Coordination study of a new class of imine imidazol-2-imine ligands to titanium(iv) and palladium(ii)

Article abstract of DOI:10.1039/c0dt00498g

A new family of ligand precursors, N-(1-(2,6-dimethylphenylimino)ethyl)-1, 3-bis(aryl)imidazol-2-imine hydrochloride, wherein aryl = 2,4,6-trimethylphenyl (3a) and 2,6-diisopropylphenyl (3b), was prepared and structurally characterised. Deprotonation of t

Full text of DOI:10.1039/c0dt00498g

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Coordination study of a new class of imine imidazol-2-imine ligands to
titanium(IV) and palladium(II)†
Sarim Dastgir and Gino G. Lavoie*
Received 18th May 2010, Accepted 9th June 2010
First published as an Advance Article on the web 25th June 2010
DOI: 10.1039/c0dt00498g
A new family of ligand precursors, N-(1-(2,6-dimethyl-
phenylimino)ethyl)-1,3-bis(aryl)imidazol-2-imine hydrochlo-
ride, wherein aryl = 2,4,6-trimethylphenyl (3a) and 2,6-
diisopropylphenyl (3b), was prepared and structurally char-
acterised. Deprotonation of the salts yields molecules with
Scheme 1 Mesomeric structures for imidazol-2-imine.
delocalised p-electrons, leading to zwitterionic mesomeric
structures. Titanium(IV) (4a,b) and palladium(II) (5a,b) com-
plexes were isolated and also structurally characterised. Two
different coordination modes were observed where the ligand
is either coordinated in a bidentate fashion as expected, or
in a monodentate fashion through the more basic aryliminic
nitrogen atom.
of imidazol-2-imines as monodentate or multidentate ligands for
coordination to transition metals offers great opportunity for a
wide range of catalytic transformations.10,11
We herein wish to report our preliminary results on the devel-
opment of a new family of ligands based on the imine imidazol-2-
imine scaffold A (Scheme 2) and its coordination to titanium(IV)
and palladium(II). The ligand structure is analogous to that
of an amidine, where one of the nitrogen atoms is substituted
with an imidazol-2-ylidene fragment for further tailoring of the
ligand electronics. This substitution effectively shifts the steric
bulk from the first to the second coordination sphere, leading to
a more open metal centre that is still protected from bimolecular
decomposition. This shift is also observed in metal complexes
coordinated by phosphidoimide ligands, and is well known to
enhance catalytic activities.9,12
Ancillary ligands with nitrogen as the coordinating atom are
used extensively in homogeneous catalysts, either as neutral or as
anionic amido and imido donors.1,2 These ligands allow systematic
variations of the electronics and sterics, leading to rational design
of the catalysts and consequently, tailoring of their performance.
Initially developed for coordination to early transition metals
as an alternative to the ubiquitous cyclopentadienyl derivatives,
amidinate [RC(NR¢)2]- and guanidinate [R2NC(NR¢)2]- ligands
now find use as bidentate ligands to metals across the periodic
table.2,3 While the sterics for both ligand systems can be easily
tuned, the presence of a third nitrogen atom on guanidinates
provides yet another mean to drastically alter the electronics of the
ligand, otherwise difficult to achieve with the related amidinates.2,3
Since the isolation of the first stable N-heterocyclic carbene
(NHC), the chemistry of these ligands has seen tremendous
growth due to their similarities in terms of thermodynamics and
coordination chemistry to tertiary phosphines.4,5 More recently,
the reactivity of these carbene centres towards azides was exploited
to prepare imidazol-2-imines.5,6 Upon deprotonation, these imines
yield a new class of highly basic monoanionic ligands (Scheme 1).7
Interestingly, the delocalisation of the electron density over the
imidazol-2-imine system leads to the formation of a zwitterionic
resonance structure, resulting in a relatively electron-rich exocyclic
iminic nitrogen atom. With their ability to act as 2s,4p-electron
donors, these new ligands are isoelectronic analogues of cyclopen-
tadienyls, phosphidoimides and even imido ligands, depending
on which resonance structure is considered.8,9 As such, the use
Scheme 2 Mesomeric structures for imine imidazol-2-imine ligands.
The new ligand takes advantage of the various possible me-
someric structures, including two distinct zwitterionic isomers
B and C (Scheme 2). Thus, although the ligand is overall
neutral, both exocyclic nitrogen atoms are electron-rich from the
delocalised negative charge (Scheme 2, D). Coordination of these
electron-rich ligands should thus lead to transition metal centres
with decreased electrophilicity.
