Solid and solution state flexibility of sterically congested bis(imino)bipyridine complexes of zinc(II) and nickel(II)

Article abstract of DOI:10.1039/b811309b

Two new sterically demanding bis(imino)bipyridine ligands, 6,6′-{(2,6-i-Pr₂C₆H₃)NCR} ₂C₁₀H₆N₂ (R = H (L1), Me (L2)), have been prepared in high yield by the condensation reaction of 2,6-diisopropylaniline with 6,6′-(OCR)₂C₁₀H ₆N₂ (R = H, Me). Palladium(0)-mediated cross coupling of 2-(Bu₃Sn)-6-{C(Me)OCH₂CH₂O}C₅H ₃N with 2-Br-6-{C(Me)OCH₂CH₂O}C ₅H₃N, followed by an acid-mediated deprotection, has been employed as an efficient route to the precursor 6,6′-bis(acetyl)-2, 2′-bipyridine. Reaction of aldimino L1 with two equivalents of MX ₂ [MX₂ = ZnCl₂, NiCl₂ or (DME)NiBr₂] in n-BuOH at elevated temperature gives the five-coordinate mononuclear complexes [(L1)MX₂] (M = Zn, X = Cl 1; M = Ni, X = Cl 2a; M = Ni, X = Br 2b) as the sole products, in which one imine group is bound and the other uncoordinated (endo-exo). In the case of diamagnetic 1, VT ¹H NMR spectroscopy reveals a fast exchange process operating between the two possible forms of the endo-exo isomer which is likely to proceed via an exo-exo intermediate (ΔH^? _{interconversion} = 35.6 ± 1.5 kJ mol^-1, ΔG^?₂₉₈ = 47.4 ± 3.3 kJ mol ^-1). In contrast, treatment of ketimino L2 with MCl₂ (MCl₂ = ZnCl₂ or NiCl₂) affords bimetallic [(L2)Zn₂Cl₄] (4) and the six-coordinate monometallic species [(L2)NiCl₂] (5), respectively; in 4, L2 adopts a bis(bidentate) bonding mode (endo-endo) while in 5 it acts as tetradentate ligand (endo-endo). Prolonged standing of 1 in chlorinated solvents results in partial hydrolysis and the formation of the 6-imino-6′-formyl-2,2′- bipyridine zinc complex, [(6-{(2,6-i-Pr₂C₆H ₃)NCMe)-6′-(CHO)C₁₀H₆N ₂)ZnCl₂] (3) (endo-imine, exo-formyl). Single crystal X-ray structures are reported for L1, 1, 2a, 3, 4 and 5. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b811309b

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www.rsc.org/dalton | Dalton Transactions
Solid and solution state flexibility of sterically congested bis(imino)bipyridine
complexes of zinc(II) and nickel(II)†
Gerry A. Griffith, Mohamed J. Al-Khatib, Kalpana Patel, Kuldip Singh and Gregory A. Solan*
Received 3rd July 2008, Accepted 17th September 2008
First published as an Advance Article on the web 5th November 2008
DOI: 10.1039/b811309b
=
Two new sterically demanding bis(imino)bipyridine ligands, 6,6¢-{(2,6-i-Pr2C6H3)N CR}2C10H6N2
(R = H (L1), Me (L2)), have been prepared in high yield by the condensation reaction of
=
2,6-diisopropylaniline with 6,6¢-(O CR)2C10H6N2 (R = H, Me). Palladium(0)-mediated cross coupling
of 2-(Bu3Sn)-6-{C(Me)OCH2CH2O}C5H3N with 2-Br-6-{C(Me)OCH2CH2O}C5H3N, followed by an
acid-mediated deprotection, has been employed as an efficient route to the precursor
6,6¢-bis(acetyl)-2,2¢-bipyridine. Reaction of aldimino L1 with two equivalents of MX2 [MX2 = ZnCl2,
NiCl2 or (DME)NiBr2] in n-BuOH at elevated temperature gives the five-coordinate mononuclear
complexes [(L1)MX2] (M = Zn, X = Cl 1; M = Ni, X = Cl 2a; M = Ni, X = Br 2b) as the sole
products, in which one imine group is bound and the other uncoordinated (endo-exo). In the case of
diamagnetic 1, VT 1H NMR spectroscopy reveals a fast exchange process operating between the two
possible forms of the endo-exo isomer which is likely to proceed via an exo-exo intermediate
(DH‡
= 35.6 1.5 kJ mol-1, DG‡ = 47.4 3.3 kJ mol-1). In contrast, treatment of ketimino
interconversion
298
L2 with MCl2 (MCl2 = ZnCl2 or NiCl2) affords bimetallic [(L2)Zn2Cl4] (4) and the six-coordinate
monometallic species [(L2)NiCl2] (5), respectively; in 4, L2 adopts a bis(bidentate) bonding mode
(endo-endo) while in 5 it acts as tetradentate ligand (endo-endo). Prolonged standing of 1 in chlorinated
solvents results in partial hydrolysis and the formation of the 6-imino-6¢-formyl-2,2¢-bipyridine zinc
=
=
complex, [(6-{(2,6-i-Pr2C6H3)N CMe)-6¢-(CH O)C10H6N2)ZnCl2] (3) (endo-imine, exo-formyl). Single
crystal X-ray structures are reported for L1, 1, 2a, 3, 4 and 5.
In recent work we have been interested in sterically con-
gested longer chain oligopyridylimines (n or m ≥ 2) and,
in particular, the impact of modifying the imino-carbon sub-
stituent (R) on the transition metal chemistry.8 With regard
to 6,6¢¢-bis(arylimino)terpyridines (A in Fig. 1) and 6-arylimi-
noterpyridines (B in Fig. 1), the nature of R plays a key role
in determining the bonding mode of the ligand exhibited in
complexes of divalent 3d metal ions. For example, neutral nickel
dihalide complexes of B with Ar = 2,6-i-Pr2C6H3 are either 5-
(imine pendant = exo) or 6-coordinate (imine bound = endo)
depending on whether the R group is H or Me, respectively.8c
Equally, the nature of the metal centre can be influential on the
bonding capacity of B with only five-coordinate (exo-imine; R =
H or Me) monometallic complexes accessible with zinc(II).
Herein as an extension to our previous studies, we are
concerned with probing the effects of R group variation on
metal coordination and lability in complexes of the symmetrical
tetraaza 6,6¢-bis(arylimino)-2,2¢-bipyridine C (in which the chain-
end pyridyl group in B has been replaced by an imino group).
To the knowledge of the authors only bis(aldimino) derivatives
of C (R = H, Ar = 2,4,6-Me3C6H2, p-tol, p-FC6H5) have
been reported for iron(II), copper(I) and palladium(II) ions.5,7,9
Specifically, we report the synthesis and structural characterisation
of nickel and zinc dihalide complexes of the novel sterically
1
Introduction
Despite the importance of 2-pyridylimines and 2,6-bis(imino)-
pyridines as N,N - and N,N,N -chelating ligands in wide variety
of chemistries including metallo-supramolecular1 and catalysis,2–4
their more extended relatives the 2,6-oligopyridylimines,
=
=
=
C5H4N(C5H3N)nCR NR¢ or R¢N RC(C5H3N)mCR NR¢ (n ≥ 1
or m ≥ 2; R = H or hydrocarbyl; R¢ = hydrocarbyl), have been
employed rather more sparingly.5–9 For example, 2,2¢-dipyridyl-
6-imines (n = 1) have served as compatible supports for late
transition metal alkene oligomerisation catalysts,5 while longer
chain members such as 6,6¢¢-bis(imino)terpyridines (m = 3) have
been successfully employed as helicands for the formation of
double helicates.6 Nevertheless, the potential amenability of this
ligand family to systematic variation of both the imino-nitrogen
(R¢) and imino-carbon substituents (R) for a given chain length
makes them particularly attractive for studies directed towards
examining the effects of electronic/steric variations on their
resultant coordination chemistry and/or catalysis. Notably, within
the oligomerisation/polymerisation arena this type of study with
the parent 2-pyridylimines and 2,6-bis(imino)pyridines has been
central to the development of the research area.10
Department of Chemistry, University of Leicester, University Road,
Leicester, UK LE1 7RH. E-mail: gas8@leicester.ac.uk
† CCDC reference numbers 692756–692761. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b811309b
=
bulky bis(aldimine), 6,6¢-{(2,6-i-Pr2C6H3)N CH)2C10H6N2 (L1),
=
and bis(ketimine), 6,6¢-{(2,6-i-Pr2C6H3)N CMe)2C10H6N2 (L2),
ligands.
