Preparation, characterization, and theoretical analysis of group 14 element(I) dimers: A case study of magnesium(I) compounds as reducing agents in inorganic synthesis

Article abstract of DOI:10.1021/ic200682p

A synthetic route to the new amidine (DipNH)(DipN)- C(C₆H ₄Bu^t-4) (ButisoH; Dip = C₆H₃Pr ⁱ ₂-2,6) has been developed. Its deprotonation with either LiBun or KN(SiMe₃)₂ yields the amidinate complexes [M(Butiso)] (M = Li or K). Their reactions with group 14 element halides/pseudohalides afford the heteroleptic group 14 complexes [(Butiso)SiCl3], [(Butiso)ECl] (E = Ge or Sn), and [{(Butiso)- Pb(μ-O ₃SCF₃)(THF)}∞], all of which have been crystallographically characterized. In addition, the synthesis and spectroscopic characterization of the homoleptic complex [Pb(Butiso)2] is reported. Reductions of the heteroleptic complexes with a soluble magnesium(I) dimer, [{(^MesNacnac)Mg}2] (^MesNacnac = [(^MesNCMe) ₂CH]; Mes = mesityl), have given moderate-to-high yields of the group 14 element(I) dimers [{(Butiso)E}₂] (E = Si, Ge, or Sn), the X-ray crystallographic studies of which reveal trans-bent structures. The corresponding lead(I) complex could not be prepared. Comprehensive spectroscopic and theoretical analyses of [{(Butiso)E}₂] have allowed their properties to be compared. All complexes possess E-E single bonds and can be considered as intramolecularly base-stabilized examples of ditetrelynes, REER. Taken as a whole, this study highlights the synthetic utility of soluble and easy to prepare magnesium(I) dimers as valuable alternatives to the harsh, and often insoluble, alkali-metal reducing agents that are currently widely employed in the synthesis of low-oxidation-state organometallic/inorganic complexes. 2011 American Chemical Society.

Full text of DOI:10.1021/ic200682p

FORUM ARTICLE
pubs.acs.org/IC
Preparation, Characterization, and Theoretical Analysis of Group 14
Element(I) Dimers: A Case Study of Magnesium(I) Compounds as
Reducing Agents in Inorganic Synthesis
Cameron Jones,*,† Simon J. Bonyhady,† Nicole Holzmann,‡ Gernot Frenking,*,‡ and Andreas Stasch*,†
†School of Chemistry, Monash University, P.O. Box 23, Clayton, Victoria 3800, Australia
‡Fachbereich Chemie, Philipps-Universit€at Marburg, 35032 Marburg, Germany
S Supporting Information
b
ABSTRACT: A synthetic route to the new amidine (DipNH)(DipN)-
C(C6H4But-4) (ButisoH; Dip = C6H3Pri2-2,6) has been developed. Its
deprotonation with either LiBun or KN(SiMe3)2 yields the amidinate
complexes [M(Butiso)] (M = Li or K). Their reactions with group 14
element halides/pseudohalides afford the heteroleptic group 14 com-
plexes [(Butiso)SiCl3], [(Butiso)ECl] (E = Ge or Sn), and [{(Butiso)-
Pb(μ-O3SCF3)(THF)}¥], all of which have been crystallographically
characterized. In addition, the synthesis and spectroscopic characteriza-
tion of the homoleptic complex [Pb(Butiso)2] is reported. Reductions
of the heteroleptic complexes with a soluble magnesium(I) dimer,
[{(MesNacnac)Mg}2] (MesNacnac = [(MesNCMe)2CH]ꢀ; Mes =
mesityl), have given moderate-to-high yields of the group 14 element(I)
dimers [{(Butiso)E}2] (E = Si, Ge, or Sn), the X-ray crystallographic
studies of which reveal trans-bent structures. The corresponding lead(I) complex could not be prepared. Comprehensive
spectroscopic and theoretical analyses of [{(Butiso)E}2] have allowed their properties to be compared. All complexes possess
EꢀE single bonds and can be considered as intramolecularly base-stabilized examples of ditetrelynes, REER. Taken as a whole, this
study highlights the synthetic utility of soluble and easy to prepare magnesium(I) dimers as valuable alternatives to the harsh,
and often insoluble, alkali-metal reducing agents that are currently widely employed in the synthesis of low-oxidation-state
organometallic/inorganic complexes.
’ INTRODUCTION
via the reduction of “normal oxidation state” precursor com-
plexes can be problematic. These syntheses generally involve
insoluble, and sometimes harsh, reducing agents, e.g., Li(s),
Na(s), K(s), KC8, and Mg(s), which can be difficult to use
stoichiometrically and which often lead to “over-reduction” of
the precursor complex and/or the occurrence of other side
reactions.5,6 Recently, the solubility and moderate reducing
nature of magnesium(I) dimers have lent them to use as
stoichiometric reagents for the syntheses of species that so far
have not been accessible by other routes. These include the
N-heterocyclic carbene (NHC) stabilized digermanium(0) com-
plex, 1 (via reduction of an NHC adduct of GeCl2),2d,7 and the
first examples of stable, neutral aluminum(II) hydride complexes,
2 and 3 (via reduction of aluminum(III) hydride precursors).2g,7
In order to further demonstrate the synthetic utility of
magnesium(I) dimers, we chose to investigate their use in the
preparation of a series of novel amidinato-coordinated group 14
element(I) dimers as a case study. The first example of such a
complex, trans-bent 4 (and a guanidinato-coordinated analogue, 5),
In 2007, the syntheses of the first room-temperature-stable
magnesium(I) complexes [LMgMgL] (L = [(DipN)2CNPri2]ꢀ,
Prisoꢀ, or[(DipNCMe)2CH]ꢀ, DipNacnacꢀ;Dip=C6H3Pri2-2,6),
were reported.1 Since that time, a variety of related dimers have
been forthcoming, and their further chemistry has begun to
develop.2ꢀ4 This work has revealed that perhaps the greatest
application potential that these “bottleable” magnesium(I) species
hold is as selective reducing agents for organic and organometallic
synthesis.2,3 In this respect, they possess a number of properties
that can make them appealing alternatives to the “traditional”
reducing agents (e.g., alkali metals, magnesium, KC8, SmI2, etc.)
that are widely employed by synthetic chemists. For example, they
are very thermally stable (decomposition > 300 ꢀC in some cases),
are only moderately air- and moisture-sensitive, are soluble in a
wide range of organic solvents, can be prepared in high yields on
gram scales, and do not present any significant fire or toxicity
hazards.
These properties are especially advantageous for the synthesis
of low-oxidation-state p-block compounds, the chemistry of
which has rapidly developed over the past 2 decades.5 Despite
the emerging importance of such compounds, their preparation
Received: April 2, 2011
Published: May 06, 2011
r
2011 American Chemical Society
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Chart 1
Scheme 1
complex crystallized from hexane and its X-ray crystal structure
was determined (see the Supporting Information). This showed
the complex to be dimeric, [Li{Li(k2-N,N0-Butiso)2}], with two
different lithium coordination environments, one chelated by two
localized Butisoꢀ ligands and one chelated by one N center from
each amidinate. This structural motif had been previously dis-
playedbya closelyrelatedlithium guanidinate complex, [Li{Li(k2-
N,N0-Giso)2}] (Gisoꢀ = [(DipN)2CN(C6H11)2]ꢀ.15a
was reported by us in 2006.8 Subsequently, Roesky et al.
described the related gauche-bent silicon(I) and germanium(I)
dimers, 69 and 7,10 respectively (Chart 1).11 Compounds 4ꢀ7
can be considered as intramolecularly base-stabilized examples of
the doubly or triply bonded trans-bent heavier group 14 alkyne
analogues, RGedGeR and RSitSiR (R = bulky terphenyl, aryl,
or silyl), the chemistry of which has been extensively explored by
the groups of Power, Sekiguchi, and others.12 Similarly, Roesky
et al.’s 1,2-ditetrelenes, 6 and 7, have begun to show their synthetic
worth, especially toward the activation of small molecules.13
Despite this, the preparations of 4ꢀ7 via the reduction of amidinato
element(II or IV) chloride precursors with potassium metal or KC8
are quite low-yielding (4, 16%; 5, 13%; cf. 6, 5%; 7, 35%), a situation
that will no doubt hamper the future expansion of their chemistry. It
seemed to us that using magnesium(I) dimers as alternative
reducing agents in similar reactions might allow us access to higher
yields of related silicon(I) and germanium(I) dimers. Moreover,
because tin(I) and lead(I) analogues of 4ꢀ7are currently unknown,
these were seen as worthwhile synthetic targets because their
properties and reactivity could be compared to those of their lighter
group 14 counterparts and to those of previously prepared dis-
tannyne and diplumbyne complexes, REER (E = Sn or Pb).12 Our
efforts in this direction are reported herein.
The preparation of the amidinato group 14 element chloride
complexes that were to be used as precursors to the target
element(I) dimers was achieved by the reaction of either SiCl4,
GeCl2 dioxane or SnCl2 with 1 equiv of the lithium or potassium
3
salt of Butisoꢀ in tetrahydrofuran (THF) or diethyl ether. The
products of these reactions, 8ꢀ10, were all obtained as colorless
crystalline solids in moderate yields (Scheme 1). Attempts
to form the analogous lead(II) complex [(Butiso)PbCl], via a
similar 1:1 reaction in THF, were not successful and instead led
to the homoleptic complex [Pb(Butiso)2] (11) and unreacted
PbCl2. Compound 11 probably originates from the initially
generated [(Butiso)PbCl], which, because of its greater solubility
than PbCl2, reacts further with [Li(Butiso)] to give 11. In order
to access a heteroleptic precursor complex related to [(Butiso)-
PbCl], 1 equiv of [K(Butiso)] was added to a THF solution of
[Pb(O3SCF3)2]. This afforded the polymeric complex
[{(Butiso)Pb(μ-O3SCF3)(THF)}¥] (12) in low isolated yield.