The ligand precursors, N-(1-(2,6-dimethylphenylimino) ethyl)-
1,3-bis(aryl)imidazol-2-imine hydrochloride, wherein aryl
=
Department of Chemistry, York University, 4700 Keele Street, Toronto,
Ontario, M3J 1P3, Canada. E-mail: glavoie@yorku.ca; Fax: +1 416 736
5936; Tel: +1 416 736 2100 ext. 77728
† Electronic supplementary information (ESI) available: Full details of
experimental procedures, analytical data and X-ray structural determina-
tion. CCDC reference numbers 778192 (3a), 778193 (3b), 778194 (4a) and
778195 (5b). For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0dt00498g
2,4,6-trimethylphenyl and 2,6-diisopropylphenyl, abbreviated as
IMesN∧Imine·HCl (3a) and IPrN∧Imine·HCl (3b) respectively,
were synthesised by the reaction of imidazol-2-imine IMesNH
(1a) and IPrNH (1b) with imidoyl chloride 2 in toluene (125 ◦C)
for 12 h (Scheme 3). The resulting amidinium salts were isolated
as air-stable white solids in 93% (3a) and 79% (3b) yields.
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Scheme 3 Synthesis of imine imidazol-2-imine ligand precursors.
The 1H NMR (CDCl3) spectrum for salt 3a shows characteristic
resonances for the mesityl para- and ortho-CH3 protons in a 1 : 2
ratio at d 1.91 and 2.36, respectively. A typical AX2 spectrum for
the isopropyl groups in 3b was observed as a set of two doublets at
d 1.12 and 1.06 integrating to 12 protons each and a septet at d 2.47
integrating to 4 protons, with vicinal coupling of 6.8 Hz observed
for all three sets. The resonances arising from the iminic methyl
protons were observed at d 2.11 for 3a and at d 2.75 for 3b. The ben-
zylic protons of the 2,6-dimethylphenyl ring appeared as a singlet
at d 1.60 (3a) and 1.53 (3b), whereas the imidazolium backbone
(–NCHCHN–) protons were observed at the characteristically
higher frequencies of d 6.91 for 3a and 6.95 for 3b, respectively. In
Fig. 1 Molecular structure for amidinium salts 3a and 3b. Thermal
ellipsoids are drawn at 50% probability. Hydrogen atoms and solvent
molecules (CH2Cl2) are omitted for clarity.
1
the 13C{ H} NMR (CDCl3), the characteristic higher frequency
resonances were observed for the exocyclic iminic carbon at d
162.3 (3a) and 162.1 (3b), while the central imidazolium carbon
1
1
resonated at d 146.8 (3a) and 149.7 (3b). Full H and 13C{ H}
assignments for 3a and 3b are detailed in the ESI.†
The solid-state structures of 3a and 3b (Fig. 1), determined
by single-crystal X-ray diffraction studies, are consistent with the
solution NMR data. X-Ray data reveal that 3a and 3b crystallise
in the monoclinic space group P21/c. The structure was solved
using SIR-9213 and refined using SHELXTL V6.114 for full-
matrix least-squares refinement based on F2. All hydrogen atoms
were included in calculated positions and allowed to refine in
riding-motion approximation with Uiso-tied to the carrier atom.
Hydrogen atoms bound to N4 nitrogen were found in difference
Fourier maps. Thus, N4 rather than N3 is unexpectedly found to be
protonated, possibly arising from a 1,3-shift of the acidic proton
through the formation of a putative four-centre transition state
(Scheme 4). Subsequent rotation around the C4–N4 bond would
Scheme 4 Proposed mechanism for the observed [1,3]-proton shift in 3a
and 3b.
lead to the observed solid-state structures. This [1,3]-proton shift
also suggests that N4 is significantly more basic than N3.
A comparison of the selected bond lengths and angles are
summarised in Table 1. The average C1–N1 and C1–N2 bond
lengths for 3a and 3b are shorter than the corresponding bond
lengths observed for IPrNH,11 but they are comparable to those
observed for the imidazolium salt IMes·HCl and IPr·HCl,15
respectively. This bond shortening arises from delocalisation of
◦
11
˚
Table 1 Comparison of selected bond lengths (A) and bond angles ( ) for compounds 3a, 3b, 4a and 5b with those reported for IPrNH.