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Fig. 1 Bis(imino)terpyridine (A), iminoterpyridine (B) and bis(imino)bipyridine; R = H or hydrocarbyl, Ar = aryl group.
route being based on a general method for preparing 2,6-
oligopyridylcarbonyls.8b
2
Results and discussion
Compounds L1 and L2 both display [M + H]+ peaks in
(a) Ligand synthesis
=
their electrospray mass spectra and exhibit n(C N)imine absorption
bands at ca. 1640 cm-1 in their IR spectra. In their 1H NMR spectra
=
The aldimino-containing ligand, 6,6¢-{(2,6-i-Pr2C6H3)N CH)2-
=
=
C10H6N2 (L1), can be prepared by treating the corre-
the CH N and CMe N protons are seen as singlets at d 8.40 and
2.32, respectively. Further confirmation of the composition of L1
was achieved in the form of a single crystal X-ray determination.
L1 crystallises as two independent molecules (A and B) with
each molecule sitting on a centre of symmetry; only minor
differences are apparent between A and B. A view of molecule
A is depicted in Fig. 2 while selected bond lengths and angles are
listed for both A and B in Table 1. The molecular structure of
L1 comprises a 2,2¢-linked bipyridine unit substituted at the 6,6¢-
positions by 2,6-diisopropylphenyl-substituted aldimino groups.
A mutually transoid conformation of the nitrogen atoms is
observed throughout the tetra-nitrogen skeleton, in a manner
similar to that observed for other 2,6-oligopyridylimines8a,d and
=
sponding carbonyl compound, 6,6¢-(O CH)2C10H6N2 with
2,6-diisopropylaniline in absolute ethanol at 50 ◦C while the
=
ketimino derivative, 6,6¢-{(2,6-i-Pr2C6H3)N CMe)2C10H6N2 (L2),
requires more forcing conditions (Scheme 1). Typically, L2
=
can be prepared by reacting 6,6¢-(O CMe)2C10H6N2 with 2,6-
diisopropylaniline (as reagent and solvent) at temperatures be-
tween 160–200 ◦C over a 45 min reaction time. The precur-
sor carbonyl compounds are not commercially available and
are synthesised by either oxidising 6,6¢-dimethyl-2,2¢-bipyridine
with selenium dioxide11 or via a palladium(0)-mediated cross-
coupling of 2-(Bu3Sn)-6-{C(Me)OCH2CH2O}C5H3N with 2-Br-
6-{C(Me)OCH2CH2O}C5H3N (Scheme 1); the latter synthetic
◦
Scheme 1 Reagents and conditions: (i) SeO2, dioxane, heat; (ii) 2 2,6-i-Pr2C6H3NH2, EtOH, 50 C, cat. H+; (iii) 2-bromo-6-(2-methyl-1,3-
dioxolan-2-yl)pyridine, Pd(PPh3)4 (4 mol%), toluene, 100 ◦C, 72 h; (iv) HCl (4 M), 60 ◦C, 12 h; (v) xs 2,6-i-Pr2C6H3NH2, 160–200 ◦C, cat. H+.
Fig. 2 Molecular structure of L1 including a partial atom numbering scheme; the atoms labelled by an additional B are generated by symmetry
(-x, -y, -z). All hydrogen atoms, apart from H13, have been omitted for clarity.
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◦
˚
dipyridylimine unit from L1 with the second imino group pendant
to give an overall endo-exo configuration of the imine groups.
Based on the magnitude of the geometric parameters (t) in
1 and 2a [t = 0.44 (1), 0.43 (2a)],13 the geometries of each
metal centre can be best described as distorted square pyra-
midal. The nitrogen–metal distances within the N,N,N-chelate
are uneven with the central N(2)pyridyl–M(1) distance being the
Table 1 Selected bond distances (A) and angles ( ) for L1
Molecule A
Molecule B
C(18)–C(18B)
C(13)–C(14)
C(13)–N(1)
1.490(4)
1.478(3)
1.267(3)
1.436(3)
1.498(4)
1.480(3)
1.269(3)
1.427(3)
121.3(2)
119.8(2)
N(1)–C(12)
N(1)–C(13)–C(14)
C(13)–N(1)–C(12)
120.9(2)
119.48(19)
˚
shortest [2.064(6) (1), 1.956(3) (2a) A] and the exterior imino
˚
[N(1)imine] group the longest [2.237(7) (1), 2.167(4) (2a) A]. Between
structures the nitrogen–nickel distances in 2a are shorter than
related 2,6-oligopyridines.12 The C–Nimine bond lengths [1.267(3)
the corresponding values in both 1 and the structurally related
˚
A] are consistent with double bond character with the N-aryl
5
=
iron complex [(6,6¢-{(2,4,6-Me3C6H2)N CH}2C10H6N2)FeCl2],
rings inclined essentially orthogonally to the imino units [tors.
consistent with the ionic radii for dipositive nickel ion being
C(13)–N(1)–C(12)–C(8) 104.9◦].
smaller than for zinc and ironhigh spin. The exo-imino group in
˚
each structure [C(24)–N(4) 1.232(10)–1.269(5) A] is almost co-
(b) Synthesis of complexes
planar with the adjacent coordinated pyridine groups [tors. N(3)–
C(23)–C(24)–N(4) 176.19◦ (1), 174.13◦ (2a)] with the N-aryl unit
essentially perpendicular to these planes [tors. C(26)–C(25)–N(4)–
C(24) 109.05◦ (1), 100.78◦ (2a)]. Similarly, the coordinated imine
Treatment of L1 with two equivalents (or greater) of MX2 [MX2 =
ZnCl2, NiCl2, (DME)NiBr2] in n-BuOH at elevated temperature
gave [(L1)MX2] (M = Zn, X = Cl 1; M = Ni, X = Cl 2a; M =
Ni, X = Br 2b) as the sole products in high yield (Scheme 2).
All three complexes have been characterised using a combination
of FAB mass spectrometry, IR and 1H NMR spectroscopy,
elemental analyses and magnetic measurements (see Table 2 and
Experimental section). In addition, crystals of 1 and 2a have been
the subject of single crystal X-ray diffraction studies.
˚
group [C(13)–N(1) 1.248(9)–1.276(5) A] is virtually co-planar with
its neighbouring pyridine unit with the N-aryl unit again tilted
towards orthogonality [tors. C(2)–C(1)–N(1)–C(13) 76.18◦ (1),
71.77◦ (2)].