The solution-state NMR spectra of 8ꢀ10 are largely consis-
tent with their proposed structures. In the case of 8, a singlet
resonance was observed in its 29Si{1H} NMR spectrum at δ = ꢀ89.4
ppm, which is in the normal region for five-coordinate silicon
complexes (cf. δ = ꢀ98.9 ppm for [{PhC(NBut)2}SiCl3]).16
That said, the 1H and 13C{1H} NMR spectra of 8 are suggestive
of the compound possessing one chemically equivalent set of
methine protons and two sets of isopropylmethyl protons. This is
not what would be expected if the compound retained a rigid
trigonal-bipyramidal structure with axial and equatorial N centers
in solution (vide infra) and implies that a fluxional process, which
is rapid on the NMR time scale, is in operation. This likely
involves a scrambling of the coordinated N and Cl atoms of the
complex between its axial and equatorial sites, via an intermediate
square-pyramidal structure. Clearly, in solution, compound 12
does not retain its polymeric structure; however, its NMR spectra
are compatible with the Butisoꢀ ligand chelating its Pb center in
an N,N0 fashion, as is the case in the solid state (vide infra).
All complexes 8ꢀ10 and 12 were crystallographically char-
acterized, and the molecular structures of 8, 9, and 12 are
depicted in Figure 1 (see also Table 1). Because the structure
of compound 10 is isomorphous to that of 9, its molecular
’ RESULTS AND DISCUSSION
The amidinate ligand used in this study, [(DipN)2C(C6H4-
But-4)]ꢀ (Butisoꢀ), was chosen for its steric bulk, which could
potentially impart significant kinetic stability to low-valent group
14 compounds incorporating it, and because we have previously
found that complexes of Butisoꢀ readily crystallize from non-
coordinating solvents.14 The conjugate acid of the ligand,
ButisoH, was prepared in good yield by the reaction of 4-tert-
butylphenyllithium with the carbodiimide DipNdCdNDip,
followed by an aqueous workup. The solution NMR spectra of
ButisoH show the compound to exist as more than one isomer/
tautomer in solution, as is common for amidines and guanidines.15
In the solid state, an X-ray crystal structure of the compound (see
the Supporting Information) revealed it to cocrystallize as a
mixture of its (Z)-anti and (E)-syn isomeric forms. The amidine
can be readily deprotonated with LiBun or KN(SiMe3)2 to give
alkali-metal amidinate complexes, [M(Butiso)] (M = Li or K),
which can be used as transfer reagents in subsequent reactions
without purification. It is of note that on one occasion the lithium
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Figure 1. Thermal ellipsoid plots (25% probability surface) of the molecular structures of (a) 8, (b) 9, and (c) 12. The H atoms of all structures and the
isopropyl groups of 12 are omitted. Select metrical parameters for these compounds and for 10 are given in Table 1.
Table 1. Selected Interatomic Distances (Å) and Angles
structure (and that of a different structural modification of 8) can
be found in the Supporting Information. Complex 8 is mono-
meric and exhibits a trigonal-bipyramidal silicon coordination
geometry, with N1 and N2 taking up equatorial and axial sites,
respectively [N2ꢀSi1ꢀCl1 = 162.52(8)ꢀ]. The NCN fragment
of the amidinate ligand appears to be only partially delocalized,
and the overall structure of the complex is very similar to that of
[{PhC(NBut)2}SiCl3].17 Complexes 9 and 10 are also mono-
meric with distorted pyramidal Ge or Sn centers, the geometries
of which [Σ angles = 257.3ꢀ (9), 249.5ꢀ (10)] indicate that they
possess high-s-character lone pairs. These structures are closely
related to previously reported amidinato and guanidinato com-
plexes, e.g., [(Giso)ECl] and [(Piso)ECl] (E = Ge or Sn; Pisoꢀ =
[(DipN)2CBut]ꢀ).8,18 Compound 12 forms one-dimensional
polymeric strands in the solid state, with each five-coordinate Pb
center being ligated by O atꢀoms from two bridging triflate anions,
one N,N0-chelating Butiso ligand, and one molecule of THF.
The structure is similar to that of the four-coordinate triflate-
bridged polymer [{(DipNacnac)Pb(μ-O3SCF3)}¥],19 and its
(deg) for 8ꢀ10 and 12
8
9
10
12
EꢀN1
EꢀN2
EꢀXa
1.7693(14)
1.9912(14)
2.074(3)
2.071(3)
2.229(2)
2.243(3)
2.289(7)
2.427(7)
2.0427(6) (Cl1) 2.249(3)
2.0526(7) (Cl2)
2.4059(19) 2.595(12) (O1)
2.727(7) (O30)b
2.1293(6) (Cl3)
2.700(8) (O4)
C1ꢀN1
C1ꢀN2
1.362(2)
1.309(2)
1.332(5)
1.342(5)
1.338(4)
1.333(4)
1.338(11)
1.315(11)
NꢀEꢀN 68.62(6)
63.32(12) 59.06(8)
108.9(3) 111.2(3)
56.2(2)
NꢀCꢀN 105.57(14)
113.9(8)
a X = Cl or O; where more than one value is given, the relevant atom label is
presented after the bond length. b Symmetry operation: 0, ꢀxþ /2, yꢀ /2,
1
1
1
ꢀz þ /2.
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Scheme 2
at ν 388 cmꢀ1 (13), 192 cmꢀ1 (14), and 165 cmꢀ1 (15).
Although the 1H and 13C{1H} NMR spectra of the compounds
are similar and consistent with their proposed structures, some
revealing information could be gained from multinuclear NMR
spectroscopic studies on 13 and 15. The 29Si NMR spectrum of
13 exhibits a singlet resonance at low field (δ = 96.9 ppm), as
might be expected for a three-coordinate silicon complex. This value is
comparable with that for the related complex 6 (δ = 76.3 ppm)9 but
is significantly downfield from the signals observed for Robin-
son’s silicon(I) chlorideꢀNHC complex, [(IPr)Si(Cl)ꢀ
Si(Cl)(IPr)] (δ = 38.4 ppm; IPr = :C{N(Dip)C(H)}2),11a and
a recently reported intramolecularly base-stabilized disilyne,
[{[k2-N,P-(C7H8)P(But)2N(Dip)]Si}2] (δ = ꢀ18.5 ppm).11b
In this respect, it is noteworthy that 29Si NMR chemical shifts for
related two-coordinate disilynes, RSitSiR, can vary in the field
by more than 70 ppm.12 The 119Sn NMR spectrum of 15 displays
a resonance at δ = 777.7 ppm (N.B.: no 117Sn satellites were
detected), which is downfield of the value for the base-stabilized
distannynes, [{[k3-C,N,N0-C6H3(CH2NMe2)2-2,6]Sn}2] (four-
coordinate, δ = 612 ppm)11e and [Ar0(RNtC)SnSn(Ct
NR)Ar0] (three-coordinate, R = But, δ = 181 ppm; R = Mes,
δ = 381 ppm; Ar0 = C6H3(C6H3Pri2-2,6)2-2,6).22 To the best of
our knowledge, the only reported 119Sn NMR chemical shifts for
two-coordinate distannynes are δ = 335 and ꢀ36 ppm from the
solid-state spectra of [Ar0SnSnAr0] and [Ar*SnSnAr*] (Ar* =
C6H3(C6H3Pri3-2,4,6)2-2,6), respectively.23 It is clear that draw-
ing any comparisons from these data should be treated with some
caution because 119Sn NMR chemical shifts are well-known to be
very sensitive to the coordination number of the metal and the
nature of the atoms bonded to it.24
The electronic spectra of the intensely colored complexes
13ꢀ15 were obtained, and all showed a moderately strong
absorption band in the visible region {toluene solutions, λmax
[nm] (ε [L molꢀ1 cmꢀ1])}: 13, 629 (7520); 14, 502 (6030); 15,
388 (2100). It is interesting that there is a decrease in the
wavelength of this absorption on going from the lighter to the
heavier group 14 elements in these complexes. This is the
opposite of what is normally observed for multiply bonded group
14 element(I) dimers (lowest energy transition: π f π*),25
which suggests that the EꢀE bonds of 13ꢀ15 have little π
character. At this stage, one cannot be certain as to what
electronic transitions give rise to the colors of these complexes,
but the energies of these transitions are comparable with the few
previously reported for related singly bonded element(I) dimers.
Pertinent examples here include the tin(I) dimer [{[k3-C,N,N0-
C6H3(CH2NMe2)2-2,6]Sn}2] [λmax = 367 nm (4300)],11e the
germanium(I) dimer [(DipNacnac)GeGe{C(Me)C(H)C(Me)-
N(Dip)}] [λmax = 501 nm (2600)],11c and the silicon(I) dimer
[(IPr)Si(Cl)ꢀSi(Cl)(IPr)] (λmax = 510 nm).11a
PbꢀO distances are in the normal ranges for leadꢀtriflate and
PbꢀTHF interactions.20 There is only one structurally charac-
terized heteroleptic amidinato lead complex in the literature to
compare with 12, and in that dimeric, four-coordinate complex,
[{(Piso)Pb(μ-Cl)}2],21 the two crystallographically indepen-
dent PbꢀN distances [2.330(4) and 2.326(4) Å] are identical.
