-block systems.
Scheme 1
The characterization data for 6 and 7 are comparable with
those for 1 and 4. For example, a hydride resonance is seen in the
1H NMR spectrum of 6 (δ 8.25 ppm) at a field similar to that of 1
(δ 8.08 ppm). In addition, both compounds exhibit GeÀH
stretching bands in their infrared spectra at wavenumbers (6, ν
1722 cmÀ1; 1, ν 1733 cmÀ1) lower than are normally seen for
germanium(IV) hydrides (1953À2175 cmÀ1).8b There is no
spectroscopic evidence for the presence of a hydride ligand in 7;
instead, its 1H NMR spectrum displays an AB coupling pattern
for its two ligand backbone protons. The asymmetrical nature of
the diamido ligand of 7 is also reflected in its 13C{1H} NMR
spectrum.
The differences between 6 and 7 are very evident from their
molecular structures, which are depicted in Figure 1. The
structure of 6 is similar to that for 1 and shows it to be monomeric
with a close to planar NC3N fragment, the bond lengths within
which imply significant π delocalization. The Ge atom is
displaced from the ligand plane by 0.65 Å and is coordinated
by two N centers and a hydride ligand, the latter of which was
located from difference maps and refined isotropically. The
angles about the Ge atom of the molecule are all close to 90°,
which possibly indicates that the Ge lone pair has high s
character. It is clear from the interatomic distances within the
NC3N fragment of 7 that C(2)ÀC(3) is a localized double bond,
while the other three interactions are single bonds. This situation
has arisen from protonation of C(1), which also causes the ligand
backbone of 7 to deviate significantly away from planarity, as was
the case for 4. It is of note that C(1) is a chiral center. Both the
GeÀN distances in 7 are considerably smaller than those in 6,
while its NÀGeÀN angle is more obtuse (cf. GeÀN = 1.865 Å
mean and NÀGeÀN = 95.84(8)° for the related diamidoger-
mylene 2).11 These differences can be explained by the lower
coordination number of the Ge center in 7 and the likely higher
degree of s character for the GeÀN interactions that would be
expected for this compound.
of 1 and 4 and the reversible hydrometalation of a phosphaalkyne by
the germanium(II) hydride compound.
’ RESULTS AND DISCUSSION
The β-diketiminate ligands chosen for this study, [{N(Mes)
C(R)}2CH]À (Mes = mesityl, R = Me (MeMesNacnac), But
(
ButMesNacnac)),14 differ from those used in the preparation of
1À4 in that they include smaller mesityl substituents at their N
centers. It was believed this would lead to germanium(II) hydride
complexes derived from the ligands displaying a greater, and
potentially broader, reactivity than 1. The germanium(II) chlo-
ride precursors to the target complexes, [(MeMesNacnac)GeCl]15
and [(ButMesNacnac)GeCl] (5), were initially prepared via reac-
tions of the appropriate β-diketiminato lithium compounds16 with
GeCl2 (dioxane). The complex [(ButMesNacnac)GeCl] (5) has
3
not been previously reported, though its characterization data are
consistent with it possessing a monomeric, three-coordinate stru-
cture similar to those of [(MeMesNacnac)GeCl],15 [(DipNacnac)-
GeCl],17 and [(ButNacnac)GeCl].18
The reactions of [(MeMesNacnac)GeCl] and [(ButMes-
Nacnac)GeCl] with 1 equiv of K[HBEt3] in toluene/THF led
to differing outcomes, namely the isolation of moderate (65%)
and low (32%) yields of orange [(MeMesNacnac)GeH] (6) and
yellow [(ButMesNacnacH)Ge:] (7) (ButMesNacnacH = [N(Mes)
C(H)(But)C(H)C(But)N(Mes)]2À), respectively (Scheme 1).
That said, spectroscopic analyses of both total reaction mixtures
prior to workup suggested that 6 and 7 are the major (>80%)
germanium-containing products. The low isolated yield of
Germanium(IV) hydrides have been widely used in synthetic
transformations, including hydrogermylations of unsaturated
substrates.20 To the best of our knowledge, the only reports
of hydrogermylations involving a germanium(II) hydride are
those mentioned above for 1. So as to compare the reactivity of
6 with 1, we chose to investigate its reactions with CO2 and the
phosphaalkyne PtCBut. It has previously been shown that 1
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Figure 1. Thermal ellipsoid plot (25% probability surface) of the molecular structures of (a) 6 and (b) 7. Hydrogen atoms (except the hydride ligand of
6 and H(1) and H(2) of 7) are omitted. Selected interatomic distances (Å) and angles (deg) for 6: Ge(1)ÀN(2) = 1.977(2), Ge(1)ÀN(1) = 1.991(2),
Ge(1)ÀH(1) = 1.569(10), N(1)ÀC(2) = 1.329(3), N(2)ÀC(4) = 1.336(3), C(2)ÀC(3) = 1.394(4), C(3)ÀC(4) = 1.394(4); N(2)ÀGe(1)ÀN(1) =
90.70(9), N(2)ÀGe(1)ÀH(1) = 90.5(13), N(1)ÀGe(1)ÀH(1) = 91.6(12). Selected interatomic distances (Å) and angles (deg) for 7: Ge(1)ÀN(1) =
1.8297(12), Ge(1)ÀN(2) = 1.8635(11), N(1)ÀC(1) = 1.4789(18), N(2)ÀC(3) = 1.4242(18), C(1)ÀC(2) = 1.510(2), C(2)ÀC(3) = 1.344(2);
N(1)ÀGe(1)ÀN(2) = 98.16(5), N(1)ÀC(1)ÀC(2) = 112.30(12), C(3)ÀC(2)ÀC(1) = 131.29(13), C(2)ÀC(3)ÀN(2) = 119.71(12).