Both 1 and 2 give fragmentation peaks in their FAB mass
spectra corresponding to the loss of one or two halides from
=
their molecular ions. In their IR spectra a n(C N)imine band at
Needle-shaped crystals of 1 and 2a suitable for the X-ray
determination were grown by slow evaporation of pre-heated chlo-
roform or dichloromethane solutions, respectively. The molecular
structures of 1 and 2a belong to an isomorphous series and will
be discussed together; in each case the complexes crystallise as
two independent molecules (A and B) with only minor variations
apparent between molecules. Fig. 3 shows a view of 2a (molecule
A); selected bond distances and angles for both molecules in 1 and
2a are collected in Table 3.
ca. 1639 cm-1 is comparable to that seen in the free ligand and
hence assignable to the exo-configured imine group. Complexes
2a/2b are both paramagnetic, displaying magnetic moments of
ca. 3 BM (Evans balance at ambient temperature) that support
the presence of two unpaired electrons.
While the 1H NMR spectra of 2a and 2b proved too broad and
paramagnetically shifted to satisfactorily assign, the H NMR
spectrum for diamagnetic 1 at ambient temperature was well
resolved but inconsistent with the structure displayed in the solid
state. In order to probe any dynamic behaviour in 1, a variable
1
The structures of 1 and 2a consist of a five-coordinate metal
centre surrounded by two chloride ligands and a chelating
Scheme 2 Reagents and conditions: (i) 2 MX2 [MX2 = ZnCl2, NiCl2, (DME)NiBr2], n-BuOH, 110 ◦C; (ii) H2O, CDCl3 (Ar = 2,6-i-Pr2C6H3).
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Table 2 Selected characterisation data for 6,6¢-bis(imino)bipyridine-containing 1, 2, 4 and 5
n(C Nimine) a(cm-1)
1H NMR (d)b
meffc/mB
=
Complex
FAB mass spectra
e
1
629 [M - Cl]+, 594
[M - 2Cl]+
1638, 1594
194 K: 1.00 (d, 3J(HH) 5.9, 6H, Nendo-ArCH(CH3)), 1.11 (d, 3J(HH) 6.8,
12H, Nexo-ArCH(CH3)), 1.21 (d, 3J(HH) 5.9, 6H, Nendo-ArCH(CH3)), 2.96
(m, 2H, Nexo-ArCH(CH3)), 3.16 (m, 2H, Nendo-ArCH(CH3)), 7.12 (s,br,
3H, Ar-H), 7.23 (s,br, 3H, Ar-H), 8.03 (d, 3J(HH) 7.2, 1H, Py-Hc), 8.29 (t,
3J(HH) 7.2, 1H, Py-Hb¢), 8.44 (s, 1H, CHd¢=N), 8.46 (d, 1H, Py-Ha¢), 8.47
(t, 1H, Py-Hb¢), 8.60 (d, 3J(HH) 8.4, 1H, Py-Ha), 8.73 (d, 3J(HH) 7.7, 1H,
Py-Hc¢), 9.22 (s, 1H, CHd=N)
303 K: 1.19 (d, 3J(HH) 6.9, 24H, CH(CH3)2), 3.19 (sept, 3J(HH) 6.9, 4H,
CH(CH3)2), 7.20 (m, 6H Ar-H), 8.36 (app d, 4H, Py-Hb/Hb¢, Py-Hc/Hc¢),
8.49 (app t, 2H, Py-Ha/Ha¢), 8.89 (s, 2H, CHd /Hd¢=N)
d
2a
2b
4
624 [M - Cl]+, 588
[M - 2Cl]+
1639, 1590
1638, 1591
1627
3.0
d
668 [M - Br]+, 588
[M - 2Br]+
2.9
795 [M - Cl]+, 695
[M - ZnCl2]+, 657
[M - ZnCl3]+, 622
[M - ZnCl4]+
1.11 (d, 3J(HH) 6.9, 12H, CH(CH3)2), 1.25 (d, 3J(HH) 6.9, 12H,
d
3
=
CH(CH3)2), 2.49 (s, 6H, CH3C N), 2.70 (sept, J(HH) 6.9, 4H,
CH(CH3)2), 7.20–7.30 (m, 6H, Ar–H), 8.32 (d, 3J(HH) 6.9, 2H, Py-Hm),
8.48 (d, 3J(HH) 6.9, 2H, Py-Hm), 8.54 (t, 3J(HH) 6.9, 2H, Py-Hp)
e
5
651 [M - Cl]+, 616
[M - 2Cl]+
1583
3.1
a Recorded on a Perkin-Elmer Spectrum One FT-IR spectrometer on solid samples. b Recorded on a Bruker DRX 400 MHz spectrometer at RT (unless
otherwise stated) in CD2Cl2 (1) or CDCl3 (4). c Recorded on an Evans balance at ambient temperature. d Spectra broad and paramagnetic shifted.
e Samples diamagnetic.
Fig. 3 Molecular structure of 2a including a partial atom numbering scheme. All hydrogen atoms, apart from H13 and H24, have been omitted for
clarity.
1
temperature H NMR study was conducted between 303 K and
194 K (see Table 2 and Fig. 4).
their non-primed counterparts). Some support for the downfield
=
shifted assignment of the CHd Nexo proton (d 9.22) is based
=
On inspection of the pyridyl-H/CH N region of the spectrum
at 194 K (Fig. 4), the presence of six independent pyridyl-H (Ha,
Hb, Hc, Ha¢, Hb¢, Hc¢) and two CH N (Hd, Hd¢) protons is consistent
with one imine group being bound to the metal centre in an endo-
exo fashion as observed in the solid state. The tentative assignment
of Hd¢ (endo-imine) versus Hd (exo-imine) at this temperature was
made using a combination of T-ROESY (350 ms spin lock) and
COSY spectroscopy (as were the assignments of Ha¢, Hb¢, Hc¢ over
on a comparison of the chemical shifts in previously reported
[(oligopyridylimine)ZnCl2] complexes in which the imine groups
adopt exo-configurations.8a,8c As the temperature is raised the
signals start to collapse and then coalesce until at 303 K a weighted
average spectrum is observed for the Ha/Ha¢, Hb/Hb¢ and Hc/Hc¢
and Hd/Hd¢ pairs of resonances. It would seem probable that a fast
exchange process operates at higher temperature involving the two
forms of the endo-exo isomer and is most likely to proceed via an
=
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˚
Table 3 Selected bond distances (A) and angles ( ) for 1, 2a and 3
1 [M = Zn, X = N(4)]
2a [M = Ni, X = N(4)]
Molecule A
Molecule B
Molecule A
Molecule B
3 [M = Zn X = O(1)]
M(1)–N(1)
M(1)–N(2)
M(1)–N(3)
M(1)–Cl(1)
M(1)–Cl(2)
C(13)–N(1)
C(24)–X
2.237(7)
2.064(6)
2.216(7)
2.226(3)
2.226(3)
1.248(9)
1.232(10)
2.228(3)
2.065(6)
2.225(6)
2.247(2)
2.238(3)
1.267(10)
1.243(9)
2.167(4)
1.956(3)
2.122(4)
2.2603(14)
2.2523(14)
1.276(5)
1.269(5)
2.184(4)
1.956(3)
2.118(3)
2.2482(14)
2.2542(15)
1.263(5)
1.253(5)
2.283(5)
2.067(6)
2.288(6)
2.231(2)
2.239(2)
1.285(8)
1.201(9)
N(2)–M(1)–N(3)
N(3)–M(1)–N(1)
N(3)–M(1)–Cl(1)
N(3)–M(1)–Cl(2)
N(2)–M(1)–N(1)
N(2)–M(1)–Cl(1)
N(2)–M(1)–Cl(2)
N(1)–M(1)–Cl(1)
N(1)–M(1)–Cl(2)
Cl(1)–M(1)–Cl(2)
C(13)–N(1)–C(1)
C(23)–C(24)–X
74.3(2)
148.8(2)
99.65(19)
93.4(2)
75.2(2)
149.0(2)
97.67(17)
95.98(18)
74.4(2)
121.5(2)
119.02(19)
92.89(18)
104.00(18)
119.46(9)
119.2(8)
121.3(8)
78.81(15)
156.34(14)
92.86(11)
93.23(11)
72.82(14)
113.23(11)
115.96(11)
99.64(10)
93.68(10)
130.69(5)
120.5(4)
78.43(14)
156.16(14)
94.85(11)
91.73(11)
77.73(14)
110.31(11)
119.60(11)
93.63(10)
99.76(10)
129.97(6)
121.3(4)
74.2(2)
149.0(2)
99.46(14)
93.40(15)
74.9(2)
124.48(15)
118.00(15)
99.45(15)
99.36(14)
117.41(8)
119.7(6)
124.3(7)
74.5(3)
117.0(2)
122.3(2)
96.10(19)
101.5(2)
120.56(11)
120.6(7)
125.6(9)
121.5(5)
122.3(4)
1
=
Fig. 4 Variable temperature H NMR spectra for 1 in the pyridyl-H/CH N region (in CD2Cl2).
