Bulky guanidinate stabilized homoleptic magnesium, calcium and zinc complexes and their catalytic activity in the Tishchenko reaction

Article abstract of DOI:10.1016/j.jorganchem.2015.02.044

Bulky guanidinate stabilized homoleptic complexes of [LM(thf)_nL] [L = {ArNC (NⁱPr₂)NAr} (Ar = 2,6- Me₂-C₆H₃)]; M = Mg, Ca, and Zn; n = 0-1) have been synthesized with three different synthetic routes. First, treatment of bis{bis(trimethylsilyl)amide}s of Mg, Ca, and Zn i.e. [M{(N(SiMe₃)₂}₂] (M = Mg, Ca(thf)₂, and Zn) with free guanidine ligand (LH) at 80 ^°C, elimination of NH(SiMe₃)₂ and formation of [LM(thf)_nL] (M = Mg, Ca and Zn; n = 0-1) (3-5) complexes were occurred. Second, the reaction of two equivalents of potassium salt of LH with metal dihalides via salt metathesis led to the formation of homoleptic complexes. Third, the reaction between MR₂ (M = Mg, Ca, Zn; R = alkyl or OTf) and LH at reflux temperature, formation of bis(guanidinate) magnesium, calcium, and zinc complexes have been observed. The solid state structures of LH(1), LK(2) and all three bis(guanidinate) magnesium, calcium and zinc complexes (3-5) were confirmed by X-ray structural analysis. Furthermore, dimerization of a range of aromatic, heteroaromatic and aliphatic aldehydes (Tishchenko reaction) has been demonstrated by using catalysts 3-5.

Full text of DOI:10.1016/j.jorganchem.2015.02.044

Journal of Organometallic Chemistry 785 (2015) 52e60
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Bulky guanidinate stabilized homoleptic magnesium, calcium and zinc
complexes and their catalytic activity in the Tishchenko reaction
*
Milan Kr Barman, Ashim Baishya, Sharanappa Nembenna
School of Chemical Sciences, National Institute of Science Education and Research (NISER), Bhubaneswar 751005, India
a r t i c l e i n f o
a b s t r a c t
i
Article history:
Bulky guanidinate stabilized homoleptic complexes of [LM(thf) L] [L ¼ {ArNC (N Pr )NAr} (Ar ¼ 2,6- Me
n
2
2
Received 17 February 2015
Accepted 25 February 2015
Available online 7 March 2015
eC
6
H
3
)]; M ¼ Mg, Ca, and Zn; n ¼ 0e1) have been synthesized with three different synthetic routes. First,
] (M ¼ Mg, Ca(thf) , and
and formation of [LM(thf) L]
M ¼ Mg, Ca and Zn; n ¼ 0e1) (3e5) complexes were occurred. Second, the reaction of two equivalents
of potassium salt of LH with metal dihalides via salt metathesis led to the formation of homoleptic
complexes. Third, the reaction between MR
treatment of bis{bis(trimethylsilyl)amide}s of Mg, Ca, and Zn i.e. [M{(N(SiMe
3
)
2
}
2
2
ꢀ
Zn) with free guanidine ligand (LH) at 80 C, elimination of NH(SiMe
3
)
2
n
(
Keywords:
Alkaline earth metal
Guanidine
Homoleptic
Tishchenko reaction
Zinc
2
(M ¼ Mg, Ca, Zn; R ¼ alkyl or OTf) and LH at reflux tem-
perature, formation of bis(guanidinate) magnesium, calcium, and zinc complexes have been observed.
The solid state structures of LH(1), LK(2) and all three bis(guanidinate) magnesium, calcium and zinc
complexes (3e5) were confirmed by X-ray structural analysis. Furthermore, dimerization of a range of
aromatic, heteroaromatic and aliphatic aldehydes (Tishchenko reaction) has been demonstrated by using
catalysts 3e5.
©
2015 Elsevier B.V. All rights reserved.
Introduction
coworkers reported the first guanidinate transition metal complex
in 1970 [17]. Since then, many guanidinate stabilized coordination
complexes involving metals from across the periodic table have
been documented [18]. Despite the significant research interest in
main group and transition metal guanidinates, only a handful of
guanidinate supported homoleptic complexes of magnesium, cal-
cium and zinc have been reported, which are stabilized by less
bulky guanidinate ligand [7,11c,19]. Very few examples of sterically
encumbered (bulky) guanidinate [20] supported alkaline earth
metal complexes have been reported in the literature; most
importantly low oxidation state magnesium(I) complex with
MgeMg bond [21]. Recently, Westerhausen and coworkers re-
ported the structurally characterized bulky guanidinate supported
homoleptic and heteroleptic heavier alkaline earth metal com-
plexes [22]. Very recently, Fortier and coworkers reported the
“super bulky” guanidinates of alkali metals [23], however, no re-
ports on alkaline earth metal guanidinates.
In recent years, the utilization of main group compounds as
catalysts [1] is an attractive area of research; especially compounds
containing Mg, Ca and Zn elements, due to their low toxicity, large
abundance and inexpensive. Accordingly, magnesium, calcium and
zinc metal complexes have been reported for their activity as cat-
alysts in a wide range of chemical transformations including,
dimerization of aldehydes [2] (the Tishchenko reaction), ring
opening polymerization [3], hydroboration [4], hydroamination [5],
guanylation [6], coupling of alkynes with carbodiimides [7] and
cross dehydrocoupling [8]. Various homoleptic and heteroleptic
magnesium, calcium and zinc complexes have been reported,
which are stabilized by a wide variety of nitrogen based ancillary
ligands, such as
b-diketiminates [9], formamidinate,[10] amidinates
[11], iminopyrroles [12], dipyrromethenes [13], amino-
troponiminates [5g,5h], 1,4-diaza-1,3 butadiene [14], amino-
pyridinates [15] tris(pyrozolyl)borate) [16] etc. Furthermore, by
using another type of anicillary ligand i.e. guanidinate, Lappert and
Herein, we report the structurally characterized new bulky
guanidine ligand LH(1) and its potassium salt i.e., LK(2) and three
bulky guanidinate supported homoleptic complexes of magnesium,
calcium and zinc(3e5), respectively. In addition, a range of aromatic,
heteroaromatic and aliphatic aldehydes was converted into corre-
sponding esters i.e. Tishchenko reaction by using precatalysts 3e5.
*
Corresponding author. Tel.: þ91 674 2304126; fax: þ91 674 2302436.