E isomer. It is clear from both reactions that 6 is significantly
more reactive than 1 toward hydrogermylation reactions, as
might be expected on steric grounds.
Scheme 2
The clean formation of the germanium phosphaalkenyl com-
plex 10 is unusual for a number of reasons. First, germylenes have
been shown to readily undergo cycloaddition reactions with
phosphaalkynes on a number of previous occasions,21 yet there is
no evidence for such reactivity here. Indeed, as was the case with
the CO2 reaction, the +2 oxidation state of germanium is retained
post-hydrogermylation of the phosphaalkyne. Second, although
hydrometalation reactions of phosphaalkynes have been
reported,22 they are rare and always give the P-phosphaalkenyl
product [LnMÀPdC(H)R], despite the established polarity of
alkyl-substituted phosphaalkynes, viz. δ+PtCδÀÀR.23 It should
also be noted that while hydrogermylations of phosphaalkynes
are unknown, one report on their hydrostannylation with tin(IV)
hydrides has suggested the formation of C-phosphaalkenyl
intermediates, R1 SnC(R2)dPH, though these were never iso-
3
lated and, instead, underwent further reactions.24
reacts with CO2 slowly at room temperature to give the
germylene ester of formic acid, [(DipNacnac)GeOC(dO)H],
in quantitative yield.9b In the current study we found that 1 does
not react with the phosphaalkyne PtCBut up to temperatures of
50 °C. The reactions of the less hindered 6 with both substrates
were carried out, and that with CO2 proceeded at temperatures
above À30 °C to give [(MeMesNacnac)GeOC(dO)H] (9)
regioselectively (Scheme 2). Although the isolated yield of this
The spectroscopic data for 9 and 10 are compatible with their
proposed formulations. Of most note are the 31P NMR spectra of
the Z and E isomers of 10, which exhibit low-field resonances at δ
253.9 and 190.0 ppm, respectively: i.e., in the known region for
phosphaalkenes and metallophosphaalkenes.23,25 The JPH cou-
pling constants for the compounds (144 and 96 Hz) are in the
normal range for one-bond interactions,23 and identical cou-
1
1
compound was 75%, monitoring the reaction by H NMR
plings were observed for the PH resonances in the H NMR
spectroscopy indicated that it is essentially quantitative. When
6 was treated with an excess of PtCBut at 20 °C, hydrogermyla-
tion of the phosphaalkyne slowly occurred (over ca. 3 days) to
give the C-phosphaalkenyl complex 10 exclusively as its Z isomer
in a good isolated yield as a dark red crystalline solid. When the
reaction was carried out at 35 °C, it was complete within 15 h,
though in that case the product cocrystallized with ca. 10% of its
spectra of a cocrystallized mixture of the complexes (at δ 8.22
and 8.02 ppm, respectively). Moreover, a band in the infrared
spectrum of a pure sample of (Z)-10 at 2357 cmÀ1 (Nujol mull)
was assigned to its PÀH stretching mode.
The molecular structures of 9 and (Z)-10 are depicted in
Figures 2 and 3, respectively. The structure of compound 9 is
very similar to that previously reported for [(DipNacnac)GeOC
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Figure 3. Thermal ellipsoid plot (25% probability surface) of the
molecular structure of (Z)-10. Hydrogen atoms (except H(1)) are
omitted. Selected interatomic distances (Å) and angles (deg): Ge-
(1)ÀN(2) = 2.0041(18), Ge(1)ÀN(1) = 2.0259(17), Ge(1)ÀC(24)
= 2.058(2), P(1)ÀC(24) = 1.676(2), P(1)ÀH(1) = 1.30(3), N(1)ÀC-
(2) = 1.328(3), N(2)ÀC(4) = 1.337(3), C(2)ÀC(3) = 1.395(3),
C(3)ÀC(4) = 1.399(3); N(2)ÀGe(1)ÀN(1) = 90.58(7), N(2)À
Ge(1)ÀC(24) = 101.14(8), N(1)ÀGe(1)ÀC(24) = 96.04(8), C(24)À
P(1)ÀH(1) = 99.9(13), P(1)ÀC(24)ÀGe(1) = 125.99(12).