intermediate exo-exo form of 1 (see inset in Fig. 4). The
presence of exo-exo-1 as the sole species in solution cannot,
however, be discounted to explain the room temperature data.
In any case, the barrier to interconversion for the process
has been determined as DH‡ 35.6 1.5 kJ mol-1 (DG‡ 47.4
298
3.3 kJ mol-1).14
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In the isopropyl region of the 1H NMR spectrum of 1 at 303 K,
the presence of signals for only one type of CHMe2 and CHMe2
protons supports free rotation about the N-aryl bond. However,
at low temperature (194 K) the signal for the CHMe2 protons
L2 with MX2 (MX2 = ZnCl2, NiCl2) under similar conditions to
that described with L1 gave [(L2)Zn2Cl4] (4) and [(L2)NiCl2] (5)
in high yield (Scheme 2). Both complexes have been characterised
using a combination of FAB mass spectrometry, IR and 1H NMR
spectroscopy, elemental analyses and magnetic measurements (see
Table 2 and experimental section). In addition, crystals of 4 and 5
have been the subject of single crystal X-ray diffraction studies.
Crystals of 4 suitable for the X-ray determination were grown
by evaporation of a saturated chloroform solution of the complex.
Molecules of 4 sit on a crystallographic symmetry element;
Fig. 6 depicts a view of 4 while selected bond distances and
angles are collected in Table 4. The structure of 4 reveals a
bimetallic complex in which the two zinc centres are supported
on the same L2 ligand frame and each bound terminally by
two chloride ligands. The geometry at each zinc centre can
be best described as distorted tetrahedral with the Nimine–Zn–
Npyridine bite angle in each pyridylimine cavity being 78.5(3)◦.
Both pyridylimine moieties are almost planar [tors. N(1)–C(13)–
C(15)–N(2) 3.44◦] and are tilted with respect to each other
[tors. N(2)–C(19)–C(19A)–N(2A) 98.09◦]; the N-aryl groups are
almost perpendicular to the pyridylimine units [tors. C(2)–C(1)–
N(1)–C(13) 79.13◦]. The Zn–N distances are similar [Zn(1)–
3
splits to form two separate septets (each J(HH) 6.8 Hz), while
the CHMe2 protons give rise to three distinct doublets in a ratio
6 : 12 : 6; the two doublets with a 6 : 6 ratio correspond to the bound
imine group in which rotation about the N–C2,6-diisopropylphenyl bond
has been restricted. Due to difficulties associated with fitting the
data in the isopropyl region at lower temperature, it is likely that
the dynamic process highlighted above is further complicated by
the consequences of restricted rotation about the N-aryl bond.15
A further feature of the 1H NMR spectrum of 1 is the presence
of a minor impurity (see Fig. 4) that, despite numerous attempts
to eliminate it (e.g., crystallisation), could not entirely be removed
from the sample. Indeed, the intensity of the signals corresponding
to the impurity relative to 1 increased on prolonged standing in
CD2Cl2 or CDCl3. After several weeks in CDCl3 small quantities of
=
pyramidal-shaped yellow crystals of [(6-{(2,6-i-Pr2C6H3)N CH}-
=
6¢-(CH O)C10H6N2)ZnCl2] (3) suitable for a single crystal X-ray
diffraction study were obtained. Fig. 5 shows a view of 3; selected
bond distances and angles are collected in Table 3. The structure
of 3 resembles 1 with a five-coordinate zinc centre surrounded by
a dipyridylimine chelate and two chloride ligands but differs in
that the pendant unit is now a formyl group (i.e., endo-imine, exo-
formyl). The presence of the formyl group has minimal effect on
the parameters within the N,N,N-chelate (e.g., slight elongation
of the exterior Npy and Nim donors) and the geometric parameter
[t = 0.41]13 at zinc is similar to that in 1. The apparent hydrolysis
of one imine group in 1 to give 3 and 2,6-diisopropylaniline during
standing is likely due to contamination by low levels of water in
the NMR solvent employed. Notably, the minor signals due to 3
present in the 1H NMR spectrum of 1 showed no major variation
during the variable temperature study.
˚
N(1) 2.055(7) vs. Zn(1)–N(2) 2.057(7) A] and comparable with
◦
˚
Table 4 Selected bond distances (A) and angles ( ) for 4
Zn(1)–Cl(1)
Zn(1)–Cl(2)
Zn(1)–N(1)
Zn(1)–N(2)
C(13)–N(1)
C(13)–C(14)
C(1)–N(1)
2.207(3)
2.172(3)
2.055(7)
2.057(7)
1.285(10)
1.516(12)
1.439(11)
1.482(15)
C(19)–C(19A)
N(1)–Zn(1)–N(2)
N(1)–Zn(1)–Cl(1)
N(1)–Zn(1)–Cl(2)
Cl(1)–Zn(1)–Cl(2)
N(1)–C(13)–C(15)
78.5(3)
108.2(2)
122.0(2)
117.17(10)
116.6(8)
Given the observed flexibility of aldimino L1 to coordination
with an MX2 unit, it was of interest to probe the effect of subtly
changing the donor properties of the imine unit would have on
the binding properties of the ligand. Thus, reaction of ketimino
Fig. 5 Molecular structure of 3 including a partial atom numbering scheme. All hydrogen atoms, apart from H13 and H24, have been omitted for clarity.
190 | Dalton Trans., 2009, 185–196 This journal is The Royal Society of Chemistry 2009
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Fig. 6 Molecular structure of 4 including a partial atom numbering scheme; the atoms labelled by an additional A are generated by symmetry (-x, y,
-z + 1/2). All hydrogen atoms have been omitted for clarity.
those seen in 1, 3 and other crystallographically characterised
˚
˚
longest [Ni(1)–N(1) 2.238(4) A, Ni(1)–N(4) 2.226(4) A]. The
two N-aryl groups are some way of orthogonality with regard
to the respective pyridylimine planes which is most significant
for the aryl group linked to N(4) [tors. C(25)–N(4)–C(27)–C(28)
64.64◦]; this uneven tilting of the N-aryl groups is likely due to
intramolecular steric hindrance between the two neighbouring
bulky aryl groups. Notably, the endo-endo configuration exhibited
by L2 in 5 contrasts with the endo-exo arrangement adopted by L1
in nickel-containing 2a but its capacity to bind in a tetradentate
pyridylimine-zinc dihalide-containing complexes.16 Inspection of
˚
the intermetallic distance [4.888 A] reveals that there is no direct
Zn ◊ ◊ ◊ Zn interaction between the two metal centres.