E-mail address: snembenna@niser.ac.in (S. Nembenna).
http://dx.doi.org/10.1016/j.jorganchem.2015.02.044
022-328X/© 2015 Elsevier B.V. All rights reserved.
0

M.K. Barman et al. / Journal of Organometallic Chemistry 785 (2015) 52e60
53
Results and discussion
Cheng et al. reported [26] the ligand exchange reaction between
guanidinate supported yttrium dihalide with allyl magnesium
halide (in ethereal solvent) allows the formation of heteroleptic
guanidinate magnesium halide. By employing above two methods,
homoleptic complexes of magnesium, calcium and zinc i.e.,
Synthesis of LH(1), LK(2) and LM(thf)
complexes
n
L (M ¼ Mg, Ca and Zn) (3e5)
e
Guanidinate ([RNC(NR
2
)NR] ) ligands are highly versatile and
LM(thf)
prepared (Scheme 2).
n
L (M ¼ Mg, Ca and Zn; n ¼ 0e1) (3e5), respectively, were
readily available systems play an important role in stabilizing
complexes of main group, transition and lanthanide elements. The
steric and electronic properties of guanidines can be readily tuned
by changing the substituents at the nitrogen atoms of the ligand
core. These ligands have displayed various coordination modes;
more dominantly are bridging and chelating modes, which depend
upon the nature and steric bulk of the substituent (R), and the
metal involved.
i) a) Direct addition of one equivalent of metal amide [M
{N(SiMe
sequent release of two equivalents NH(SiMe
homoleptic complexes b) Deprotonation of LH (two equiv) upon
3
)
2
}
2
] (M ¼ Mg, Ca(thf)
2
and Zn) to LH (two equiv), sub-
3 2
)
and afforded
n
treatment with magnesium/zinc dialkyls(one equiv) viz. Bu
2
Mg/
Et Zn, evolution of butane/ethane gas and formation homoleptic
2
magnesium and zinc complexes (Scheme 2). And also, LCaL(4a) was
1
In 2009 Jones and co-workers reported the bulky guanidine L H
obtained by the deprotonation of LH (two equiv) upon treatment
ꢀ
1
ꢁ
i
i
or (PrisoH) [L ¼ Priso ¼ {ArNC (N Pr
2
)NAr} (Ar ¼ 2,6- Pr
2
-C
6
H
3
)}]
with Ca(OTf)
2
at 100 C for 2 d in C
6
D
6
in J Young valve NMR tube. ii)
dipp
by treating metallated amide with
carbodiimide i.e. ArN]C]
Easy deprotonation of LH by treating with metal amide i.e.
KN(SiMe led to the formation of LK, in situ generated two
equivalents of LK and subsequent treatment with one equivalent of
metal dihalide MX
i
NAr (Ar ¼ 2,6- Pr
our hands, deprotonation of L H upon treatment with MR
M ¼ Mg, Ca; R ¼ N(SiMe or salt metathesis reaction between
potassium salt of L H i.e., L K with MX
-C
2 6
H
3
) and followed by aqueous work up [24]. In
3 2
)
1
2
(
3
)
2
2
(M ¼ Mg, Ca, Zn; X ¼ Cl or I) led to the for-
1
1
2
(M ¼ Mg, Ca;
mation of LM(thf)L complexes (Scheme 3).
X ¼ halide) led to the formation of mixture of products; major
Furthermore, efforts were made to prepare homoleptic guani-
1
product free ligand (L H) was noticed by NMR analysis of reaction
dinate complexes of magnesium, calcium and zinc, by insertion of
xyl
mixture. Moreover, during the course of this study, Westerhausen
and coworkers have reported [22] the homoleptic strontium
MR
2
i.e. [M{N(SiMe
3
)
2
}
2
] (M ¼ Mg, Ca(thf)
2
, Zn) into carbodii-
i
mide and turned to be unsuccessful. And also the reaction of Pr
2
NH
1
1
,
1
1
xyl
complex L SrL and unsuccessful to produce L CaL . Although, a
3 2 2
(two equiv), [Mg{N(SiMe ) } ] (one equiv) and
carbodiimide
1
saturated hexane solution of heteroleptic complex, L Ca(N(Si-
either in tetrahydrofuran or hexane at room temperature or
elevated temperature was failed to produce homoleptic magne-
sium complex.
3 2
Me ) (THF), was kept at room temperature for several months led
1
1
to the formation of homoleptic L CaL . This is presumably due to
the steric nature of the ligand, 2,6- Pr
i
2 6 3
-C H or dipp substituent
present on the nitrogen atoms. We presumed less bulky group i.e.
xyl or 2,6- Me eC H instead of dipp (2,6- Pr -C H ) substituent,
2 6 3 2 6 3
Spectroscopic characterization
i
Both compounds LH(1) and LK(2) were characterized by H, 13C
1
which is attached to the chelating nitrogen atoms of the guanidine
core, might be suitable in isolation of homoleptic alkaline earth
metal complexes. Thus, we targeted to prepare a new bulky gua-
{1
H} NMR, IR and mass spectrometry analyses. The H NMR spec-
trum exhibits a singlet at 4.79 ppm for the NeH resonance. The
H} NMR spectrum, the carbon atom of the guanidine core i.e., N3C
resonates at 148.7 ppm, that is well in agreement with other re-
ported bulky guanidines (N3C 140e159 ppm) [24].
Guanidines often display several isomeric and tautomeric forms,
due to CeN bond rotation and C]N bond isomerization (Eanti, Esyn
anti, Zsyn) [27], this can lead to complicated NMR spectra for such
compounds. However, H and C{ H} NMR spectra for LH clearly
indicate the existence of one isomer in solution. The infrared
spectrum of the guanidine LH displays an NeH stretching fre-
quency at 3366 cm and C]N stretching frequency at 1623 cm . A
complete disappearance of NeH resonance was observed in the H
NMR spectrum for compound LK(2). In the C{ H} NMR, N3C peak
resonates at 157 ppm which is shifted downfield in comparison to
LH (N3C 148.7 ppm).
Bis(guanidinate) metal complexes (3e5) are freely soluble in
organic solvents and that are moderately air and moisture sensitive.