This is not the case for 12, in which the Pb1ꢀN2 separation is
considerably greater than the Pb1ꢀN1 distance.
The magnesium(I) reducing agent that was chosen for this
study was [{(MesNacnac)Mg}2] (MesNacnac = [(MesNCMe)2-
CH]ꢀ; Mes = mesityl),2e because we previously showed this to
have a greater reactivity than more sterically hindered dimers,
e.g., [{(DipNacnac)Mg}2].2 The reaction of the silicon(IV) com-
plex 8 with 1.5 equiv of [{(MesNacnac)Mg}2] in toluene led to
the formation of a deep-blue solution and a colorless precipitate
of [{(MesNacnac)Mg(μ-Cl)}2]. Workup of this reaction mixture
afforded a high isolated yield (91%) of the deep-blue silicon(I)
dimer 13 after its recrystallization from pentane (Scheme 2).
Similarly, the related reaction between the group 14 element(II)
precursor 9 or 10 with 0.5 equiv of [{(MesNacnac)Mg}2] gave
red-purple 14 and dichroic orangeꢀgreen 15 in isolated crystal-
line yields of 71% and 36%, respectively. An attempt to form the
lead(I) analogues of 13ꢀ15 via the treatment of 12 with
[{(MesNacnac)Mg}2] resulted in the deposition of lead metal
and the formation of the amidine ButisoH. It is of note that
reacting 8 with 1 equiv of [{(MesNacnac)Mg}2] did not generate
a silicon(II) compound, [(Butiso)SiCl], but instead yielded a
mixture of 13 and unreacted 8. This contrasts with Roesky et al.’s
treatment of [{PhC(NBut)2}SiCl3] with 2 equiv of potassium
metal, which gave [{PhC(NBut)2}SiCl], albeit in very low yield
(10%).16
In order to compare the efficacy of [{(MesNacnac)Mg}2] to
that of other reducing agents, the silicon(IV) complex 8 was
reacted with an excess of lithium metal, 3 equiv of lithium
naphthalenide, or 1.5 equiv of [(THF)3Mg(anthracene)]. The
first reaction led to a blue solution at ꢀ50 ꢀC, but upon warming
to 20 ꢀC, this color was lost and [Li(Butiso)] was recovered from
the reaction mixture. Therefore, “over-reduction” of 8 by the
excess lithium metal had almost certainly occurred. The other
two reactions did yield persistent blue solutions at 20 ꢀC, but
during their workup, it proved very difficult to remove the
generated naphthalene or anthracene, and only low yields of
impure 13 could be recovered from the mixtures.
The isolation of 13ꢀ15 allowed us to compare the physical
and spectroscopic properties of this series of compounds. For
example, although all are extremely air- and moisture-sensitive,
their thermal stability decreases with an increase in the size of the
group 14 element, presumably because of a concomitant de-
crease in the EꢀE bond strength as the group is descended. This
is reflected in their Raman spectra, an analysis of which allowed
the tentative assignment of bands due to EꢀE stretching modes
The X-ray crystal structures of 13ꢀ15 were obtained, and all
were found to be isostructural, while 13 and 14 are also
isomorphous. As a result, only the molecular structure of 13 is
depicted in Figure 2, although relevant geometric parameters for
all complexes can be found in Table 2. It is clear that all
complexes are dimeric and possess trans-bent structures similar
to that of 4 but different from the gauche-bent structures of the
less sterically hindered species, 6 and 7. In the case of 7, density
functional theory (DFT) calculations implied that the gauche-
bent form of the complex is favored by ca. 3.5 kcal molꢀ1 over its
trans-bent isomer.10 The EꢀE distances in the compounds are
fully consistent with singly bonded interactions and increase with
heavier E. Indeed, these bonds are similar in length to other
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Inorganic Chemistry
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for ditetrelynes, REER.12 The bond lengths within the NCN
fragments of the amidinate ligands in all complexes indicate that
they are delocalized.
In order to shed light on the nature of the bonding in 13ꢀ15
and their synthetically inaccessible lead analogue, [{(Butiso)Pb}2]
(16), DFT calculations (RI-BP86/def2-TZVPP) were carried
out on the full molecules in the gas phase (13aꢀ16a). Similar
calculations were carried out on the sterically condensed mol-
ecules, [{[PhC(NPh)2]E}2] (13bꢀ16b), in which substituted
aryl groups were replaced by phenyl substituents. The geome-
try optimization of all molecules yielded trans-bent structures
(see Figure S4 in the Supporting Information) similar to those
experimentally observed but different from the gauche-bent
structure previously calculated for 7.10 There is general agreement
between the calculated (Table 3) and experimental geometrical
parameters for the compounds, except for the EꢀE distances of
13aꢀ15a, which are ca. 0.1 Å longer than the values measured for
13ꢀ15 in the solid state. In contrast, the calculated EꢀE bonds of
the model compounds 13bꢀ15b, which are shorter than those in
13aꢀ15a, agree quite well with the experimentaldata. This may be
partly caused by fortuitous error cancellation because the experi-
mental EꢀE distances given by X-ray structure analyses can be
affected by crystal packing forces, which tend to shorten long and
weak bonds.27 It is thus conceivable that the calculated values for
13aꢀ15a are a faithful representation of the free molecules, while
13bꢀ15b better mimic the EꢀE bonding situation in the solid
state. As a component of this theoretical study, the vibrational
frequencies for the EꢀE stretching modes of 13aꢀ16a were
calculated using the harmonic approximation, and the values
obtained [ν 358.5 cmꢀ1 (13a), 193.8 cmꢀ1 (14a), 131.5 cmꢀ1
(15a), 102.5 cmꢀ1 (16a)] were found to be close to the available
experimentally observed anharmonic frequencies for 13ꢀ15
mentioned above [N.B.: the calculated values for the “cut-down”
model systems, 13bꢀ16b, are only slightly higher; ν 380.9 cmꢀ1
(13b), 197.2 cmꢀ1 (14a), 144.3 cmꢀ1 (15a), 108.5 cmꢀ1 (16a)].
The electronic structure of the calculated molecules was
examined and found to be similar for all compounds. Therefore,
only the molecular orbitals (MOs) of 13a that are most closely
associated with its SiꢀSi bond are depicted in Figure 3. The
highest occupied molecular orbital (HOMO) is clearly a σ-type
SiꢀSi bonding orbital, which is largely derived from the overlap
of two Si-centered p orbitals. An natural bond order (NBO)
analysis of 13a supported this proposition and showed the σ
bond to have 88.7% p character and 11.1% s character. Not
surprisingly, the p character of the EꢀE bonds of 13aꢀ16a
increases down the group (14a, 90.0%; 15a, 92.5%; 16a, 96.4%).
The HOMOꢀ1 and HOMOꢀ16 of 13a represent the plus and
minus combinations of the σ-type lone-pair MOs at Si, and these
are high in s character (NBO: 72.6%). Moreover, the percentage
of s character of the lone-pair orbital of the E centers in 13aꢀ16a
increases with heavier E (14a, 81.7%; 15a, 88.0%; 16a, 94.0%).
While the lowest unoccupied molecular orbital (LUMO) and
LUMOþ1 for 13a are predominantly ligand-based orbitals,
LUMOþ2 is essentially a π-type SiꢀSi bonding orbital. The
nature and ordering of the frontier orbitals for the germanium(I)
dimer 14a are equivalent, although the empty π-type EꢀE
bonding orbitals of the heavier species 15a and 16a are associated
with their LUMOs.
Figure 2. Thermal ellipsoid plot (25% probability surface) of the
molecular structure of 13. H atoms are omitted. Symmetry operation:
0, ꢀx, ꢀy, ꢀz. Select metrical parameters for 13ꢀ15 are given in Table 2.
