Synthesis, structure, and reactivity of a dimeric zinc(I) Compound stabilized by a sterically demanding diiminophosphinate ligand

Article abstract of DOI:10.1002/chem.201202560

The new sterically demanding aminoiminophosphorane Ph₂P(=NDip) (NHDip) (Dip=C₆H₃-2,6-iPr₂; LH, 1) has been prepared as a precursor to the potassium complex [LK] (2) and a series of heteroleptic zinc(II) complexes, namely [(LZnBr)₂] (3), [LZnMe] (4), [LZnEt] (5), and [(LZnI)₂] (6). The products have been obtained either through a salt metathesis route by using complex 2 and ZnBr₂ to give compound 3, through a direct reaction of ligand precursor 1 and ZnR ₂ (R=Me or Et) yielding complexes 4 or 5, respectively, or through iodination of complexes 4 or 5 by using I₂ to afford compound 6. Reduction of the heteroleptic zinc(II) halide complexes 3 or 6 by using a dimeric magnesium(I) compound as a selective, stoichiometric, and soluble reducing agent afforded the new zinc(I) dimer [(LZn)₂] (7) in good yield. Compounds 1-7 were crystallographically and spectroscopically characterized and the coordination behavior of the diiminophosphinate ligand has been investigated and compared with related CN-based ligands. An initial reactivity study has been carried out on [(LZn)₂] (7) by using small-scale reactions and the oxidative addition of small alkyl halides across the Zn-Zn bond has been found to generate equimolar amounts of the alkyl complexes 4 or 5 and the halide complexes 3 or 6, respectively.

Full text of DOI:10.1002/chem.201202560

FULL PAPER
DOI: 10.1002/chem.201202560
Synthesis, Structure, and Reactivity of a Dimeric Zinc(I) Compound
Stabilized by a Sterically Demanding Diiminophosphinate Ligand
Andreas Stasch*[a]
Dedicated to Professor Cameron Jones on the occasion of his 50th birthday
Abstract: The new sterically demand-
ing aminoiminophosphorane Ph2P-
(=NDip)ACHTUNGTRENNUNG(NHDip) (Dip=C6H3-2,6-iPr2;
plexes 4 or 5, respectively, or through
iodination of complexes 4 or 5 by using
I2 to afford compound 6. Reduction of
the heteroleptic zinc(II) halide com-
plexes 3 or 6 by using a dimeric magne-
sium(I) compound as a selective, stoi-
chiometric, and soluble reducing agent
afforded the new zinc(I) dimer
[(LZn)2] (7) in good yield. Compounds
1–7 were crystallographically and spec-
troscopically characterized and the co-
ordination behavior of the diimino-
phosphinate ligand has been investigat-
ed and compared with related CN-
based ligands. An initial reactivity
study has been carried out on [(LZn)2]
(7) by using small-scale reactions and
the oxidative addition of small alkyl
LH, 1) has been prepared as a precur-
sor to the potassium complex [LK] (2)
and a series of heteroleptic zinc(II)
complexes, namely [(LZnBr)2] (3),
[LZnMe] (4), [LZnEt] (5), and
[(LZnI)2] (6). The products have been
obtained either through a salt metathe-
sis route by using complex 2 and ZnBr2
to give compound 3, through a direct
À
halides across the Zn Zn bond has
been found to generate equimolar
amounts of the alkyl complexes 4 or 5
and the halide complexes 3 or 6, re-
spectively.
Keywords: magnesium
· N,P li-
gands · reduction · subvalent com-
pounds · zinc
reaction of ligand precursor
1 and
ZnR2 (R=Me or Et) yielding com-
À
Introduction
ed by neutral substituted pyridines.[3f] The majority of Zn
Zn-bonded complexes however feature a range of sterically
Dimeric zinc(I) compounds of the general formula
[RZnZnR] (R=monoanionic ligand) are known as stable
compounds since Carmona and co-workers presented the
synthesis and structure of [Cp*ZnZnCp*] (Cp*=C5Me5) in
2004.[1] Initially, this complex was obtained by the reaction
of [Cp*2Zn] with ZnEt2 and subsequently, an improved syn-
thesis from [Cp*K], KH, and ZnCl2 was reported.[2] Since
then, a range of ligands were successfully employed to yield
demanding anionic N,N’-chelating ligands such as the b-di-
ketiminate examples [{(Dipnacnac)Zn}2]
(
Dipnacnac=HC-
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
of unusual metal cluster compounds having a central transi-
tion metal that are rich in zinc have been investigated[4] and
À
first examples with Zn Zn-bonded fragments in the coordi-
nation sphere of transition metal ions have been prepared.[5]
2+
À
À
stable complexes of the Zn2
ion with a Zn Zn single
The chemistry of Zn Zn-bonded complexes has already
bond.[3] These complexes are typically obtained by reduction
of zinc(II) precursor complexes with strong reducing agents
such as Na, K, KC8, and so on, or by ligand substitution re-
been reviewed[6] and a limited account of their further reac-
tivity has already been forthcoming.[6,7] An exciting develop-
ment is furthermore the first catalytic use of a zinc(I) dimer
in hydroamination reactions.[8] Here, we report on the syn-
thesis of some heteroleptic zinc(II) complexes, a new zinc(I)
dimer, stabilized by a new sterically demanding diimino-
phosphinate ligand, and an initial investigation of its reactiv-
ity.
À
actions on [Cp*ZnZnCp*]. Notable examples of Zn Zn-
bonded complexes include the terphenyl-stabilized complex
[Ar’ZnZnAr’] (Ar’=C6H3-2,6-Dip2) (Dip=C6H3-2,6-iPr2) as
part of a series of dimeric Group 12 metal(I) complexes
with two-coordinated linear metal atoms,[3o] and a complex
featuring the isolated dication [Zn2ACTHNUTRGNEUNG
(dmap)6]2+ (DMAP=
C5H4N-4-NMe2) in where the Zn atoms are solely coordinat-
Results and Discussion
The new diiminophosphinate ligand [Ph2PACTHNUTRGNEUNG
(NDip)2]À (i.e.,
[a] Dr. A. Stasch
LÀ) was prepared with the target of introducing a new steri-
cally demanding N,N’-chelating ligand with favorable prop-
erties such as good solubility and crystallizing properties to
the fields of complexes with rare oxidation states and coor-
dination modes, and unusually bonded molecular fragments.