Fig. 7 shows a view of 5; selected bond distances and angles
are collected in Table 5. The molecular structure comprises a
single nickel atom bound by both L2 and two terminal chloride
ligands so as to form a distorted octahedral geometry. The
chloride ligands are disposed mutually trans [Cl(1)–Ni(1)–Cl(2)
175.75(5)◦] with all the four nitrogen donor atoms of L2 filling the
equatorial belt to give an endo-endo configuration for both imines.
The nitrogen–nickel distances are unequally disposed with the
internal pyridyl distances being the shortest [Ni(1)–N(2) 1.989(4)
fashion does resemble that found in the structurally related 1 : 1
8a
=
complexes trans-[{(2,6-i-Pr2C6H3N CMe)2terpy}NiBr2], trans-
8c
=
[{(2,6-i-Pr2C6H3N CMe)terpy}NiBr2] and trans-[(quaterpyri-
dine)Ni(L)2](NCA) (L = NCMe, NCA = PF6;12a L = OH2, NCA =
˚
˚
BF417).
A, Ni(1)–N(3) 1.997(4) A] and the external imino lengths the
Fig. 7 Molecular structure of 5 including a partial atom numbering scheme. All hydrogen atoms have been omitted for clarity.
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Scheme 3 Proposed pathway for bimetallic assembly of ZnCl2 on L2 (Ar = 2,6-i-Pr2C6H3).
ketimine > aldimine18) which are further supplemented by steric
effects associated with the nature of the imine-carbon substituents.
For example, an isomer of 5 containing endo-exo configuration of
the imine groups (cf. 2a) is unlikely as the exo-imino-methyl group
would undergo significant steric hindrance with the MX2 unit.
Indeed DFT calculations have been used to support a similar
endo-preference observed in [(6-ketiminoterpyridine)NiX2] (X =
Br, Cl).8c For zinc a similar steric argument prevents an endo-
exo configuration of the imine groups in L2 from being adopted
(Scheme 3), but in this case there is also a significant electronic
barrier to endo-endo formation (i.e., electron-electron repulsions
between the fully occupied d-shell and incoming second imine-
nitrogen donor8c). The result is that one pyridine unit in the endo-
exo intermediate dissociates to give a vacant pyridylimine cavity
(a in Scheme 3) that can now accommodate a second molecule
of ZnCl2 to give 4. Notably, in [(6-ketiminoterpyridine)ZnCl2]
the imine group does adopt an exo configuration but with some
significant twisting of the imine unit away from co-planarity with
the adjacent pyridine.8c Attempts to prepare a mono-zinc complex
of L2 using equimolar ratios of L2 and ZnCl2 in CDCl3 at room
temperature gave only unreacted L2 or 4.
◦
˚
Table 5 Selected bond distances (A) and angles ( ) for 5
Ni(1)–Cl(1)
Ni(1)–Cl(2)
Ni(1)–N(1)
Ni(1)–N(2)
2.4178(15)
2.4121(15)
2.238(4)
Ni(1)–N(3)
Ni(1)–N(4)
C(13)–N(1)
C(25)–N(4)
1.997(4)
2.226(4)
1.271(6)
1.288(6)
1.989(4)
N(1)–Ni(1)–N(2)
N(1)–Ni(1)–N(3)
N(1)–Ni(1)–N(4)
N(1)–Ni(1)–Cl(1)
N(1)–Ni(1)–Cl(2)
N(2)–Ni(1)–N(3)
N(2)–Ni(1)–N(4)
N(2)–Ni(1)–Cl(1)
76.43(16)
154.73(16)
128.76(15)
90.21(11)
92.32(11)
78.76(17)
154.62(16)
90.64(12)
N(2)–Ni(1)–Cl(2)
N(3)–Ni(1)–N(4)
N(3)–Ni(1)–Cl(1)
N(3)–Ni(1)–Cl(2)
N(4)–Ni(1)–Cl(1)
N(4)–Ni(1)–Cl(2)
Cl(1)–Ni(1)–Cl(2)
86.63(12)
76.34(16)
85.30(12)
90.97(12)
92.17(11)
88.92(11)
175.75(5)
Both 4 and 5 give fragmentation peaks in their FAB mass
spectra corresponding to the loss of chlorides from their molecular
ions; in the case of bimetallic 4 this is accompanied by the
=
loss of a ZnCl2 unit. In their IR spectra, the n(C N) bands
for the coordinated imines are shifted (cf. the free ligand) to
lower wavenumber by 11 cm-1 for 4 and 55 cm-1 for 5. Nickel(II)
complex 5 exhibits magnetic moment of 3.1 BM (Evans balance
and ambient temperature) which is consistent with the presence of
two unpaired electrons. The 1H NMR spectrum of 4 supports the
solid state structure being maintained in solution with a single type
3
Conclusions
=
of CMe N group and three types of pyridyl-H protons evident;
the expected restricted rotation of the bound N-aryl groups is
confirmed with two sets of CHMe2 doublets (3J(HH) = 6.9 Hz).
From the results described in this work, bis(imino)bipyridine
Lx represents a highly flexible ligand frame that, depending on
the nature of imino-carbon substituent and the metal centre
employed, can exhibit at least four different bonding modes viz.
as an Npy,Npy,Nim-chelate, Npy,Npy-chelate, bis(Npy,Nim)-chelate
and as a Nim,Npy,Npy,Nim-chelate and in turn three different
relative configurations of the imines: endo-exo, exo-exo and endo-
endo. While its nickel chemistry mirrors that observed with the
unsymmetrical counterpart 6-iminoterpyridine (B in Fig. 1) with
the aldimino L1 preferring to form 5-coordinate complexes (with
one exo-imine) and ketimino L2 favouring 6-coordinate complexes
(both endo-imines),8c the zinc chemistry shows some differences
which is best exemplified by its ability to form a bimetallic complex
(4). These structural variations are likely due to electronic effects
of the metal centres [d8 Ni(II) vs. d10 Zn(II)] coupled with the
donor capability of nitrogen donors (donor strength: pyridine >
Sterically bulky 6,6¢-bis(arylimino)-2,2¢-bipyridine compounds
(Lx, aryl = 2,6-diisopropylphenyl) have been successfully prepared
and their adaptability to coordination of divalent zinc and nickel
halides has been revealed. With the aldimine derivatives (L1),
irrespective of the metal centre employed, a preference for the
ligand to bind as tridentate ligand with one imino group exo is
observed (1, 2). In the case of the zinc derivative, 1, the lability of
the M–Naldimine bond in solution and consequent interconversions
between bonding modes has been demonstrated. On the other
hand the ketimine derivatives (L2) can bind in an alternative
manner, the precise mode of binding being dependent on the
metal centre employed. Zinc(II) ions prefer to fill two bidentate
sites within the ligand frame while nickel forms a 1 : 1 complex
with the ligand employing all four donor nitrogen donor atoms. It
is likely that the steric/electronic attributes of the particular imine
donor (ketimine vs. aldimine) coupled with stabilisation effects
imparted by specific dn configurations are likely to be influential
factors on product distribution.