Table 2. Selected Interatomic Distances (Å) and Angles
(deg) for 13ꢀ15a
13
14
15
EꢀE
EꢀN
2.4885(15)
1.872(2)
1.877(2)
2.6136(9)
2.025(2)
2.028(3)
3.0141(8)
2.243(3)
2.237(3)
2.240(3)
2.244(3)
1.338(5)
1.328(5)
1.332(5)
1.338(5)
CꢀN
1.346(3)
1.360(3)
1.338(4)
1.350(4)
NꢀEꢀN
NꢀCꢀN
69.36(10)
104.0(2)
64.68(10)
107.5(3)
59.04(12)
58.84(12)
111.8(3)
111.2(3)
a Where more than one value is given, the order follows the numbering
scheme for N atoms. Compound 15 does not sit on a crystallographic
center of inversion and therefore possesses two crystallographically
independent Sn atoms and four N centers (see the Supporting Informa-
tion for the ORTEP drawing).
singly bonded element(I) dimers in the literature, e.g., 2.413(2)
Å for 6,9 2.6380(8) Å for 4,8 and 2.9712(12) Å for [{[k3-C,N,N0-
C6H3(CH2NMe2)2-2,6]Sn}2].11e In line with the EꢀE single
bonds in 13ꢀ15 is the pyramidal coordination geometry of their
E centers, which indicates that they each possess a lone pair of
electrons. As the degree of pyramidalization increases for heavier
E[Σangles about E = 267.5ꢀ (13), 254.9ꢀ (14), 248.8ꢀ (mean;15)],
it can be assumed that the s character of the lone pair also
increases, in line with what would be expected from the inert pair
effect.26 Concomitantly, the degree of trans bending for the
compounds increases down the group [C(backbone)EE angle:
103.9ꢀ (13), 98.5ꢀ (14), 97.0ꢀ (mean; 15)], as is well-known
The frontier orbitals of 13aꢀ16a are strikingly similar to those
previously calculated for a model of the germanium(I) dimer 48
and are plainly consistent with the compounds having EꢀE
single bonds with negligible π character. In this respect, the
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Inorganic Chemistry
FORUM ARTICLE
Table 3. Select Bond Lengths (Å), Angles/Torsion Angles (deg), and Calculated EꢀE BDEs De (kcal molꢀ1) for the Optimized
Geometries of 13aꢀ16a and 13bꢀ16b (RI-BP86/def2-TZVPP)
Si
Ge
Sn
Pb
E
13a
2.560
13b
2.515
14a
2.731
14b
2.657
15a
3.111
15b
3.038
16a
3.240
16b
3.176
EꢀE
EꢀN
NꢀC
1.880ꢀ1.893
1.364ꢀ1.371
1.900ꢀ1.903
1.355ꢀ1.360
2.044ꢀ2.075
1.347ꢀ1.358
2.055
2.265ꢀ2.278
1.344ꢀ1.351
2.265ꢀ2.267
1.345ꢀ1.347
2.383ꢀ2.400
1.342ꢀ1.348
2.383ꢀ2.385
1.343ꢀ1.344
1.348ꢀ1.350
NꢀEꢀN
69.6
68.7
64.5
64.0
58.9
58.6
56.0
55.9
NꢀCꢀN
103.9
108.8
180.0
108.4
104.5
108.6
107.7
93.3
111.9
99.4
111.0
113.2
98.5
112.6
CꢀEꢀE
CꢀEꢀEꢀC
NꢀEꢀEꢀN
98.5ꢀ98.6
180.0
102.6
91.1ꢀ91.2
179.9
90.6ꢀ90.8
179.9
180.0
180.0
115.9
180.0
120.7
179.9
123.6
110.8ꢀ110.9
114.7ꢀ114.8
121.3ꢀ121.5
124.0ꢀ124.2
De
16.6
30.7
18.1
31.8
19.9
29.7
19.5
28.9
Figure 3. (a) LUMOþ2, (b) HOMO, (c) HOMOꢀ1, and (d) HOMOꢀ16 of 13a.
experimental compounds 13ꢀ15 are different to the trans-bent,
two-coordinate ditetrelynes, REER (E = Si, Ge, or Sn), which
have been calculated to have EꢀE multiple bonds with orders
ranging from ca. 2 for E = Ge or Sn to ca. 3 for E = Si.12,25,28 That
said, it is now clear that the potential energy surface relating to
the trans bending of distannynes is very shallow, and such systems
with relatively acute CꢀSnꢀSn angles (ca. 98ꢀ) and long SnꢀSn
single bonds (ca. 3.08 Å) are synthetically accessible.25 These
singly bonded distannynes, and their diplumbyne analogues,
RPbPbR, which are invariably void of multiple bond character,
are therefore directly comparable with 13ꢀ15. It should be noted
that although two-coordinate disilynes and digermynes always
possess considerable multiple bond character, several recent
reports have shown that when they form adducts with isonitriles,
their bond orders are reduced to unity.29 Consequently, they are
also related to 13ꢀ15, which can be considered to be intramole-
cularly base-stabilized ditetrelynes.
13bꢀ16b (yielding two [PhC(NPh)2]E fragments) were also
calculated. The theoretically predicted BDE (De) values are
shown in Table 3. For 13aꢀ16a, these lie between 16.6 and
19.9 kcal molꢀ1, thus suggesting that their EꢀE bonds are indeed
rather weak. Remembering that the calculated EꢀE bond lengths
for 13aꢀ15a were overestimated by ∼0.1 Å, it might be expected
that their calculated BDEs would be smaller than the BDEs of the
experimental compounds 13ꢀ15. Considering that the EꢀE
bond lengths of the model “cut-down” compounds 13bꢀ15b are
in much better agreement with the experimental values for
13ꢀ15, their EꢀE BDEs may be better suited for predictions
of the BDEs in the experimental compounds in the solid state.
Table 3 shows that the BDEs of the EꢀE bonds in 13bꢀ16b are
between 28.9 and 31.8 kcal molꢀ1, i.e., considerably greater than
those for 13aꢀ16a. All of these values may be compared with the
calculated BDEs of the unsupported EꢀE single bonds in the
parent systems HEEH (E = SiꢀPb), which possess planar trans-
bent structures with HꢀEꢀE angles of ca. 90ꢀ.28a The π orbitals
in these two-coordinate species are empty, and the atoms, E,
carry σ lone-pair orbitals. The BDEs of these compounds,
calculated at a level of theory (BP86/QZ4P) similar to that
used here, are 40.3 (Si), 37.2 (Ge), 33.0 (Sn), and 31.0 (Pb)
The high p character of the EꢀE bonds of 13aꢀ16a indicates
that these interactions may be weaker than generally expected for
EꢀE single bonds. We calculated the bond dissociation energies
(BDEs) of each compound, yielding two (Butiso)E fragments in
the electronic doublet state. The BDEs of the model compounds
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Inorganic Chemistry
kcal molꢀ1 28a
A comparison of these values with the data in
FORUM ARTICLE
.
Preparation of ButisoH. A solution of 1-bromo-4-tert-butylben-
zene (3.39 g, 15.9 mmol) in diethyl ether (20 mL) was added slowly to a
slurry of lithium powder (200 mg, 28.9 mmol) and diethyl ether
(40 mL), which was heated at reflux. After the addition, the solution
was heated at reflux for an additional 1 h. The purple/red mixture was
subsequently filtered into a solution of the carbodiimide DipN=C=N-
Dip (5.80 g, 16.0 mmol) in diethyl ether (40 mL) at ꢀ50 ꢀC. The
resultant solution was subsequently warmed to room temperature and
stirred for 1 h. The reaction mixture was quenched with H2O (15 mL)
and the organic layer collected. The aqueous layer was extracted with
dichloromethane (2 ꢁ 30 mL), and the combined organic layers were
dried over MgSO4. Volatiles were removed in vacuo, and the residue was
washed with hexane (8 mL), leaving ButisoH as a colorless solid (5.05 g,
Table 3 shows that the EꢀE bonds in three-coordinate 13aꢀ16a
and 13bꢀ16b are somewhat weaker than those in the unsup-
ported two-coordinate parent systems. It is noteworthy that all of
these BDEs are significantly lower and more uniform than those
reported for the molecules H3EꢀEH3.30 Their calculated BDEs
(BP86/TZ2P) are 72.4 (Si), 67.5 (Ge), 57.4 (Sn), and 46.8 (Pb)
kcal molꢀ1. The percentages of s character of the EIIIꢀEIII bonds
in H3EꢀEH3 were calculated to be considerably higher than
those for the EIꢀEI bonds of the molecules described herein.
A full analysis of the factors that determine the comparative
strengths of EIꢀEI and EIIIꢀEIII single bonds will form the basis
of a forthcoming theoretical study by us.
1
70%). Mp: 162ꢀ164 ꢀC. H NMR (400 MHz, 298 K, C6D6): main
isomer, δ 0.89 (d, 3JH,H = 6.8 Hz, 6H, CH(CH3)2), 0.92 (d, 3JH,H = 6.8
Hz, 6H, CH(CH3)2), 1.05 (s, 9H, C(CH3)3), 1.39 (d, 3JH,H = 6.8 Hz,
6H, CH(CH3)2), 1.40 (d, 3JH,H = 6.8 Hz, 6H, CH(CH3)2), 3.30 (sept,
’ CONCLUSIONS
3JH,H = 6.8 Hz, 2H, CH(CH3)2), 3.50 (sept, JH,H = 6.8 Hz, 2H,
3
In summary, the utility of a dimeric magnesium(I) compound
as a soluble and stoichiometric reducing agent for inorganic
synthesis has been demonstrated by its use in the facile prepara-
tion of a series of amidinato coordinated group 14 element(I)
dimers. These include the first tin(I) example of such species.
Access to these compounds has allowed a detailed comparison of
their properties using a variety of spectroscopic, crystallographic,
and theoretical techniques. Given the emerging importance of
group 14 element(I) dimers to small-molecule activation, etc.,
there is much scope to examine the further chemistry of the
compounds prepared in this study. Moreover, we believe that the
advantageous properties of easy to prepare magnesium(I) com-
pounds will lead to them finding widespread use as more selective
alternatives to the harsh and often insoluble alkali-metal reducing
agents that are commonly used in the preparation of low-
oxidation-state p-block systems. We continue to explore the
reactivity of magnesium(I) compounds and will report on this in
a series of forthcoming publications.
