Influence of pyrido-annulation on N,N′-dineopentyl-imidazolin-2-ylidene and associated transition metal complexes; comparison with benzo-, naphtho- and quinoxalino-annulation

Journal of Organometallic Chemistry 890 (2019) 43e57

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Journal of Organometallic Chemistry

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Inﬂuence of pyrido-annulation on N,N⁰-dineopentyl-imidazolin-2-

ylidene and associated transition metal complexes; comparison with

benzo-, naphtho- and quinoxalino-annulation

,

Farman Ullah ^{a 1}, Saravanakumar Shanmuganathan ^{a 2}, Claudia Schindler ^a,

Peter G. Jones ^b, Joachim W. Heinicke ^a

Universitat Greifswald, Institut für Biochemie, F.-L.-Jahn-Str. 18c, 17487, Greifswald, Germany

,

*

a

€

b

€

Technische Universitat Braunschweig, Institut für Anorganische und Analytische Chemie, Hagenring 30, D-38106, Braunschweig, Germany

a r t i c l e i n f o

a b s t r a c t

The syntheses of isolable N,N⁰-dineopentyl-substituted non- and pyrido-annulated imidazolin-2-ylidenes

and selected Ag, Rh and Pd complexes thereof, in part earlier communicated, are now reported in full

detail. The compounds were structurally characterized by multinuclear NMR and in part by crystal

structure analyses. Diagnostic structural and NMR data were compared with those of analogously N,N⁰-

dineopentyl-substituted benzo-, naphtho- and quinoxalino-annulated compounds, and the inﬂuence of

the annulation was tested by L/Pd-and L₂/Pd-catalyzed Suzuki-Miyaura couplings of PhB(OH)₂with p-

bromotoluene.

Article history:

Received 20 February 2019

Received in revised form

14 March 2019

Accepted 1 April 2019

Available online 6 April 2019

Dedicated to the memory of Prof. Dr. Dr. h.c.

W. Keim

Keywords:

N-heterocyclic carbenes

Pyridine annulation

Rhodium

Silver

Palladium complexes

Suzuki-Miyaura coupling /

1. Introduction

material sciences [7].

We studied a variety of annulated imidazole-2-ylidenes (Chart

The chemistry of N-heterocyclic carbenes (NHCs) has attracted

considerable attention in the last two decades. After the initially

investigations of imidazol-2-ylides (imy), a variety of related

annulated systems and their complexes were studied [1,2]. The

exceptionally high interest in the NHCs is attributed to their mul-

tiple use as nucleophilic reagents, organocatalysts or ligands for

main group elements, f-block and, in particular, transition metal

compounds and catalysts. Examples are NHC silver complexes,

applied in transmetalation reactions, catalysis and medicinal

chemistry [3] and Rh or Pd NHC complexes, allowing a broad

spectrum of homogenous transition-metal catalyzed organic

transformations [2e5]. Furthermore, various transition-metal NHC

complexes were found to be useful in medicinal chemistry [6] or

1) [8,9] as part of long-standing investigations of carbo- and het-

erocyclically annulated -excess-aromatic N-heterocycles with (p-

p) -bonded heavier main group elements (As, P [10]; Si, Ge, Sn

p

[11]). Results for non- and N-pyrido-annulated imidazolin-2-

ylidenes and some complexes thereof were partly communicated

earlier with a focus on the different electronic structure of pyrido-

annulated N-heterocyclic carbenes and silylenes [9]. This ﬁnal

report will provide a detailed description of the syntheses, struc-

ture, screening in a Suzuki-Miyaura test-coupling and comparison

of the results with those of closely related benzo- [12], naphtho-,

quinoxaline and phenanthrene-annulated NHCs [8] to illustrate the

inﬂuence of annulation.

2. Results and discussion

* Corresponding author.

2.1. Syntheses

E-mail address: heinicke@uni-greifswald.de (J.W. Heinicke).

CanAm Bioresearch Inc., Winnipeg, Manitoba, Canada.

1

2

Alagappa University, Department of Industrial Chemistry, Karaikudi, India.

For comparison with our previously studied annulated NHC

https://doi.org/10.1016/j.jorganchem.2019.04.001

44

F. Ullah et al. / Journal of Organometallic Chemistry 890 (2019) 43e57

the pyridoimidazolin-2-ylidenes 1pb and 1pc. Both carbenes are

monomeric at r.t., without noticeable interactions between the N-

donor atom of the pyridine ring and the divalent carbon atom. The

thermal stability is sufﬁcient to allow distillation of the colorless

oils under vacuum without detectable decomposition. Thus they

differ from their higher homologues with more electropositive

group 14 elements, which are less stable as monomers [16].

Nevertheless they are highly reactive and very sensitive to moisture

or air. For this reason it is more convenient to use the crude car-

benes for application in further reactions immediately after sepa-

ration from excess KH and insoluble potassium salts. The free

carbenes 1 pb and 1pc, crude or distilled, are suitable for direct

conversion with various transition metal compounds. Examples are

the reactions with ionic silver salts such as AgO₃SCF₃or with

chloro-bridged complexes such as [Rh(1,5-COD)Cl]₂and [Pd(Allyl)

Cl]₂in THF, or replacement of the labile -donor/acceptor-ligands

m-

p

from Pd(dvds)_n(8e10% Pd, dvds ¼ divinyltetramethyldisiloxane) in

hexane, leading to 3pb_OTfand 3pc_OTf, 4pb and 4pc, 5pb and 5pc,

and 6pc (Scheme 2). In the reaction of 1pb and 1pc with

[Rh(PPh₃)₂(CO)Cl], the replacement of one Ph₃P ligand was strongly

preferred but was accompanied by competing slow replacement of

Cl as a minor side reaction [9a].

Chart 1. Types of non-, carbo- and heterocyclically annulated imidazole-2-ylidenes 1,

included in our studies [8,9].

Alternative routes to transition metal complexes of 1pb and 1pc

are realized via the synthesis of the AgCl complexes 7pb and 7pc,

readily available from the pyridoimidazolium chlorides 2pb_Cland

2pc_Clby reaction with Ag₂O in the presence of a molecular sieve

(according to the protocol of Wang and Lin [17]) and subsequent

transmetalation in DCM. Examples are the conversions with

[Rh(1,5-COD)Cl]₂or with Pd(MeCN)₂Cl₂to 4pb and 4pc or to 8pb

and 9pc, respectively (Scheme 3).

In all these complexes, the transition metal coordinates at the

nucleophilic NHC carbene center, as shown by the upﬁeld coordi-

nation chemical shifts of the ¹³C(II) nuclei in the ¹³C{¹H} solution

NMR spectra (Table 1) and in the solid state by crystal structure

analyses of 3pb_OTf, 4pb, 4pc, 7pb and 7pc.

systems, we focussed on N,N⁰-dineopentyl-substituted compounds.

These are advantageous because of the facile accessibility of the

precursors, signiﬁcant steric stabilization of the benzo- [12a] and

naphtho[b]-annulated [8a] NHCs as monomers and, last but not

least, by straightforward NMR reaction monitoring and product

identiﬁcation. Attempts to synthesize non-annulated dineopentyl-

imidazolium salts did not succeed by classic procedures, such as

bis-alkylation of sodium imidazolide with neopentyl bomide in

DMF, or by one-pot condensation of the primary amine NpNH₂with

glyoxal and formaldehyde [13]. However, a protocol introduced by

Glorius et al. for the synthesis of bis-oxazoline-annulated imida-

zolium salts [14] allowed us to obtain 2i_OTfand 2i_BF4in reasonable

yields (ca. 70%) by reaction of glyoxal bis(neopentylimine) [15] with

AgOTf or AgBF₄and chloromethyl pivalate. Conversion of 2i_OTfwith

excess KH in THF and of 2i_BF4with Ag₂O and [Rh(COD)Cl]₂in CH₂Cl₂

(DCM) led to the dineopentylimidazolin-2-ylidene 1i and its silver

and rhodium complexes 3i_BF4and 4i, respectively (Scheme 1).

The pyrido[b]- and pyrido[c]-annulated imidazolium salts

2pb_PF6/Cland 2pc_PF6/Clwere obtained in the classic way by heating

the corresponding diaminopyridines with triethylorthoformate in

the presence of NH₄PF₆or NH₄Cl, and allowing ethanol to distill off.

Reaction of the hexaﬂuorophosphates 2pb_PF6and 2pc_PF6with

excess potassium hydride (at least 2 equivalents) in dry THF under

strictly inert conditions provides a high-yield (90e95%) access to

Almost all of the complexes were readily soluble in CD₂Cl₂,

CDCl₃or d₈-THF. Only 9pc was insoluble in these solvents and

sparingly soluble even in DMSO‑d₆. Single crystals that would have

provided more direct information on its structure and possible

coordinative interactions of the pyridine N-atom could therefore

not be obtained. For evaluation of effects of the pyridine N-atom on

structural data and properties of pyrido-imy compounds, the ben-

zannulated carbene 1b and a few benzanellated complexes were

included in the comparison. The new compounds 5b, 8b and 10/11b

were prepared by transmetalation of the bimy-AgCl complex 7b

[8b] with appropriate Pd-precursor complexes (Scheme 4). The

transmetalation of 7b with palladium acetate in a 2:1 M ratio

(referred to monomer formula unit) led to a mixture of two bimy-

Pd(OAc)₂complexes 10b and 11b, characterized by ¹H and ¹³C NMR

data with typical chemical shifts for C^IIand COO^ꢀ,

d

¹³C ¼ 181.58,

183.75 (C^II, integral 5:3), 175.73, 175.80 (CO), and with the HRMS

isotopic pattern of the [Pd(1b)₂(OAc)]^þcation. However, separation

and/or growing single crystals for structure proof by XRD have so

far failed. A literature report on related compounds suggests facile

isomerization and control of the cis/trans-preference by steric fac-

tors. Thus, for the reaction of the trans-(R₂bimy)₂PdX₂complexes

(R ¼ Me, X ¼ I and R ¼ iPr, X ¼ Br) with two equivalents AgOAc in

acetonitrile, rapid isomerization of the initially formed trans-

(Me₂bimy)₂Pd(OAc)₂to the thermodynamically more stable cis-

isomer was observed, whereas for the more bulky (iPr₂bi-

my)₂Pd(OAc)₂a ﬁnal trans/cis-ratio of 3.5:1 was determined by ¹H

NMR spectroscopy. The major trans-isomer, also characterized by

crystal structure analysis, displays only one set of signals for the N-

isopropyl group, indicating free rotation around the Pd-C^IIbonds

[18]. The same behavior was observed for the N-neopentyl signals

Scheme 1. Synthesis of Np₂imy salts 2i_OTf/BF4, 1i and the complexes 3i_BF4and 4i.