School of Chemistry, Monash University
PO Box 23, Melbourne, VIC, 3800 (Australia)
Fax : (+61)399054597
E-mail: Andreas.Stasch@monash.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202560.
Chem. Eur. J. 2012, 18, 15105 – 15112
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15105

ferent Dip substituents and suggest a localized aminoimino
configuration at room temperature or at 608C in solution.
The crystal structure of compound 1 (see Figure 1) shows lo-
À
calized P=N (ꢀ1.54 ꢁ) and P N(H) (ꢀ1.68 ꢁ) bonds in 1’
(see Table 1) although a polymorph (1’’) has been character-
ized (see the Supporting Information for an image) that con-
À
A central phosphorus atom was desirable to allow additional
reaction monitoring by 31P NMR spectroscopy on small
scale reactions. The overall geometry of the diiminophosphi-
nate unit is comparable to the related sterically demanding
amidinate and guanidinate class of ligands[9] and is expected
to show some similarities with the chemistry of those and re-
lated CN-based ligand systems.
tains two P N distances that average (ꢀ1.61 ꢁ) the P=N
À
and P N(H) bonds in the localized polymorph 1’ (see
Table 1). The central phosphorus(V) center shows a distort-
ed tetrahedral arrangement with a determined N-P-N angle
range of approximately 108–1178 (see Table 1). This geome-
try contrasts with the large range of anionic ligands that are
À
based on predominantly planar unsaturated C N systems,
The target aminoiminophosphorane 1 (LH) is prepared
such as related amidinates and guanidinates.[9] However, re-
by a Staudinger reaction[10] of a phosphinoamine[11] with
lated isoelectronic sila-bisamido ligands with the general for-
[12]
2À
DipN3
in a straightforward manner (see Scheme 1) in
mula R2Si
(NAr)2 (e.g., R=Me, Ph Ar=Dip) are known
good yield, similar to closely related diiminophosphinate
ligand precursors.[13,14] The compound is a crystalline solid
that is well soluble in hydrocarbons including hexane. The
1H and 13C{1H} NMR spectra display resonances for two dif-
and have previously been successfully employed to stabilize
[3l]
À
Zn Zn-bonded complexes. Direct comparisons in here are
mainly made with complexes carrying ligands of similar
steric bulk (i.e., with NDip groups).
Compound 1 is readily de-
protonated with standard re-
agents to give complexes of the
anion LÀ. For example, the re-
action of equimolar LH (1) and
[K{NACHTNURTGNEG(UN SiMe3)2}] cleanly affords
the potassium salt [LK] (2), in
very good yield (see Scheme 1).
The compound has a good solu-
bility in THF, but shows only
poor solubility in aromatic sol-
vents such as benzene and tol-
uene and crystallizes from the
latter as a solvent-free complex
in one-dimensional polymeric
chains, see Figure 1 for an ex-
cerpt of the polymer. Notable is
the coordination of the potassi-
Scheme 1. Synthesis of compounds 1–6. a) DipN3, ÀN2; b) [K{N
d) ZnR2, ÀRH; e) I2, ÀRI.
G
U
Table 1. Selected interatomic distances [ꢁ] and angles [8] for compounds 1–8.
1’
1’’
2
3·C7H8
4·0.5C6H14
5
6·C6H6
7·0.5C6H6 (7’) 7·1.5C6H14 (7’’)
8
À
P N
1.6842(15)
1.5428(16)
1.6855(14)[a]
1.5446(16)[a]
–
1.6068(14) 1.5681(19) 1.6187(19) 1.612(3)
1.6110(14) 1.623(2)
1.6154(14) 1.627(2)
1.6099(17)
1.6017(19)
1.6061(18)
1.6133(18)
2.0163(18)
2.0517(17)
2.0270(17)
2.0383(18)
1.6051(16)
1.6086(16)
1.6129(16)
1.6127(16)
2.0489(15)
2.0287(15)
2.0410(15)
2.0267(15)
2.3147(6)
1.627(3)
1.630(3)
1.6169(14) 1.5703(17) 1.6266(19) 1.619(3)
1.613(3)[a]
1.616(4)[a]
À
Zn N
–
–
–
–
1.9980(18) 2.014(3)
2.0133(19) 2.041(3)
2.031(4)[a]
2.0292(14) 2.015(2)
2.0197(14) 2.022(2)
–
2.009(3)[a]
–
2.4052(6)
2.5018(6)
1.941(4)
1.950(2)
(E=C)
2.6668(6) 2.3132(9)
2.6021(6) (E=Zn)
(E=I)
2.5325(7)[b]
2.5776(6)[b]
2.6869(7)[c]
2.6640(6)[c]
À
Zn E
1.932(5)[a]
N
A
ACHTUNGTRNE(NUNG E=Zn)
A
N
ACHTUNGTRENNUNG
AHCTUNGTERG(NNUN E=I)
N-P-N 117.34(9)
111.40(8)[a]
108.08(7)
2.609
125.29(10) 98.68(10)
2.787 2.462
99.08(18)
98.80(18)[a]
2.458
98.51(7)
2.444
97.94(11) 99.07(9)
99.21(9)
99.15(8)
99.13(8)
2.446
108.04(13)
2.636
N···N
2.757
2.451
2.443
2.452
2.670[a]
2.452[a]
2.455
[a] From a second independent molecule in the asymmetric unit. [b] Terminal. [c] Bridging.