3
CH(CH3)2), 5.92 (s, 1H, NH), 6.93 (d, JH,H = 8.0 Hz, 2H, ArH),
6.99ꢀ7.30 (m, 6H, ArH), 7.64 (d, JH,H = 8.0 Hz, 2H, ArH); minor
3
isomer, δ 0.65 (br, 6H, CH(CH3)2), 0.97 (br, 6H, CH(CH3)2, 1.05
(s, 9H, C(CH3)3), 1.11 (br, 6H, CH(CH3)2), 1.24 (vbr, 6H, CH-
(CH3)2), 3.31 (vbr, 2H, CH(CH3)2), 3.58 (vbr, 2H, CH(CH3)2), 5.68
(s, 1H, NH), 7.01ꢀ7.31 (m, 10H, ArH). 13C{1H} NMR (100 MHz, 298 K,
C6D6): main isomer, δ 21.1 (CH(CH3)2), 21.6 (CH(CH3)2), 23.2
(CH(CH3)2), 23.5 (CH(CH3)2), 27.4 (CH(CH3)2), 27.8
(CH(CH3)2), 29.7 (C(CH3)3), 33.2 (C(CH3)3), 122.0 (ArC), 122.4
(ArC), 122.6 (ArC), 123.4 (ArC), 127.9 (ArC), 131.3 (ArC), 133.7
(ArC), 137.9 (ArC), 143.6 (ArC), 143.7 (ArC), 151.2 (ArC), 151.3
(ArC), CN2 resonance not observed. IR (Nujol, cmꢀ1): ν 3424s (NH),
3360s (NH), 1651s, 1621s, 1587s, 1563m, 1407m, 1326m, 1298m,
1269m, 1105m, 1081m, 936m, 848m, 826m, 780m, 760s. MS(ES, 35 eV):
m/z 497.3 (MHþ, 100%).
N.B.: In C6D6 solutions, ButisoH exists as two isomeric forms in an
approximate ratio of 1:2. Only the 1H NMR spectroscopic data are given
for both isomers.
Preparation of [(Butiso)SiCl3] (8). LiBun (1.76 mL, 2.83 mmol,
1.6 M solution in hexane) was added to a solution of ButisoH (1.33 g,
2.83 mmol) in diethyl ether (40 mL) at ꢀ78 ꢀC. The resultant solution
was warmed to room temperature, whereupon it was cooled again to
ꢀ78 ꢀC. To this was added SiCl4 (0.34 mL, 2.83 mmol) over 2 min. The
mixture was warmed to room temperature and stirred for 1 h, and
volatiles were removed in vacuo. The pale-yellow residue was extracted
into hexane (25 mL), and the extract was concentrated to ca. 15 mL and
cooled to ꢀ30 ꢀC overnight to give colorless crystals of 8 (775 mg,
43%). Mp: 180ꢀ182 ꢀC (melts, no visible decomposition up to 300 ꢀC).
1H NMR (400 MHz, 298 K, C6D6): δ 0.74 (s, 9H, C(CH3)3), 0.95
’ EXPERIMENTAL SECTION
General Methods. All manipulations were carried out using
standard Schlenk and glovebox techniques under an atmosphere of
high-purity dinitrogen. THF, hexane, pentane, and toluene were distilled
over potassium, while diethyl ether was distilled over a Na/K alloy. 1H
and 13C{1H} NMR spectra were recorded on either Bruker DPX300 or
AvanceIII 400 spectrometers and were referenced to the resonances of
the solvent used. 29Si{1H} and 119Sn{1H} NMR spectra were recorded
on a Bruker AvanceIII 400 spectrometer and were referenced to external
SiMe4 and SnMe4, respectively. Mass spectra were obtained from the
EPSRC National Mass Spectrometric Service at Swansea University
(Swansea, Wales, U.K.). IR spectra were recorded using a Perkin-Elmer
RX1 FT-IR spectrometer as Nujol mulls between NaCl plates. UVꢀvis
spectra were recorded on a Cary 1E spectrometer. Microanalyses were
carried out by Campbell Microanalytical Laboratory, University of
Otago (Dunedin, New Zealand). Raman spectra were recorded on
crystalline samples of 13ꢀ15 sealed in glass capillaries under dinitrogen
using a Renishaw RM2000 micro-Raman spectrometer with 514 nm
excitation from a diode laser. Melting points were determined in sealed
glass capillaries under dinitrogen and are uncorrected. The compounds
3
3
(d, JH,H = 6.8 Hz, 12H, CH(CH3)2), 1.43 (d, JH,H = 6.8 Hz, 12H,
CH(CH3)2), 3.75 (sept, 3JH,H = 6.8 Hz, 4H, CH(CH3)2), 6.81 (d, 3JH,H
=
8.8 Hz, 2H, ArH), 7.02ꢀ7.15 (m, 6H, ArH), 7.34 (d, 3JH,H = 8.8 Hz, 2H,
ArH). 13C{1H} NMR (100 MHz, 298 K, C6D6): δ 22.6 (CH(CH3)2),
23.9 (CH(CH3)2), 28.0 (CH(CH3)2), 29.0 (C(CH3)3), 33.5
(C(CH3)3), 121.1 (ArC), 123.6 (ArC), 124.1 (ArC), 127.4 (ArC),
130.1 (ArC), 133.6 (ArC), 145.1 (ArC), 156.6 (ArC), 169.7 (CN2).
29Si NMR (80 MHz, 298 K, C6D6): δ ꢀ89.4. IR (Nujol, cmꢀ1): ν 1664m,
1609s, 1570m, 1543m, 1501m, 1421s, 1379s, 1261s, 1204m, 1178m,
1097s, 934m, 862m, 792s. MS (EI, 70 eV): m/z 628.3 (Mþ, 15%), 585.2
(Mþ-Pri, 25%), 496.3 (ButisoHþ, 5%), 453.3 (ButisoHþ-Pri, 5%), 320.2
(Butisoþ-NDip, 100%). HREI. Acc. mass calcd for C35H47Cl3N2Si: m/z
628.2569. Found: m/z 628.2565. Anal. Calcd for C35H47Cl3N2Si: C,
66.70; H, 7.52; N, 4.45. Found: C, 66.42; H, 7.79; N, 4.28.
Pb(O3SCF3)2 and [{(MesNacnac)Mg}2]2e were prepared by varia-
31
tions of the literature procedures. [K(Butiso)] was prepared by treating a
toluene solution of ButisoH with KN(SiMe3)2. The resultant reaction
mixture was then filtered, volatiles were removed in vacuo, and
[K(Butiso)] was used without further purification. All other reagents
were used as received.
Preparation of [(Butiso)GeCl] (9). A solution of [Li(Butiso)]
(0.300 g, 0.60 mmol) in THF (20 mL) was added to a solution of
GeCl2 dioxane (0.138 g, 0.60 mmol) in THF (40 mL) at ꢀ78 ꢀC. The
3
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Inorganic Chemistry
FORUM ARTICLE
resultant mixture was slowly warmed to room temperature and stirred
overnight. Volatiles were removed in vacuo, and the residue was
extracted into pentane (30 mL). The extract was concentrated to
15 mL and cooled to ꢀ30 ꢀC overnight to give colorless crystals of 9
(0.134 g, 37%). Mp: 197ꢀ199 ꢀC (dec). 1H NMR (400 MHz, 298 K,
Preparation of [{(Butiso)Pb(μ-O3SCF3)(THF)}¥] (12). A solu-
tion of [K(Butiso)] (0.400 g, 0.75 mmol) in THF (20 mL) was added
to a solution of Pb(O3SCF3)2 (0.430 g, 0.85 mmol) in THF (20 mL) at
ꢀ78 ꢀC. The mixture was slowlywarmedto room temperature and stirred
overnight. Volatiles were removed in vacuo, and the residue was extracted
into pentane (2 ꢁ 15 mL). The pale-yellow extract was concentrated to
10 mL and placed at ꢀ30 ꢀC overnight to give yellow crystals of 12 (0.154
g, 24%). Mp: 124ꢀ126 ꢀC. 1H NMR (400 MHz, 298 K, C6D6): δ 0.83 (s,
3
C6D6): δ 0.79 (s, 9H, C(CH3)3), 0.98 (d, JH,H = 6.8 Hz, 6H,
3
CH(CH3)2), 1.02 (d, 3JH,H = 6.8 Hz, 6H, CH(CH3)2), 1.19 (d, JH,H
= 6.8 Hz, 6H, CH(CH3)2), 1.46 (d, 3JH,H = 6.8 Hz, 6H, CH(CH3)2),
3.62 (sept, 3JH,H = 6.8 Hz, 2H, CH(CH3)2), 4.05 (sept, 3JH,H = 6.8 Hz,
2H, CH(CH3)2), 6.81 (d, 3JH,H = 8.8 Hz, 2H, ArH), 6.95ꢀ7.15 (m, 6H,
ArH), 7.20 (d, 3JH,H = 8.8 Hz, 2H, ArH). 13C{1H} NMR (100 MHz, 298
K, C6D6): δ 21.5 (CH(CH3)2), 21.7 (CH(CH3)2), 25.0 (CH(CH3)2),
25.1 (CH(CH3)2), 27.5 (CH(CH3)2), 28.0 (CH(CH3)2), 29.2
(C(CH3)3), 33.3 (C(CH3)3), 122.5 (ArC), 123.3 (ArC), 123.9 (ArC),
125.6 (ArC), 125.7 (ArC), 128.6 (ArC), 136.0 (ArC), 142.3 (ArC), 144.6
(ArC), 154.0 (ArC), 169.9 (CN2). IR (Nujol, cmꢀ1): ν 1651m, 1610m,
1463m, 1456m, 1435w, 1402w, 1261s, 1099s (br), 1020s (br), 864m,
847w, 803s. MS (EI, 70 eV): m/z 604.3 (Mþ, 10%), 561.2 (Mþ-Pri, 5%),
496.3 (ButisoHþ, 8%), 453.3 (ButisoHþ-Pri, 43%), 320.2 (Butisoþ-
NDip, 100%). HREI. Acc. mass calcd for C35H47Cl70GeN2: m/z
600.2665. Found: m/z 600.2662. Anal. Calcd for C35H47ClGeN2: C,
69.62; H, 7.85; N, 4.64. Found: C, 69.05; H, 7.92; N, 4.69.