192 | Dalton Trans., 2009, 185–196
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protected compound. Mp: 179–181 ◦C. 1H NMR (CDCl3): d 1.85
(s, 6H, C(CH3)OCH2CH2-), 3.95 (m, 4H, OCH2CH2-), 4.15 (m,
4H, OCH2CH2-), 7.04 (d, 3J(HH) 6.7, Py-H), 7.71 (t, 3(HH) 6.7,
4
Experimental
4.1 General
Py-H), 8.45 (d, (HH) 6.7, Py-H). 13C { H} NMR (CDCl3): d
25.0 (CH3), 65.1 (-OCH2CH2-), 108.9 (C(CH3)O), 119.2 (Py),
120.6 (Py), 137.3 (Py), 155.8 (Py), 160.0 (Py). IR (cm-1): 2880 (w),
1574 (s), 1374 (s), 1204 (s), 1144 (w), 1036 (w), 952 (w), 877 (s),
751 (w), 685 (w). ESIMS: m/z 329 [M + H]+. HRMS (FAB): calcd
for C18H21N2O4 [M + H]+ 329.14961, found 329.14969.
3
1
All operations, unless otherwise stated, were carried out under
an inert atmosphere of dry oxygen-free nitrogen using standard
Schlenk and cannular techniques or in a nitrogen purged glove box.
Solvents were distilled under nitrogen from appropriate drying
agents and degassed prior to use19 or were employed directly from
a Solvent Purification System (Innovative Technology, Inc). The
electrospray (ESI) mass spectra were recorded using a micromass
Quattra LC mass spectrometer with dichloromethane or methanol
as the matrix. FAB mass spectra (including high resolution)
were recorded on Kratos Concept spectrometer with NBA as
matrix. The infrared spectra were recorded in the solid state
with Universal ATR sampling accessories on a Perkin Elmer
Spectrum One FTIR instrument. NMR spectra were recorded
on a Bruker DPX 300 spectrometer operating at 300.03 (1H) and
75.4 MHz (13C) or a Bruker DRX400 spectrometer at 400.13 (1H)
and 100.61 MHz (13C) at ambient temperature unless otherwise
stated; chemical shifts (ppm) are referred to the residual protic
solvent peaks and coupling constants are expressed in hertz (Hz).
Assignments of 1H NMR and 13C NMR signals were made where
possible, using COSY, DEPT, HMQC and HMBC experiments.
Low temperature NMR spectra were recorded using the Bruker
DRX400 spectrometer equipped with a nitrogen evaporator unit
and Eurotherm temperature control unit. Sample temperatures
were corrected against a calibration curve obtained from neat
methanol (Wilmad) using the method of Ammann et al.20 Line
shape analysis of the variable temperature 1H NMR spectra
using the program gNMR21 was used to obtain rate constants
Step 2. The diacetal-protected compound was stirred over-
night at 60 ◦C in 4M HCl (20 ml), cooled to room temperature and
carefully neutralised by addition of 2M NaHCO3. The suspension
was extracted with CHCl3 (3 ¥ 30 ml) and the organic phase
separated, washed with water (3 ¥ 30 ml), brine (1 ¥ 40 ml) and
dried over magnesium sulfate. Following filtration, the solvent was
removed under reduced pressure and the crude product crystallised
from ethanol at -30 ◦C. The resulting precipitate was collected by
filtration to afford 6,6¢-diacetyl-2,2¢-bipyridine (0.611 g, 66%) as
an off-white solid. Mp: 161–164 ◦C. 1H NMR (CDCl3): d 2.74 (s,
3
3
6H, CH3C(O)), 8.01 (t, J(HH) 6.8, 2H, Py-H), 8.11 (d, J(HH)
6.8, 2H, Py-H), 8.72 (d, 3J(HH) 6.8, 2H, Py-H). 13C { H} NMR
1
(CDCl3): d 25.7 (CH3), 121.9 (CH, Py), 124.3 (CH, Py), 138.0
-1
=
(CH, Py), 153.1 (C, Py), 154.7 (C, Py), 200.0 (C O). IR (cm ):
=
2964 (w), 1699 (s, C O), 1575 (s), 1436 (w), 1411 (s), 1356 (w),
1149 (s), 738 (w). ESIMS: m/z 241 [M + H]+. HRMS (FAB):
calcd for C14H13N2O2 [M + H]+ 241.09738, found 241.09745.
4.3 Synthesis of 6,6¢-bis(aldimino)-2,2¢-bipyridine
(2,6-diisopropylanil) (L1)
=
for the CH N protons in 1. Magnetic susceptibility studies were
6,6¢-Diformyl-2,2¢-bipyridine (0.500 g, 2.55 mmol) was suspended
in absolute ethanol (10 ml) and 2,6-diisopropylaniline (2.2 equiv.)
introduced along with a catalytic amount of glacial acetic acid. The
mixture was stirred and heated at 50 ◦C for 18 h. After cooling to
room temeperature the resulting precipitate collected by filtration
and washed with ethanol to give L1 as a bright yellow solid
(0.960 g, 71%). Slow evaporation of a dichloromethane solution
of L1 gave yellow blocks. Mp: 226–228 ◦C. 1H NMR (CDCl3): d
1.19 (d, 3J(HH) 6.9, 24H, CH(CH3)2), 3.00 (sept, 3J(HH) 6.9, 4H,
CH(CH3)2), 7.15 (t, 3J(HH) 6.9, 2H, Ar-p-H), 7.20 (d, 3J(HH) 6.9,
performed using an Evans Balance (Johnson Matthey) at room
temperature. The magnetic moments were calculated following
standard methods22 and corrections for underlying diamagnetism
were applied to the data.23 Melting points (mp) were measured
on a Gallenkamp melting point apparatus (model MFB-595) in
open capillary tubes and were uncorrected. Elemental analyses
were performed at the Science Technical Support Unit, London
Metropolitan University.
The reagents 2,6-diisopropylaniline, nickel dichloride, nickel
dibromide 1,2-dimethoxyethane and zinc chloride were purchased
from Aldrich Chemical Co. and used without further pu-
rification. The compounds 2-bromo-6-(2-methyl-1,3-dioxolan-2-
yl)pyridine,12c,24 tetrakis(triphenylphosphine)palladium(0),25 6,6¢-
diformyl-2,2-bipyridine11 and 6-tributylstannyl-2-(2-methyl-1,3-
dioxolan-2-yl)pyridine8b were prepared using literature proce-
dures. All other chemicals were obtained commercially and used
without further purification.
3
3
4H, Ar-m-H), 7.98 (t, J(HH) 6.9, 2H, Py-p-H), 8.31 (d, J(HH)
3
=
6.9, 2H, Py-m-H), 8.40 (s, 2H, CH N), 8.63 (d, J(HH) 6.9, 2H,
Py-m-H). 13C { H} NMR (CDCl3): d 23.9 (CH3), 28.4 (CH(CH3)2),
1
121.7 (CH, Py/Ar), 123.2 (CH, Py/Ar), 123.5 (CH, Py/Ar), 124.9
(C, Py/Ar), 137.7 (C, Py/Ar), 138.1 (CH, Py), 148.9 (C, Py/Ar),
-1
=
154.4 (C, Py/Ar), 156.0 (C, Py/Ar), 163.4 (C N). IR (cm ):
=
=
2959 (m), 1637 (s, C Nim), 1572 (s, C Npy), 1440 (s), 1383 (m),
1361 (m), 1306 (w), 1256 (w), 1181 (m), 1108 (w), 1074 (m), 991 (w),
802 (s), 799 (s), 742 (s). ESIMS: m/z 553 [M + Na]+, 531 [M +
H]+.