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Chem. Eur. J. 2012, 18, 15105 – 15112

Synthesis, Structure, and Reactivity of a Dimeric Zinc(I) Compound
FULL PAPER
for metrical data. For compari-
son, closely related heteroleptic
zinc halide complexes with ZnII
in four-membered chelates are
known in [{(Piso)ZnX}2] com-
plexes
(Piso=tBuCACHTUNGTRENNUNG(NDip)2,
X=Cl, Br, I)[17] that are broad-
ly isostructural and show very
À
À
comparable Zn N and Zn
halide distances though their
N···N separations are signifi-
cantly smaller (see below).
The heteroleptic diimino-
phosphinato zinc alkyl com-
plexes [LZnR] (R=Me (4), Et
(5)) can be obtained in good
yields by treating the phosphor-
ane 1 with ZnR2 (R=Me or Et,
respectively). The complexes
are monomeric with three-coor-
dinate ZnII centers in the solid
state, see Figure 1 and the Sup-
porting information for pictures
of compounds 4 and 5, respec-
tively, and Table 1 for metrical
data. The overall structures as
À
À
well as the Zn N and Zn C
bond lengths compare well with
related sterically demanding
complexes with a similar ligand
environment,[17,18] though it is
worth to point out that [(Pi-
so)ZnMe][17a] shows
(arene)-complexation
a
N,C-
mode,
G
Figure 1. Molecular structures of compounds 1’, 2, 4, 6, 7’, and 7’’ (30% thermal ellipsoids). Hydrogen atoms
(except NH in 1’) and solvent molecules are omitted for clarity.
likely due to the smaller N···N
separation in the amidinate
ligand (ꢀ2.19–2.20 ꢁ in ZnII
um ion to the anionic ligand. It involves no P···N contacts
and the K+ is entirely coordinated by arene···K interactions
from two Dip substituents and two phenyl groups from a
neighboring ligand. The only other crystallographically char-
acterized diiminophosphinato potassium complex features
an N,N’-coordination mode and additional THF complexa-
tion.[15] For comparison, some homoleptic (solvent-free) po-
tassium complexes of related monoanionic N,N’-chelating li-
gands bearing the Dip substituent have been crystallographi-
cally characterized and all show at least one K···N contact.[16]
The coordination mode in compound 2 can be described as
approximately h4 and h6 to Dip and h1 and h6 to Ph arene
rings with the K···Dip interactions generally being shorter
presumably due to a more negative charge accumulation on
the Dip groups. Complex 2 is suitable for salt metathesis
chemistry and reacts with ZnBr2 to form the heteroleptic
complex [(LZnBr)2] (3) with a N,N’-chelating diiminophos-
phinate ligand. The reaction has been carried out in THF
and work-up with toluene afforded a THF-free dimeric com-
plex with bridging bromide ligands, see Figure 1 and Table 1
complexes) compared with the diiminophosphinate Zn com-
plexes of this work (ꢀ2.44–2.46 ꢁ). The latter values are
still considerably shorter than those for comparable five- or
six-membered N,N’-chelating Zn complexes,[18] and slightly
shorter than those in comparable related dianionic Me2Si-
2À
G
Zn complexes.[3l]
Treatment of the alkyl complexes 4 and 5 with iodine in
an aromatic solvent affords the diiminophosphinato zinc
iodide complex [(LZnI)2] (6) in good yields. Small-scale ex-
periments followed by multinuclear NMR spectroscopy
show that this reaction is facile, high yielding, and produces
RI (R=Me or Et) as the expected volatile byproduct. The
reaction has been carried out on a preparative scale in good
yield by using [LZnEt] (5). This reaction provides a second
convenient access to a mixed diiminophosphinato zinc
halide complex and the desired precursors for reduction re-
actions. Generally, the heteroleptic zinc(II) complexes 3–6
are stable in solution and show no signs of ligand redistribu-
tions to a homoleptic complex, that is, [L2Zn].
Chem. Eur. J. 2012, 18, 15105 – 15112
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A. Stasch
1
Reduction reactions of [(LZnX)2] (X=Br (3), I (6)) with
an excess of K or KC8 largely afforded the potassium salt
[LK] (2). Dimeric magnesium(I) complexes[19] have previ-
ously been successfully used as hydrocarbon-soluble, selec-
tive, and stoichiometric reducing agents towards a range of
inorganic/organometallic substrates.[19a,20] The reaction of
complexes 3 or 6 with [{(Mesnacnac)Mg}2][21] afforded the di-
meric zinc(I) compound [(LZn)2] (7) in good yield (see
Scheme 2). When the reaction is followed by multinuclear
[(LZnH)2] complex formulation was found in the H NMR
spectrum[23] and a molecular ion peak for compound 7 was
observed in the EI mass spectrum and supports the compo-
sition as a metal–metal-bonded species.
The metal complexes of the diiminophosphinate ligand
presented here, compounds 2–7, all show one doublet and
one septet only for the protons of the isopropyl groups at
room temperature in their solution 1H NMR spectra. This
contrasts with related complexes of comparable sterically
demanding N,N’-chelating ligands with Dip substitu-
ents.[3,6,18] To investigate this, we have chosen [LZnEt] (5) as
the most soluble compound of this series for a low-tempera-
1
ture H NMR study. [LZnEt] shows one sharp doublet for
the methyl protons of the isopropyl groups at room temper-
ature that starts to broaden below À30 to À508C and splits
into two broad resonances (at d=0.76 and 1.34 ppm) below
the coalescence temperature of approximately À708C that
are not fully resolved at À858C, the limit of this experiment,
but were well separated. This splitting of the doublet reso-
nances at very low temperature for PN-based species com-
pared with related CN-based ligands suggest a more flexible
coordination behavior compared to the former class and im-
plies a low energy process for the rotation of the iPr groups
or another fluxional process of the anionic ligand. This sug-
gests that the ligand in compounds 2–7 behaves more like a
true anion compared with a more “heterocycle”-like behav-
ior found for CN-based ligands. This difference in the coor-
dination behavior of the diiminophosphinate ligand likely
impacts on the further chemistry of the metal complexes. It
is possible that this flexibility allows reduction of the precur-
sor complexes 3 and 6 with sterically demanding magnesiu-
m(I) dimers at relatively low temperatures and thus facili-
tates the formation of complex 7.