9H, C(CH3)3), 1.01 (d, 3JH,H = 6.8 Hz, 12H, CH(CH3)2), 1.39(d, 3JH,H
=
6.8 Hz, 12H, CH(CH3)2), 1.41ꢀ1.47 (m, 4H, O(CH2CH2)2), 3.51ꢀ
3.57 (m, 4H, O(CH2CH2)2), 3.64 (sept, 3JH,H = 6.8Hz, 4H, CH(CH3)2),
6.80ꢀ7.14 (m, 10H, ArH). 13C{1H} NMR (100 MHz, 298 K, C6D6):
δ 22.0 (CH(CH3)2), 24.3 (CH(CH3)2), 25.0 (O(CH2CH2)2), 27.2
(CH(CH3)2), 29.3 (C(CH3)3), 33.1 (C(CH3)3), 66.8 (O(CH2CH2)2),
121.9 (ArC), 123.4 (ArC), 125.0 (ArC), 128.8 (ArC), 137.0 (ArC), 138.2
(ArC), 142.9 (ArC), 152.4 (ArC), 171.7 (CN2).19F NMR (377 MHz, 298 K,
C6D6): δ ꢀ77.4. IR (Nujol, cmꢀ1): ν 1650m, 1619s, 1609s, 1587m, 1414s,
1362vs, 1316s, 1200s, 1056s, 1017vs, 873m, 846s, 824s, 804s, 775s. MS
(EI, 35 eV): m/z 497.7 (ButisoH2þ, 100%).
Preparation of [{(Butiso)Si}2] (13). A solution of [{(MesNacnac)-
Mg}2] (0.350 g, 0.49 mmol) in toluene (20 mL) was added to a solution
of 8 (0.200 g, 0.32 mmol) in toluene (20 mL) at 20 ꢀC. The mixture was
stirred for 1 h, during which time it gradually changed from yellow to deep
blue. Volatiles were removed in vacuo, and the residue was extracted into
pentane (30 mL). The extract was concentrated to 20 mL and placed at
ꢀ30 ꢀC overnight to give deep-blue crystals of 13 (0.151 g, 91%). Mp:
235ꢀ237 ꢀC (dec). 1H NMR (400 MHz, 298 K, C6D6): δ 0.86 (s, 18H,
C(CH3)3), 0.99 (d, br, 12H, CH(CH3)2), 1.08 (d, br, 12H, CH(CH3)2),
1.16 (v tr of two overlapping d, br, 24H, CH(CH3)2), 3.82 (sept, br, 4H,
CH(CH3)2), 3.87 (sept, br, 4H, CH(CH3)2), 6.77 (d, 3JH,H = 8.8 Hz, 4H,
ArH), 7.00ꢀ7.16 (m, 16H, ArH). 13C{1H} NMR (100 MHz, 298 K,
C6D6): δ 21.6 (CH(CH3)2), 22.5 (CH(CH3)2), 22.7 (CH(CH3)2), 24.4
(CH(CH3)2), 27.4 (CH(CH3)2), 27.8 (CH(CH3)2), 29.4 (C(CH3)3),
33.1 (C(CH3)3), 122.9 (ArC), 123.2 (ArC), 123.7 (ArC), 125.4 (ArC),
127.7 (ArC), 137.2 (ArC), 142.7 (ArC), 143.2 (ArC), 146.9 (ArC), 151.0
(ArC), 159.7 (CN2). 29Si NMR (80 MHz, 298 K, C6D6): δ 96.9. IR
(Nujol, cmꢀ1): ν 1651m, 1620m, 1362m, 1261s, 1096s, 1024s, 803s.
Raman (solid under N2, 514 nm excitation, cmꢀ1): ν 388 (SiꢀSi str).
UVꢀvis (toluene solution): λmax = 629 nm (ε ≈ 7520 L molꢀ1 cmꢀ1).
MS (EI. 70 eV): m/z 1046.7 (Mþ, 2%), 523.3 ([M/2]þ, 4%), 496.4
(ButisoHþ, 9%), 453.4 (ButisoHþ-Pri, 16%), 320.2 (Butisoþ-NDip,
100%). HREI. Acc. mass calcd for C70H94N4Si2: m/z 1046.7012. Found:
m/z 1046.7008.
Preparation of [(Butiso)SnCl] (10). A solution of [K(Butiso)]
(0.550 g, 1.03 mmol) in THF (20 mL) was added to a slurry of SnCl2
(0.196 g, 1.03 mmol) in THF (40 mL) at ꢀ78 ꢀC. The reaction mixture
was slowly warmed to room temperature and stirred overnight. Volatiles
were removed in vacuo, and the residue was extracted into pentane
(30 mL). The extract was concentrated to 15 mL and placed at ꢀ30 ꢀC
overnight to yield colorless crystals of 10 (0.456 g, 68%). Mp:
199ꢀ201 ꢀC (melts, no visible decomposition up to 300 ꢀC). 1H
NMR (400 MHz, 298 K, C6D6): δ 0.82 (s, 9H, C(CH3)3), 0.96ꢀ1.06
(m of overlapping d, br, 12H, CH(CH3)2), 1.12 (d, br, 6H, CH(CH3)2),
1.48 (d, br, 6H, CH(CH3)2), 3.55 (sept, br, 2H, CH(CH3)2), 4.08 (sept,
3
br, 2H, CH(CH3)2), 6.84 (d, JH,H = 8.8 Hz, 2H, ArH), 6.90ꢀ7.12
(m, 6H, ArH), 7.14 (d, 3JH,H = 8.8 Hz, 2H, ArH). 13C{1H} NMR (100
MHz, 298 K, C6D6): δ 21.4 (CH(CH3)2), 21.7 (CH(CH3)2), 24.9
(CH(CH3)2), 25.6 (CH(CH3)2), 27.5 (CH(CH3)2), 27.8 (CH(CH3)2),
29.3(C(CH3)3), 33.2 (C(CH3)3), 122.1(Ar-C), 122.6 (Ar-C), 123.2(Ar-
C), 123.5 (ArC), 125.0 (ArC), 128.5 (ArC), 137.5 (ArC), 141.7 (ArC),
143.9 (ArC), 153.0 (ArC), 172.4 (CN2). 119Sn NMR (150 MHz, 298 K,
C6D6): δ 28.4. IR (Nujol, cmꢀ1) ν 1651m, 1619s, 1586m, 1364s, 1322s,
1281s, 1261m, 1098m, 1054m, 1016m, 968m, 934m, 827m, 802m, 776s.
MS (EI, 70 eV): m/z 650.3 (Mþ, 2%), 496.4 (ButisoHþ, 15%), 453.4
(ButisoHþ-Pri, 18%), 320.2 (Butisoþ-NDip, 100%). HREI. Acc. mass
calcd for C35H47Cl116SnN2: m/z 646.2440. Found: m/z 646.2441.
Preparation of [Pb(Butiso)2] (11). A solution of [Li(Butiso)]
(0.500 g, 0.99 mmol) in THF (20 mL) was added to a slurry of PbCl2
(0.350 g, 1.26 mmol) in THF (20 mL) at room temperature over 5 min.
The mixture was stirred overnight before volatiles were removed in
vacuo. The residue was extracted into hexane (30 mL) and the extract
concentrated to 15 mL. Cooling the solution to ꢀ30 ꢀC overnight
yielded pale-yellow crystals of 11 (0.45 g, 75% based on Butiso). Mp:
137ꢀ139 ꢀC (dec). 1H NMR (400 MHz, 298 K, C6D6): δ 0.60ꢀ1.60
(v br, 48H, CH(CH3)2), 0.84 (s, 18H, C(CH3)3), 3.10ꢀ4.10 (v br, 8H,
Preparation of [{(Butiso)Ge}2] (14). A solution of [{(MesNacnac)-
Mg}2] (0.119 g, 0.09 mmol) in toluene (20 mL) was added to a solution
of 9 (0.100 g, 0.17 mmol) in toluene (20 mL) at 20 ꢀC. The mixture was
stirred for 1 h, during which time it gradually changed from yellow to
red-purple. Volatiles were then removed from the reaction mixture in
vacuo, and the residue was extracted into pentane (30 mL). The extract
was concentrated to ca. 15 mL and placed at ꢀ30 ꢀC overnight to give
red-purple crystals of 14 (0.069 g, 71%). Mp: 178ꢀ179 ꢀC (dec). 1H
NMR (400 MHz, 298 K, C6D6): δ 0.84 (s, 18H, C(CH3)3), 1.02ꢀ1.08
(m of three overlapping d, 3JH,H = 6.8 Hz, 36H, CH(CH3)2), 1.21 (d,
3JH,H = 6.8 Hz, 12H, CH(CH3)2), 3.87 (m of two overlapping sept,
3
3
3JH,H = 6.8 Hz, 8H, CH(CH3)2), 6.81 (d, JH,H = 8.8 Hz, 4H, ArH),
CH(CH3)2), 6.88 (d, JH,H = 8.8 Hz, 4H, ArH), 6.82ꢀ7.16 (m, 12H,
ArH), 7.14 (d, 3JH,H = 8.8 Hz, 4H, ArH). 13C{1H} NMR (100 MHz, 298 K,
C6D6): δ 23.1 (v br, CH(CH3)2), 28.3 (v br, CH(CH3)2), 30.7
(C(CH3)3), 34.2 (C(CH3)3), 123.0 (br, ArC), 123.9 (ArC), 124.9 (br,
ArC), 125.5 (br, ArC), 130.6 (ArC), 135.8 (ArC), 142.3 (br, ArC), 143.4
(br, ArC), 144.4 (br, ArC), 151.9 (ArC), 167.8 (CN2). IR (Nujol, cmꢀ1): ν
1636m, 1619s, 1609m, 1588m, 1563m, 1398s, 1380s, 1265m, 1178m,
1104m, 1018m, 947m, 935m, 844m, 824m, 801m. MS (EI, 70 eV): m/z
1198.8 (Mþ, 1%), 703.3 (Mþ-Butiso, 21%), 496.3 (ButisoHþ, 16%),
453.3 (ButisoHþ-Pri, 20%), 320.2 (Butisoþ-NDip, 100%).