F. Ullah et al. / Journal of Organometallic Chemistry 890 (2019) 43e57

45

Scheme 2. Syntheses of 1pb and 1pc and direct conversion to Ag, Rh and Pd complexes.

detailed information on the molecular structures of these pbimy

and pcimy complexes, and also on interactions between neigh-

boring molecules. The complexes 4pb and 4pc are not isotypic, nor

are 7pb and 7pc. For the LAgCl complexes 7pb and 7pc m-chloro-

bridged dimers were observed. A short HLN# contact (2.57 Å) was

observed in 7pc (C4-H4LN5), whereas other short distances to

neighboring molecules in 4pb and 4pc and in 7pb and 7pc were

observed mainly between C-H groups and the chloro ligands.

Coordinative interactions between the pyridinic N atom and the

transition metal of neighboring molecules were not present in

these complexes.

The cationic silver complex 3pb contains two formula units and

one molecule of DCM in the asymmetric unit. Both cations display

linear coordination at the metal (176.85(18), 178.33(17) ^ꢁ), typical

for more spatially demanding ligands, but differ in their structural

parameters and adopt different ligand interplanar angles of ca. 61^ꢁ

and 6^ꢁ(Fig. 1, left and right respectively). The Ag-C(II) bonds

(2.106(5)-2.118(5) Å) are slightly longer than in the cationic

[(IMes₂)₂Ag]CF₃SO₃complex (2.067, 2.078 (4) Å) [19] and in the

carbocyclically annulated LAgCl complexes 7b (2.0845(14) Å, [8b])

and 7n (2.079(3) Å [8a]) (see below), but comparable with those of

the pyrido-annulated AgCl complexes 7pb and 7bc. The N-C^IIbonds

of 3pb are on average slightly longer (1.364(6)) than in the annu-

lated LAgCl complexes of type 7 (average ca. 1.355 Å [8a,b]). Crystals

of the complex 3pc were also studied; an analogous structure was

conﬁrmed qualitatively but disorder, particularly of the anion, did

not allow a detailed analysis.

Scheme 3. Synthesis and transmetallation reactions of 7pb and 7pc.

of 10b, assigned as the trans-isomer, whereas AB-splitting of the

NCH₂protons of 11b hint at the cis-isomer.

2.2. Structural aspects

The electronic structure of the pyrido[b]-, pyrido[c]- and ben-

zimidazolin-2-ylidenes 1pb, 1pc and 1b was determined by He(I)-

photoelectron studies and correlation of the experimental IP_v

values with calculated ionization potentials (ROVGF/cc-pVDZ on

geometries optimized at the B3LYP/cc-pVTZ level of theory) of the

respective N,N⁰-di-tert-butyl substituted model compounds [9b].

These studies reveal that the C-lone pair occupies the HOMO (IP_v

exp./calc. 7.9/7.72, 8.0/7.83, 7.9e8.5(superimp.)/7.57 eV), while the

pyridine N-lone pair of 1pb and 1pc is much lower in energy

(HOMO-3) and requires higher energies for ionization (IP_vexp./calc.

9.65/9.53, 9.3/9.53 eV) and interactions with empty orbitals of

electrophiles. This may explain the strongly preferred coordination

of the transition metals at carbon.

The Rh-Cl and Rh-C^IIbonds of 4pb are slightly longer than those

of 4pc and also of the analogous quinoxalino-annulated complex

4q, whereas the bond lengths and angles within the ligands alter

only slightly for the different complexes, even with other metals

(Table 1). Within the ﬁve-membered ring the N-C^IIbonds are

shorter than the N-C bonds to the bridging carbon atoms, indicating

an increased N-C double bond character in the diaminocarbene

substructure, though less than that of C-N-C in the pyridine ring.

The N-C^II-N and other angles in the ﬁve-membered ring are

controlled by the ring size and thus considerably smaller than

typical values for sp²hybridization in acyclic or six-membered ring

structures. The rhodium atom in the complexes 4pb and 4pc ex-

hibits the typical slightly distorted square-planar coordination,

involving the centers of the two C¼C bonds of 1,5-COD, chloride

2.2.1. Crystal structures

XRD analyses of 3pb, 4pb, 4pc, 7pb and 7pc (Figs. 1e5) give

46

F. Ullah et al. / Journal of Organometallic Chemistry 890 (2019) 43e57

Table 1

Selected bond lengths (Å), angles and torsion angles (^ꢁ) for 3pb, 4pb, 4pc, 7pb and 7pc and comparison with related carbo- or heterocyclically annulated imidazolin-2-ylidene

complexes.^a

Compd.

M-Cl;

C^II-N;

C_bridge-N;

C-N_pyr(average)

C^II-M-C_trans

;

M-C^II;

C^II-M-Cl

N-C^II-N

C^II-M-Cent;abs. N1-C^II-M-Cl

3pb

-;

1.350(6) - 1.377(6);

105.5(5) e 107.0(4)

1.383(6) e

1.398(6);

1.332(6) e

1.345(8)

176.85(18),

178.33(17);

e

2.106(5), 2.118(5),

2.106(5), 2.116(5);

e

4pb

2.4042(6);

2.032(2);

86.17(6)

1.375(3),

1.374(3);

105.77(18)

1.394(3),

1.389(3);

1.340(3)

156.69(9),

167.19(9);

91.5;

105.2(2)

159.66(8),

162.91(8);

89.0;

4pc

2.3773(5);

2.0269(18);

96.22(5)

1.372(2),

1.383(2);

106.06(15)

1.383(2)

1.393(2);

1.346(3)

106.74(14)

158.36(12),

164.97(11);

90.3;

99.8

-;

4q [9a]

7pb

2.3756(9);

1.998(3);

91.08(8)

1.395(3),

1.381(3);

107.2(2)

1.375(3),

1.393(3);

1.344

2.3863(14);^b

2.106(5);

164.69(14),

Cl#1 106.88(14)

2.3770(7);^c

2.107(3);

166.94(8),

Cl#1 100.75(8)

2.3356(4);

2.0845(14);

175.88(4)

1.361(6),

1.349(6);

106.2(4)

1.388(6),

1.395(6);

1.339(7)

-;

7pc

1.358(4),

1.351(4);

106.3(3)

1.391(4),

1.395(4);

1.346(4)

-;

7b [8b]

7n [8a]

1.3572(18),

1.3586(17);

106.21(12)

1.351(3),

1.355(3);

107.1(2)

1.3921(18),

1.3956(18);

e

-;

e

2.3178(7);

2.079(3);

176.56(7)

1.403(3),

1.401(3);

e

-;

e

a

C^IIis C2; C_bridgeis C3A or C7A; N_pyris N4 or N5; N without sufﬁx are the carbenic nitrogens N1 and N3; Cent is the midpoint of the coordinated double bond cis to the

chlorine.

b

c

Cl-bridged dimer: Ag-Cl#1 2.9729(15) Å, Cl-Ag-Cl#1 88.05(4), Ag-Cl-Ag#1 91.95(4)^ꢁ.

Cl-bridged dimer: Ag-Cl#1 3.0180(7) Å, Cl-Ag-Cl#1 92.06(2), Ag-Cl-Ag#1 87.94(2)^ꢁ.

allowing two rotamers. 4 pb displays the rotamer with one CMe₃

above, the other below the ring plane, whereas in 4pc both CMe₃

groups are on the same side, thus conveying (very) approximate

mirror symmetry on the molecule.

The NHC ligands affect the coordination geometry of the 1,5-

COD ligand. The C¼C bond trans to C(II) (1.365(3), 1.367(3) Å) is

signiﬁcantly shorter than cis-C¼C (1.399(3), 1.401(4) Å) and in-

dicates, together with longer trans-C¼C/Rh distances (2.198(2),

2.202(2) for 4 pb and 2.214(2), 2.242(2) Å for 4pc) than cis-C¼C/Rh

(2.108(2), 2.123(2) and 2.1056(19), 2.1184(18) Å), a strong trans-

inﬂuence of the pyrido-imy ligands. The more electron-

withdrawing and

p-acidic cationic pyridine-N-methylated

[Ir(NMe-4pb)(COD)Cl]^þcomplex [1f] exhibits similar values for the

trans-C¼C and trans-C¼C/Rh bond lengths, whereas the cis-C¼C

(1.416(6) Å) and cis-C¼C/Rh bonds (2.118(4), 2.149(4) Å) are slightly

lengthened. This trend for the C¼C/Rh bonds is observed also in the

quinoxaline-annulated LRh(COD)Cl complex 4q (trans 2.206(3),

2.253(3); cis 2.125(3), 2.155(3) Å) [9a]. The largely unchanged trans-

inﬂuence but weakening of the cis-C¼C/Rh and the C(II)-N bonds

Scheme 4. Transmetalation of 7b [8b] to 5b, 8b and 10b/11b.

with

p-bonding contributions might be accounted for in terms of

and C^IIof the carbene; the coordination plane is approximately

perpendicular to the NHC ring plane (Figs. 2 and 3; interplanar

angles 70^ꢁfor 4 pb and 83^ꢁfor 4pc). The asymmetry within the

ligand (position of the pyridine-N) and the coordination planes (cis-

Cl/COD) might imply the presence of two isomers. The fact that only

one is observed in the solid state is probably associated with a low

barrier to rotation around the C2-Rh axis and crystallization of the

more stable form. Also the orientation of the CMe₃termini of the

neopentyl groups (CH₂lies in the ring plane) could be different,

Rh-C^II(d-

p*)p-back bonding and repulsion of p-density from the

C^II-N bonds towards the pyridine ring. Unfortunately, no crystal-

lographic data could be obtained for the analogous non-annulated

rhodium complex 4i.

The silver chloride complexes 7pb and 7pc differ from the

strictly monomeric and almost linear benzo- and naphtho-

annulated complexes 7b and 7n (C-Ag-Cl 175.88(4), [8b]

176.56(7)^ꢁ[8a]) by formation of weakly associated dimers, at least

in the crystal (Figs. 4 and 5). The polarity of the

p-density within

F. Ullah et al. / Journal of Organometallic Chemistry 890 (2019) 43e57

47

Fig. 1. The two independent cations of 3pb in the crystal (ellipsoids with 50% probability).

Fig. 2. Molecular structure of 4pb in the crystal (ellipsoids with 50% probability).