Scheme 2. Synthesis
of
compound
7.
a) [{Mg(Mesnacnac)}2],
À[{Mg(Mesnacnac)
ACHTUNGTRENNUNG(m-X)}2].
NMR spectroscopy at room temperature in deuterated ben-
zene, it is evident that the reaction proceeds rapidly and in
good yield (ꢀ85–90%). Small amounts of the free ligand 1
and Zn metal are also formed. The formation of these by-
products can be suppressed by carrying out the reaction in
toluene at low temperatures with slow warming to room
temperature. Work up and recrystallization from benzene
afforded colorless blocks of [(LZn)2] (7) in up to 76% yield
of the isolated product. For this reduction, even the more
sterically
demanding
magnesium(I)
dimer
[{(Dipnacnac)Mg}2][19b,22] can be used with the precursors 3 or
6 and the reaction proceeds slowly at room temperature and
faster at elevated temperatures to yield complex 7. Attempts
to prepare related zinc(I) dimers with sterically demanding
amidinate or guanidinate ligands through reduction of
zinc(II) halide complexes have failed so far,[17a,b] even de-
spite the use of dimeric magnesium(I) compounds as reduc-
ing agents, and the degradation of the ligand systems has
been observed in one case.[17b]
Access to the soluble zinc(I) dimer 7 with a central P
atom in its ligand framework has allowed an initial investi-
gation of its reactivity by small-scale reactions followed by
1H and 31P{1H} NMR spectroscopy. The addition of the
strong pyridine donor DMAP to 7 led to no changes of the
1H and 31P{1H} NMR spectra at room temperature nor to
any color change. For comparison, DMAP addition to
[Cp*ZnZnCp*] gave the unsymmetrically substituted com-
Compound 7 crystallizes with a full molecule in the asym-
metric unit for two structurally characterized solvates, 7’
from benzene and 7’’ from hexane, respectively, that are
very similar in their overall geometry (see Figure 1). Two
plex [h5-Cp*ZnZn
G
(h1-Cp*)][3j] and DMAP forms the
2ACHTUNGTRENNUNG
adducts
(Arnacnac)]
ACHTUNGTREN(UNGN dmap)MgACHTUNTRGEG(NNUN dmap)CAHTNUGTRENNUGN
(Arnacnac=Mesnacnac or Dipnacnac)[21,22] with the dimeric
2+
diiminophosphinate ligands N,N’-coordinate the Zn2 ion
magnesium(I) compounds [{(Arnacnac)Mg}2] with lengthened
À
À
with a Zn Zn bond length of 2.3132(9) (7’) or 2.3147(6) ꢁ
M M bond lengths and a color change for the coordinated
complexes compared with the uncoordinated ones.
(7’’). This is in the range (ꢀ2.29–2.44 ꢁ) of previously char-
acterized zinc(I) dimers,[1–3,6] but is slightly shorter than
those of other N,N’-chelating examples (ꢀ2.33–2.44 ꢁ).[3,6]
The reaction of the zinc(I) dimer 7 with a stoichiometric
amount of iodine in deuterated benzene afforded the ex-
pected zinc(II) iodide complex 6 at room temperature. A
small-scale reaction of 7 with an excess of iodine led to a
product mixture and the formation of a few crystals of a salt
2+
Some Zn2 complexes bearing N,N’-chelating ligands con-
taining P=N fragments have recently been prepared from
[Cp*ZnZnCp*] as a starting material.[3e] The least-square
planes of the two NPNZn four-membered chelate rings in
complex 7 are twisted by 56.3 (7’) or 54.18 (7’’) relative to
each other and thus are oriented between an orthogonal
and a co-planar arrangement. As expected, no resonance
of the doubly protonated ligand [LH2]2ACHTUNGRTENNG[U I2ZnACHTUGNTREN(NUGN m-I)2ZnI2] (8)
that was crystallographically characterized. Metrical param-
eters of compound 8 are included in Table 1 and an image
can be found in the Supporting Information. Treating com-
plex 7 with an excess of the small alkyl halides RX (MeI,
À
that could be assigned to a Zn H moiety for a possible
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Chem. Eur. J. 2012, 18, 15105 – 15112

Synthesis, Structure, and Reactivity of a Dimeric Zinc(I) Compound
FULL PAPER
EtI, or EtBr) in deuterated benzene led to an oxidative ad-
dition reaction at the Zn2 fragment with an umpolung of the
alkyl group from electrophilic to nucleophilic character.
This reaction forms both the zinc alkyl complexes 4 or 5 and
the zinc halide complexes 3 or 6, respectively, as shown in
Scheme 3. This reaction is reminiscent of the first generation
the formation of organozinc bromides from activated zinc
metal and the reaction rate for the insertion of activated
zinc metal into organic bromides can be faster for tertiary
over secondary over primary organic bromides. A one-elec-
tron transfer process is involved in the rate-determining
step.[25–27] In contrast, the reaction of the diamagnetic com-
plex 7 with alkyl halides may
not be dominated by radical-
type mechanisms and may be
largely dependent upon the
steric profile of the reagents. A
similar general trend has been
found for the reactivity of sub-
strates with related dimeric
magnesium(I) compounds.[19a]
For reactions involving com-
Scheme 3. Reaction of compound 7 with alkyl halides. a) MeI, EtI, or EtBr.