7.00ꢀ7.10 (m, 12H, ArH), 7.20 (d, 3JH,H = 8.8 Hz, 4H, ArH). 13C{1H}
NMR (100 MHz, 298 K, C6D6): δ 22.9 (CH(CH3)2), 23.4 (CH(CH3)2),
24.1 (CH(CH3)2), 25.7 (CH(CH3)2), 28.4 (CH(CH3)2), 28.6
(CH(CH3)2), 30.5 (C(CH3)3), 34.1 (C(CH3)3), 123.8 (ArC), 123.9
(ArC), 124.5 (ArC), 125.8(ArC), 129.6(ArC), 139.8(ArC), 143.5(ArC),
143.7 (ArC), 152.5 (ArC), 156.8 (ArC), 170.0 (CN2). IR (Nujol, cmꢀ1): ν
1653m, 1618m, 1396m, 1364m, 1322m, 1260m, 1177m, 1098s, 1018s,
934m, 830m, 800m, 776m, 744m. Raman (solid under N2, 514 nm
excitation, cmꢀ1): ν 192 (GeꢀGe str). UVꢀvis: λmax = 502 nm
12322
dx.doi.org/10.1021/ic200682p |Inorg. Chem. 2011, 50, 12315–12325

Inorganic Chemistry
FORUM ARTICLE
Table 4. Summary of Crystallographic Data for Two Structural Modifications of 8, 9, 10, 12ꢀ15, ButisoH, and [Li{Li(k2-N,N0-
Butiso)2}]
8
8 0.5hexane
9
10
12
13 hexane
3
3
empirical formula
fw
C35H47Cl3N2Si
630.19
C38H54Cl3N2Si
673.27
orthorhombic
Pbca
C35H47ClGeN2
603.79
orthorhombic
Pbcn
C35H47ClN2Sn
649.89
orthorhombic
Pbcn
C40H55F3N2O4PbS
924.11
C76H108N4Si2
1133.84
triclinic
P1
cryst syst
space group
a (Å)
monoclinic
P21/n
monoclinic
C2/c
10.0389(5)
20.1943(11)
17.6677(11)
90
20.2749(6)
18.6319(5)
20.4170(6)
90
16.414(3)
22.426(5)
18.926(4)
90
16.504(3)
22.298(5)
18.868(4)
90
29.304(6)
12.205(2)
27.204(5)
90
11.9565(7)
12.0716(7)
14.1529(8)
86.468(2)
70.557(2)
65.810(2)
1750.46(17)
1
b (Å)
c (Å)
R (deg)
β (deg)
γ (deg)
V (Å3)
99.898(3)
90
90
90
90
94.18(3)
90
90
90
90
3528.4(3)
4
7712.7(4)
8
6966(2)
8
6944(2)
8
9704(3)
8
Z
F(calcd) (g cmꢀ3
)
1.186
1.160
1.151
1.243
1.265
1.076
μ (mmꢀ1
)
0.319
0.296
0.979
0.836
3.566
0.094
F(000)
1344
2888
2560
2704
3728
620
reflns collected
unique reflns
Rint
47 702
85 341
7561
17 308
6126
24 616
6813
14 820
22 474
7689
8605
7885
0.0437
0.1265
0.0531
0.1400
0.41, ꢀ0.40
816557
0.0457
0.0689
0.2092
1.64, ꢀ0.41
816559
0.0529
0.0448
0.1295
0.66, ꢀ0.55
816552
0.0456
0.0753
R1 indices [I > 2σ(I)]
wR2 indices (all data)
largest peak and hole (e Aꢀ3
CCDC no.
0.0387
0.0685
0.0570
0.1015
0.1923
0.1133
)
0.33, ꢀ0.36
816558
4.42 (near Pb1), ꢀ1.36
816553
0.29, ꢀ0.24
816554
14 hexane
15
ButisoH
[Li{Li(k2-N,N0-Butiso)2}]
3
empirical formula
fw
C
76H108Ge2N4
C70H94N4Sn2
1228.87
monoclinic
P21/n
C35H48N2
496.75
triclinic
P1
C70H94Li2N4
1005.37
triclinic
P1
1222.84
triclinic
P1
cryst syst
space group
a (Å)
11.968(2)
12.134(2)
14.323(3)
86.60(3)
70.97(3)
65.86(3)
1787.9(6)
1
16.627(3)
19.181(4)
21.647(4)
90
9.899(2)
10.533(2)
15.683(3)
84.35(3)
80.53(3)
71.89(3)
1531.1(5)
2
11.698(2)
13.099(3)
24.937(5)
87.85(3)
76.53(3)
65.22(3)
3365.9(12)
2
b (Å)
c (Å)
R (deg)
β (deg)
γ (deg)
V (Å3)
Z
103.36(3)
90
6717(2)
4
F(calcd) (g cmꢀ3
)
1.136
1.215
1.078
0.992
μ (mmꢀ1
)
0.883
0.784
0.062
0.056
F(000)
656
2568
544
1096
reflns collected
unique reflns
Rint
13 284
7002
19 653
12 308
6652
17 297
11 808
11 494
0.0348
0.0414
0.0962
0.53, ꢀ0.39
816555
0.0377
0.0446
0.1061
1.41, ꢀ0.69
816556
0.0355
0.0555
0.1513
0.19, ꢀ0.19
816550
0.0333
R1 indices [I > 2σ(I)]
wR2 indices (all data)
largest peak and hole (e Aꢀ3
CCDC no.
0.0792
0.2435
)
0.47, ꢀ0.24
816551
(ε ≈ 6030 L molꢀ1 cmꢀ1). MS (EI, 70 eV): m/z 496.3 (ButisoHþ, 10%),
453.4 (ButisoHþ-Pri, 17%), 320.2 (Butisoþ-NDip, 100%).
extracted into pentane (30 mL). The extract was concentrated to ca.
15 mL and placed at ꢀ30 ꢀC to yield dichroic orangeꢀgreen crystals of
15 (0.080 g, 36%). Mp: 144ꢀ146 ꢀC (dec). 1H NMR (400 MHz, 298 K,
Preparation of [{(Butiso)Sn}2] (15). A solution of [{(MesNacnac)-
Mg}2] (0.119 g, 0.18 mmol) in toluene (20 mL) was added to a solution
of 10 (0.230 g, 0.35 mmol) in toluene (20 mL) at 20 ꢀC. The mixture
was stirred for 1 h, during which time it gradually changed from yellow to
orange-brown. Volatiles were removed in vacuo, and the residue was
3
C6D6): δ 0.81 (d, JH,H = 6.8 Hz, 12H, CH(CH3)2), 0.82 (s, 18H,
C(CH3)3), 1.00 (d, 3JH,H = 6.8 Hz, 12H, CH(CH3)2), 1.23 (d, 3JH,H
=
6.8 Hz, 12H, CH(CH3)2), 1.50 (d, 3JH,H = 6.8 Hz, 12H, CH(CH3)2),
3.99 (sept, 3JH,H = 6.8 Hz, 4H, CH(CH3)2), 4.13 (sept, 3JH,H = 6.8 Hz,
12323
dx.doi.org/10.1021/ic200682p |Inorg. Chem. 2011, 50, 12315–12325

Inorganic Chemistry
FORUM ARTICLE
4H, CH(CH3)2), 6.87ꢀ7.08 (m, 16H, ArH), 7.27 (d, 3JH,H = 8.8 Hz, 4H,
ArH). 13C{1H} NMR (100 MHz, 298 K, C6D6): δ 22.9 (CH(CH3)2),
23.5 (CH(CH3)2), 24.5 (CH(CH3)2), 26.4 (CH(CH3)2), 28.4
(CH(CH3)2), 28.9 (CH(CH3)2), 30.7 (C(CH3)3), 34.3 (C(CH3)3),
123.5 (ArC), 123.8 (ArC), 124.3 (ArC), 125.8 (ArC), 130.3 (ArC),
130.4 (ArC), 140.9 (ArC), 143.5 (ArC), 144.1 (ArC), 152.6 (ArC),
162.8 (CN2). 119Sn NMR (150 MHz, 298 K, C6D6): δ 777.7. IR
(Nujol, cmꢀ1): ν 1652m, 1620m, 1587m, 1563w, 1360m, 1324m,
1314m, 1262s, 1176m, 1100s, 1019m, 844m, 826m. Raman (solid under
Chem., Int. Ed. 2009, 48, 9701. (e) Bonyhady, S. J.; Jones, C.; Nembenna,
S.; Stasch, A.; Edwards, A. J.; McIntyre, G. J. Chem.—Eur. J. 2010,
16, 938. (f) Jones, C.; McDyre, L.; Murphy, D. M.; Stasch, A. Chem.