1
13
C
i
1
nidine ligand i.e. LH(1) [L ¼ {ArNC (N Pr
2
)NAr} (Ar ¼ 2,6-
{
1
Me
guanidine ligand i.e., LH(1) can be prepared by using carbodii-
eC ) and following the
2 6 3
eC H )], which is less bulky in comparison to L H. New bulky
xyl
mide i.e. ArN]C]NAr (Ar ¼ 2,6- Me
2
6 3
H
same method reported by Jones and coworkers (Scheme 1). Very
recently, our group reported the structurally characterized low
valent germanium and tin amide complexes by utilizing bulky
guanidine LH [25].
,
Z
1
13
1
Deprotonation of LH (1) ligand upon treatment with one
equivalent of potassium bis(trimethylsilyl)amide in tetrahydro-
furan at ambient temperature led to the formation LK(2)
-
1
-1
1
13
1
(Scheme 1).
Generally, three procedures allow the synthesis of guanidinates
of alkaline earth metals. i) Metallation of neutral guanidines with
MR
(R ¼ amide, alkyl etc.) ii) Salt metathesis reaction of alkali
metal guanidinates with MX to
(X ¼ halide) iii) Addition of MR
carbodiimide allows the synthesis of guanidinates. Furthermore,
2
2
2
1
13
1
All these compounds are characterized by H, C{ H} NMR, IR and
Scheme 1. Syntheses of bulky guanidine LH(1) and its potassium salt LK(2).

54
M.K. Barman et al. / Journal of Organometallic Chemistry 785 (2015) 52e60
1
equiv MR2,
Toluene, Benzene
(THF)n
M
N
N
N
N
N
N
o
RT, 80
C
N
N
N
2
H
M = Mg [R = N(SiMe ) , nBu]
3
2
Ca(thf) [R = (N(SiMe ) ,(OTf)]
2
3 2
Zn [R = N(SiMe ) , Et]
3
2
1
M = Mgn=0(3), Can=1(4), Znn=0(5)
2
Scheme 2. Syntheses of 3e5; by metalation of neutral guanidine (LH) with MR .
(
THF)n
M
N
N
K
1 equiv MX2
THF, RT, 12h
N
N
N
N
N
N
N
2
M = Mg, Zn (X = Cl)
Ca, (X = I)
2
M = Mgn=0(3), Can=1(4), Znn=0(5)
2
Scheme 3. Syntheses of 3e5; by salt metathesis reaction between LK and MX .
mass spectrometry analyses. Reproducible microanalyses on com-
pounds 2e5 could not be obtained owing to their air and moisture
symmetrical). There is a downfield shift of the guanidine backbone
13
1
NCN resonance to 170e171 ppm in the C{ H} NMR spectra of
these complexes compared to the free guanidine and its potassium
salt values. This suggests that the metal atom is coordinated by the
two N-donor guanidinate ligands. ESI mass spectra for compounds
2e5 show peak at m/z 352 corresponds to molecular ion peak for
free guanidine ligand, this suggests that the decomposition of these
air and moisture sensitive compounds. Therefore, no molecular ion
peaks for compounds 2e5 were detected in the ESI mass spectra.
sensitive nature. The lack of n(NeH) absorption in the infrared and
1
the absence of an NeH resonance in the H NMR spectra indicate
complete deprotonation of the guanidine in isolated homoleptic
1
magnesium, calcium and zinc complexes (3e5). The H NMR
spectra of 3, 4 and 5 display one chemically equivalent set of iso-
propyl methyl groups, one isopropyl methine resonance and one
aryl methane resonance, which can be attributed to the symmet-
rical coordination of two ligand systems to the metal site (C
2
Crystallographic characterization
Molecular structures for compounds 1e2 are depicted in Fig. 1
and Fig. 2, respectively.
The compounds 1 and 2 display solid state structures compa-
1
rable to those of previously characterized bulky guanidine (L H)
1
and its alkali salt (L K)
n
compounds [24]. Four different isomeric
forms exist for guanidine and related amidines, due to CeN bond
rotation and C]N bond isomerization (Eanti, Esyn, Zanti, Zsyn). The
guanidine ligand 1 exists in the Z-anti form which is most common
for (bulky)guanidines [24]. The carbon atom of the guanidine core
N3C, is connected to one imino and two amino nitrogen atoms,
which are confirmed by the CeN bond distances (C23e N3
1.2815(15), C23eN2 (1.3912(15) and C23eN1 1.3789(15) Å). Solid
state structure of LK reveals that it is in polymeric in form and
adopts Z-anti configuration, but with more localized coordinated
NCN fragment (C39eN11.342(4) C39eN3 1.321(4) Å). Furthermore,
6
the areneeK interaction in the compound (LK)
n
close to
h
, this
leads to a one dimensional polymeric structure. The bond length of
1
N(3)eK(1) ¼ 2.749(2) Å is well in agreement with that of (L K)
(
(K(1) eN(1) 2.755(3) Å).
The molecular structure for compounds 3, 4 and 5 are repre-
Fig. 1. Molecular structure of 1 (ORTEP view, 20% probability level) Hydrogen atoms,
except H30 are omitted for clarity. Selected bond lengths[Å] and bond angles[deg]:
C1eN2 1.4260(15), C11eN3 1.4071(16), C23eN1 1.3789(15), C23eN2 1.3912(15),
C23eN3 1.2815(15), N2eH30 0.842(14); N3eC23eN2 121.94(11), N3eC23eN1
sented in Fig. 3, Fig. 4 and Fig. 5, respectively. The asymmetric unit
of compound 3 contains two crystallographically independent and
almost identical molecules. Sterically encumberd bis(guanidinate)
120.1510, N1eC23eN2 117.90(10), C23eN3eC11 121.98(10), C23eN2eC1 130.69(10).

M.K. Barman et al. / Journal of Organometallic Chemistry 785 (2015) 52e60
55
Fig. 4. Molecular structure of 4 (ORTEP view, 20% probability level) Hydrogen atoms
are omitted for clarity. Selected bond lengths [Å] and bond angles[deg]: Ca1eN1
2
.412(2), Ca1eN4 2.380(2), Ca1eN2 2.349(2), Ca1eN3 2.3976(19), Ca1eO1 2.3646(17),
C45eN3 1.339(3), C45eN2 1.348(3), C45eN5 1.394(3), C46eN1 1.337(3), C46eN4
.345(3), C46eN6 1.399(3); N1eCa1eN4 56.239(66), N2eCa1eN3 56.72(7),
N1eC46eN4 114.7(2), C45eN3eCa1 93.43(13), C45eN2eCa1 95.33(14), C46eN4eCa1
5.09(14), C46eN1eCa1 123.8(2), N(1) eC(46) eN(4) 114.7(2), N(2) eC(45) e N(3)
1
9
Fig. 2. Molecular structure of 2 (ORTEP view, 20% probability level) Hydrogen atoms
are omitted for clarity. Selected bond lengths [Å] and bond angles[deg]: K1eN3
114.12(19), N(1) eCa(1) eO(1) 89.53(6), N2eCa1eO1 98.08(6).