IPrNHa
IMesN∧Imine·HCl (3a) IPrN∧Imine·HCl (3b) Ti(IMesN∧Imine)Cl4 (4a) [Pd(IPrN∧Imine)2Cl2]2 (5b)
˚
Bond lengths/A
C1–N1, C1–N2
N1–C2, N2–C3
C2–C3
N1–Cipso, N2–Cipso 1.434(2), 1.433(2)
C1–N3
1.3815(15), 1.4004(16) 1.354(4), 1.356(4)
1.353(5), 1.354(5)
1.397(5), 1.403(5)
1.345(5)
1.450(5), 1.438(5)
1.343(5)
1.365(5), 1.349(5)
1.386(5), 1.400(5)
1.341(6)
1.455(5), 1.449(5)
1.354(5)
1.360(7), 1.381(8)
1.395(7), 1.401(7)
1.327(9)
1.438(7), 1.447(8)
1.313(7)
1.382(2), 1.387(2)
1.335(2)
1.396(4), 1.397(4)
1.336(5)
1.448(4), 1.450(3)
1.338(4)
1.289(2)
—
C4–N3, C4–N4
1.311(4), 1.330(4)
1.307(5), 1.328(5)
1.361(5), 1.312(5)
1.321(7), 1.332(7)
Bond angles/◦
N1–C1–N2
N1,2–C1–N3
C1–N3–C4
N3–C4–N4
104.19(10)
124.05(11), 131.75(11) 123.3(3), 129.6(3)
—
—
—
109.0(3)
109.1(3)
128.7(3), 123.4(3)
124.4(3)
120.0(3)
123.5(3)
106.5(3)
127.2(4), 126.2(4)
122.3(4)
110.7(4)
122.3(3)
105.7(4)
124.8(5), 128.7(5)
124.2(5)
120.3(5)
118.8(5)
122.7(3)
118.8(3)
125.3(3)
=
C4–N4 C6
a Ref. 11.
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the electrons over the whole p-system, which results in the
development of a partial positive charge on the five-membered
ring and an increase in the bond order between C1 and N3
when compared with IPrNH.11 The wider N1–C1–N2 bond angles
observed for the imidazole ring in 3a and 3b, when compared
with that for IPrNH11 are also due to the effective stabilisation
of the positive charge within the imidazole ring. The presence of
the zwitterionic behaviour is further supported by the prominent
titanium(IV) centre. The bond angles around the titanium atom
are in the range of 60.50(12)–177.72(5)◦, with the smallest angle
attributed to the amidine N3–Ti–N4 bite angle. The lengthening
˚
˚
of the C1–N3 and N3–C4 bond to 1.354(5) A and 1.361(5) A,
respectively, and the reduction of the N1–C1–N2 bond angle to
106.5◦ compared to those observed for 3a are due to the loss
of delocalisation of the electron density over the C1–N3–C4–
N4 p-system and are consistent with the coordination of the
ligand in a bidentate mode. Furthermore, greater double bond
character for the C4–N4 bond is also supported by the bond
lengthening of C1–N3 and C4–N4 bonds, when compared with
16
the respective bond lengths reported for IPrNH,11 Ph2C NPh
=
and C∧Imine·HCl.17
length of 1.312(5) A, which is shorter than the values observed
˚
The angles between the best planes formed by the imidazole ring
and each of the aryl rings at the 1,3-positions are 61.9◦, 80.6◦ (3a)
and 70.6◦, 80.3◦ (3b). The angles between the best planes formed
by either the imidazole ring or the 2,6-dimethylphenyl group and
that passing through C1, N3, C4, C5, N4, C6 are 64.9◦, 70.5◦ (3a)
and 72.5◦, 74.1◦ (3b), respectively. All the remaining bond lengths
and angles are unexceptional and lie within the expected range.
Ti(IMesN∧Imine)Cl4 (4a) and Ti(IPrN∧Imine)Cl4 (4b) were
synthesised as very air- and moisture-sensitive bright red solids
in 70–90% yields by the reaction of (THF)2TiCl4 with the neutral
ligand generated in situ by deprotonation of the corresponding
hydrochloride salt 3 in benzene (Scheme 5). The crystal structure
for 4a (Fig. 2) shows a distorted octahedral geometry around the
for C1–N3 and N3–C4 bonds. This is further evidence of the
electron density being predominantly localised over the C1–N3–
C4 bonds. The angles between the best plane passing through
the diazametallacycle and either the imidazole ring or the 2,6-
dimethylphenyl groups are 56.7◦ and 80.4◦, respectively. The
angles between the diazametallacycle and the mesityl groups on
either side of the imidazole rings are 64.9◦ and 76.7◦, illustrating
the fan-type structure of imidazol-2-imine fragments18 and the
shift of the steric bulk from the first to the second coordination
sphere. Remaining bond lengths and angles are within the expected
range.