Commun. 2010, 46, 1511. (g) Bonyhady, S. J.; Collis, D.; Frenking, G.;
Holzmann, N.; Jones, C.; Stasch, A. Nat. Chem. 2010, 2, 865. (h) Platts,
J. A.; Overgaard, J.; Jones, C.; Stasch, A.; Iversen, B. B. J. Phys. Chem. A
2011, 115, 194.
(3) Reviews: (a) Jones, C. Coord. Chem. Rev. 2010, 254, 1273.(b)
Stasch, A.; Jones, C. Dalton Trans. 2011, 40, DOI:10.1039/c0dt01831g.
(4) Liu, Y.; Li, S.; Yang, X.-J.; Yang, P.; Wu, B. J. Am. Chem. Soc. 2009,
131, 4210.
(5) For example, see:(a) Yao, S.; Xiong, Y.; Driess, M. Organome-
tallics 2011, 30, DOI:org/10.1021/om200017h. (b) Asay, M.; Jones, C.;
Driess, M. Chem. Rev. 2011, 111, 354. (c) Power, P. P. Nature 2010,
463, 171. (d) Fischer, R. C.; Power, P. P. Chem. Rev. 2010, 110, 3877. (e)
Schn€ockel, H. Chem. Rev. 2010, 110, 4125. (f) Mizuhata, Y.; Sasamori,
T.; Tohkitoh, N. Chem. Rev. 2009, 109, 3479. (g) Nagendran, S.; Roesky,
H. W. Organometallics 2008, 27, 457 and references cited therein.
(6) For example, see: Woodul, W. D.; Richards, A. F.; Stasch, A.;
Driess, M.; Jones, C. Organometallics 2010, 29, 3655 and references cited
therein.
(7) N.B.: The silicon(0) and boron(I) analogues of 1 and 2 were
previously prepared via the KC8 reduction of NHC adducts of SiCl4 and
BBr3, respectively. See: (a) Wang, Y.; Xie, Y.; Wei, P.; King, R. B.;
Schaefer, H. F., III; Schleyer, P. v. R.; Robinson, G. H. Science 2008,
321, 1069. (b) Wang, Y.; Quillian, B.; Wei, P.; Wannere, C. S.; Xie, Y.;
King, R. B.; Schaefer, H. F., III; Schleyer, P. v. R.; Robinson, G. H. J. Am.
Chem. Soc. 2007, 129, 12412.
N2, 514 nm excitation, cmꢀ1): ν 165 (SnꢀSn str). UVꢀvis: λmax
=
388 nm (ε ≈ 2100 L molꢀ1 cmꢀ1). MS (EI, 70 eV): m/z 1110.6
(Mþ-Sn, 1%); 615.3 ([M/2]þ, 3%), 496.3 (ButisoHþ, 20%), 453.3
(ButisoHþ-Pri, 26%), 320.2 (Butisoþ-NDip, 100%).
X-ray Crystallography. Crystals of two structural modifications of
8, 9, 10, 12ꢀ15, ButisoH, and [Li{Li(k2-N,N0-Butiso)2}] suitable for
X-ray structural determination were mounted in silicone oil. Crystal-
lographic measurements were made using a Nonius Kappa CCD
diffractometer using a graphite monochromator with Mo KR radiation
(λ = 0.710 73 Å). The structures were solved by direct methods and
refined on F2 by full-matrix least squares (SHELX97)32 using all unique
data. All non-H atoms are anisotropic with H atoms included in
calculated positions (riding model). Crystal data, details of data collec-
tion, and refinement are given in Table 4.
Theoretical Studies. Geometry optimizations were carried out
using the TurboMole optimizer33 and gradients at the BP8634/def2-
TZVPP35 level of theory. The resolution-of-identity method has been
applied.36 Stationary points were characterized as minima by calculating
the Hessian matrix analytically at this level of theory. NBO orbital
analyses were performed using the program packages Gaussian0337 and
NBO 5.038 at the BP86/def2-TZVPP level.
(8) Green, S. P.; Jones, C.; Junk, P. C.; Lippert, K.-A.; Stasch, A.
Chem. Commun. 2006, 3978.
(9) Sen, S. S.; Jana, A.; Roesky, H. W.; Schulzke, C. Angew. Chem., Int.
Ed. 2009, 48, 8536.
(10) Nagendran, S.; Sen, S. S.; Roesky, H. W.; Koley, D.; Grubm€uller,
H.; Pal, A.; Herbst-Irmer, R. Organometallics 2008, 27, 5459.
(11) Several related dimeric group 14 element(I) dimers bearing
unsupported EꢀE bonds have been reported. See, for example, E = Si:
(a) Reference 7a. (b) Gau, D.; Rodriguez, R.; Kato, T.; Saffron-
Merceron, N.; de Cozar, A.; Cossio, F. P.; Baceiredo, A. Angew. Chem.,
Int. Ed. 2011, 50, 1092. See E = Ge: (c) Wang, W.; Inoue, S.; Yao, S.;
Driess, M. Chem. Commun. 2009, 2661. (d) Leung, W.-P.; Chiu, W.-K.;
Chong, K.-H.; Mak, C. W. Chem. Commun. 2009, 6822. See E = Sn: (e)
Jambor, R.; Kasna, B.; Kirschner, K. N.; Sch€urmann, M.; Jurkschat, K.
Angew. Chem., Int. Ed. 2008, 47, 1650.
’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic data as CIF
b
files for two structural modifications of 8, 9, 10, 12ꢀ15, ButisoH,
and [Li{Li(k2-N,N0-Butiso)2}], ORTEP diagrams for 8 0.5hexane,
3
10, 14, 15, ButisoH, and [Li{Li(k2-N,N0-Butiso)2}], select
metrical parameters for ButisoH and [Li{Li(k2-N,N0-Butiso)2}],
and further details of the calculations. This material is available
free of charge via the Internet at http://pubs.acs.org.
(12) For recent reviews, see: (a) Power, P. P. Organometallics 2007,
26, 4362. (b) Sekiguchi, A. Pure Appl. Chem. 2008, 80, 447. (c) Sasamori,
T.; Han, J.-S.; Hironaka, K.; Takagi, N.; Nagase, N.; Tokitoh, N. Pure
Appl. Chem. 2010, 82, 603 and references cited therein.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: cameron.jones@monash.edu (C.J.), frenking@chemie.
uni-marburg.de (G.F.), andreas.stasch@monash.edu (A.S.).
(13) For example, see: (a) Sen, S. S.; Khan, S.; Roesky, H. W.;
Kratzert, D.; Meindl, K.; Henn, J.; Stalke, D.; Demers, J.-P.; Lange, A.
Angew. Chem., Int. Ed. 2011, 50, 2322. (b) Yeong, H.-X.; Xi, H.-W.; Lim,
K. H.; So, C.-W. Chem.—Eur. J. 2010, 16, 12956. (c) Tavcar, G.; Sen,
S. S.; Roesky, H. W.; Hey, J.; Kratzert, D.; Stalke, D. Organometallics
2010, 29, 3930. (d) Sen, S. S.; Roesky, H. W.; Meindl, K.; Stern, D.;
Henn, J.; Stueckl, A. C.; Stalke, D. Chem. Commun. 2010, 46, 5873. (e)
Sen, S. S.; Tavcar, G.; Roesky, H. W.; Kratzert, D.; Hey, J.; Stalke, D.
Organometallics 2010, 29, 2343. (f) Yeong, H.-X.; Lau, K.-C.; Xi, H.-W.;
Hwa Lim, K.; So, C.-W. Inorg. Chem. 2010, 49, 371.
’ ACKNOWLEDGMENT
This research was supported by the Australian Research
Council (fellowships for C.J. and A.S. and Discovery Project
Grant DP0879508), the U.S. Air Force Asian Office of Aerospace
Research and Development, and the Deutsche Forschungsge-
meinschaft (Grant FR641/25-1 to G.F.). The EPSRC is also
thanked for access to the U.K. National Mass Spectrometry
Facility.
(14) Jones, C.; Schulten, C.; Fohlmeister, L.; Stasch, A.; Murray,
K. S.; Moubaraki, B.; Kohl, S.; Ertem, M. Z.; Gagliardi, L.; Cramer, C. J.
Chem.—Eur. J. 2011, 17, 1294.
(15) (a)Jin, G.; Jones, C.; Junk, P. C.; Lippert, K.-A.; Rose, R. P.; Stasch,
A. New J. Chem. 2009, 33, 64. (b) Coles, M. P. Dalton Trans. 2006, 985.
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(17) Lu, L.-D.; Li, J.-J.; Yang, W.; Ren, C.-X.; Ding, Y.-Q. Acta
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dx.doi.org/10.1021/ic200682p |Inorg. Chem. 2011, 50, 12315–12325

Inorganic Chemistry
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