Figure 2. Thermal ellipsoid plot (25% probability surface) of the
molecular structure of 9. Hydrogen atoms (except H(24)) are omitted.
Selected interatomic distances (Å) and angles (deg): Ge(1)ÀO(1) =
1.9514(18), Ge(1)ÀN(1) = 1.963(2), Ge(1)ÀN(2) = 1.965(2), O-
(1)ÀC(24) = 1.286(3), O(2)ÀC(24) = 1.202(3), N(1)ÀC(2) =
1.324(4), N(2)ÀC(4) = 1.338(4), C(2)ÀC(3) = 1.393(4), C(3)ÀC-
(4) = 1.382(4); N(1)ÀGe(1)ÀN(2) = 90.52(9), O(1)ÀGe(1)ÀN(1) =
88.79(8), O(1)ÀGe(1)ÀN(2) = 88.36(8), C(24)ÀO(1)ÀGe(1) =
120.87(17), O(2)ÀC(24)ÀO(1) = 128.0(3).
1
[(MeMesNacnac)GeH] was seen in the H NMR spectrum of
(dO)H],9b while the structure of (Z)-10 represents the first for a
C-metallophosphaalkene derived from a hydrometalation reaction.
The NC3N backbones of the β-diketiminate ligands of both 9 and
(Z)-10 appear to be largely delocalized. Their Ge centers have
similar coordination geometries with GeÀN distances and
NÀGeÀN angles which are comparable with each other and to
those in the structureof 6. The phosphaalkenyl fragment of (Z)-10
exhibits a PÀC distance (1.676(2) Å) which is fully consistentwith
a localizeddouble-bondedinteraction,23,25 while the locationofthe
hydride ligand from difference maps, and its subsequent unrest-
rained isotropic refinement, allowed the assignment of the stereo-
chemistry of the compound. It is clear from Figure 3 that, for steric
reasons, the Z isomer of 10 should be the preferred form, though
even in this case the hydride ligand, H(1), has a relatively close
interaction with Ge(1) (3.13(3) Å).
the sample after cooling. Given these observations, it can be
proposed that upon heating the close proximity of the hydride
ligand of (Z)-10 to its germanium center allows an intramole-
cular nucleophilic attack at that center to give PtCBut and
[(MeMesNacnac)GeH] via a β-hydrogen elimination-like pro-
cess. The two compounds then react to regenerate (Z)-10 upon
cooling. It seems that the elimination of PtCBut from (E)-10 is
not facile, probably because its hydride ligand is directed away
from its germanium center.
When the sample of 10 was heated to 80 °C, a number of other
changes to its 31P{1H} NMR spectrum occurred. After 40 min at
this temperature, the signals corresponding to PtCBut and both
isomers of 10 had almost vanished and a new signal (A) appeared
at δ 334.4 ppm (N.B. a trace of this signal is present at 60 °C). In
the corresponding proton-coupled spectrum, this appears as a
doublet with a JPH coupling constant of 21 Hz, which is within the
two-bond range for phosphaalkenes.23 We propose that this
signal corresponds to the P-phosphaalkenyl germanium complex
[(MeMesNacnac)GePdC(H)(But)] (11), which on the basis of
steric arguments should preferentially exist in its E isomeric form.
In fact, the 2JPH coupling constant observed for this compound
would be expected to be larger (ca. 30À50 Hz) for its Z isomer,23
in which the alkenic proton and P lone pair are cis to each other.
When the solution containing (E)-11 was cooled to room
temperature and let stand for 1 h, there was no change in its
31P{1H} NMR spectrum, which indicates that it is stable to
isomerization back to 10.