Reaction of the in situ generated free ligand with
(CH3CN)2PdCl2 in benzene at room temperature over 2 h
produced the palladium complexes [Pd(IMesN∧Imine)Cl2]2 (5a)
and [Pd(IPrN∧Imine)Cl2]2 (5b) as orange-brown solids in 60–
70% yields. Single-crystal X-ray diffraction analysis for 5b shows
the unexpected dimeric structure with a perfectly square-planar
palladium metal centre bridged by two chloride ligands (Fig. 3).
The imine imidazol-2-imine ligand is coordinated in a monoden-
tate fashion through N4, in further agreement with our previous
observation through the amidinium salts 3a and 3b that the iminic
nitrogen N4 is more basic than N3. As expected the bond lengths
between palladium and the bridging chloride ligands (2.3335(16)
˚
and 2.3413(18) A) are greater than between the metal centre and
˚
the terminal chloride Cl2 (2.2920(18) A). Attempts to enforce
cyclisation through the abstraction of a chloride in 5a and 5b
with AgBF4, AgPF6 and AgOTf, or through reaction of the free
Scheme 5 Synthesis of titanium(IV) and palladium(II) complexes.
Fig. 2 Molecular structure of 4a. Thermal ellipsoids are drawn at 50%
probability. Hydrogen atoms and solvent molecule (CHCl3) have been
removed for clarity.
Fig. 3 Molecular structure of 5a. Thermal ellipsoids are drawn at 50%
probability. Hydrogen, solvent molecules (CH2Cl2) and disorder have been
removed for clarity.
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ligand with [Pd(COD)(CH3CN)Cl]BF4 resulted in the formation
of palladium black in all cases.
References
1 R. R. Schrock, Angew. Chem., Int. Ed., 2008, 47, 5512; R. R. Schrock
and A. H. Hoveyda, Angew. Chem., Int. Ed., 2003, 42, 4592; R. R.
Schrock, J. Mol. Catal. A: Chem., 2004, 213, 21; L. H. Gade and P.
Mountford, Coord. Chem. Rev., 2001, 216–217, 65; R. A. Eikey and
M. M. Abu-Omar, Coord. Chem. Rev., 2003, 243, 83; T. E. Mu¨ller,
K. C. Hultzsch, M. Yus, F. Foubelo and M. Tada, Chem. Rev., 2008,
108, 3795; D. J. Mindiola, Angew. Chem., Int. Ed., 2009, 48, 6198; V. C.
Gibson, C. Redshaw and G. A. Solan, Chem. Rev., 2007, 107, 1745;
S. D. Ittel, L. K. Johnson and M. Brookhart, Chem. Rev., 2000, 100,
1169.
˚
The short C1–N3 bond in 5b (1.313(7) A), compared to that
˚
observed in 3b (1.343(5) A), indicates a greater double-bond
character for the exocyclic imine of the imidazole-2-imine ring.
Structure A (Scheme 2) more closely represents the actual nature
of the ligand once coordinated to palladium, despite the slightly
elongated C1–N3 bond over that reported for IPrNH.11 This
assessment is further supported by the longer C1–N1 and C1–
N2 bond in 5b compared to the corresponding ones in 3b.
2 F. T. Edelmann, Chem. Soc. Rev., 2009, 38, 2253.
˚
Similarly, the N3–C4 bond in 5b at 1.321(7) A is also slightly
3 P. J. Bailey and S. Pace, Coord. Chem. Rev., 2001, 214, 91; J.
Barker and M. Kilner, Coord. Chem. Rev., 1994, 133, 219; F. T.
Edelmann, in Advances in the Coordination Chemistry of Amidinate
and Guanidinate Ligands, Adv. Organometal. Chem., ed. F. H. Anthony
and J. F. Mark, Academic Press, Oxford, 2008, vol. 57, p. 183.
4 R. H. Crabtree, J. Organomet. Chem., 2005, 690, 5451; S. Diez-Gonzalez
and S. P. Nolan, Coord. Chem. Rev., 2007, 251, 874; W. A. Herrmann,
Angew. Chem., Int. Ed., 2002, 41, 1290; D. M. Khramov, V. M. Lynch
and C. W. Bielawski, Organometallics, 2007, 26, 6042.