pound 7, traces of activated
of air-sensitive organometallics by Frankland in 1849.[24] The
reaction of 7 proceeds slowly at room temperature or more
rapidly at elevated temperatures (50–708C) and appears
faster for iodides over bromides and methyl- over ethyl-sub-
stituted halides. Temperatures of up to approximately 758C
in sealed NMR tubes can be employed though long reaction
times and higher temperatures will eventually lead to partial
decomposition of complex 7 and the formation of some
finely divided zinc metal, which is expected to have an influ-
ence on the activation reaction of RX. Compound 7 also
reacts with diiodomethane, CH2I2, and initially after partial
conversion at room temperature, some [(LZnI)2] (6) and an-
other complex, which shows a resonance at d=16.6 ppm
(31P NMR spectrum), presumably [LZnCH2I], is formed, but
over time converts to a product mixture with a main product
having a resonance at d=14.2 ppm. This type of activation
may potentially be of interest for Zn–carbenoid reactivity,
for example, the Simmons–Smith reaction.[25] A product
mixture was also found for the reaction of 7 with CBr4. No
reaction of compound 7 has been observed with PhF and
PhI, even at elevated temperatures. Treatment of complex 7
with the secondary halide 2-bromobutane or the tertiary
halide 1-bromoadamantane (AdBr) showed no reaction at
zinc metal, for example, formed during partial decomposi-
tion of 7 (as well as possible traces of Lewis acids, e.g., ZnII
species), may likely have an influence on the activation
chemistry of the alkyl halides, especially at elevated temper-
atures. The apparent different selectivity between activated
zinc metal and complex 7, and presumably other dimeric
zinc(I) compounds, with organic halides could be harnessed
for their selective activation and may lead to complementa-
ry synthetic methods. We are currently investigating the fur-
ther chemistry of compound 7 and related species towards
this goal.
Conclusion
We have synthesized and characterized a potassium complex
and a series of heteroleptic zinc(II) complexes of the new
diiminophosphinate ligand [Ph2PACTHNUTRGNEUNG
(NDip)2]À and compared
the coordination behavior with related CN-based ligand sys-
tems. The reduction of zinc(II) halide complexes with stoi-
chiometric amounts of dimeric magnesium(I) reagents af-
À
forded a new dimeric zinc(I) complex with a Zn Zn bond in
good yield. We performed an initial investigation of the re-
activity of the zinc(I) dimer by using small-scale reactions
and found an activation of small alkyl halides with an umpo-
lung of the alkyl groups yielding 1:1 mixtures of heteroleptic
zinc alkyl and zinc halide complex fragments.
1
room temperature as judged by H and 31P NMR spectrosco-
py. However, after heating to approximately 708C (in case
of 2-bromobutane) or about 808C (in case of AdBr) for sev-
eral hours, the formation of [(LZnBr)2] (3) and another
product (d=13.6 or 13.1 ppm for 2-bromobutane or AdBr,
respectively), likely LZnR, has been observed by
31P{1H} NMR spectroscopy after some decomposition of 7
with zinc metal formation had occurred in both cases. It is
possible that freshly generated and highly active zinc metal,
known to react with the bromoalkanes (RBr) to form
RZnBr,[18,25–27] activates the RBr and subsequently reacts
with compound 7 to form the products.
The reaction of activated zinc metal (e.g., Rieke zinc)
with organic halides (RX) to form organozinc halides has
been investigated as a useful synthetic tool capable of differ-
entiating between unsymmetrical dibromides.[25–27] A pro-
nounced structure–reactivity relationship has been found for
Experimental Section
General considerations: All manipulations were carried out by using
standard Schlenk and glove box techniques under an atmosphere of high
purity nitrogen. Benzene, toluene, hexane, and THF were dried and dis-
tilled over molten potassium whereas pentane was dried and distilled
from Na/K alloy. H, 13C{1H}, and 31P{1H} NMR spectra were recorded on
1
a Bruker DPX 300 or Bruker Avance 400 spectrometer in deuterated
benzene, toluene, and/or tetrahydrofuran and were referenced to the re-
sidual 1H or 13C{1H} resonances of the solvent used, or external aqueous
H3PO4 solutions. IR spectra were recorded by using a Perkin–Elmer RXI
FT-IR spectrometer as Nujol mulls between NaCl plates. Melting points
Chem. Eur. J. 2012, 18, 15105 – 15112
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15109

A. Stasch
were determined in sealed glass capillaries under nitrogen and are uncor-
rected. Elemental analyses were performed by the Elemental Analysis
Service at London Metropolitan University. DipN3,[12] DipNHPPh2,[11]
and [{(Mesnacnac)Mg}2][21] were prepared according to literature proce-
dures. All other reagents were used as received (Aldrich chemical com-
pany). Small-scale reactions (typically on ꢀ15 mg complex samples) were
carried out in 5 mm NMR tubes with J. Young stopcock in dried deuter-
ated benzene (ꢀ0.55 mL). The reactivity is qualitatively described in the
1.06 equiv). The mixture was stirred briefly at À808C then with slow
warming to room temperature overnight. Toluene (4 mL) was added, all
volatiles removed, and the residue was dried under vacuum. The residue
was extracted into warm (ꢀ508C) toluene (20 mL) and filtered. Concen-
tration (to ꢀ10 mL) and cooling to 48C yielded compound 3 as a color-
less crystalline material that was dried under vacuum. A second crop was
obtained after further concentrating (to ꢀ4 mL) and cooling of the solu-
tion. Yield: 0.38 g (64%); m.p. 278–2828C (melts); 1H NMR (C6D6,
main text. A few small crystals of [LH2]
tained from the reaction of [(LZn)2] (7) with an excess of I2.
LH (Ph2P(=NDip)(NHDip)) (1): A solution of DipN3 (4.59 g, 22.6 mmol,
2ACHTUNGTERNNUG[I2ZnAHCUTGNTREN(NUGN m-I)2ZnI2] (8) were ob-
400.1 MHz, 303 K): d=1.00 (d, J
(sept, J(H,H)=6.8 Hz, 8H; CH(CH3)2), 6.71–7.71 ppm (m, 32H; Ar-H);
13C{1H} NMR (C6D6, 75.5 MHz, 303 K): d=24.6 (CH
(CH3)2), 28.9 (CH-
(CH3)2), 124.1 (d, J(C,P)=1.5 Hz, Ar-C), 124.4 (d, J(C,P)=2.0 Hz, Ar-
C), 131.0 (d, J(C,P)=2.0 Hz, Ar-C), 132.4 (d, J(C,P)=6.6 Hz, Ar-C),
134.6 (d, J(C,P)=73.1 Hz, Ar-C), 139.8 (Ar-C), 146.9 ppm (d, J(C,P)=
ACHUTGTNRNE(NGU H,H)=6.8 Hz, 48H; CHAHCTUNGTRENN(UNG CH3)2), 3.98
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
1.15 equiv) in toluene (30 mL) was slowly added to a cooled (À808C) sol-
ution of Ph2PNHDip (7.09 g, 19.6 mmol, 1.0 equiv) in toluene (20 mL).