Fig. 3. Molecular structure of 4pc in the crystal (ellipsoids with 30% probability).

the ring plane, caused by the pyridine-N atom, seems to favor a

slight bending of Cl away from the C(II)-Ag axis and dimerization to

an Ag₂Cl₂rhombus with a slightly lengthened polar covalent and a

greatly lengthened coordinative Ag-Cl contact (bond lengths and

angles see Table 1). The planar ﬁve-membered ring of 7pb, average

deviation 0.01 Å, adopts in the crystal an interplanar angle of 57^ꢁ

towards the Ag₂Cl₂plane (58^ꢁin 7pc) The packing of 7pc also in-

volves the short intermolecular contacts H7LCl 2.71 and H4LN5

2.57 Å. The benzanellated 7b under the same conditions involves

stabilization of the monomer by a weak ClLH hydrogen bond to

CH₂Cl₂and displays a completely different packing involving single

molecules [8b]. The naphtho-anellated complex 7n shows in the

packing a pairwise p-stacking arrangement with the AgCl groups at

opposite ends [8a]. It is well known that NHC-AgCl complexes,

depending on the substituents, adopt various structure types [20],

also structures with (NHC)₂Ag^þand AgCl₂^ꢀunits, whose ¹³C NMR

spectra display a broad singlet instead of doublets of doublets. In

this context it should be mentioned that 7pc shows a broadened

Fig. 4. An inversion-symmetric dimer in the structure of 7pb in the crystal (ellipsoids

with 50% probability).

48

F. Ullah et al. / Journal of Organometallic Chemistry 890 (2019) 43e57

3.06 ppm [8a], 1bH₂d¹H ¼ 2.53 ppm [12]).

Sharing of the C^IIlone pair with a transition metal extinguishes

or diminishes the

s-p electron repulsion in the complexes and

causes signiﬁcant upﬁeld shifts of the ¹³C^IIsignal. For the pyrido-

and some related non-, carbo- or quinoxalino-annulated imidazo-

lin-2-ylidene Rh(I), Ag(I) and Pd(II) complexes, these values and the

¹³C^II-coordination chemical shifts

(

Dd¹³C^II

)

are

complex-ligand

compiled for comparison in Table 3. The Dd¹³C^IIvalues range

from ꢀ30 to ꢀ51 ppm and depend mainly on the metal and its

substituents, but to

a minor extent also on the type of

annulation. Within comparable complex/ligand pairs with

identical N-substituents (3i, 3pb, 3pc / 4i, 4pb, 4pc, 4n / 5pb,

5pc, 5b / 7pb, 7pc, 7b, 7n / 8pb, 8b, 8n), the upﬁeld coordination

chemical shifts increase in most cases slightly in the order

imy < bimy z pcimy < pbimy z nimy. Replacement of the N-neo-

pentyl by isopropyl or mesityl groups has a low impact on Dd¹³C^II.

For complexes of the more electron-withdrawing 4-Mepbimy^þ[1f]

Fig. 5. An inversion-symmetric dimer in the structure of 7pc in the crystal (ellipsoids

with 50% probability).

and qimy [9a] ligands the

d

¹³C^IIsignals were observed further

downﬁeld, but low stability of these ligands prevented evaluation

doublet of doublets for C(II) with a markedly decreased Ag-C

coupling constant, which hints at stronger intermolecular in-

teractions in 7pc than in 7pb.

of the coordination shift values Dd¹³C of C^II.

Further aspects worth mentioning for the structures of the

complexes in solution are the hindered rotation of the ligand

around the C^II-metal axis and the impact of the asymmetry within

the pyrido-imidazolin-2-ylidene plane. Even for identical N1- and

N3-substituents and for free rotation around the N-CH₂R axes, two

isomers or diastereoisomers could be possible for the linear L₂Ag^þ

and L₂Pd(0) or the square planar Rh(I) and Pd(II) complexes in

solution. However, as for the LAgCl complexes 7pb and 7pc, only

one data set was found in the ¹³C and ¹H NMR spectra of the

cationic silver complexes 3pb and 3pc and in the Pd(0) complex

6pc. Also, the square-planar LRh(COD)Cl complexes each display

only one data set for 4pb and 4pc. In contrast to 7pb, 7pc and 6pc,

each with two (in the latter case broad) singlets for N(1)CH₂and

N(3)CH₂, the Rh-complexes exhibit four AB-doublets for the four

NCH₂protons. In the case of the allylpalladium chloride complexes

5pb and 5pc two very broad signal ranges were observed for the

four NCH₂protons. This and the occurrence of major and minor a-

and c-allyl proton signals hint at dynamic processes in solutions of

5pb and 5pc. The observation of four sharp singlets for the NCH₂

protons of the L₂PdCl₂complex 8pb, each two close together,

suggest trans-conﬁguration, free rotation of the neopentyl groups

around the N-CH₂axes but two obviously non-averaged pop-

ulations with N1/N3 of the two 1pb ligands in the same and the

opposite orientation.

2.2.2. Solution NMR characterization

The molecular structures of the pyridoimidazolium salts, the

pyridoimidazolin-2-ylidenes and their complexes in solution were

conﬁrmed by conclusive ¹³C{¹H} and proton solution NMR data.

The most typical feature of the “free” carbenes 1 pb and 1pc is their

strong C^IIdownﬁeld shift [21]. This increases within the series of

related carbenes 1i < 1ph < 1b < 1pb < 1pc < 1n much more

strongly than

particularly with linear extension of the annulated

inclusion of electron-withdrawing

²N-atoms, whereas the effect

of phenanthrene-annulation [8d,22], with a Y-shaped -system, is

much smaller. This may be attributed to a push-pull polarization of

-density diminishing at the N₂C(II)-end in favor of the annulated

heterocycle at the opposite end. Driving forces are -electron

repulsion at C(II) and electron withdrawal by the annulated N-

deﬁcient-aromatic heterocycle. The result is increased -acidity

d

¹³C(H)-2 of their azolium precursors (cf. Table 2),

-system and

p

s

p

s/p

p

and destabilization of the NHC, preventing isolation of the

quinoxalino-annulated 1q [9a] at room temperature. An indication

of the N-C(II)-N double bond character and cyclodelocalization

(ring current effect) is provided by the strong downﬁeld shifts of

the NCH₂protons of 1pb and 1b (Table 2) compared to the NCH₂

signals of the C2-hydrogenated derivatives (1pbH₂

d

¹H ¼ 2.77,

Table 2

¹³C^II/¹³CH-2 and ¹H(NCH₂) NMR data of non-, carbo- or heterocyclically annulated imidazolin-2-ylidenes 1 and their precursors 2_X

(d in ppm).

Compd.

1

d

DdC^II

2_X

¹³CH-2,

DdCH-2_(ann-np2imy)

¹³C^II,

d

¹H (NCH₂)

d

¹H (NCH₂)

(ann-np2imy)

1i, 2i_BF4[9a]

1b [12], 2b_PF6[8b]

217 (br)

231.5,

14.5

3.72

3.98

136.8

143.6,

6.8

4.03

4.40

1pb, 2pb_PF6

235.2,

18.2

235.8,

18.8

239.9,

22.9

3.86, 4.43

3.86, 3.91

4.06

148.4,

11.6

145.8,

9.0

147.6,

10.8

4.39, 4.42

4.38, 4.47

4.48

1pc, 2pc_PF6

1n, 3n [8a]

1q, 3q_PF6[9a,8b]

1ph, 3ph_PF6[8d]

nBu₂phimy, [nBu₂phimy]BF₄[22]

e

153.2 (¹J_CH221 Hz),

16.4

141.5, 141.8,

4.7, 5.0

141.6,

(4.8)

4.49

224.5, 224.9,

7.5, 7.9

225.1,

e

4.70

4.93

8.1

F. Ullah et al. / Journal of Organometallic Chemistry 890 (2019) 43e57

49

Table 3

¹³C(II) and ¹H(NCH₂) NMR data of 3e9 and some related non-, carbo- or heterocyclically annulated imidazolin-2-ylidene complexes (

d

[ppm], J [Hz]).

¹H (NCH₂)

Metal/Complex

d

¹³C(II)

Dd_(complex-L)

¹J_CRhor ¹J_CAg

d

Rh(I)/

4i

4pb

184.0

202.9

ꢀ33

52.2

52.7

4.60, 5.01^b

4.51, 5.21,

4.73, 5.15^c

4.43, 4.46,

5.32, 5.37^c

ꢀ32.3

4pc

205.2

ꢀ30.5

51.9

[3,4-Me₂-1-Np-pbimy-Rh(COD)Cl]PF₆[1f]

4n [8a]

4q [9a]

212.95

208.1

219.5

e

53.5

52.8

52

4.64, 5.23,^c(3-Me: 5.01, 5.14, 2 s)

4.43, 5.49^b

ꢀ31.8

e

4.86, 5.25^b

Ag(I)/

3i [9a]

3pb

3pc

[(iPr₂qimy)₂Ag]PF₆[9a]

7pb

7pc

7b [8b]

7b [8a]

Pd(0)/

6pc

181.9

192.2

193.9

197.4

193.7

195.8

193.2

197.5

ꢀ35.1

ꢀ41.5

ꢀ39.9

e

213.8, 185.8

218.8, 189.4

219.7, 190.5

214.9, 185.8

270.6, 234.7

273.1, 236.1

271.0, 234.9

269.1, 233.1

3.94 (s)

4.34, 4.46 (2 s)

CH: 5.41 (sep)

4.29, 4.41 (2 s)

4.31, 4.38 (2 s)

4.27 (s)

ꢀ43.1

ꢀ41.8

ꢀ38.3

ꢀ42.4

4.37 (s)

202.3

ꢀ33.5

e

4.26 (br),

4.31 (br)

Pd(II)/

[(iPr₂imy)Pd(Allyl)Cl] [23]

5pb

177.2(iPr₂imy 211.9 [24])

197.9

ꢀ34.7

ꢀ37.3

e

CH: 4.98 (br)

4.04.3 (vbr),

4.5e4.9 (vbr)

4.14.4 (vbr),

4.5e4.9 (vbr)

4.0e4.9 (vbr)

4.90, 4.91,

5pc

200.0

ꢀ35.8

e

5b

8pb

194.4

185.6

ꢀ37.1

ꢀ49.6

e

5.00, 5.01^d

4.91 (s)

8b

8n [8a]

9pc

182.6

188.7

ꢀ48.9

ꢀ51.2

ꢀ47.1

e

5.06 (br)

4.94 (br),

5.05,5.15 (br)

a 2

J

.

PC(II)

b

Two doublets, ²J ¼ 13.5e13.8 Hz.

Four doublets, ²J ¼ 13.5e14.0 Hz).

Four singlets.

c

d

2.3. Catalytic ligand screening

investigations with the easily available 2b_PF6showed that K₃PO₄

and dry 1,4-dioxane are suitable as base and solvent, respectively.

Cs₂CO₃gave lower yields of p-methylbiphenyl (pTolPh); KH,

applied in the synthesis of 1 pb, 1pc and the non- or carbocyclic

annulated dineopentyl-imidazolin-2-ylidenes [8], was found to be

unsuitable for catalyst generation because of competing reduction

reactions, e.g. of p-bromotoluene and PhB(OH)₂to toluene and

benzene, respectively. Moisture in DMF, dioxane or the base low-

ered the coupling yields (Table 4, entries 1e5). A 2:1 M ratio of the

ligand precursor and Pd(OAc)₂led (in comparison to a 1:1 M ratio)

to increased cross-coupling yields with the benzo- and naphtho-

annulated catalysts, whereas 1pc/Pd catalysts generated with

these precursor ratios gave almost the same results (entries 3, 6, 7

and 10, 12, 13). Within the 1:1 ligand precursor/Pd series (entries

9e17), the yields of pTolPh are similar with non-, benzo- and pyrido

[c]-annulated catalysts (74e76%) and markedly higher than with

naphtho- and pyrido[b]-annulation (62, 59%). With catalysts

generated from Pd(OAc)₂and 2q_PF6(1:2 or 1:1), the yields are

comparable to the yields in blind tests without a ligand precursor.