The molecular structure of 10 could provide an explanation
for an unusual observation that was made when a crystalline
sample of the cocrystallized major (Z) and minor (E) isomers of
the compound was dissolved in C6D6 and gently heated (see
Figure 4). Even at temperatures as low as 30 °C a small resonance
(D) appeared at δ À67 ppm in the 31P{1H} NMR spectrum, in
addition to the signals for (Z)-10 (B) and (E)-10 (C). Resonance
D corresponds to the free phosphaalkyne PtCBut. This,
1
in combination with the fact that the H NMR spectrum of
the sample at that temperature showed the generation of
[(MeMesNacnac)GeH] (6), implies that the hydrogermylation
reaction that gave 10 is reversible under mild conditions. Indeed,
as the sample was heated to 60 °C, the signal for the phosphaal-
kyne grew at the expense of that for (Z)-10. Interestingly, the
relative intensity of the signal for (E)-10 changed little over the
temperature range. Moreover, when the sample was cooled back
to 20 °C and let stand for 1 h, the signal for the phosphaalkyne
vanished and that for (Z)-10 increased in intensity, while no
It is unknown how (E)-11 is formed, but one possibility is that
it is the thermodynamic product of the original phosphaalkyne
hydrogermylation, whereas 10 is the kinetic product. If so, at
temperatures above 60 °C the free phosphaalkyne PtCBut and
[(MeMesNacnac)GeH] react to give (E)-11, thereby altering the
equilibrium of the system, which leads to the eventual consumption
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Figure 4. Solution 31P{1H} NMR spectra of a cocrystallized mixture of (Z)-10 and (E)-10 recorded over the temperature range 30À80 °C (spectra
recorded after ca. 35 min at each temperature).
31P NMR chemical shifts for the three experimentally observed
isomers are reassuringly close to those recorded in the variable-
temperature NMR experiment, whereas no signal was seen in
those spectra at a field similar to that calculated for (Z)-11. This
strongly suggests that this isomer is not formed in significant
quantities during the thermal isomerization of 10.
Table 1. Calculated Relative Energies (kcal/mol) and Cal-
culated (Gas-Phase) and Observed (C6D6 Solution) 31P NMR
Chemical Shifts (ppm) for (Z/E)-10 and (Z/E)-11
31P NMR chemical shift
compd
rel energy
calcd
measd
(Z)-10
(E)-10
(Z)-11
(E)-11
+19.45
+20.18
+6.09
0.00
267.1
201.5
359.0
327.0
253.9
’ CONCLUSIONS
190.0
In summary, the second example of a monomeric N-heterocyclic
germanium(II) hydride compound, [(MeMesNacnac)GeH], has
been prepared and shown to be more reactive than the previously
reported and sterically more bulky system [(DipNacnac)GeH].
Attempts to prepare and isolate the slightly bulkier system
[(ButMesNacnac)GeH] were not successful. This compound was,
however, spectroscopically observed and found to rearrange via a
1,3-hydrogen migration to give the novel diamido germylene
[(ButMesNacnacH)Ge:]. These results show that small differences
in the steric bulk of β-diketiminate ligands can lead to major
differences in their ability to stabilize low-oxidation-state germanium
systems.
not obsd
334.4
of all (Z)-10. One explanation for the loss of (E)-10 from the
system at 80 °C is that it thermally isomerizes to (Z)-10 at that
temperature, which then converts to (E)-11 by the process
mentioned above. Similar thermal isomerizations of phosphaalk-
enes, at even lower temperatures, have been reported.26 Although
these proposals are compelling, above 60 °C the integration of
signal A is much lower than would be expected for the clean
conversion of 10 to (E)-11. This, combined with the fact that the
1H NMR spectrum of the sample revealed several unidentifiable
products, suggests that other processes are involved at higher
temperatures.
In order to ascertain the relative energies of the four isomers
(Z/E)-10 and (Z/E)-11, their gas-phase structures were opti-
mized using DFT calculations (RI-DFT/BP86/def2-SVP). Once
optimized, the isotropic 31P NMR chemical shift was calculated
for each isomer using the Turbomole program. The results of
these calculations are summarized in Table 1. It is first worth
noting that the calculated optimized geometry of (Z)-10 is close
to that of the compound in the solid state (see the Supporting
Information). From Table 1 it can be seen that the two isomers of
the C-germaphosphaalkene 10 are significantly higher in energy
than those of the P-germaphosphaalkene 11. Within these two
sets of isomers, (Z)-10 and (E)-11 lie at the lowest energies. Both
of these results are consistent with our proposal that (Z)-10 is the
kinetic product of the original phosphaalkyne hydrometalation,
whereas (E)-11 is the thermodynamic product. The calculated
Preliminary studies of the reactivity of [(MeMesNacnac)GeH]
toward the hydrogermylation of unsaturated substrates have
been carried out. It has been shown that reactions with CO2
and PtCBut proceed at or below ambient temperature, in the
absence of a catalyst, to give the hydrogermylation products
[(MeMesNacnac)GeOC(dO)H] and [(MeMesNacnac)GeC-
(But)dPH] with complete regioselectivity. When a solution of
the latter product was gently heated, the hydrogermylation
reaction was partially reversed, regenerating [(MeMesNacnac)-
GeH] and PtCBut. Heating to higher temperatures led to the
irreversible isomerization of [(MeMesNacnac)GeC(But)=PH] to
its P-germaphosphaalkenyl regioisomer (E)-[(MeMesNacnac)-
GePdC(H)(But)], theoretical studies of which indicate is the
thermodynamic product of the original hydrometalation reac-
tion. This study has highlighted the potential versatility of
[(MeMesNacnac)GeH] as an effective hydrogermylation re-
agent. This potential is currently being further developed in
our laboratory.