4.2 Synthesis of 6,6¢-diacetyl-2,2¢-bipyridine
Step 1. 2-Bromo-6-(2-methyl-1,3-dioxolan-2-yl)pyridine(0.941 g,
4.13 mmol), 6-tributylstannyl-2-(2-methyl-1,3-dioxolan-2-yl)-
pyridine (2.052 g, 4.54 mmol, 1.1 eq.) and tetrakis(triphen-
ylphosphine)palladium(0) (0.191 g, 0.165 mmol, 0.04 eq.) were
loaded in a Schlenk vessel under nitrogen and the contents stirred
in dry toluene (30 ml) for 72 h at 90 ◦C. After removal of the
solvent under reduced pressure, the reaction mixture was dissolved
in ethanol and left in the freezer overnight to yield the diacetal-
4.4 Synthesis of 6,6¢-bis(ketimino)-2,2¢-bipyridine
(2,6-diisopropylanil) (L2)
6,6¢-Diacetyl-2,2¢-bipyridine (0.375 g, 1.56 mmol) was suspended
in an excess of 2,6-diisopropylaniline (2.8 ml, 10 eq.) and stirred
for 15 min at 160 ◦C on a heating mantle. A catalytic amount of
formic acid was added, and the reaction mixture was stirred for an
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additional 20 min at temperatures between 160–200 ◦C. Following
removal of the excess 2,6-diisopropylaniline under reduced pres-
sure (130 ◦C, 0.5 mmHg), the resulting brown residue was stirred in
ethanol at room temperature and the resultant precipitate filtered
0.189 mmol, 0.5 eq.) afforded [(L1)NiBr2] (2b) as a pale red solid.
-1
=
Yield: 0.114 g, 81%. IR (cm ): 2924 (m), 1638 (m, n(C N)), 1591
=
(w, n(C N)), 1573 (m), 1461 (s), 1434 (m), 1360 (w), 1264 (w),
1166 (m), 1109 (m), 1014 (w), 799 (vs), 770 (s) and 738 (s). Anal
Calc. for (C36H42Br2Ni): C, 57.72; H, 5.61; N, 7.48. Found: C,
57.99; H, 5.81; N, 7.35%.
and washed with ethanol to give L2 as an off-white solid (0.410 g,
◦
1
3
47%). Mp: 288–289 C. H NMR (CDCl3): d 1.15 (d, J(HH)
3
=
6.9, 24H, CH(CH3)2), 2.32 (s, 6H, CH3C N), 2.70 (sept, J(HH)
3
6.9, 4H, CH(CH3)2), 7.10 (t, J(HH) 6.9, 2H, Ar-p-H), 7.10 (d,
4.6 Synthesis of [(L2)Zn2Cl4] (4)
3J(HH) 6.9, 4H, Ar-m-H), 7.94 (t, 3J(HH) 6.9, 2H, Py-p-H), 8.38
(d, 3J(HH) 6.9, 2H, Py-m-H), 8.63 (d, 3J(HH) 6.9, 2H, Py-m-H).
An oven-dried Schlenk flask equipped with a magnetic stir bar was
evacuated and backfilled with nitrogen. The flask was charged with
ZnCl2 (0.049 g, 0.358 mmol) in n-BuOH (10 ml) and the contents
stirred at 100 ◦C until the zinc salt had completely dissolved. L2
(0.100 g, 0.179◦mmol, 0.5 eq.) was added and the reaction mixture
stirred at 100 C overnight. After cooling to room temperature,
the solvent was removed under reduced pressure and the residue
dried. Hexane was added and the precipitate filtered, washed with
more hexane and dried overnight under reduced pressure to afford
[(L2)Zn2Cl4] (4) as a pale orange powder. Yield: 0.111 g, 75%.
13C { H} NMR (CDCl3): d 17.3 (CH3), 22.9 (CH3), 23.2 (CH3),
1
28.3 (CH(CH3)2), 121.3 (CH, Py/Ar), 123.0 (CH, Py/Ar), 123.6
(CH, Py/Ar), 135.8 (C, Py/Ar), 137.3 (CH, Py), 146.6 (C, Py/Ar),
154.9 (C, Py/Ar), 155.7 (C, Py/Ar), 167.1 (C=N). IR (cm-1):
=
=
2961 (m), 1638 (s, C Nim), 1572 (s, C Npy), 1437 (s), 1383 (m),
1359 (s), 1306 (w), 1243 (w), 1195 (w), 1116 (s), 1077 (m), 814 (s),
780 (s), 745 (s), 686 (w). ESIMS: m/z 559 [M + H]+. HRMS (FAB):
calcd for C38H47N4 [M + H]+ 559.37890, found 559.37908.
Recrystallisation from warm chloroform gave 4 as pale yellow
-1
4.5 Synthesis of [(L1)MX2] (1, 2)
=
=
blocks. IR (cm ): 2963 (m), 1627 (s, C Nim), 1587 (s, C Npy),
1463 (s), 1447 (s), 1384 (w), 1368 (s), 1326 (w), 1255 (s), 1199 (s),
1116 (s), 1057 (w), 1021 (m), 808 (s), 790 (s), 746 (s). Anal Calc.
for (C38H46Cl4Zn2): C, 54.89; H, 5.54; N, 6.74. Found: C, 55.04; H,
5.61; N, 6.86%.
(a) M = Zn, X = Cl (1). An oven-dried Schlenk flask equipped
with a magnetic stir bar was evacuated and backfilled with
nitrogen. The flask was charged with ZnCl2 (0.039 g, 0.283 mmol)
in n-BuOH (10 ml) and the contents stirred at 100 ◦C until the zinc
salt had completely dissolved. L1 (0.075 g, 0.142 mmol, 0.5 eq.)
4.7 Synthesis of [(L2)NiCl2] (5)
◦
was added and the reaction mixture stirred at 100 C overnight.
After cooling to room temperature, the solvent was concentrated
under reduced pressure and hexane added to induce precipitation.
Following filtration, washing with hexane and drying under under
reduced pressure, complex [(L1)ZnCl2] (1) was obtained as a yellow
powder. Yield: 0.083 g, 88%. Slow evaporation of a chloroform
solution of 1 gave pale yellow needles; on prolonged standing of
the filtrate a second crop of 1 were obtained along with pyramidal-
shaped crystals of 3. Complex 1: IR (cm-1): 2962 (m), 1638 (m,
An oven-dried Schlenk flask equipped with a magnetic stir bar
was evacuated and backfilled with nitrogen. The flask was charged
with anhydrous NiCl2 (0.046 g, 0.358 mmol) in n-BuOH (10 ml)
and the contents stirred at 100 ◦C until the nickel salt had
partially dissolved. L2 (0.100 g, 0.179 mmol, 0.5 eq.) was added
and the reaction mixture stirred at 100 ◦C overnight. After
cooling to room temperature, the solvent was concentrated under
reduced pressure and hexane added to induce precipitation of the
product. The precipitate was filtered, washed with hexane and
dried overnight under reduced pressure to afford [(L2)NiCl2] (5)
as a pale orange powder. Yield: 0.105 g, 85%. Recrystallisation
=
=
n(C N)), 1594 (m, n(C N)), 1574 (m), 1462 (m), 1438 (s), 1361 (m),
1239 (w), 1170 (m), 1024 (m), 812 (m), 799 (vs), 769 (s) and 737 (s).