The low stability of 1q, not isolable at r.t. [9a], may have prevented

formation of the corresponding Pd complex in this case. The lower

coupling yields with the di-o- and di-p-tolyl-phenanthreno-annu-

lated catalysts may be attributable both to different N-substituents

and annulation types.

Countless reports on the use of NHC ligands in a wide variety of

transition-metal-catalyzed conversions have been published in the

last three decades, usually with the aim of optimizing a certain

transformation with respect to suitable metals, NHC ligand types,

reaction conditions, applicability of less reactive but cheaper sub-

strates, of water as solvent, etc. [2e5]. Electronic annulation effects

were demonstrated by Organ and coworkers [25] for the Suzuki-

Miyaura coupling of p-tolylboronic acid with p-methoxy-chloro-

benzene by 5,6-diﬂuoro- (ꢀM), 5,6-non- and 5,6-dimethoxy-

substituted (þM) 1,3-diadamantyl-benzimidazolin-2-ylidene li-

gands. The increasing yields (53, 72, 83%) with increasing p-density

at the bimy ligand/Pd catalyst might be explained by increasing

rates of the oxidative additions of the Pd(bimy)-catalysts at the aryl

halide as the rate determining step. Similar electronic annulation

effects were observed in Heck coupling reactions catalyzed by 5,6-

diﬂuoro-, -non- and -dibutoxy-substituted nBu₂bimy/Pd catalysts

(L/Pd 2:1) [26] and in Suzuki-Miyaura and Mizoroki-Heck couplings

catalyzed by o-xylylene-N,N-bridged bis(N⁰-Mebimy)Pd-catalysts

with 5,6-dibutoxy-substituents on the benzene ring [27]. In this

context we were interested to examine whether annulation of the

imidazolin-2-ylidene ligands by the pyridine ring or related

carbocyclic aromatic rings, previously studied in our group, effects

the Suzuki-Miyaura cross-coupling of p-bromotoluene with phenyl

boronic acid (Scheme 5), chosen as model reaction.

Since NMR spectra of CD₂Cl₂-soluble components, formed dur-

ing heating of 2b_PF6with Pd(OAc)₂(2:1) with dry K₃PO₄in 1.4-

dioxane (the ﬁrst step of the catalytic protocol), displayed roughly

Most tests were performed with in-situ generated catalysts. Pre-

50

F. Ullah et al. / Journal of Organometallic Chemistry 890 (2019) 43e57

Scheme 5. Suzuki-Miyaura cross-coupling of p-bromotoluene with phenyl boronic acid.

Table 4

Suzuki-Miyaura reaction catalysed by annulated imy/Pd catalysts.^a

entry

Lig. precursor, L/Pd (mol%) or PdL complex (mol%);

solvent, (reaction time unless 16 h)

pTolBr (mmol),

PhB(OH)₂(mmol);

Base (mmol)

pTolPh (%)^b

In situ catalyst, L/Pd ratio 2:1

1

2

3

4

5

6

7

2b_PF6, 6/3; DMF/5%H₂O

2b_PF6, 6/3; 1.4-dioxane

2pc_PF6, 6/3; 1.4-dioxane

2n_PF6, 6/3; 1.4-dioxane

2q_PF6, 6/3; 1.4-dioxane

1, 1.5; K₃PO₄

1, 1.5; Cs₂CO₃

2

2^[d]

29

1, 1.5; K₃PO₄

1, 1.5; Cs₂CO₃

1, 1.5; KH 2

1, 1.5; K₃PO₄

86

68^d

1e8^c

74

2

92

12

8

In situ catalyst, L/Pd ratio 1:1

9

10

11

12

13

14

15

16

2i_BF4, 3/3; 1,4-dioxane

2b_PF6, 3/3; 1,4-dioxane

2pb_PF6, 3/3; 1,4-dioxane

2pc_PF6, 3/3; 1,4-dioxane

2n_PF6, 3/3; 1,4-dioxane

2ph_PF6, 3/3; 1,4-dioxane

pTol₂phimyHPF₆, 3/3; 1,4-dioxane

2q_PF6, 3/3; 1,4-dioxane

-, 0/3; 1,4-dioxane

1, 1.5; K₃PO₄

2

74

75

59

76

62

42

38

10e18

14e18

17

One-component precatalyst

18

19

20

21

22

23

5pb, 3; 1,4-dioxane

1, 1.5; K₂CO₃

1, 1.5; K₃PO₄

2

65

60

69

89

52

26^d

5pb, 3; 1,4-dioxane, (2 h)

5pc, 3; 1,4-dioxane, (2 h)

5b, 3; 1,4-dioxane, (2 h)

5ph [8d], 3; 1,4-dioxane, (2 h)

8b, 3; 1,4-dioxane

Reaction conditions: 1. Precatalyst formation from the respective annulated imyH^þX^ꢀprecursor, Pd(OAc)₂and base in the given solvent (5 mL) by heating for 30 min at

80 ^ꢁC; 2. heating the in-situ formed or one-component precatalyst with p-bromotoluene and PhB(OH)₂for ca. 16 h (or 2h) at 80 ^ꢁC; 3. quenching with aqueous NH₄Cl solution,

addition of external standard and GC analysis (for more details see exp. part).

a

b

Average of two experiments.

Reduction products 15% toluene and 14% benzene.

Single experiment.

c

d

equally intense signals of unconverted 2b_PF6and the cis- and trans-

(bimyPd)₂Pd(OAc)₂complexes 10b and 11b, supplementary ex-

periments with the LPd(Allyl)Cl complexes 5pb, 5pc, 5b and 5ph

be vacuum-distilled. The C^IIresonances of 1pb and 1pc are down-

ﬁeld shifted compared to 1i and 1b but upﬁeld towards 1n and hint

at repulsion of

with the linear extension of the

electron-withdrawing

²N-atoms in the annulated rings. This effect

p

-electrons by the C^IIelectron lone pair, increasing

were performed. The weak

p

-coordination of the

s

- and

p

-bound

p-system and the number of

bidentate allyl group makes them suitable as single-component

catalyst precursors and gave rise to comparable or somewhat

higher cross-coupling yields compared to the respective in-situ

reactions. In contrast, the L₂PdCl₂complex 8b is too stable and

unsuitable for catalysis. It led to only slightly more cross-coupling

product pTolPh than the blind tests. The amounts of the homo-

coupling product diphenyl were generally low (<10%). In sum, the

conversions decrease somewhat from non- and benzo-to pyrido[c]-

and pyrido[b]-annulated Np₂imy Pd catalysts, and stronger for the

phenanthrene- and quinoxaline-annulated imy Pd catalysts and

don't reach the high performance of current state of art catalysts

with bulkier N-substituents and higher carbene basicity [2,5].

s

is strongly reduced by coordination of Lewis-acidic transition

metals, which causes strong upﬁeld C^II-coordination chemical

shifts. The DdC^IIvalues of the Rh(I) complexes 4pb and 4pc were

almost the same as in the analogous non- or naphtho-annulated

imy-complexes, whereas for Ag(I) and Pd(II) complexes with

annulated imy-ligands the DdC^IIvalues were slightly larger than for

non-annulated analogues. The crystal structure analyses of 7pb and

7pc display noticeable bending of the C^II-Ag-Cl angle and crystal-

lization as dimer aggregates, reﬂecting changes of the

p-density

distribution within the ligand ( -electron repulsion) compared

s/p

to the linear monomer benzanalogue 7b, lacking the N-atom in the

annulated ring. The C^II-Ag bond is then slightly lengthened

(weakened) compared to 7b (and also 7n) and correlates with the

slight decrease of the HOMO energies (C^IIlone pair) in the order

1b > 1 pb > 1pc. The decrease of the C^II-Rh bond lengths in the

order 4pb > 4pc > 4q hints at back bonding effects, conﬁrmed also

by the inﬂuence on the cis-C¼C/Rh and C^II-N bond lengths. The

3. Conclusions

Pyrido[b]- and pyrido[c]-annulated imidazol-2-ylidenes are

accessible by deprotonation of the corresponding azolium salts, are

stable for 1pb and 1pc under inert gas at room temperature and can

F. Ullah et al. / Journal of Organometallic Chemistry 890 (2019) 43e57

51

orbital energy for the pyridine-N electron lone-pair is only slightly

inﬂuenced by the annulation with the imy-ring and even somewhat

raised for the ligand 1pc, but much lower compared to the C^IIlone

pair. This disfavors coordination of transition metals at the N-atom,

which has not yet been observed in the crystallographically char-

acterized complexes.o However, interactions between the N-atom

and the metal might play a role in the sparingly soluble Pd(pcimy)

Cl₂complex 9pc. This suggests an investigation whether 1pc is able

to form mixed C^II,N-coordinated transition metal complexes or

MOF-materials. In the tested Suzuki-Miyaura coupling of p-bro-

motoluene and phenylboronic acid in dioxane the yields of the

cross-coupling product were similar for non-, benzo- and pyrido

[c]-imy and slightly lower for pyrido[b]-imy 1:1-Pd-catalysts. For

2:1 bimy/Pd and nimy/Pd catalysts and the 5b precatalyst complex

the conversion was noticeably higher than for the 2:1 pcimy/Pd

catalyst and the precatalysts 5pc and 5pb. The better performance

of the pcimy than the pbimy catalysts might hint at stabilizing in-

teractions of the pyridine N atoms of the pcimy ligand and possibly

(over)compensate a slightly weaker C^II-Pd coordination.

b) A suspension of KH in THF was prepared from 30% KH

(105 mg, 0.79 mmol) in mineral oil by repeated washing/ﬁltration

with THF to remove the oil. Then 2i_OTf(175 mg, 0.488 mmol) was

added at ꢀ78 ^ꢁC, the mixture allowed to come to room temperature

and stirred overnight. After ﬁltration the solvent was removed

under vacuum. The residue was extracted with diethyl ether and

the ether evaporated to give 56 mg (55%) of NMR spectroscopically

almost pure oily 1i. ¹H NMR (C₆D₆):

d

¼ 0.90 (s, 18 H, CMe₃), 3.72 (s,

4 H, NCH₂), 6.48 (s, 2 H, H-4, H-5). ¹³C{¹H} NMR (C₆D₆):

d

¼ 28.62

(CMe₃), 33.37 (CMe₃), 63.06 (NCH₂), 120.48 (CH-4, CH-5), 217 (br,

C_q-2). The compound is highly sensitive to moisture and air.