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’ EXPERIMENTAL SECTION
extracted with hexane (10 mL), the extract was filtered, and the filtrate
was placed at À30 °C to yield pale yellow crystals of 7 (0.06 g, 32%). Mp:
1
General Methods. All manipulations were carried out using
standard Schlenk and glovebox techniques under an atmosphere of
high-purity dinitrogen. Hexane and toluene were distilled over potas-
sium, while diethyl ether was distilled over Na/K alloy. 1H, 13C{1H} and
31P{1H} NMR spectra were recorded on either Bruker DPX300 or
AvanceIII 400 spectrometers and were referenced to the resonances of
the solvent used or external H3PO4 (85% solution in D2O). Mass spectra
were obtained from the EPSRC National Mass Spectrometric Service at
Swansea University. IR spectra were recorded using a Perkin-Elmer RX1
FT-IR spectrometer as Nujol mulls between NaCl plates. Microanalyses
were carried out by Campbell Microanalytical, Otago, New Zealand, or
at the Science Centre, London Metropolitan University. Melting points
were determined in sealed glass capillaries under dinitrogen and are
uncorrected. The compounds [(MeMesNacnac)GeCl]15 and (ButMes-
Nacnac)H14 were prepared by variations of the literature procedures. The
phosphaalkyne PtCBut was synthesized by the [Li{N(SiMe3)2}]-cata-
lyzed elimination of hexamethyldisiloxane from (Me3Si)PdC(But)
(OSiMe3).27 All other reagents were used as received.
164À167 °C dec. H NMR (300 MHz, C6D6, 298 K): 1.06 (s, 9H,
C(CH3)3), 1.09 (s, 9H, C(CH3)3), 2.11, 2.16, 2.25, 2.41, 2.52, 2.57 (6 Â
s, 6 Â 3H, 6 Â ArÀCH3), 4.03 (d, 1H, 3JHH = 6.9 Hz, CHBut), 4.87 (d,
3
1H, JHH = 6.9 Hz, NCCHCN), 6.75À7.03 (m, 4H, Ar-H). 13C{1H}
NMR (400 MHz, C6D6, 300 K): 20.0, 20.8, 20.9, 21.1, 21.2, 21.8 (6 Â
Ar-CH3), 27.3, 31.8 (2 Â C(CH3)3), 38.4, 40.2 (2 Â C(CH3)3), 72.6
(CHBut), 102.2 (NCCHCN), 129.6, 129.7, 130.2, 131.0, 132.5, 133.2,
135.3, 136.7, 139.2, 143.5, 147.6, 149.2 (Ar-C), NC(But)CH signal
not observed. IR ν/cmÀ1 (Nujol): 1377 s, 1260 s, 864 m, 721 m, 665 m,
660 m. MS (EI 70 eV) m/z (%): 491.2 (M+, 3), 228.1 (MesNC
(But)C(H)C(H)+, 100). Anal. Calcd for C29H42GeN2: C, 70.90; H,
8.62; N, 5.70. Found: C, 70.81; H, 8.68; N, 5.80.
N.B. An aliquot of the reaction mixture was taken when it reached
À30 °C. Volatiles were removed from this in vacuo, and the residue was
dissolved in C6D6. A 1H NMR spectrum of this was sample was
immediately acquired and showed an approximately 80:20 mixture of
[(ButMesNacnac)GeH] (8) and [(ButMesNacnacH)Ge:] (7). After a
further 10 min at 20 °C all [(ButMesNacnac)GeH] had converted to
[(ButMesNacnacH)Ge:]. Attempts to isolate and crystallize [(ButMes-
Nacnac)GeH] were not successful. Reliable 1H NMR data for the
compound were, however, extractable from the aforementioned initial
Preparation of [(ButMesNacnac)GeCl] (5). To as solution of
(
ButMesNacnac)H (1.0 g, 2.37 mmol) in diethyl ether (15 mL) at
À78 °C was added a 1.6 M solution of n-butyllithium in hexane (1.6 mL,
2.56 mmol), whereupon the reaction solution was warmed to room
temperature and stirred for 1 h. The resultant solution was added over
5 min to a precooled (À78 °C) suspension of GeCl2.(dioxane) (0.55 g,
2.37 mmol) in diethyl ether (15 mL). The reaction mixture was warmed
to ambient temperature and stirred for 15 h. All volatiles were subse-
quently removed in vacuo, the residue was extracted into hexane
(20 mL), the extract was filtered, and the filtrate was stored at
À30 °C. Pale yellow crystals of 5 deposited overnight (1.02 g, 82%).