˚
longer than that observed in the precursor salt (1.307(5) A). The
˚
N3–C4 and C4–N4 bond length of 1.321(7) and 1.332(7) A in
5b are consistent with the coordination with palladium, which
also results in a smaller C4–N4–C6 bond angle of 118.8(5)◦ in
5b compared to 123.5(3)◦ observed for 3b. In contrast to that
for 3b, the 2,6-diisopropylphenyl rings attached to N1 and N2
in 5b are approximately crystallographically equivalent due to a
pseudo-mirror plane that is orthogonal to the imidazole ring and
that passes through the exocyclic iminic bond. The angles between
the best plane passing through C1, N3, C4, C5, N4, C6 and those
formed by the imidazole ring and the 2,6-dimethylphenyl group
are 75.6◦ and 82.4◦, respectively. The plane passing through the
bimetallic palladium bridge is tilted off by 65.8◦ from the best
plane formed by N3, C4, C5, N4, C6. The remaining bond lengths
and bond angles are normal and lie within the expected range.
In summary, we have synthesised and structurally characterised
the new family of imine imidazol-2-imine ligands and their
corresponding titanium(IV) and palladium(II) complexes. Two
different coordination modes were observed depending on the
nature of the metal centre itself. We are currently exploring the
coordination chemistry of these new complexes and expanding
the scope of the ligands to other early and late transition metals.
Catalytic activities in various organic transformations are also
being evaluated.
5 N. Kuhn and A. Al-Sheikh, Coord. Chem. Rev., 2005, 249, 829.
6 N. Kuhn, R. Fawzi, M. Steimann, J. Wiethoff and D. Bla¨ser, Z. Natur-
forsch., B: Chem. Sci, 1995, 50, 1779; N. Kuhn, M. Grathwohl, M.
Steimann and G. Henkel, Z. Naturforsch., B: Chem. Sci, 1998, 53, 997.
7 G. Frison and A. Sevin, J. Chem. Soc., Perkin Trans. 2, 2002, 1692; M.
Tamm, D. Petrovic, S. Randoll, S. Beer, T. Bannenberg, P. G. Jones and
J. Grunenberg, Org. Biomol. Chem., 2007, 5, 523.
8 D. W. Stephan, Organometallics, 2005, 24, 2548.
9 K. Dehnicke, M. Krieger and W. Massa, Coord. Chem. Rev., 1999, 182,
19.
10 S. Beer, C. G. Hrib, P. G. Jones, K. Brandhorst, J. Grunenberg and M.
Tamm, Angew. Chem., Int. Ed., 2007, 46, 8890; W. P. Kretschmer, C.
Dijkhuis, A. Meetsma, B. Hessen and J. H. Teuben, Chem. Commun.,
2002, 608.
11 T. K. Panda, A. G. Trambitas, T. Bannenberg, C. G. Hrib, S. Randoll,
P. G. Jones and M. Tamm, Inorg. Chem., 2009, 48, 5462.
12 D. White and N. J. Coville, Quantification of Steric Effects
in Organometallic Chemistry, in Adv. Organomet. Chem., ed.
F. G. A. Stone and R. West, Academic Press, Oxford, 1994, vol. 36, p.
95.
13 A. Altomare, G. Carascano, C. Giacovazzo, A. Guagliardi, M. C. Burla
and G. M. Polidori Camalli, J. App. Cryst., 1994, 27, 435.
14 G. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008, 64,
112.
15 A. J. Arduengo, III, R. Krafczyk, R. Schmutzler, H. A. Craig, J. R.
Goerlich, W. J. Marshall and M. Unverzagt, Tetrahedron, 1999, 55,
14523.
Acknowledgements
16 P. A. Tucker, A. Hoekstra, J. M. Ten Carte and A. Vos, Acta Crystallogr.,
Sect. B: Struct. Crystallogr. Cryst. Chem., 1975, 31, 733.
17 A. C. Badaj, S. Dastgir, A. J. Lough and G. G. Lavoie, Dalton Trans.,
2010, 39, 3361; K. S. Coleman, S. Dastgir, G. Barnett, M. J. P. Alvite,
A. R. Cowley and M. L. H. Green, J. Organomet. Chem., 2005, 690,
5591.
18 A. C. Hillier, W. J. Sommer, B. S. Yong, J. L. Petersen, L. Cavallo and
S. P. Nolan, Organometallics, 2003, 22, 4322; N. M. Scott and S. P.
Nolan, Eur. J. Inorg. Chem., 2005, 1815.
GGL gratefully acknowledges financial support from the Ontario
Ministry of Research and Innovation for an Early Researcher
Award, the Natural Sciences and Engineering Research Council
for a Discovery Grant and a Research and Tools Instrumentation
Grant, the Canadian Foundation for Innovation and the Ontario
Research Fund for infrastructure funding, and York University
for start-up funds.
6946 | Dalton Trans., 2010, 39, 6943–6946
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The Royal Society of Chemistry 2010
©