4.3. Preparation of bis(1,3-dineopentyl-imidazol-2-ylidene) silver

tetraﬂuoroborate (3i_BF4

)

a) Reaction of glyoxal bis(neopentylimine) (435 mg, 2.22 mmol),

AgBF₄(519 mg, 2.67 mmol) and chloromethyl pivalate (0.45 mL,

3.12 mmol) in DCM (10 mL) as described for 2i_OTffurnished 464 mg

(71%) of the ionic liquid 2i_BF4

CMe₃), 4.03 (s, 4 H, NCH₂), 7.38 (br s, 2 H, H-4, H-5), 8.69 (br s, 1 H,

H-2). ¹³C{¹H} NMR (CDCl₃):

.

¹H NMR (CDCl₃):

d

¼ 0.98 (s, 18 H,

4. Experimental

d

¼ 26.69 (CMe₃), 32.19 (CMe₃), 60.69

(NCH₂), 123.35 (CH-4, CH-5), 136.78 (CH-2).

4.1. General

b) Ag₂O (133 mg, 0.574 mmol) was added to a solution of 2i_BF4

(170 mg, 0.574 mmol) in DCM (10 mL). The suspension was stirred

for 24 h at r.t., ﬁltered and washed with DCM. The ﬁltrate was

concentrated under vacuum (to 1e2 mL) and hexane (4 mL) was

added. The resulting white precipitate was separated and crystal-

lized from a small amount of DCM to give 224 mg (63%) white

All syntheses were carried out in carefully dried, freshly distilled

solvents under an argon atmosphere using Schlenk techniques.

Glyoxal bis(neopentylimine) [15], 2,3- and 3,4- di(neopentylamino)

pyridine were prepared as reported earlier [11b], other chemicals

were purchased. Potassium hydride (in oil) was repeatedly washed

with dry THF and dried under vacuum before use. NMR spectra

were recorded on a multinuclear FT-NMR spectrometer ARX300

crystals of 3i_BF4. ¹H NMR (CD₂Cl₂):

d

¼ 1.01 (s, 18 H, CMe₃), 3.94 (s,

4 H, NCH₂), 7.11 (d br, ⁴J(^107/109Ag¹H) ¼ 1.8 Hz, 2 H, H-4, H-5). ¹³C

{¹H} and 135-DEPT NMR (CD₂Cl₂):

d

¼ 28.10 (CMe₃), 32.97 (CMe₃),

(Bruker) at 300.1 (¹H), 75.5 (¹³C), and 121.5 (³¹P) MHz.

given in ppm. Shift references are tetramethylsilane for ¹H and ¹³

d

-Values are

63.71 (NCH₂), 122.99 (d, ³J(^107/109Ag¹H) ¼ 5.9 Hz, CH-4, CH-5),

181.93 (dd, ¹J(¹³C-^107/109Ag) ¼ 185.8, 213.8 Hz, C_q-2). Elemental

analysis calcd. for C₂₆H₄₈AgBF₄N₄(611.38): C, 51.02, H, 7.91, N, 9.17;

found: C, 50.77, H, 8.25, N, 9.19.

C

and H₃PO₄(85%) for ³¹P or calibrated solvent signals. Assignment

numbers for NMR data follow ring-atom numbering and are illus-

trated for 5pb/5pc in Scheme 2. Coupling constants refer to J_HH

unless stated otherwise. For small ¹⁰⁷Ag¹³C and ¹⁰⁵Ag¹³C coupling

constants the two doublets are unresolved and are referred to as

J_AgC. EI(þ) Mass spectra were measured on a single focusing sector-

ﬁeld spectrometer AMD40, FAB (NBA) spectra on a Finnigan MAT-

95XP (Thermo). High-resolution mass spectra were recorded on

double-focussing sector ﬁeld mass spectrometers Finnigan MAT

8200 or MAT-95XP (Thermo) with EI (70 eV) or ESI. Assignments L

in mass spectra of NHC complexes or in NHC metal catalysts mean

the respective ligand. Melting points (uncorrected) were deter-

4.4. Preparation of (1,3-dineopentylimidazole-2-ylidene)

rhodium(cyclooctadiene-1,5) chloride (4i)

The suspension of 1,3-dineopentylimidazolium tetra-

ﬂuoroborate (184 mg, 0.620 mmol) and [Rh(1,5-COD)Cl]₂(153 mg,

0.310 mmol) in THF was added at ꢀ78 ^ꢁC to a suspension of 30% KH

in mineral oil (19 mg, 0.992 mmol). The mixture was allowed to

come to room temperature and stirred overnight. The solvent was

removed in vacuum, the residue was subjected to chromatography

on silica gel (hexane, elution with DCM ([Rh(1,5-COD)Cl]₂, 1%

MeOH/DCM) to give the pure substance, yield 35 mg (23%). ¹H NMR

mined with

a Sanyo Gallenkamp melting point apparatus,

elemental analysis with a CHNS-932 analyzer (in some cases cali-

bration problems). Chloride was determined by potentiometrically

controlled titration. GC analyses were carried out using a gas

chromatograph Hewlett Packard 5890, column HP-5(30 m)

(crosslinked 5%PhMe silicone), 30 ^ꢁC 5 min, 10 ^ꢁC/min or 20^ꢁ/min,

250 ^ꢁC 5 min, external reference n-nonane or tridecane.

(CDCl₃):

d

¼ 1.45 (s,18 H, CH₃), 2.24 (m, 4 H, CH₂), 2.70 (m, 4 H, CH₂),

3.55 (br q, 2 H, ¼CH), 4.60 (d, ⁴J(¹⁰³Rh¹H) ¼ 13.7 Hz, 2 H, CH₂), 5.01

(d, ⁴J(¹⁰³Rh¹H) ¼ 13.7 Hz, 2 H, CH₂), 5.33 (br m, 2 H, ¼CH), 7.59 (s,

2 H, CHCH) ppm. ¹³C NMR (CDCl₃):

d

¼ 28.83 (CH₂), 28.89 (CMe₃),

32.55 (CH₂), 32.78 (CMe₃), 62.24 (NCH₂), 68.12 (d,

J(¹⁰³Rh¹³C) ¼ 14.6 Hz, CH ¼ ), 97.47 (d, J(¹⁰³Rh¹³C) ¼ 7 Hz, CH ¼ ),

121.10 (4,5-CH), 183.97 (d, ¹J(¹⁰³Rh¹³C) ¼ 52 Hz, C_q-2) ppm.

4.2. Preparation of 1,3-dineopentyl-imidazol-2-ylidene (1i)

C

₂₁H₃₆ClN₂Rh (454.88), HRMS (ESI in MeCN): calc. for [M ꢀ Cl]^þ

a) Glyoxal bis(neopentylimine) (1.10 g, 5.60 mmol), AgCF₃SO₃

(1.73 g,6.73 mmol) and then chloromethyl pivalate (1.13 mL,

7.84 mmol) were added to DCM (10 mL). The pressure tube was

sealed and stirred in the dark at 50 ^ꢁC for ca. 24 h. After cooling to r.t.

the mixture was ﬁltered, the solvent evaporated under vacuum and

the resulting oil washed several times with ether to give 1.45 g (72%)

of ionic liquid 1,3-dineopentyl-imidazolium triﬂate (2i_OTf). ¹H NMR

419.1928; found: 419.1926.

4.5. Preparation of 1,3-dineopentyl-imidazolinium[4,5-b]pyridine

hexaﬂuorophosphate (2pb_PF6

)

2,3-Diaminopyridine (3.0 g, 12.03 mmol) and NH₄PF₆(1.960 mg,

12.02 mmol) in triethyl orthoformate (20 mL) were heated in a

rectiﬁcation apparatus for 7 h at 140 ^ꢁC (oil-bath) to remove

ethanol formed in the reaction. The precipitate was ﬁltered off at

r.t., washed with Et₂O and dissolved in CH₃CN. Insoluble

(CDCl₃):

4, H-5), 8.87 (br s, 1 H, H-2). ¹³C{¹H} NMR (CDCl₃):

32.31 (CMe₃), 61.02 (NCH₂), 123.28 (CH-4, CH-5), 137.23 (CH-2).

d

¼ 0.98 (s,18 H, CMe₃), 4.02 (s, 4 H, NCH₂), 7.32 (br s, 2 H, H-

d

¼ 26.79 (CMe₃),

52

F. Ullah et al. / Journal of Organometallic Chemistry 890 (2019) 43e57

contamination was separated and volatiles were removed under

14.20, Cl 11.98; found: H 8.41, N 13.79, Cl 11.5.

vacuum to give 4.46 g (91%) crystalline 2pb_PF6. ¹H NMR (DMSO‑d₆):

d

¼ 1.00 (br s, 18 H, 2 CMe₃), 4.39, 4.42 (2 s, 4 H, NCH₂), 7.78 (dd,

4.9. Preparation of 1,3-dineopentyl-imidazolino[4,5-b]pyridin-2-

ylidene (1pb)

³J ¼ 8.2, ³J ¼ 4.4 Hz, 1 H, H6), 8.66 (d, ³J ¼ 8.2 Hz, 1H, H7), 8.79 (d,

³J ¼ 4.2 Hz, 1 H, H5), 9.97 (s, 1 H, H2). ¹³C{¹H} NMR (DMSO‑d₆):

d

¼ 26.94 (CMe₃), 27.22 (CMe₃), 32.89, 33.15 (2 CMe₃), 55.55 (1-

A suspension of 2pb_PF6(300 mg, 0.74 mmol) in THF (25 mL) was

added at room temperature to a suspension of excess KH (74 mg,

1.85 mmol). Evolution of hydrogen took place, and the color of the

solution became red-brown after stirring for 2 h. The mixture was

stirred overnight at room temperature and then ﬁltered. The sol-

vent was removed from the ﬁltrate under vacuum, and the residue

was extracted with benzene for separation from salts soluble in

THF. Evaporation of the solvent afforded 170 mg (88%) of crude 1pb,

colored red-brown by trace impurities. Distillation at 10^ꢀ4torr/

105 ^ꢁC (bath) provided almost colorless viscous oily 1 pb. ¹H NMR

NCH₂), 57.64 (3-NCH₂), 122.33 (CH6), 124.03 (CH7), 125.33 (C_q7a),

143.30 (CH5), 145.10 (C_q3a), 148.35 (C2). ³¹P{¹H} NMR (DMSO‑d₆):

1

d

¼ ꢀ143.20 (sept, J_PF¼ 711.2 Hz). MS (EI, 70 eV, 345 ^ꢁC): m/z

(%) ¼ 260 (17) [M ꢀ PF₆]^þ, 259 (24), 244 (16), 204 (14), 203 (100)

[M-PF₆-C₄H₉]^þ, 202 (26), 174 (29), 133 (79), 132 (54), 85 (65), 43

(35). Elemental analysis calcd. for C₁₆H₂₆F₆N₃P (405.36): C, 47.41, H,

6.46, N, 10.37; found: C, 47.81, H, 6.78, N, 10.34.