1
spectrum. Data for [(ButMesNacnac)GeH] (8): H NMR (300 MHz,
C6D6, 298 K) 1.10 (s, 18H, C(CH3)3), 2.08, 2.25, 2.42 (3 Â s, 3 Â 6H,
o-CH3, p-CH3), 5.94 (s, 1H, NCCHCN), 6.67 (br s, 4H, Ar-H), 7.94
(s, 1H, GeH).
N.B. When solutions of 7 in C6D6 were left to stand at ambient
temperature for 30 h, they decomposed to give mixtures of the new
azabutadiene MesNdC(But)C(H)dC(H)But and other unidentifiable
products. The 1H and 13C{1H} NMR spectroscopic data for this
compound could be reliably assigned from analyses of the spectra of
the mixture and are comparable to those previously reported for
DipNdC(But)C(H)dC(H)But.13 Data for MesNdC(But)C(H)dC-
(H)But: 1H NMR (300 MHz, C6D6, 298 K) 0.66 (s, 9H, C(CH3)3), 1.27
(s, 9H, C(CH3)3), 1.98 (s, 6H, o-CH3), 2.15 (s, 3H, p-CH3), 5.47 (d, 1H,
3JHH= 17.8 Hz, CHBut), 5.65 (d, 1H, 3JHH= 17.8 Hz, NCCHCN), 6.72
(s, 2H, Ar-H); 13C{1H} NMR (400 MHz, C6D6, 300 K) 16.8 (o-CH3),
19.5 (p-CH3), 27.2, 27.5 (2 Â C(CH3)3), 31.8, 38.9 (2 Â C(CH3)3),
116.2 (CHBut), 123.5 (NCCHCN), 127.3, 129.4, 146.2, 147.2 (Ar-C),
172.9 (NC(But)CH).
1
Mp: 173À175 °C. H NMR (300 MHz, C6D6, 298 K): 1.09 (s, 18H,
C(CH3)3), 2.00, 2.06, 2.70 (3 Â s, 3 Â 6H, o-CH3, p-CH3), 6.25 (s, 1H,
NCCHCN), 6.63 (s, 2H, Ar-H), 6.67 (s, 2H, Ar-H). 13C{1H} NMR
(400 MHz, C6D6, 300 K): 19.9, 20.6, 21.4 (o-CH3, p-CH3), 31.1
(C(CH3)3), 41.4 (C(CH3)3), 104.9 (NCCHCN), 128.9, 130.3, 133.6,
134.7, 135.4, 142.2 (Ar-C), 171.3 (NCCH); IR ν/cmÀ1 (Nujol): 1377 s,
1260 s, 861 w, 722 w, 682 w, 660 w. MS (EI 70 eV) m/z (%): 526.2 (M+,
45), 491.3 (M+ À Cl, 28), 202.1 (MesNCBut+, 100). Anal. Calcd for
C29H41ClGeN2: C, 66.25; H, 7.86; N, 5.33. Found: C, 66.23; H, 7.94;
N, 5.24.
Preparation of [(MeMesNacnac)GeOC(dO)H] (9). A solution
of [(MeMesNacnac)GeH] (0.12 g, 0.30 mmol) in toluene (10 mL) at
À80 °C in a 250 mL Schlenk flask was placed under an atmosphere of
CO2 gas (predried by passage over P2O5). The orange reaction solution
was stirred and warmed slowly to ambient temperature, during which
time (at ca. À30 °C) the solution become colorless. The resultant
colorless solution was concentrated in vacuo to ca. 4 mL and layered
with hexane (15 mL) to afford colorless crystals of 9 (0.10 g, 75%); Mp:
175À178 °C. 1H NMR (400 MHz, C6D6, 298 K): 1.52 (s, 6H, NCCH3),
2.06, 2.09, 2.36 (3 Â s, 3 Â 6H, o-CH3, p-CH3), 5.04 (s, 1H,
NCCHCN), 6.72 (s, 2H, Ar-H), 6.75 (s, 2H, Ar-H), 8.64 (1H, OCH).
13C{1H} NMR (400 MHz, C6D6, 300 K): 18.5, 19.0, 20.7 (o-CH3,
p-CH3), 22.3 (NCCH3), 99.1 (NCCHCN), 129.4, 130.4, 132.5, 135.1,
136.2, 140.2 (Ar-C), 164.5 (NCCH), 164.7 (O(CO)H). IR ν/cmÀ1
(Nujol): 1654 m (CdO), 1376 s, 1260 s, 862 w, 722 m. MS (EI 95 eV)
m/z (%): 452.2 (M+, 20), 407.2 (M+ À CO2H, 100). Anal. Calcd for
C24H30GeN2O2: C, 63.90; H, 6.70; N, 6.21. Found: C, 63.64; H, 7.08;
N, 6.20.