2
.749(2), K1eAr centroid 2.9565(12), K2eN1 2.694(3), K2eAr centroid 2.8867(13),
C39eN1 1.342(4), C39eN3 1.321(4), C39eN2 1.402(4), C6eN1 1.389(4), C14eN3
.406(4); C39eN1eK2 129.9(2), C39eN3eK1 157.0(2), N1eC39eN3 120.2(3), N1eC39
ZnNCN four membered heterocycles are planar and twisted with
respect to each other. The average MgeN and ZneN bond distances
in compounds (3 and 5) 2.045 Å and 2.0097 Å are similar. The
1
eN2 121.4(3), N2eN3eC39 118.4(3), C39eN2eC33 115.4(3), C39eN2eC36 116.9(3),
C33eN2eC36 121.4(3).
o
NC(backbone)N bond angles in compounds 3 and 4112.2(2) and
o
1
10.67(14) , respectively are well in agreement with bis(guanidi-
i
i
magnesium complex 3 crystallizes as unsolvated, in contrast to less
bulky bis(guanidinate) magnesium complexes, in those one thf
molecule is coordinated to magnesium atom [Mg{( PrN)
N Pr
Like the magnesium complex 3, the zinc atom is in a distorted
tetrahedral environment. In both compounds 3 and 5, MgNCN and
nate) magnesium and zinc complexes in [Mg{( PrN)
[19d],[Mg{(Me
[19e], [Zn{(Me
In contrast to molecular structure of L SrL , the LCaL (4) is sol-
vated by one molecule of tetrahydrfuran and it is pentacoordinated
and distorted square pyramidal geometry (Fig. 4). The CaeN bond
2
CN Pr
Si) NC(NCy
NC(NCy )} ] [19a].
2
}(THF)]
i
3
Si)
Si)
2
NC(N Pr
2
)}
)}
2
(THF)] [7],[Zn{(Me
] and [Zn{Et
3
2
2
)}
2
]
i
i
2
C-
3
2
NC(N Pr
2
2
2
2
2
i
i
1
1
2
}(THF)] [19d] and [Mg{(Me
3
Si)
2
NC(N Pr
2
)}
2
(THF)] [7].
Fig. 3. Molecular structure of 3 (ORTEP view, 20% probability level) Hydrogen atoms
are omitted for clarity. Selected bond lengths [Å] and bond angles[deg]: Mg1eN1
Fig. 5. Molecular structure of 5 (ORTEP view, 20% probability level) Hydrogen atoms
are omitted for clarity. Selected bond lengths [Å] and bond angles[deg]: Zn1eN1
2.040(3), Mg1eN2 2.051(2), Mg1eN3 2.042(2), Mg1eN4 2.047(2), C45eN1 1.346(3),
i
ii
iii
C45eN2, C45eN5 1.390(4), C46eN4 1.349(3), C46eN3 1.353(3), C46eN6 1.383(3);
N1eMg1eN2 66.67(10), N3eMg1eN4 66.51(9), N1eC45eN2 113.1(2), N4eC46eN3
2.0097(10), Zn1eN1 2.0097(10), Zn1eN1 2.0097(10), Zn1eN1 2.0097(10), N1eC9
i
i
1.3521(13), N1 eC9 1.3521(13), N2eC9 1.377(2); N1 eZn1eN1 67.20(6),
ii
iii
i
ii
ii
iii
112.2(2), C45eN1eMg1 90.38(17), C45eN2eMg1 89.82(17), C46eN4eMg1 90.60(16),
N1 eZn1eN1 67.20(6), N1 eC9eN1 110.67(14), N1 eC9 eN1 110.67(14), C9e N1e
i
C46eN3eMg1 90.68(16).
Zn1 91.06(8), N1 eC9eN2 124.66(7).

56
M.K. Barman et al. / Journal of Organometallic Chemistry 785 (2015) 52e60
lengths are in average 2.40 Å and CaeO bond length is 2.3646(17)
Å. The CaeN bond distance av. 2.384, which is 0.339 Å longer than
that of MgeN and ZneN bond distances (av. 2.027), this is expected
due to the large ionic radii of Ca(II) ion in comparison to Mg(II) and
Zn(II) ions.
room temperature conditions, however, for 5 catalyzed reaction at
80 C.
ꢀ
During the initial 4 h, in contrast to compound 5, which is
showing less than 50% yield of benzylbenzoate, compounds 3 and 4
produce more than 70%. After 18 h, compounds 3 and 4 exhibit
catalytic activities for the formation of ester is more than 90% when
compared with 5 (75%). This represents a marked increase in the
catalytic activity of compounds 3 and 4 over the time period when
compared with 5. Thus, both compound 3 and 4 are exhibiting
almost same catalytic activity.
In view of the above results, a variety of aromatic, hetero-
aromatic and aliphatic aldehydes, were examined, using 3 and 6 as
catalysts at 2 mol% loading under solvent free conditions (Table 2,
entries 1e24). Complexes 3 and 6 show good to excellent catalytic
activity for the dimerization of various aldehydes (Table 2, entries
Catalytic activity
Catalytic studies: Bis(guanidinate) supported Mg(II), Ca(II) and Zn(II)
complexes as catalysts for Tishchenko reaction.. The dimerization of
aldehydes to form the corresponding symmetric esters, or Tish-
chenko reaction (or ClaiseneTishchenko reaction), has been known
for more than a century. This reaction exemplifies an atom-efficient
synthesis of esters and it is industrially viable. A large number of
compounds containing main group [28], transition [29] and
lanthanide [30] elements have been used as catalysts for this re-
1-24, except entries 13-14 and 23e24). In all cases, the preparative
3 2 2 2
action. Hill et al., have shown [Ca{N(SiMe ) } (thf) ] acts as pre-
scale yields paralleled those observed by NMR (aliquot of reaction
catalyst for the intramolecular and intermolecular dimerization of
aldehydes [2a]. Coles and coworkers have reported cyclic guanidi-
nate metal complexes as active precatalysts for the Tishchenko
reaction [2b,2c]. Very recently, Lappert and coworkers reported the
dimerization of aldehydes by using various zinc complexes,
including bis(guanidinate) zinc complexes [19a]. To the best of our
knowledge, there have been no reports on bis(guanidinate) mag-
nesium and calcium compounds use as catalysts for the Tishchenko
reaction.