4.6. Preparation of 1,3-dineopentyl-imidazolinium[4,5-c]pyridine

hexaﬂuorophosphate (2pc_PF6

)

(C₆D₆):

d

¼ 0.90 (s, 9 H, CMe₃), 1.10 (s, 9 H, CMe₃), 3.86 (s, 2 H, 1-

NCH₂), 4.43 (s, 2 H, 3-NCH₂), 6.65 (dd, ³J ¼ 8.0, 4.8 Hz, 1 H, H6),

Reaction of 3,4-diaminopyridine (1.5 g, 6.01 mmol) andNH₄PF₆

(979 mg, 6.01 mmol) with triethyl orthoformate (20 mL) and work-

up as described for 2pb_PF6gave 2.2 g (90%) of white crystalline

6.99 (dd, ³J ¼ 8.0, ⁴J ¼ 1.4 Hz, 1 H, H7), 8.20 (dd, ³J ¼ 4.8, ⁴J ¼ 1.4 Hz,

1 H, H5). ¹³C{¹H} and DEPT-135 NMR (C₆D₆):

d

¼ 29.02 (CMe₃), 29.13

(CMe₃), 34.28/34.42 (2 CMe₃), 57.30 (1-NCH₂), 60.06 (3-NCH₂),

116.67 (CH6), 117.84 (CH7), 128.3 (C_q7a), 142.39 (CH5), 149.72

(C_q3a), 235.23 (C2). MS of moisture (H₂O) adduct (EI, 70 eV, 100 ^ꢁC):

m/z (%) ¼ 277 (24) [M þ H₂O]^þ, 275 (28), 261 (28), 260 (49) [MþH]^þ,

220 (100) [M þ H₂O-C₄H₉]^þ, 219 (33), 204 (24), 192 (98) [M ꢀ C₅

H₁₁]^þ, 134 (36), 120 (26), 43 (48). HRMS (EI 70 eV): C₁₆H₂₅N_3,calcd.

for [MþH]^þ260.2121; found 260.2110 (100%).

2pc_PF6

.

¹H NMR (DMSO‑d₆):

d

¼ 0.99 (s, 9 H, CMe₃), 1.02 (s, 9 H,

CMe₃), 4.38, 4.47 (2 s, 4 H, NCH₂), 8.20 (d, ³J ¼ 5.8 Hz, 1 H, H7), 8.77

(d, ³J ¼ 5.8 Hz, 1 H, H6), 9.50 (s, 1 H, H4), 9.85 (s, 1 H, H2). ¹³C{¹H}

NMR (DMSO‑d₆):

d

¼ 26.91 (2 CMe₃), 33.10 (CMe₃), 33.21 (CMe₃),

57.31 (3-NCH₂), 57.70 (1-NCH₂), 108.95 (CH7), 129.58 (C_q3a), 137.56

(CH6), 137.96 (CH4), 145.16 (C_q7a), 145.82 (C-2). ³¹P{¹H} NMR

(DMSO‑d₆):

d

¼ ꢀ143.2 (sept, ¹J_PF¼ 711.4 Hz). MS (EI, 70 eV, 200 ^ꢁC):

m/z (%) ¼ 260 (29) [M ꢀ PF₆]^þ, 259 (24), 203 (100) [M-PF₆-C₄H₉]^þ,

174 (20), 133 (57), 132 (33), 120 (18), 106 (48), 92 (41), 91 (57), 85

(65), 43 (44). Elemental analysis calcd. for C₁₆H₂₆F₆N₃P (405.36): C,

47.41, H, 6.46, N, 10.37; found: C, 47.63, H, 6.56, N, 10.34.

4.10. Preparation of 1,3-dineopentyl-imidazolino[4,5-c]pyridin-2-

ylidene (1pc)

Reaction of a suspension of 2pc_PF6(500 mg, 1.23 mmol) and KH

(123 mg, 3.07 mmol) in THF and work-up as described above for the

preparation of 1pb furnished 268 mg (84%) of red-brown oil.

Colorless 1pc was separated from trace impurities by vacuum

distillation at 10^ꢀ4torr/100 ^ꢁC bath temperature. ¹H NMR (C₆D₆):

4.7. Preparation of 1,3-dineopentyl-imidazolinium[4,5-b]pyridine

chloride (2pb_Cl)

Reaction of 2,3-diaminopyridine (1.5 g, 6.01 mmol) and NH₄Cl

(322 mg, 6.01 mmol) with triethyl orthoformate (15 mL) and work-

up as described for 2pb_PF6gave 1.6 g (90%) white solid of 2pb_Cl. ¹H

d

¼ 0.88 (s, 9 H, CMe₃), 0.90 (s, 9 H, CMe₃), 3.86 (s, 2 H, 3-NCH₂), 3.91

(s, 2 H, 1-NCH₂), 6.78 (dd, ³J ¼ 5.5, ⁵J ¼ 0.9 Hz, 1 H, H7), 8.41 (d,

³J ¼ 5.5 Hz, 1 H, H6), 8.71 (d, ⁵J ¼ 0.9 Hz, 1 H, H4). ¹³C{¹H} NMR

NMR (CD₃OD):

d

¼ 1.09 (s, 9 H, CMe₃), 1.10 (s, 9 H, CMe₃), 4.48, 4.51

(C₆D₆):

d

¼ 28.95 (2 CMe₃), 34.22 (CMe₃), 34.43 (CMe₃), 59.43, 59.81

(2 s, 4 H, NCH₂), 7.78 (dd, ³J ¼ 8.5, 4.7 Hz, 1 H, H6), 8.57 (dd, ³J ¼ 7.9,

(2 NCH₂), 106.30 (CH7), 134.28, 134.39 (C_q3a, C_q7a), 141.48 (CH6),

142.27 (CH4), 235.76 (C2). MS (EI, 70 eV, 165 ^ꢁC): m/z (%) ¼ 259 (18)

[M]^þ, 257 (15), 220 (41), [M ꢀ C₃H₉]^þ, 210 (18), 204 (13), 203 (99)

[M ꢀ C₄H₈]^þ, 192 (38), 174 (20), 152 (100), 133 (77), 121 (92), 120

(22), 92 (14), 43 (83). HRMS (EI 70 eV): C₁₆H₂₅N_3,calcd. for [MþH]^þ

260.2121; found 260.2110 (100%); calcd. for [M^þ] 259.2043; found

259.2032 (71%).

⁴J ¼ 1.3 Hz, 1 H, H7), 8.81 (dd, ³J ¼ 4.5, ⁴J ¼ 1.3 Hz, 1 H, H5). ¹³C{¹H}

NMR (CD₃OD):

d

¼ 27.66 (CMe₃), 27.81 (CMe₃), 34.08, 34.36 (2

CMe₃), 57.28 (1-NCH₂), 59.62 (3-NCH₂), 123.78 (CH6), 124.98 (CH7),

126.82 (C_q7a), 144.97 (C_q3a), 150.08 (CH5); CD-2 at noise level. MS

(EI, 70 eV, 345 ^ꢁC): m/z (%) ¼ 261 (18), 260 (61) [M ꢀ Cl]^þ, 259 (28),

258 (9), 244 (18), 204 (20), 203 (100) [[M-Cl-C₄H₉]^þ, 202 (22), 174

(25), 145 (10), 134 (20), 133 (79), 132 (46), 120 (26), 43 (29).

Elemental analysis: calcd. for C₁₆H₂₆N₃Cl (295.86): H 8.85, N 14.20,

Cl 11.98; found: H 8.51, N 14.39, Cl 11.7.

4.11. Preparation of rhodium(1,5-cyclooctadiene)(1,3-dineopentyl-

imidazolino[4,5-b]pyridin-2-ylidene)chloride (4pb)

4.8. Preparation of 1,3-dineopentyl-imidazolinium[4,5-c]pyridine

chloride (2pc_Cl)

a) [Rh(1,5-COD)Cl]₂(146 mg, 0.295 mmol) was added to a

ﬁltered solution of free carbene 1pb in THF, generated from 2pb_PF6

(300 mg, 0.74 mmol), and KH (60 mg, 1.50 mmol) at room temper-

ature. The mixture was stirred for 12 h, ﬁltered, and the solvent was

removed in vacuum. The residue was washed thoroughly with

hexane and recrystallized from a small amount of DCM to give

187 mg (63%) of yellow crystals of 4pb. Selected bond lengths and

angles are compiled in Table 1, crystal data in Table 5. ¹H NMR

Reaction of 3,4-diaminopyridine (1.5 g, 6.01 mmol) and NH₄Cl

(322 mg, 6.01 mmol) with triethyl orthoformate (15 mL) and work-

up as described for 2pb_PF6gave 1.72 g (96%) white solid of 2pc_Cl. ¹H

NMR (CD₃OD):

d

¼ 1.10 (s, 9 H, CMe₃), 1.12 (s, 9 H, CMe₃), 4.46, 4.55

(2 s, 4 H, NCH₂), 8.15 (dd, ³J ¼ 5.9, ⁵J ¼ 0.7 Hz, 1 H, H7), 8.80 (d,

³J ¼ 5.9 Hz, 1 H, H6), 9.43 (br, 1 H, H4). ¹³C{¹H} NMR (CD₃OD):

(CDCl₃):

d

¼ 1.23 (s, 9 H, CMe₃), 1.24 (s, 9 H, CMe₃), 1.88e2.03 (m,

d

¼ 27.59 (2 CMe₃), 34.29 (CMe₃), 34.42 (CMe₃), 59.31, 59.72 (2

4 H, CH₂), 2.35e2.51 (m, 4 H, CH₂), 2.98e3.10 (m, 2 H, ¼CH), 4.51 (d,

²J ¼ 14.0 Hz, 1 H, NCH₂), 4.73 (d, ²J ¼ 13.5 Hz, 1 H, NCH₂), 5.15 (d,

²J ¼ 13.5 Hz, 1 H, NCH₂), 5.21 (d, ²J ¼ 14.0 Hz, 1 H, NCH₂), 5.22 (br m,

2 H, ¼CH), 7.12 (dd, ³J ¼ 8.1, ³J ¼ 4.8 Hz, 1 H, H6), 7.64 (dd, ³J ¼ 8.1,

⁴J ¼ 1.3 Hz, 1 H, H7), 8.25 (dd, ³J ¼ 4.8, ⁴J ¼ 1.3 Hz, 1 H, H5). ¹³C{¹H}

NCH₂), 110.26 (CH7), 131.22 (C_q3a), 138.98 (CH6), 139.44 (C_q7a),

146.56 (CH4); CD-2 in noise. MS (EI, 70 eV, 345 ^ꢁC): m/z (%) ¼ 260

(5) [M ꢀ Cl]^þ, 259 (22), 258 (13), 244 (16), 204 (15), 203 (100) [M-

Cl-C₄H₉]^þ, 202 (20), 174 (24), 146 (12), 133 (72), 132 (37), 120 (10),

43 (35). Elemental analysis calcd for C₁₆H₂₆N₃Cl (295.86): H 8.85, N

NMR (CDCl₃):

d

¼ 28.70, 28.80 (2 CH₂), 29.86 (CMe₃), 29.94 (CMe₃),

F. Ullah et al. / Journal of Organometallic Chemistry 890 (2019) 43e57

53

Table 5

a) Crystal data and structure reﬁnement of 3pb, 4 pb and 4pc.