The mixture was stirred and slowly warmed to room temperature over-
night and nitrogen evolved. The mixture was further stirred for another
day at room temperature, stirred for 3 h at approximately 608C and sub-
sequently all volatiles were removed under reduced pressure. The residue
was dried under vacuum at approximately 508C yielding a honey-like
paste. Recrystallization from pentane (ꢀ40 mL, 30 to À308C) afforded
colorless crystals of ligand 1. A second crop was obtained after further
concentrating and cooling. Yield: 7.48 g (71%); m.p. 102–1048C (melts),
liquid >3408C; 1H NMR (C6D6, 400.1 MHz, 303 K): d=0.97 (d, J-
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
3.9 Hz, Ar-C) (N.B.: one resonance is missing and is believed to be
hidden by the strong residual solvent peak at around d=128 ppm);
31P{1H} NMR (C6D6, 162.0 MHz, 303 K): d=14.4 ppm(s); IR (nujol): n˜ =
1620 (w, br), 1589 (w), 1460 (s), 1436 (s), 1377 (s), 1365 (m), 1260 (s),
1185 (m), 1111 (s), 1024 (s), 797 (s), 750 (m), 695 cmÀ1 (m); elemental
analysis calcd (%) for C72H88Br2N4P2Zn2 (1362.03 gmolÀ1, vacuum dried
sample): C 63.49, H 6.51, N 4.11; found: C 62.87, H 6.40, N 4.09.
AHCTUNGTERNG[NUN LZnMe] (4): Me2Zn (0.440 mL of a 1.0m solution in heptane,
A
ACHTUGTNRNENUG(CH3)2), 1.07 (d, JACTHUNGTRENNNUG
0.440 mmol, 1.07 equiv) was added to a cooled (À808C) solution of 1
(0.220 g, 0.410 mmol, 1.0 equiv) in hexane (8 mL). The mixture was
warmed to room temperature and stirred for 30 min. The mixture was
warmed to 408C and concentrated under reduced pressure at this temper-
ature to approximately 4 mL. Slow cooling to room temperature afforded
a crop of colorless crystals of compound 4. Further concentrating (to
ꢀ2 mL) and cooling of the supernatant solution to 48C gave a second
crop of 4. Crystals suitable for X-ray diffraction were obtained from cy-
clohexane. Yield: 0.20 g (79%); m.p. slow decomp. above about 1508C,
full decomp. above about 2008C; 1H NMR (C6D6, 400.1 MHz, 303 K):
A
J
A
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
143.8 (Ar-C), 147.5 ppm (Ar-C); 31P{1H} NMR (C6D6, 162.0 MHz, 303 K):
d=À15.7 ppm (s); 31P{1H} NMR (CDCl3, 121.5 MHz, 303 K): d=
À13.5 ppm (s); IR (nujol): n˜ =3373 (m, NH), 1654 (w, br), 1588 (m), 1455
(s), 1436 (s), 1366 (s,), 1330 (m), 1298 (m), 1258 (m), 1189 (m), 1116 (s),
1062 (m), 1029 (m), 906 (m), 795 (m), 754 (s), 696 cmÀ1 (s); elemental
analysis calcd (%) for C36H45N2P (536.73 gmolÀ1): C 80.56, H 8.45, N
5.22; found: C 79.93, H 8.38, N 5.17.
d=À0.04 (s, 3H; ZnCH3), 1.01 (d, J
ACHUTGTNRNE(GUN H,H)=6.8 Hz, 24H; CHACHTUNGTRENNUNG(CH3)2),
3.81 (sept, JACTHNUTRGENN(UG H,H)=6.8 Hz, 4H; CHACHTUGNTRENN(UNG CH3)2), 6.74–7.34 ppm (m, 16H; Ar-
H); 13C{1H} NMR (C6D6, 75.5 MHz, 303 K): d=À13.4 (ZnCH3), 24.1
(CHACTHUNGNERT(NNUG CH3)2), 29.3 (CHACHTUNRTGEN(GUNN CH3)2), 124.0 (d, JCAHTNUGTREN(GUNN C,P)=2.3 Hz, Ar-C), 124.2 (d,
J
N
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
N.B.: Compound preparation and storage was carried under inert condi-
tions under nitrogen, but the crystallized product may be stored in a
closed vial in air.
JACHTUNGTRENNUNG
31P{1H} NMR (C6D6, 162.0 MHz, 303 K): d=14.8 ppm (s); IR (nujol): n˜ =
1588 (m), 1573 (w), 1461 (s), 1432 (s), 1380 (s), 1360 (s), 1318 (s), 1283
(m), 1258 (s), 1239 (m), 1211 (s), 1188 (m), 1158 (m), 1115 (s), 1056 (m),
1043 (s), 995 (s), 932 (m), 813 (m), 803 (m), 784 (s), 753 (m), 743 (m),
715 (m), 696 (s), 666 (s), 648 (m), 616 (m), 598 cmÀ1 (s); elemental analy-
sis calcd (%) for C37H47N2PZn (616.15 gmolÀ1): C 72.13, H 7.69, N 4.55;
found: C 72.04, H 7.73, N 4.44.
[LK] (2): Toluene (20 mL) was added to
a mixture of 1 (2.01 g,
3.74 mmol, 1.0 equiv) and [K{N(SiMe3)2}] (0.784 g, 3.93 mmol, 1.05 equiv)
AHCTUNGTRENNUNG
and the mixture was vigorously stirred for 2 h and a precipitate formed.