1
mixture) and products were isolated and characterized by H and
13
C NMR spectroscopy methods. (See Supporting Information). This
study showed that 3 was an effective catalyst for both inter and
intramolecular Tishchenko reaction. Various electron donating and
electron withdrawing aromatic, heteroaromatic and aliphatic al-
dehydes have been used for the intermolecular dimerization.
However, only one example has been shown for the intra molecular
Tishchenko reaction.
Our initial trial reaction was done by choosing benzaldehyde as
a substrate, and bis(guanidinate) magnesium, calcium and zinc
complexes were screened as catalysts for the Tishchenko reac-
tion(Table 1). From the Table 1, it is very clear that good to excellent
yields (85e98%) were obtained when each 3e5 was used as catalyst
for the dimerization of benzaldehyde. The catalytic activity of
bis(guanidinate) magnesium, calcium and zinc complexes for this
reaction is in the order of Mg z Ca > Zn. These compounds 3e5
show catalytic activity in line with the [Ca{N(SiMe ) } (THF) ] and
3 2 2 2
better than Lappert's homoleptic zinc guanidine complex. Then,
our attention turned to compare the catalytic activities of these
3 2 2 3 2 2
compounds with [Mg{N(SiMe ) } ](6) and [Zn{N(SiMe ) } ] for the
dimerization of benzaldehyde at same reaction conditions. Sur-
prisingly, we noticed very poor catalytic activity from [Zn
Conclusions
Synthesis and crystal structures of bulky guanidine ligand (LH)
and its potassium salt (LK) have been described. Utilizing these
precursors three homoleptic complexes of magnesium, calcium and
zinc i.e., LM (thf) L (3e5) have been synthesized with two standard
synthetic procedures i.e., i) deprotanation of LH with MR
metathesis reaction between LK and MX . However, insertion MR
into bulky aryl carbodiimide reaction was unsuccessful. X-ray
crystal structural analysis revealed that both complexes of Mg and
Zn, crystallize as C
2
and ii)
2
2
2
symmetric with coordination number four, but
symmetric with mono etherate
the complex of Ca crystallizes as C
2
with coordination number five. Furthermore, we have shown the
efficacy of these compounds in organic transformation reaction i.e.
Tishchenko reaction. All these compounds 3e5 exhibit as good to
excellent catalysts for the conversion of aldehydes into esters.
{
3 2 2 3 2 2
N(SiMe ) } ], however Mg{N(SiMe ) } (6) shows excellent cata-
lytic activity. Furthermore, the esterification of benzaldehyde was
examined on a time scale vs. yield using 5 mol% 3, 4 and 5 (Fig. 6) at
Table 1
Homoleptic Mg(II), Ca(II) and Zn(II) catalyzed dimerization of benzaldehyde.
Catalyst
Temp.
Mol %
of cat.
Time (h)
Yielda (%)
Ref.
o
(
C)
3
3
4
4
5
5
25
25
25
25
80
80
25
25
80
25
80
2
5
2
5
2
5
1
2
2
1
5
21
18
23
20
36
24
6
95
98
90
93
85
85
98
98
<10
97
b
b
b
b
b
b
b
b
[
[
[
[
[
Mg{N(SiMe
Mg{N(SiMe
Zn{N(SiMe
Ca{N(SiMe
Zn{Et NC(NCy)
3
)
)
2
}
}
2
](6)
](6)
]
3
2
2
4
3
)
2
}
2
12
24
12
b
3
)
2
}
2
(THF)
2
]
[2a]
[19a]
2
2
}
2
]
96
Fig. 6. NMR monitoring of the reaction progress: benzyl benzoate formation vs. time;
b
This work, solvent free.
NMR yield.
catalyst 3, 4 and 5 (5 mol %) and benzaldehyde was stirred at room temperature and
ꢀ
a
80 C for 5.

M.K. Barman et al. / Journal of Organometallic Chemistry 785 (2015) 52e60
57
Preparation of LH (1) nBuLi (1.6 M solution in hexane)
Table 2
a
Compounds 3 and 6 catalyzed dimerization of various aldehydes.
(
Pr
11.29 mL, 18.07 mmol, 1.04 equiv) was added to a solution of
ꢀ
o
Yield (%)d
i
Entry
Cat.
Product
Temp.( C)
Time (h)
2
NH (1.82 g, 18.07 mmol, 1.04 equiv) in THF (30 mL) at 0 C and
the resultant solution stirred for 1 h. ArN]C]NAr (Ar ¼ 2,6-Me
2
-
1
2
3
6
25
25
21
4
95(92)
98
6 3
C H
) in THF (30 mL) (4.303 g, 17.21 mmol, 1.0 equiv) was then
added, the resultant solution was stirred for overnight. All volatiles
were removed under reduced pressure and diethyl ether and water
added to the residue. The mixture was stirred for 30 min to give two
clear phases. The organic layer was separated and the aqueous layer
was extracted with dichloromethane. The combined organic layers
3
4
3
6
80
80
8
4
95(90)
96
5
6
3
6
80
80
8
4
96(92)
97
4
were dried over MgSO , filtered, and the volatiles evaporated under
vacuum to get yellow solid. The solid residue was recrystallized as
colorless crystals from hexane. Yield: (4.80 g, 79.4%).