Compound number

3pb

4pb [9b]

4pc [9b]

CCDC number

Empirical formula

Formula weight

Temperature

Wavelength

Crystal system

Space group

1880308

617669

C₂₄H₃₇ClN₃Rh

505.93

133(2) K

0.71073 Å

monoclinic

P2₁/c

617670

C₂₄H₃₇ClN₃Rh

505.93

133(2) K

0.71073 Å

Monoclinic

P2₁/c

C

_33.5H₅₁AgClF₃N₆O₃S

818.18

100(2) K

0.71073 Å

triclinic

P1

Unit cell dimensions:

a

b

c

a

b

g

11.9279(5) Å

14.3530(10) Å

22.6918(15) Å

88.315(5)^ꢁ

15.3603(14) Å

12.8518(12) Å

12.0112(12) Å

90^ꢁ

14.4491(12) Å

10.6453(8) Å

16.2096(12) Å

90^ꢁ

89.393(5)^ꢁ

92.731(3)^ꢁ

104.126(4)^ꢁ

90^ꢁ

77.965(5)^ꢁ

90^ꢁ

Volume

Z

Density (calculated)

Absorption coefﬁcient

F(000)

3797.8(4) Å³

4

2368.4(4) Å³

4

2417.9(3) Å³

4

1.431 Mg/m³

0.71 mm^ꢀ1

1.419 Mg/m³

0.85 mm^ꢀ1

1056

1.390 Mg/m³

0.83 mm^ꢀ1

1056

1700

Crystal size

2

0.25 ꢂ 0.10 ꢂ 0.10 mm³

0.20 ꢂ 0.15 ꢂ 0.05 mm³

0.35 ꢂ 0.25 ꢂ 0.15 mm³

q_max

52.7^ꢁ

61^ꢁ

Reﬂections collected

Independent reﬂections

Completeness to theta

Abs. correction

66948

45163

44785

15106 [R_int¼ 0.067]

(26.37^ꢁ) 97.3%

multi-scan

1.00 and 0.92

15106/49/898

0.93

7225 [R_int¼ 0.055]

(30^ꢁ) 100.0%

multi-scan

0.96 and 0.86

7225/6/284

1.06

7358 [R_int¼ 0.031]

(30^ꢁ) 99.8%

multi-scan

0.89 and 0.78

7358/35/286

1.09

Transmissions

Data/restraints/parameters

Goodness-of-ﬁt on F²

Final R indices [I > 2

R indices (all data)

Largest diff. peak and hole

s

(I)]

R1 ¼ 0.056, wR2 ¼ 0.131

R1 ¼ 0.098, wR2 ¼ 0.139

2.20 and ꢀ0.77 e$Å^ꢀ3

R1 ¼ 0.032,wR2 ¼ 0.065

R1 ¼ 0.032, wR2 ¼ 0.065

1.02 and ꢀ0.46 e$Å^ꢀ3

R1 ¼ 0.027, wR2 ¼ 0.059

R1 ¼ 0.039, wR2 ¼ 0.066

0.61 and ꢀ0.33 e$Å^ꢀ3

Identiﬁcation code

7pb

7pc

2b_PF6·H₂O

CCDC number

Empirical formula

Formula weight

617671

C₁₆H₂₅AgClN₃

402.71

1880310

C₁₆H₂₅AgClN₃

402.71

1880311

C₁₇H₂₉ClN₂O

312.87

Temperature

Wavelength

Crystal system

Space group

133(2) K

0.71073 Å

monoclinic

P2₁/n

100(2) K

1.54184 Å

monoclinic

P2₁/n

100(2) K

0.71073 Å

monoclinic

P2₁/c

Unit cell dimensions:

a

b

12.249(2) Å

9.439(2)

12.8550(5)

9.9307(4)

12.1550(2)

8.7127(2)

c

a

b

g

15.201(3)

13.7454(6)

16.6474(3)

90^ꢁ

90.897(6)^ꢁ

90^ꢁ

92.458(4)

93.369(2)^ꢁ

90^ꢁ

Volume

Z

Density (calculated)

Absorption coefﬁcient

F(000)

1757.2(6) Å³

4

1753.11(12) Å³

4

1759.95(6) Å³

4

1.522 Mg/m³

1.30 mm^ꢀ1

824

1.526 Mg/m³

10.6 mm^ꢀ1

824

1.181 Mg/m³

0.22 mm^ꢀ1

680

Crystal size

2

0.12 ꢂ 0.12 ꢂ 0.06 mm³

0.15 ꢂ 0.10 ꢂ 0.01 mm³

0.45 ꢂ 0.25 ꢂ 0.15 mm³

q_max

56.6^ꢁ

140.6^ꢁ

61^ꢁ

Reﬂections collected

Independent reﬂections

Completeness to theta

Absorption correction

Transmissions

16929

26233

67596

4350 [R_int¼ 0.134]

(28) 99.8%

none

3072 [R_int¼ 0.047]

(67.5^ꢁ) 95.6%

Numerical

0.88 and 0.40

3072/0/196

1.05

5278 [R_int¼ 0.027]

(30.5^ꢁ) 98.0%

multi-scan

0.97 and 0.91

5278/1/208

1.04

Data/restraints/parameters

4350/0/196

1.08

Goodness-of-ﬁt on F²

Final R indices [I > 2

R indices (all data)

Largest diff. peak and hole

s

(I)]

R1 ¼ 0.076, wR2 ¼ 0.099

R1 ¼ 0.117,wR2 ¼ 0.106

1.68 and ꢀ1.57 e$Å^ꢀ3

R1 ¼ 0.031,wR2 ¼ 0.075

R1 ¼ 0.038, wR2 ¼ 0.078

1.17 and ꢀ0.41 e$Å^ꢀ3

R1 ¼ 0.031,wR2 ¼ 0.080

R1 ¼ 0.036, wR2 ¼ 0.083

0.42 and ꢀ0.24 e$Å^ꢀ3

32.71 (br, 2 CH₂), 33.30, 33.73 (2 CMe₃), 58.51 (1-NCH₂), 60.89 (3-

NCH₂), 69.66, 69.92 (2 d, J_RhC¼ 14.4, 14.5 Hz, 2 CH), 99.92, 100.01

(2 d, J_RhC¼ 1.7 Hz, 2 CH), 117.05 (CH6), 118.20 (CH7), 128.06 (C_q7a),

142.25 (CH5), 148.05 (C_q3a), 202.94 (d, J_RhC¼ 52.7 Hz, C_q2). HRMS

(EI 70 eV, 200 ^ꢁC), M^þcalcd for C₂₄H₃₇ClN₃Rh: 505.1731 (100),

506.1765 (27), 507.1702 (32); found: m/e ¼ 505.1694 (100),

506.17248 (23), 507.1642 (33).

b) The silver complex 7pb (300 mg, 0.74 mmol) was dissolved in

DCM (20 mL) and [Rh(1,5-COD)Cl]₂(183 mg, 0.37 mmol) was

added. The mixture was stirred in the dark at room temperature for

24 h and ﬁltered to give a clear yellow solution. The solvent was

partly removed, and the saturated solution was layered with hex-

ane. Storage at ꢀ20 ^ꢁC for 24 h gave yellow crystals of 4pb,

contaminated by a small amount of residual silver complex, which

54

F. Ullah et al. / Journal of Organometallic Chemistry 890 (2019) 43e57

was removed by chromatography using DCM; yield 355 mg (94%).

The NMR data correspond to those given above.

204 (52), 189 (64). HRMS (EI, 70 eV): C₁₉H₃₀ClN₃Pd, calcd. for

strongest peak of [M]^þ441.1157; found: 411.1165; the observed

isotopic pattern ﬁts with the calculated pattern.

4.12. Preparation of rhodium(1,5-cyclooctadiene)(1,3-dineopentyl-

imidazolino[4,5-c]pyridin-2-ylidene) chloride (4pc)

4.14. Preparation of (h

³-Allyl)(1,3-dineopentyl-imidazolino[4,5-c]

pyridin-2-ylidene) palladium(II) chloride (5pc)

a) Compound 4pc was prepared as described for 4pb from

[Rh(1,5-COD)Cl]₂(121 mg, 0.24 mmol) and crude 1pc in THF,

generated from 2pc_PF6(250 mg, 0.62 mmol) and KH (50 mg,

1.25 mmol) at room temperature. The yield of the yellow crystals

was 149 mg (60%). Selected bond lengths and angles are listed in

Potassium hydride (98 mg, 2.44 mmol) was added at room

temperature to a suspension of 2pc_PF6(500 mg, 1.23 mmol) in THF

(25 mL). The crude NHC was converted with [Pd(Allyl)Cl]₂(203 mg,

0.55 mmol, calcd. for 90% NHC generation) and worked up as

described above for 5pb affording 430 mg (88%) of red-brown solid

Table 1, crystal data in Table 5. ¹H NMR (CDCl₃):

d

¼ 1.22 (s, 9 H,

CMe₃), 1.25 (s, 9 H, CMe₃, 1.91e2.05 (m, 4 H, CH₂), 2.37e2.53 (m,

4 H, CH₂), 2.93 (br, 2 H, ¼CH), 4.43, 4.46 (2 d, ²J ¼ 14.0, 13.9 Hz, 2 H,

NCH₂), 5.24 (br m, 2 H, ¼CH), 5.32, 5.37 (2 d, ²J ¼ 14.0, 14.0 Hz, 2 H,

NCH₂), 7.32 (d br, ³J ¼ 5.7 Hz, 1 H, H7), 8.37 (d, ³J ¼ 5.7 Hz, 1 H, H6),

5pc. ¹H NMR (CD₂Cl₂, ref. solvent 5.32 ppm):

d

¼ 1.08 (br s, 2 CMe₃),

2.57 (d br, ³J ¼ 11.7 Hz, ca. 1H, CH_trans-a), 3.17 (d, ³J ¼ 12.1 Hz, minor,

CH_trans-a), 3.31 (d, ³J ¼ 13.4 Hz, 1 H, CH_trans-c), 3.60 (d br, ³J ¼ 5.7 Hz,

ca. 1H, CH_{cis -a}), 4.02 (d br, ³J ¼ 6.8 Hz, minor, CH_cis-c), 4.25 (d br,

³J ¼ 7.4 Hz, ca. 1H, CH_cis-c), 4.1e4.4, 4.5e4.9 (vbr, 2 NCH₂), 5.37 (m,

1 H, CH_b), 7.44 (d br, ³J ¼ 5.5 Hz, 1 H, H-7), 8.44 (d br, ³J ¼ 5.5 Hz, 1 H,

H-6), 8.94 (s br, 1 H, H-4); assignment of allyl protons in analogy to

8.79 (d, ³J ¼ 0.6 Hz, 1 H, H4). ¹³C{¹H} NMR (CDCl₃):