The mixture was concentrated to approximately 10 mL under reduced
pressure and hexane (15 mL) was added. The precipitate of 2 was filtered
off, dried under vacuum and the solution was cooled to À308C yielding a
small second crop of 2. Yield: 2.02 g (94%); m.p. slow decomp. above ap-
proximately 2708C; full decomp. around 298–3028C; 1H NMR (C6D6,
AHCTUNGTRENNUNG
a
1
1.73 mmol, 1.0 equiv) in hexane (30 mL). The mixture was warmed to
room temperature and stirred for 1 h. The mixture was stirred at 408C
for 20 min and concentrated under reduced pressure at this temperature
to approximately 6 mL. Cooling to 48C afforded a crop of colorless crys-
tals of complex 5. Further concentrating (to ꢀ2–3 mL) and cooling of the
supernatant solution gave a second smaller crop of 5. Yield: 0.78 g
(71%); m.p. slow decomp. above 2008C; full decomp. around 3008C;
[D8]THF (ꢀ4:1), 400.1 MHz, 303 K): d=1.27 (d, J
ACHTUNGTREN(NUGN H,H)=6.8 Hz, 24H;
CH(CH3)2), 4.24 (sept, J(H,H)=6.8 Hz, 4H; CH(CH3)2), 6.94–7.00 (m,
G
N
ACHTUNGTRENNUNG
2H; Ar-H), 7.18–7.37 (m, 10H; Ar-H), 7.98–8.06 ppm (m, 4H; Ar-H);
13C{1H} NMR (C6D6, [D8]THF (ꢀ4:1), 100.6 MHz, 303 K): d=24.0 (CH-
A
ACHTUNGTRENNUNG(CH3)2), 116.7 (d, JACHTUTGNREN(NUNG C,P)=2.1 Hz, Ar-C), 122.7 (Ar-C),
A
ACHTUNGTRENNUNG
1H NMR (C6D6, 400.1 MHz, 303 K): d=0.89 (q, J
ZnCH2CH3), 0.97 (d, (H,H)=6.8 Hz, 24H; CH(CH3)2), 1.45 (t, J-
(H,H)=8.0 Hz, 3H; ZnCH2CH3), 3.84 (sept, J(H,H)=6.8 Hz, 4H; CH-
(CH3)2), 6.68–7.36 ppm (m, 16H; Ar-H); 13C{1H} NMR (C6D6, 75.5 MHz,
303 K): d=1.5 (ZnCH2CH3), 12.5 (ZnCH2CH3), 24.0 (CH(CH3)2), 29.3
(CH(CH3)2), 124.0 (d, J(C,P)=2.3 Hz, Ar-C), 124.2 (d, J(C,P)=3.0 Hz,
Ar-C), 127.9 (d, J(C,P)=11.6 Hz, Ar-C), 131.1 (d, J(C,P)=2.7 Hz, Ar-C),
131.9 (d, J(C,P)=8.6 Hz, Ar-C), 135.3 (d, J(C,P)=96.0 Hz, Ar-C), 141.2
(Ar-C), 146.2 ppm (d,
(C,P)=5.2 Hz, Ar-C); 31P{1H} NMR (C6D6,
162.0 MHz, 303 K): d=14.2 ppm (s); IR (nujol): n˜ =1588 (m), 1574 (w),
1462 (s), 1454 (s), 1434 (s), 1378 (s), 1360 (s), 1333 (m), 1317 (s), 1260 (s),
1187 (m), 1115 (s), 1098 (m), 1043 (m), 996 (s), 952 (m), 931 (m), 814
(m), 791 (s), 773 (m), 754 (m), 742 (m), 717 (m), 697 (s), 666 (m), 616
AHCTUNGTREG(NNUN H,H)=8.0 Hz, 2H;
N
ACHTUNGTRENNUNG
Ar-C) (N.B.: one resonance is missing and is believed to be hidden by
the strong residual solvent peak at around d=128 ppm); 31P{1H} NMR
(C6D6, [D8]THF (ꢀ4:1), 162.0 MHz, 303 K): d=À25.7 ppm (s);
31P{1H} NMR (C6D6, 162.0 MHz, 303 K): d=À21.9 (s); IR (nujol): n˜ =
1577 (m), 1460 (s), 1427 (s), 1376 (s), 1362 (s), 1351 (s), 1298 (m), 1289
(m), 1259 (s), 1173 (m), 1153 (m), 1135 (m), 1114 (m), 1103 (s), 1072 (m),
1057 (m), 1045 (m), 1030 (m), 994 (m), 792 (m), 778 (m), 764 (m), 756
(s), 749 (s), 709 (s), 690 (m), 587 cmÀ1 (m); elemental analysis calcd (%)
for C36H44KN2P (574.82 gmolÀ1): C 75.22, H 7.72, N 4.87; found: C 75.19,
H 7.83, N 4.91.
J
G
ACHTUNGTRENNUNG
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
G
A
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
JACHTUNGTRENNUNG
ACHTUNGTRENNUNG
2
15110
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 15105 – 15112

Synthesis, Structure, and Reactivity of a Dimeric Zinc(I) Compound
FULL PAPER
(m), 594 cmÀ1 (s); elemental analysis calcd (%) for C38H49N2PZn
(630.17 gmolÀ1): C 72.43, H 7.84, N 4.45; found: C 72.62, H 7.69, N 4.51.