7
8
3
6
25
25
18
4
90(84)
93
ꢀ
M.p. ¼ 128e130 C. ESI-HRMS: calcd for C23
H
34
N
3
[MþH]þ:
1
ꢀ
3
52.2753; found: 352.2747. H NMR (400 MHz, C
6
D
6
, 25 C):
d
¼ 7.14 - 6.93 (t, 2H, ArH), 6.82 (d, 2H, J ¼ 8 Hz, ArH), 6.76 (t, 2H,
9b
1
3
6
25
25
12
2
98(91)
99
ArH), 4.79 (s, 1H, NH), 3.44 (sept, J ¼ 6.6 Hz, 2H, CH(CH
3 2
)
), 2.42 (s,
0c
6
H, CH(CH
3
)
2
), 2.06 (s, 6H, CH(CH
3
)
2
), 1.21 (d, J ¼ 8 Hz, 12H, CH
3
)
1
3
1
ꢀ
ppm. C{ H}NMR (100 MHz, C
(AreC), 139.3 (AreC), 134.9 (AreC), 130.3 (AreC), 129.2 (AreC),
1
6
D
6
, 25 C):
d
¼ 148.7 (NCN), 148.5
1
1
1
2
3
6
80
25
24
4
88(82)
90
i
29.1 (AreC), 125.8 (AreC), 122.2 (AreC), 48.2 (N- Pr-CH), 22.4 (Ar-
i
i
3 3 3
CH ), 19.6 ( Pr-CH ), 19.4 ( Pr-CH ) ppm; IR (KBr) n (cm-1): 3366 (m,
1
1
3
4
3
6
80
25
24
6
15
70
NeH), 2966 (m), 1623 (s, C]N), 1587 (s), 1341 (s), 1188 (m), 1135
m), 1090 (m), 777 (m), 758 (m).
Preparation of (LK) (2) Tetrahydrofuran (30 mL) was added
to a mixture of KN(SiMe (0.090 g, 0.439 mmol, 1.03 equiv) LH
0.150 g, 0.426 mmol, 1.0 equiv) and the resulting solution stirred
(
n
1
1
5
6
3
6
80
25
24
6
94(89)
90
3 2
)
(
vigorously for 12 h at room temperature. All volatiles were
removed under reduced pressure and residue washed with n-
hexane (10 mL). Solid extracted from toluene (40 mL). The solu-
1
1
7
8
3
6
25
25
12
4
92(90)
93
tion was concentrated from reduced pressure ca. 15 mL and
cooled 0 C to obtain colorless crystals suitable for X-ray diffrac-
1
2
9
0
3
6
80
25
20
6
90(83)
96
ꢀ
tion analysis.
ꢀ
1
Yield: (0.160 g, 96%). M. p. >300 C. H NMR (400 MHz, C
6
D
6
,
2
2
1
2
3
6
80
80
1
2
99(95)
99
ꢀ
2
(
5
C):
d
¼ 6.89 (d, J ¼ 8Hz, 4H, AreH), 6.57 (s, 2H, AreH), 3.70
sept, J ¼ 6.66 Hz, 2H, CH(CH
3
)
2
), 2.24 (s, 12H, CH
). C { H} NMR (100 MHz, C
3
), 1.235 (d,
1
3
1
ꢀ
J ¼ 4Hz, 12H, CH(CH
3
)
2
6
D
6, 25 C):
2
2
3
4
3
6
80
80
24
12
20
20
d
(
(
¼ 157.0 (NCN), 155.5 (AreC), 130.9 (AreC), 128.9 (AreC), 117.6
i
i
1
AreC), 47.7 (N- Pr-CH), 23.5 (Ar-CH
3
), 20.3 ( Pr-CH
¼ 6.78 (d, J ¼ 4 Hz, 4H, AreH), 6.32 (t,
3
) ppm. H NMR
ꢀ
400 MHz, THF-d
8
, 25 C):
d
a
b
c
2
mol% catalyst, Solvent free.
J ¼ 8 Hz, 2H, AreH), 3.62 (sept, J ¼ 6.6 Hz, 2H, CH(CH
3 2
)
), 2.23 (s,
) ) ppm. C{ H} NMR
3 2
Solvent, toluene.
C D .
6 6
NMR yield(Isolated Yield).
1
3
1
12H, CH
(100 MHz, THF-d
27.9(AreC), 115.5(AreC), 47.4(N- Pr-CH), 23.1( Pr-CH3), 20.1 (Ar-
3
), 1.20 (d, J ¼ 8 Hz, 12H, CH(CH
d
8
):
d
¼ 156.4(NCN), 143.5(AreC), 130.3(AreC),
i
i
1
ꢁ
1
Experimental section
CH3) ppm. IR (KBr)
457(w), 1376(m), 1305(m), 722(s).
n (cm ): 2924(w), 2725(s), 1616(s), 1587(s),
1
General
i
2 2 6 3 2 2
Preparation of Mg[ Pr NC{N-2,6-Me -C H } ] (3)
All manipulations were carried out under dry nitrogen using
standard Schlenk-line and cannula techniques, or nitrogen filled
MBraun glove box. Solvents were dried with appropriate drying
agents and degassed prior to use. Chemicals were purchased from
Method A:
A Schlenk tube charged with LH (0.200 g,
0.568 mmol, 2.0 equiv) and [Mg{N(SiMe ) } ] (0.098 g, 0.284 mmol,
3
2 2
1.0 equiv). To the above mixture benzene (15 mL) was added and
ꢀ
SigmaeAldrich and used as received unless otherwise stated.
heated at 80 C for 24 h. After removal of all the volatiles, the res-
i
NH, nBuLi (1.6 M in hexane), KN(SiMe
Pr
2
3
)
2
, [Zn{N(SiMe
3
)
2
}
2
],
(1 M solution in
(99.9þ%), were purchased from
SigmaeAldrich and used as received. Benzaldehyde was freshly
distilled and stored under nitrogen. [Mg{N(SiMe ] [31] and [Ca
N(SiMe (THF) ] [32] were prepared according to the literature
procedures. H, C{ H} NMR spectra were recorded on Bruker AV-
idue was extracted with n-hexane (20 mL) and concentrated to
n
ꢀ
ZnCl
heptane), MgCl
2
, ZnEt
2
(1.0 M solution in hexane), Mg Bu
, CaI , Ca(OTf)
2
about 5 mL and finally stored in ꢁ30 C. Colorless crystals of
2
2
2
compound 3 suitable for X-ray diffraction analysis are formed after
one day. Yield: (0.165 g, 80%).
3
)
2
}
2
Method B: LH (0.500 g, 1.42 mmol, 1.0 equiv) and KN(SiMe3)2
(0.297 g, 1.49 mmol, 1.05 equiv) were placed in a Schlenk tube and
THF (10 mL) was added at room temperature and stirring was
continued overnight. The resulting solution was added drop by
drop to a stirred suspension of MgCl2 (0.035 g, 0.355 mmol, 0.5
equiv) in THF (5 mL) at room temperature under stirring for
another 24 h. Removal of all volatiles and recrystallized from n-
hexane (20 mL) gave 3 Yield: (0.417 g, 81%).