Synthesis of [(MeMesNacnac)GeH] (6). To a solution of
[(MeMesNacnac)GeCl] (0.20 g, 0.45 mmol) in toluene (20 mL) at
À80 °C was added a 1.0 M solution of K[HBEt3] in THF (0.45 mL, 0.45
mmol) over 5 min. The resultant mixture was warmed to ambient
temperature and stirred overnight. All volatiles were subsequently
removed in vacuo, the residue was extracted into hexane (30 mL),
and the extract was filtered and stored at À30 °C overnight to yield
orange crystals of 6 (0.12 g, 65%). Mp: 160À162 °C dec. 1H NMR (300
MHz, C6D6, 298 K): 1.52 (s, 6H, NCCH3), 2.09, 2.29, 2.31 (3 Â s, 3 Â
6H, o-CH3, p-CH3), 4.92 (s, 1H, NCCHCN), 6.67 (s, 2H, Ar-H), 6.70
(s, 2H, Ar-H), 8.25 (s, 1H, GeH). 13C{1H} NMR (300 MHz, C6D6,
300 K): 18.3, 18.6, 20.7 (o-CH3, p-CH3), 22.0 (NCCH3), 97.5
(NCCHCN), 129.3, 129.7, 132.7, 134.9, 135.2, 142.0 (Ar-C), 166.3
(NCCH). IR ν/cmÀ1 (Nujol): 1722 s (GeÀH), 1525 s, 1376 s, 1147 m,
1015 m, 859 m, 706 m. MS (EI, 70 eV) m/z (%): 407.2 (M+, 100). HREI
m/z: acc mass calcd for C23H3070GeN2, 403.1571; found, 403.1568.
Anal. Calcd for C23H30GeN2: C, 67.86; H, 7.43; N, 6.88. Found: C,
67.71; H, 7.34; N, 6.82.
Preparation of [(ButMesNacnacH)Ge:] (7). A solution of
[(ButMesNacnac)GeCl] (0.20 g, 0.38 mmol) in toluene (20 mL) at
À78 °C was treated with a 1.0 M solution of K[HBEt3] in THF (0.4 mL,
0.40 mmol). The resultant mixture was warmed to room temperature,
whereupon all volatiles were removed in vacuo. The yellow residue was
Preparation of [(MeMesNacnac)GeC(But)dPH] (10). Neat
PtCBut (0.17 mL, 1.03 mmol) was added to a solution of [(MeMesNacnac)-
GeH] (0.143 g, 0.35 mmol) in toluene (10 mL) at 20 °C. The reaction
solution was warmed to 35 °C and stirred for 15 h, during which time the
mixture slowly turned deep red. Volatiles were removed in vacuo, and
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ARTICLE
Table 2. Summary of Crystallographic Data for 6, 7, 9, and (Z)-10
6
7
9
(Z)-10
empirical formula
formula wt
cryst syst
space group
a (Å)
C
23H30GeN2
C29H42GeN2
491.24
C24H30GeN2O2
451.09
orthorhombic
P212121
7.5831(4)
14.1863(5)
20.696(2)
90
C28H39GeN2P
507.17
monoclinic
P21/n
407.08
monoclinic
P21/n
triclinic
P1
7.1930(14)
24.351(5)
12.147(2)
90
9.1645(3)
11.2956(2)
12.9117(4)
87.869(2)
88.142(2)
85.141(2)
1330.30(6)
2
11.739(2)
7.4260(15)
31.153(6)
90
b (Å)
c (Å)
α (deg.)
β (deg)
99.41(3)
90
90
94.95(3)
90
γ (deg)
vol (Å3)
90
2099.0(7)
4
2226.4(3)
4
2705.6(9)
4
Z
F (calcd) (g cmÀ3
)
1.288
1.226
1.346
1.245
μ (mmÀ1
F(000)
T (K)
)
1.468
1.169
1.398
1.209
856
524
944
1072
150(2)
8572
123(2)
21 157
123(2)
8233
123(2)
11 114
no. of rflns collected
no. of unique rflns
Rint
4627
5812
4352
5960
0.0505
0.0467
0.1116
0.67, À0.81
842376
0.0249
0.0306
0.0313
0.0636
0.37, À0.27
842378
0.0288
0.0339
0.0895
0.64, À0.41
842379
R1 indices (I > 2σ(I))
wR2 indices (all data)
largest peak and hole (e ÅÀ3
CCDC no.