Anal Calc. for (C36H42Cl2Zn): C, 64.83; H, 6.30; N, 8.40. Found: C,
65.01; H, 6.28; N, 8.19%. Complex 3: FABMS: m/z 471 [M-Cl]+,
from warm acetonitrile gave 5 as orange blocks; from CHCl3
436 [M-2Cl]+. H NMR (CD2Cl2): d 1.22 (d, J(HH) 6.9, 12H,
-1
1
3
=
gave orange plates. IR (cm ): 2961 (m), 1583 (s, C Nim), 1571
3
CH(CH3)2), 3.3 (sept, J(HH) 6.8, 2H, CH(CH3)2), 7.3 (m, 3H,
=
(s, C Npy), 1459 (s), 1432 (s), 1381 (w), 1369 (s), 1325 (w), 1253 (s),
Ar-H), 8.07 (d, 3J(HH) 7.6, 1H, Py-H), 8.29 (d, 3J(HH) 7.7,
1194 (s), 1037 (w), 1023 (m), 808 (s), 783 (s), 747 (s). Anal Calc.
for (C38H46N4Cl2Ni): C, 66.31; H, 6.69; N, 8.14. Found: C, 66.11;
H, 6.82; N, 8.02%.
1H, Py-H), 8.31 (d, 3J(HH) 5.2, 1H, Py-H), 8.33 (t, J(HH) 7.7,
3
3
=
1H, Py-H), 8.49 (s, 1H, CH N), 8.51 (t, J(HH) 7.5, 1H, Py-H),
3
4
=
8.59 (d, J(HH) 7.8, 1H, Py-H), 11.01 (d, J(HH) 0.7, 1H, CH O).
4.8 Crystallographic studies
(b) M = Ni, X = Cl (2a). Using the approach outlined in
4.5(a) using anhydrous NiCl2 (0.049 g, 0.377 mmol) and L1
(0.100 g, 0.189 mmol, 0.5 eq.) afforded [(L1)NiCl2] (2a) as a pale
red solid. Yield: 0.093 g, 75%. Layering of a solution of 2a in
dichloromethane with diethylether gave red needles. IR (cm-1):
Data for 1, 2a, 3, 4 and 5 were collected on a Bruker APEX 2000
CCD diffractometer while data for L1 were collected on Bruker P4
diffractometer. Details of data collection, refinement and crystal
data are listed in Table 6. The data were corrected for Lorentz and
polarisation effects and empirical absorption corrections applied.
Structure solution by direct methods and structure refinement
based on full-matrix least-squares on F2 employed SHELXTL
version 6.10.26 Hydrogen atoms were included in calculated
=
2924 (m), 1639 (m, n(C N)), 1590 (w), 1573 (m), 1462 (s), 1434 (m),
1361 (w), 1262 (w), 1167 (m), 1109 (m), 1012 (w), 799 (vs), 769 (s)
and 737 (s). FABMS: m/z 623 [M - Cl]+, 588 [M - 2Cl]+. meff
(Evans Balance): 3.0 BM. Anal Calc. for (C36H42Cl2Ni): C, 65.48;
H, 6.37; N, 8.49. Found: C, 65.71; H, 6.40; N, 8.25%.
˚
positions (C–H = 0.96 A) riding on the bonded atom with isotropic
displacement parameters set to 1.5 Ueq(C) for methyl H atoms and
1.2 Ueq(C) for all other H atoms. All non-H atoms were refined
with anisotropic displacement parameters. Disordered solvent was
(c) M = Ni, X = Br (2b). Using the approach outlined in
4.5(a) using (DME)NiBr2 (0.116 g, 0.377 mmol) and L1 (0.100 g,
194 | Dalton Trans., 2009, 185–196
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Table 6 Crystallographic and data processing parametersa for L1, 1, 2a, 3, 4 and 5
Complex
Formula
L1
1b
2ab
3b
4
5
C36H42N4
530.74
C36H42Cl2N4Zn. C36H42Cl2N4Ni.
0.5CH2Cl2.2H2O H2O
745.50
C24H25Cl2N3OZn. C38H46Cl4N4Zn2. 2CHCl3. C38H46Cl2N4Ni.
CHCl3.CH2Cl2
712.03
2(CH3CH2CH2CH2OH) 3CHCl3
1218.30
M
678.36
1046.50
Crystal size (mm)
Temperature (K)
Crystal system
Space group
0.50 ¥ 0.48 ¥ 0.17 0.21 ¥ 0.17 ¥ 0.15 0.26 ¥ 0.20 ¥ 0.17 0.27 ¥ 0.18 ¥ 0.09 0.25 ¥ 0.12 ¥ 0.07
0.23 ¥ 0.22 ¥ 0.05
200(2)
150(2)
150(2)
150(2)
150(2)
150(2)
Triclinic
P1
Orthorhombic
P212121
13.701(3)
16.571(3)
33.355(6)
90
90
90
7573(3)
8
Orthorhombic
P212121
13.5748(17)
16.500(2)
32.768(13)
90
Tetragonal
I4
20.148(5)
20.148(5)
15.683(5)
90
90
90
6366(3)
8
Monoclinic
C2/c
Triclinic
¯
P1
¯
¯
˚
a (A)
8.942(3)
9.685(3)
18.211(5)
90.00(2)
80.88(2)
88.617(18)
1556.7(8)
2
24.436(8)
14.694(5)
16.839(6)
90
112.374(6)
90
10.8776(17)
14.496(2)
17.156(3)
65.944(3)
88.962(3)
77.909(3)
2408.6(6)
2
1.443
1076
1.047
19018
˚
b (A)
˚
c (A)
a(◦)
b(◦)
g (◦)
90
90
3
˚
U (A )
7339.4(16)
8
5591(3)
4
Z
Dc (Mg m-3)
F(000)
1.132
572
1.308
3128
1.228
2864
1.486
2896
1.447
2520
m(Mo-Ka) (mm-1)
Reflections collected
Independent reflections
Rint
0.067
6642
0.897
59507
0.706
57619
1.384
24706
1.376
19671
5497
0.0445
0/361
14881
0.1839
0/781
14424
0.1161
0/791
6227
0.1569
0/284
4910
0.1937
0/222
9350
0.0779
0/524
Restraints/parameters
Final R indices (I > 2s(I)) R1 = 0.0567
R1 = 0.0788
wR2 = 0.1404
R1 = 0.1749
wR2 = 0.1710
0.883
R1 = 0.0597
wR2 = 0.0954
R1 = 0.0954
wR2 = 0.1053
0.883
R1 = 0.0651
wR2 = 0.1035
R1 = 0.1238
wR2 = 0.1190
0.809
R1 = 0.0808
wR2 = 0.1782
R1 = 0.2068
wR2 = 0.2126
0.774
R1 = 0.0665
wR2 = 0.1164
R1 = 0.1402
wR2 = 0.1371
0.886
wR2 = 0.1265
All data
R1 = 0.0963
wR2 = 0.1457
Goodness of fit on F2 (all 1.015
data)
ꢀ
ꢀ
ꢀ
ꢀ
a
2
Data in common: graphite-monochromated Mo-Ka radiation, l = 0.71073 A; R1 = ꢀFo| - |Fcꢀ/ |Fo|, wR2 = [ w(Fo - Fc2)2/ w(Fo2)2]1/2
,
˚
ꢀ
w-1 = [s (Fo)2 + (aP)2], P = [max(Fo2,0) + 2(Fc2)]/3, where a is a constant adjusted by the program; goodness of fit = [ (Fo - Fc2)2/(n - p)]1/2 where n
2
2
is the number of reflections and p the number of parameters. b Flack parameter = 0.065(18) (1), 0.046(13) (2a), 0.08(2) (3).
omitted using the SQUEEZE option in PLATON for 1, 2a, 3
and 4.27
CCDC reference numbers 692756–692761.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b811309b
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Acknowledgements
We thank the University of Leicester for financial assistance.
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