{
3
)
2
}
2
2
1
13
1
1
13
1
4
00 ( H: 400 MHz, C{ H}: 100.6 MHz) and were referenced to the
resonances of the solvent used. IR Spectra were recorded in Per-
kineElmer FT-IR Spectrometer. Mass spectra were recorded on
Bruker micrOTOF-Q II Spectrometer. Melting points were taken on
an electro thermal apparatus and are uncorrected.

58
M.K. Barman et al. / Journal of Organometallic Chemistry 785 (2015) 52e60
Table 3
Crystal data for compounds 1e5.
Compounds
1
2
3
4
5
CCDC
984875
984876
984877
984878
984879
Formula
Mol.mass
Size (mm)
Crystal system
Space group
a (Å)
C
23
H
33
N
3
C
46
H
64
K
2
N
6
C
92
H
128Mg
2
N
12
C
50
H
72CaN
6
O
46 64 6
C H N Zn
766.42
0.22 ꢂ 0.15 ꢂ 0.1
Orthorhombic
Fddd
8.577(2)
28.275(8)
34.657(10)
90.000
90.000
90.000
8405(5)
8
351.52
779.23
1450.68
813.22
0.09 ꢂ 0.06 ꢂ 0.034
Triclinic
P1
0.04 ꢂ 0.028 ꢂ 0.018
Monoclinic
P2(1)/c
11.070(6)
18.648(9)
22.237(13)
90
103.351(4)
90
4466(3)
4
0.093 ꢂ 0.075 ꢂ 0.053
Triclinic
P1
0.12 ꢂ 0.08 ꢂ 0.052
Monoclinic
P2(1)/n
11.497(8)
29.855(2)
14.776(9)
90.000
111.477(4)
90.000
4720(3)
4
8.7245(2)
10.7448(2)
12.4415(3)
110.4650(10)
91.8350(10)
97.2020(10)
1080.46(4)
2
13.394(7)
16.798(8)
21.985(11)
111.455(3)
93.475(3)
105.182(3)
4375(2)
2
b (Å)
c (Å)
ꢀ
a
b
g
( )
ꢀ
( )
ꢀ
( )
V (Å3)
Z
r
(gcmꢁ3)
(Mo-K
) (mm 1
1.081
0.064
1.159
0.250
1.101
0.078
1.144
0.175
1.211
0.623
ꢁ
m
a
)
T (K)
(max.)
296
27.88
100
25.74
100
26.05
100
27.55
100
25.5
q
Unique reflections
5141
8530
17171
10829
1959
F(000)
R(int)
Parameters
R1
wR2
GOF
384
0.0256
247
0.0518
0.1433
1.064
1680
0.1248
550
0.0566
0.1104
1.006
1576
0.0744
987
0.0605
0.1444
1.000
1768
0.1420
539
0.0508
0.1082
1.014
3296
0.0403
126
0.0263
0.0810
1.047
Method C: To a solution of LH (0.200 g, 0.568 mmol, 2 equiv) in
toluene (20 mL) was slowly added drop by drop a solution of
the formation of LCaL (4a); LH (0.020 g, 0.0568 mmol, 2.0 equiv)
1
and Ca(OTf)
2
(0.011 g, 0.0284 mmol, 1.02 equiv) H NMR (400 MHz,
n
ꢀ
Bu
2
Mg (1M solution in heptane) (0.35 mL, 1.25 equiv) at room
C
6
D
6
, 25 C):
d
¼ 6.86-6.84 (m, 12H, AreH), 2.78 (sept, 4H, J ¼ 6.66
temperature. The stirring was continued for 12 h. The solvent was
removed and solid compound 3 obtained. Yield: (0.173 g, 84%)
Hz, CH(CH
3
)
2
), 2.28 (s, 24H, CH
3
), 0.955 (d, J ¼ 4 Hz, 24H, CH(CH
3 2
) )
ppm.
ꢀ
1
ꢀ
3
: M. p. ¼ 90e95 C. H NMR (400 MHz, C
J ¼ 8Hz, 8H, AreH), 6.91-6.87 (m, 4H, AreH), 3.86 (sept, J ¼ 6.66 Hz,
H, CH(CH ), 2.23 (s, 24H, CH
), 0.68 (d, J ¼ 8 Hz, 24H, CH(CH
ppm. C{ H} NMR (100 MHz, C ):
¼ 171.3 (NCN), 148.0 (AreC),
6
D
6
, 25 C):
d
¼ 6.99 (d,
i
Preparation of Zn[ Pr
2
NC{N-2,6-Me
2
C
6
H
3
}
2
]
2
(5)
4
3
)
2
3
3 2
) )
13
1
6
D
6
d
The compound 5 was synthesized by following similar pro-
cedures to that employed for the preparation of 3 and using
appropriate precursors.
1
CH
1
33.1 (AreC), 128.8 (AreC), 122.9 (AreC), 51.0 (N-iPr-CH), 24.5 (Ar-
i
ꢁ1
3
), 19.3 ( Pr-CH
3
) ppm. IR (KBr)
n
(cm ): 2927(w), 2726(s),
456(m), 1376(s), 1303(m), 722(s).
Method A: LH (0.2 g, 0.568 mmol, 2.0 equiv) and [Zn
{
N(SiMe
81%).
Method B: LH (0.250 g, 0.711 mmol, 1.0 equiv) and K{N(SiMe
0.149 g, 0.746 mmol, 1.05 equiv) in THF (10 mL) at room temper-
ature and stirring was continued overnight. The resulting solution
was added drop by drop to a stirred suspension of ZnCl (0.048 g,
0.355 mmol, 0.5 equiv) in THF (5 mL); Yield: (0.204 g, 75%).
Method C: LH (0.5 g, 1.42 mmol, 2 equiv) and Et Zn(1M solution
in hexane) (0.86 mL, 1.2 equiv); Yield: (0.452 g, 83%).
3 2 2
) } ] (0.114 mL, 0.284 mmol, 1.0 equiv). Yield: (0.176 g,
i
i
Preparation of Ca[ Pr
2
NC{N-2,6-Me
2
C
6
H
3
}
2
]
2
(THF) (4) & Ca[ Pr
2
NC
{
N-2,6-Me (4a)
2
C
6
H
3
}
2
]
2
3 2
) }
(
The compound 4 was synthesized by following similar pro-
cedures to that employed for the preparation of 3 and using
appropriate precursors.
2
Method A: LH (0.250 g, 0.711 mmol, 2.0 equiv) and [Ca
2
{
8
N(SiMe
3%).