der in two isopropyl groups each in 5 and 7·0.5C6H6 have been modeled
with two positions for each carbon atom. Semi-empirical (multi-scan) ab-
sorption corrections were performed on all datasets. The relatively low
data completeness for 4·0.5C6H14 and 7·1.5C6H14 are due to the limita-
tions of the experimental set-up (one phi scan) at the synchrotron. Two
datasets on a crystal of 7·1.5C6H14 in different orientations were merged,
but still yield relatively low data values. The Supporting Information con-
tains selected crystallographic details in Table S1 and images of the mo-
lecular structures of 1’’, 3·C7H8, 5, and 8. CCDC-890439 (1’), 890440 (1’’),
890441 (2), 890442 (3·C7H8), 890443 (4·0.5C6H14), 890444 (5), 890445
(6·C6H6), 890446 (7·0.5C6H6), 890447 (7·1.5C6H14), and 890448 (8) con-
tain the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
ACHTUNGTRENNUNG[(LZnI)2] (6): A solution of I2 (0.370 g, 1.46 mmol, 1.22 equiv) in toluene
(15 mL) was added dropwise to a cooled (08C) solution of 5 (0.75 g,
1.19 mmol, 1.0 equiv) in toluene (8 mL) until the iodine color does not
disappear any more upon addition (or the process is slowed considera-
bly). The mixture was stirred briefly at room temperature, concentrated
under reduced pressure to approximately 12 mL and hexane (10 mL) was
added. After cooling to À308C, a crop of colorless crystalline material of
compound 6 was obtained and dried under vacuum. The supernatant sol-
ution was concentrated at approximately 408C to about 1–2 mL and
hexane (3 mL) was added. Cooling afforded a smaller second crop of 6.
Crystals suitable for X-ray diffraction were obtained from benzene.
Yield: 0.74 g (85%); m.p. about 235–2418C (melts); decomp. above
3108C; 1H NMR (C6D6, 400.1 MHz, 303 K): d=1.01 (d, J
48H; CH(CH3)2), 3.83 (sept, J(H,H)=6.8 Hz, 8H; CH
7.70 ppm (m, 32H; Ar-H); 13C{1H} NMR (C6D6, 75.5 MHz, 303 K): d=
24.7 (CH(CH3)2), 29.3 (CH(CH3)2), 124.2 (d, (C,P)=2.1 Hz, Ar-C),
124.9 (d, J(C,P)=2.8 Hz, Ar-C), 131.4 (d, J(C,P)=2.7 Hz, Ar-C), 132.3
(d, J(C,P)=8.8 Hz, Ar-C), 134.1 (d, J(C,P)=97.0 Hz, Ar-C), 139.7 (Ar-
C), 146.7 ppm (d, J(C,P)=5.1 Hz, Ar-C) (N.B.: one resonance is missing
and is believed to be hidden by the strong residual solvent peak at
around d=128 ppm); 31P{1H} NMR (C6D6, 162.0 MHz, 303 K): d=
17.1 ppm (s); IR (nujol): n˜ =1630 (w, br), 1587 (w), 1573 (m), 1462 (s),
1384 (s), 1366 (s), 1316 (m), 1256 (m), 1233 (m), 1202 (m), 1106 (s), 1043
(m), 984 (m), 930 (m), 822 (m), 795 (s), 745 (m), 716 (m), 698 (s),
670 cmÀ1 (m); elemental analysis calcd (%) for C78H94I2N4P2Zn2
(C72H88I2N4P2Zn2·C6H6, 1534.14 gmolÀ1): C 61.07, H 6.18, N 3.65; found:
C 60.95, H 6.22, N 3.73.
ACHTUNGTRENNUNG
G
R
ACHTUNGTRENNUNG
Acknowledgements
G
E
JACHTUNGTRENNUNG
G
ACHTUNGTRENNUNG
A.S. is grateful to the Australian Research Council for support and a fel-
lowship. The Monash University Research Accelerator Program is highly
acknowledged. Part of this research was undertaken on the MX1 beam-
line at the Australian Synchrotron, Victoria, Australia.
G
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
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ACHTUNGTRENNUNG
3
90.8 mmol, 1.08 equiv) and the mixture was slowly warmed to room tem-
perature overnight with vigorous stirring. The volatiles were removed
under reduced pressure and the residue was extracted with benzene (
ꢀ6 mL) and filtered. The solution was slowly concentrated (to ꢀ1 mL)
under reduced pressure at approximately 408C and slowly cooled to
room temperature until crystallization started. Then the mixture was fur-
ther stored at 48C to afford colorless blocks of complex 7·0.5C6H6. Crys-
tals of 7·1.5C6H14 were obtained from an extraction into warm hexane.
Yield: 80 mg (76%); m.p. slow decomp. above about 1378C (turns grey
at higher temperatures); 1H NMR (C6D6, 400.1 MHz, 303 K): d=1.00 (d,
J
ACHTUNGTRENNUNG
G
ACHTUNGTERNNUG(CH3)2), 3.79 (sept, JACHTUNGTREN(NNGU H,H)=6.8, 8H; CH-
E
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
J
R
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
is believed hidden by the strong residual solvent peak at around d=
128 ppm); 31P{1H} NMR (C6D6, 162.0 MHz, 303 K): d=12.3 ppm (s); IR
(nujol): n˜ =1620 (w, br), 1589 (m), 1556 (m), 1459 (s), 1435 (s), 1379 (s),
1366 (s), 1318 (m), 1261 (s), 1233 (m), 1212 (s), 1118 (s), 1101 (m), 1043
(m), 992 (s), 931 (m), 809 (m), 775 (s), 750 (m), 716 (m), 699 (s),
660 cmÀ1 (m); MS (EI): m/z (%): 1202.7 [M]+ (8, correct isotope pat-
tern), 536.4 [Ph2PACHTUNGTRENNUNG
(NDip)2H]+ (33), 360.2 [Ph2PNDip]+ (100); elemental
analysis calcd (%) for C72H88N4P2Zn2 (1202.22 gmolÀ1, vacuum dried
sample): C 71.93, H 7.38, N 4.66; found: C 71.87, H 7.46, N 4.73.
X-ray crystallography: Suitable crystals were mounted in silicone oil and
were either measured by using a Nonius Kappa CCD diffractometer (1’,
2, 7·0.5C6H6), a Bruker X8 Apex II diffractometer (2), or an Oxford Xca-
libur Gemini Ultra diffractometer (5, 6·C6H6) with MoKa radiation (l=
0.71073 ꢁ), or at the MX1 beamline at the Australian Synchrotron
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Received: July 18, 2012
Published online: October 5, 2012
15112
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 15105 – 15112