0.0267
0.0681
)
0.34, À0.27
842377
the residue was extracted into hexane (5 mL). The extract was filtered
and the filtrate stored at À30 °C overnight to yield dark red crystals of 10
(0.12 g, 67%). Mp: 140À142 °C. Data for (Z)-10: 1H NMR (300 MHz,
C6D6, 298 K) 1.29 (s, 9H, C(CH3)3), 1.53 (s, 6H, NCCH3), 2.08, 2.38,
2.48 (3 Â s, 3 Â 6H, o-CH3, p-CH3), 4.65 (s, 1H, NCCHCN), 6.75
of 9 is À0.027(9). Crystal data and details of data collection and refine-
ment for 6, 7, 9 ,and (Z)-10 are given in Table 2.
Theoretical Studies. Quantum-chemical calculations were carried
out with the Turbomole program package,29 employing the BeckeÀ
Perdew 86 functional30 at the RI-DFT level31 with the def2-SVP basis
set.32 Isotropic NMR shieldings were calculated with the NMR script of
Turbomole; PH3 (calculated isotropic shielding 599.7 ppm; chemical shift
δ À238 ppm) was used as a standard for the 31P NMR calculations.
1
(s, 2H, Ar-H), 6.78 (s, 2H, Ar-H), 8.22 (d, 1H, JPH = 144 Hz, PH);
13C{1H} NMR (400 MHz, C6D6, 300 K): 18.7, 19.5, 20.0 (o-CH3,
2
p-CH3) 21.8 (NCCH3), 29.9 (C(CH3)3), 44.4 (d, JCP = 17 Hz,
C(CH3)3), 94.2 (NCCHCN), 128.7, 128.8, 131.9, 133.9, 134.1, 140.4
(Ar-C), 164.6 (NCCH), PdC resonance not observed; 31P NMR
’ ASSOCIATED CONTENT
1
(400 MHz, C6D6, 300 K) δ 253.9 (d, JPH = 144 Hz); IR ν/cmÀ1
(Nujol) 2357 m (PÀH), 1524 s, 1377 s, 1260 s, 862 m, 722 m, 660 w;
MS (EI 70 eV) m/z (%) 508.2 (M+, 5), 407.1 (M+ À HPCBut, 100).
Anal. Calcd for C28H39GeN2P: C, 66.30; H, 7.75; N, 5.52. Found: C,
66.42; H, 8.05; N, 5.32.
S
Supporting Information. CIF files giving crystallographic
b
data for 6, 7, 9, (Z)-10, [(MeMesNacnac)Li(tmeda)], and
[(MeMesNacnac)SnCl] tables and figures giving crystal data and
details of data collection and refinement and ORTEP diagrams
for [(MeMesNacnac)Li(tmeda)] and [(MeMesNacnac)SnCl],
and tables giving further details of the calculations. This material
is available free of charge via the Internet at http://pubs.acs.org.
N.B. Some NMR spectroscopic data for (E)-10 and (E)-11 could be
reliably extracted from the spectra of the compound mixtures resulting
from the variable-temperature NMRexperiment. These compoundscould
not be isolated, despite several attempts. Data for (E)-10: 1H NMR (300
MHz, C6D6, 298 K) 1.25 (s, 9H, C(CH3)3), 1.59 (s, 6H, NCCH3), 2.06,
2.42, 2.45 (3 Â s, 3 Â 6H, o-CH3, p-CH3), 4.95 (s, 1H, NCCHCN), Ar-H
’ AUTHOR INFORMATION
signals hidden below that of (Z)-10, 8.02 (d, 1H, 1JPH = 96 Hz, PH); 31
P
Corresponding Author
*E-mail: cameron.jones@monash.edu.
NMR (400 MHz, C6D6, 300 K) δ 190.0 (d, 1JPH = 96 Hz). Data for (E)-
11: 31P NMR (400 MHz, C6D6, 300 K) δ 334.4 (2JPH = 21 Hz).
X-ray Crystallography. Crystalsof6, 7, 9, (Z)-10, [(MeMesNacnac)
Li(tmeda)], and [(MeMesNacnac)SnCl] suitable for X-ray structural
determination were mounted in silicone oil. Crystallographic measure-
ments were made with an Oxford Gemini Ultra diffractometer using a
graphite monochromator with Mo Kα radiation (λ = 0.710 73 Å). The
structures were solved by direct methods and refined on F2 by full-matrix
least squares (SHELX97)28 using all unique data. All non-hydrogen atoms
are anisotropic with hydrogen atoms included in calculated positions
(riding model), except the hydride ligands of 6 and (Z)-10, which were
refined isotropically. The absolute structure factor for the crystal structure
’ ACKNOWLEDGMENT
C.J. thanks the Australian Research Council (No. DP0665057),
the donors of The American Chemical Society Petroleum Re-
search Fund, and the U.S. Air Force Asian Office of Aerospace
Research and Development for financial support. C.S. thanks the
Alexander von Humboldt Foundation for a Feodor-Lynen Fellow-
ship. The EPSRC Mass Spectrometry Service at Swansea Uni-
versity is also thanked.
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ARTICLE
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