Method B: LH (0.500 g, 1.42 mmol, 1.0 equiv), K{N(SiMe
0.297 g, 1.49 mmol, 1.05 equiv), CaI (0.211 g, 0.711 mmol, 0.5
equiv) in THF (10 mL). Yield: (0.482 g, 83.5%).
3 2 2 2
) } (thf) ] (0.178 g, 0.355 mmol, 1.0 equiv); Yield: (0.239 g,
ꢀ
1
ꢀ
5 M p. ¼ 155e159 C. H NMR (400 MHz, C
(d, J ¼ 8Hz, 8H, AreH), 6.91-6.88 (m, 4H, AreH), 3.88(sept, J ¼ 7.33
Hz, 4H, CH(CH ), 2.28 (s, 24H, CH
), 0.68 (d, J ¼ 4Hz, 24H,
) ppm. C { H} NMR (100 MHz, C ):
¼ 170.2 (NCN),
147.3 (AreC), 133.9 (AreC), 128.7 (AreC), 123.3 (AreC), 51.3 (N-iPr-
6
D
6
, 25 C):
d
¼ 6.98
3 2
) }
(
2
3
)
2
3
1
3
1
CH(CH
3
)
2
6
D
6
d
ꢀ
1
ꢀ
4
: M. p. ¼ 145e148 C. H NMR (400 MHz, C
d, J ¼ 8 Hz, 8H, AreH), 6.87 (t, J ¼ 8 Hz, 4H, AreH), 3.83 (sept, 4H,
J ¼ 6.66 Hz, CH(CH ), 3.25 (m, 4H, OCH CH (THF)), 2.25 (s, 24H,
CH ), 1.13 (m, 4H, OCH CH
, (THF)), 0.79 (d, J ¼ 8 Hz, 24H,
CH(CH ), C{ H} NMR (100 MHz, C ):
¼ 170.0 (NCN), 150.8
AreC), 131.8 (AreC), 128.8 (AreC), 121.0 (AreC), 68.8 (THF), 50.5
6
D
6
, 25 C):
d
¼ 7.01
ꢁ1
(
CH), 24.4 (Ar-CH ), 19.2 (iPr-CH ) ppm. IR (KBr) n (cm ): 2925(w),
2726(s), 1457(m), 1376(s), 1305(m), 1154(m), 722(s).
N.B. Reproducible microanalyses on compounds 2e5 could not
be obtained owing to their air and moisture sensitive nature.
3 3
3
)
2
2
2
3
2
2
13
1
3
)
2
6
D
6
d
(
(
(
i
i
3 3
N- Pr-CH), 25.6 (THF), 25.0 (Ar-CH ),19.8 ( Pr-CH ). IR (KBr) n
cm ): 2925(w), 2726(s), 1459(m), 1377(s), 1304(m), 722(s).
General procedure for the Tishchenko reaction
ꢁ
1
The required amount of catalyst and aldehyde both were placed
in a dry sample vial with a stirring bar inside the glove box. Sub-
i
Method C for Ca[ Pr
2
NC{N-2,6-Me
2
-C
6
H
3
2
} ]
2
(4a)
sequently, sealed vial brought outside the glove box and stirred at
ꢀ
To a mixture of LH and Ca(OTf)
2
in a J Young valve NMR tube was
room temperature or heated to 80 C till completion of the reaction.
The reaction was monitored by thin layer chromatography (TLC)
ꢀ
added 0.6 mL C and further heated to 100 C for 48 h and led to
6 6
D

M.K. Barman et al. / Journal of Organometallic Chemistry 785 (2015) 52e60
59
and aliquot of reaction mixture by NMR spectroscopy. Final product
was purified by column chromatography (Hexane/diethyl ether
(e) J.F. Dunne, D.B. Fulton, A. Ellern, A.D. Sadow, J. Am. Chem. Soc. 132 (2010)
7680e17683;
1
(f) S.R. Neal, A. Ellern, A.D. Sadow, J. Organomet. Chem. 696 (2011) 228e234;
(g) S. Datta, P.W. Roesky, S. Blechert, Organometallics 26 (2007) 4392e4394;
(h) S. Datta, M.T. Gamer, P.W. Roesky, Organometallics 27 (2008)
(
98:2))
1
207e1213.
6] A. Baishya, M.K. Barman, T. Peddarao, S. Nembenna, J. Organomet. Chem. 769
2014) 112e118.
[7] R.J. Schwamm, M.P. Coles, Organometallics 32 (2013) 5277e5280.
X-ray structure determinations
[
(
The crystal data for the compounds have been collected on a
[
8] J.F. Dunne, S.R. Neal, J. Engelkemier, A. Ellern, A.D. Sadow, J. Am. Chem. Soc.
33 (2011) 16782e16785.
Bruker SMART CCD diffractometer (MoK
All the structures were solved by direct methods using the program
a
radiation,
l
¼ 0.71073 Å).
1
[
9] (a) S. Nembenna, H.W. Roesky, S. Nagendran, A. Hofmeister, J. Magull, P.-
J. Wilbrandt, M. Hahn, Angew. Chem. Int. Ed. 46 (2007) 2512e2514;
(b) S.P. Sarish, S. Nembenna, S. Nagendran, H.W. Roesky, Acc. Chem. Res. 44
SHELXS-97 and refined by full-matrix least-squares methods
against F2 with SHELXL-97 [33]. Hydrogen atoms were fixed at
(
(
2011) 157e170;
calculated positions and their positions were refined by a riding
model. All non-hydrogen atoms were refined with anisotropic
displacement parameters. The crystal data and the cell parameters
for compounds 1e5 are summarized in Table 3.
c) S.J. Bonyhady, C. Jones, S. Nembenna, A. Stasch, A.J. Edwards, G.J. McIntyre,
Chem. Eur. J. 16 (2010) 938e955;
d) C. Ruspic, S. Nembenna, A. Hofmeister, J. Magull, S. Harder, H.W. Roesky,
J. Am. Chem. Soc. 128 (2006) 15000e15004;
(
(
(
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The authors thank the National Institute of Science Education
and Research (NISER), Department of Atomic Energy (DAE, Govt. of
India) for supporting this work. A. B. acknowledges University
Grants Commission (UGC) Govt. of India for his fellowship. We
thank Mr. S. Pardhasaradhi for his help in solving X-ray structures.
D.L. Kays, Dalton Trans. 43 (2014) 4838e4846;
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