¼ 29.75, 28.83

d

(2 CH₂), 29.88, 29.93 (2 CMe₃), 32.68, 32.77 (2 CMe₃), 33.71, 33.86 (2

CH₂), 60.26, 60.53 (2 NCH₂), 70.13, 70.32 (2 d, J_RhC¼ 14.1,

14.1 Hz, ¼CH), 100.54, 100.58 (2 d, J_RhC¼ 6.7, 6.8 Hz, ¼CH), 106.24

(CH7), 133.42 (C_q3a), 133.67 (CH6), 140.87 (C_q7a), 142.04 (CH4),

205.22 (d, J_RhC¼ 51.9 Hz, C_q2). MS (EI, 70 eV, 370 ^ꢁC): m/z (%) ¼ 507

(6) [M^þ], 506 (3), 505 (24) [M^þ], 361 (55) [M^þeCODeHCl], 260

(100) [M^þeRh(COD)Cl], 203 (64), 172 (25), 133 (50), 132 (28), 43

(45). HRMS, M^þcalcd for C₂₄H₃₇ClN₃Rh: 505.1731 (100), 506.1765

(27), 507.1702 (32); found (70 eV, 200 ^ꢁC): m/e ¼ 505.16868 (100),

506.1726 (17), 507.17305 (32).

ref. [28]. ¹³C{¹H} NMR (CD₂Cl₂, ref. solvent 53.8 ppm):

d

¼ 29.33 (2

CM-e₃), 34.20 (CMe₃), 34.41 (CMe₃), 51.74 (s br, CH₂-a), 59.60, 60.00

(2 NCH₂), 72.45 (CH₂-c), 107.20 (CH-7), 115.18, (CH-b), 133.32 (C_q-

3a), 135.10 (CH-6), 140.90 (C_q-7a), 143.01 (CH-4), 199.96 (C_q-2). MS

(EI, 70 eV, 165 ^ꢁC): m/z (%) ¼ 446 (5) [M(¹¹⁰Pd)þH]^þ, 444 (9)

[M(¹⁰⁸Pd)þH]^þ, 442 (8) [M(¹⁰⁶Pd)þH]^þ, 441 (6) [M(¹⁰⁵Pd)þH]^þ,

370 (32), 368 (65), 367 (17), 366 (79), 365 (62), 364 (29), 259 (100)

[M^þ-AllylPdCl)], 204 (52), 189 (64). HRMS (EI): C₁₉H₃₀ClN₃Pd, calcd.

for strongest peak of [M]^þ441.1157; found: 441.1163; the observed

isotopic pattern ﬁts with the calculated pattern.

b) Compound 4pc was also obtained in analogy to 4pb from the

silver complex 7pc (250 mg, 0.62 mmol) and [Rh(1,5-COD)Cl]₂

(153 mg, 0.31 mmol) in DCM (20 mL). The mixture was stirred in

the dark at room temperature for 12 h and ﬁltered to give a clear

yellow solution. Removal of solvent under vacuum gave 300 mg

(96%) crude product. A small amount of residual 7b was removed by

column chromatography on silica gel using DCM. The NMR data

correspond to those given above.

4.15. Preparation of bis(1,3-dineopentyl-imidazolino[4,5-c]pyridin-

2-ylidene)palladium(0) (6pc)

A

solution of bis(tetramethyldivinyldisiloxane)palladium(0)

(8e10% Pd(0)) (0.4 mL, 0.52 mmol) was added to a solution of 1pc

(280 mg, 1.03 mmol) in hexane at room temperature. Hexane was

partially removed in vacuum and the concentrated solution was

stored at ꢀ20 ^ꢁC for 3 days. The resulting crystals were ﬁltered off

and washed several times with hexane to give 205 mg (64%) of

brown crystals of 6pc. ¹H NMR (D₈-THF, ref. solvent 1.72 ppm):

4.13. Preparation of (h

³-Allyl)(1,3-dineopentyl-imidazolino[4,5-b]

pyridin-2-ylidene) palladium(II) chloride (5pb)

Potassium hydride (78 mg, 1.94 mmol) was added at room

temperature to a suspension of 2pb_PF6(400 mg, 0.99 mmol) in THF

(25 mL). The mixture was stirred for 3 h, ﬁltered to remove potas-

sium salts, and [Pd(Allyl)Cl]₂(161 mg, 0.44 mmol) was then added

as a solid at 0 ^ꢁC. After stirring overnight at room temperature and

ﬁltration the solvent was removed under vacuum. The residue was

washed several times with hexane and then crystallized from a

concentrated solution in THF by overlayering with hexane yielding

334 mg (86%) of orange-red solid 5pb ¹H NMR (CD₂Cl₂, ref. solvent

d

¼ 1.06 (br s, 18 H, CMe₃), 1.08 (br s, 18 H, CMe₃), 4.26 (br s, 4 H,

NCH₂), 4.31 (br s, 4 H, NCH₂), 7.50 (d, ³J ¼ 5.7, 2 H, H7), 8.31 (d,

³J ¼ 5.7 Hz 2 H, H6), 8.80 (s, 2 H, H4). ¹³C{¹H} NMR (D₈-THF, ref.

solvent 25.2 ppm):

d

¼ 29.01, 29.06 (2 CMe₃), 34.69 (CMe₃), 34.92

(CMe₃), 59.11, 59.49 (2 NCH₂), 107.30 (CH-7), 133.69 (C_q-3a), 135.01

(CH-6), 141.02 (C_q-7a), 142.96 (CH-4), 202.26 (C_q-2). Elemental

analysis: Calcd. for C₃₂H₅₀N₆Pd (625.20): C 61.48, H 8.06; found: C,

61.51, H, 8.43.

5.32 ppm):

d

¼ 1.08 (s br, 2 CMe₃), 2.56 (d br, ³J ¼ 11.6 Hz, ⁸/₉H,

CH_trans-a), 3.06 (d br, ³J ¼ 13.2 Hz, ¹/₉H, CH_trans-a), 3.31 (d,

³J ¼ 13.6 Hz, 1 H, CH_trans-c), 3.57 (d br, ³J ¼ 5.3 Hz, ca. 1H, CH_cis-a),

4.23 (dd, ³J ¼ 7.5, J ¼ 2.0 Hz, ca. 1H, CH_cis-c), 4.45 (d br, ³J z 6 Hz,

minor signal, CH_cis)), 4.0e4.3, 4.5e4.9 (2 superimposed vbr s, 2

NCH₂), 4.36, 4.47 (2 s, minor signals, NCH₂), 5.37 (m, 1 H, CH_b), 7.23

(dd, ³J ¼ 7.9, 4.9 Hz, ⁸/₉H, 6-H), 7.24 (superimposed dd, ³J ¼ 8.1,

4.7 Hz, ¹/₉H, 6-H), 7.76 (dd br, ³J ¼ 7.9, ⁴J ¼ 1 Hz, 1 H, 7-H), 8.32 (dd,

³J ¼ 4.7, ⁴J ¼ 1.3 Hz, ¹/₉H, 5-H), 8.35 (dd, ³J ¼ 4.9, ⁴J ¼ 1.5 Hz, ⁸/₉H, 5-

H); assignment of allyl protons in analogy to ref. [28]. ¹³C{¹H} NMR

4.16. Preparation of bis(1,3-dineopentyl-imidazolino[4,5-b]pyridin-

2-ylidene)silver triﬂate (3pb)

Silver triﬂate (198 mg, 0.77 mmol) was added to a THF solution

of carbene 1pb (400 mg, 1.54 mmol) at room temperature. The

mixture was stirred for 14 h and ﬁltered. After removal of the sol-

vent from the ﬁltrate under vacuum the residue was washed

several times with hexane. It was soluble in DCM and crystallized

from a concentrated solution by cooling (ꢀ5 ^ꢁC), with slow addition

of hexane; yield 480 mg (80%) colorless crystals of 3pb. Selected

bond lengths and angles are compiled in Table 1, crystal data in

(CD₂Cl₂, ref. solvent 53.8 ppm):

d

¼ 29.35 (2 CMe₃), 33.98, 34.38 (2

CMe₃), 51.21 (CH₂-a), 57.68 (br, 1-NCH₂), 59.98 (vbr, 3-NCH₂), 72.23

(CH₂-c), 115.01 (CH-b), 118.16 (CH-6), 119.64 (CH-7), 128.07 (C_q-7a),

143.56 (CH-5), 147.94 (C_q-3a), 197.94 (C_q-2). MS (EI, 70 eV, 165 ^ꢁC):

m/z (%) ¼ 446 (4) [M(¹¹⁰Pd)þH]^þ, 444 (8) [M(¹⁰⁸Pd)þH]^þ, 442 (8)

[M(¹⁰⁶Pd)þH]^þ, 441 (5) [M(¹⁰⁵Pd)þH]^þ, 370 (30), 369 (14), 368 (61),

366 (70), 365 (56), 364 (27), 260 (24), 259 (100) [MꢀallylPdCl)]^þ,

Table 5. ¹H NMR (CD₂Cl₂):

d

¼ 1.12 (s, 18 H, CMe₃), 1.13 (s, 18 H,

CMe₃), 4.34 (s, 4 H, NCH₂), 4.46 (s, 4 H, NCH₂), 7.46 (dd, ³J ¼ 8.3,

4.7 Hz, 2 H, H6), 8.03 (dd, ³J ¼ 8.3, ⁴J ¼ 1.3 Hz, 2 H, H7), 8.55 (dd,

³J ¼ 4.7, ⁴J ¼ 1.3 Hz, 2 H, H5). ¹³C{¹H} NMR (CD₂Cl₂):

d

¼ 28.86 (2

CMe₃), 33.96, 34.27 (2 CMe₃), 58.75 (d, ³J_AgC¼ 1.1 Hz,1-NCH₂), 61.65

F. Ullah et al. / Journal of Organometallic Chemistry 890 (2019) 43e57

55

3

(d, J_AgC¼ 1.4 Hz, 3-NCH₂), 120.05 (CH6), 121.61 (CH7), 121.33 (q,

34.30 (CMe₃), 60.46, 60.92 (2 NCH₂), 107.61 (CH7), 132.23 (C_q3a),

135.78 (CH6), 139.94 C_q7a), 143.46 (CH4), 195.82 (dd, ¹J_AgC¼ 273.1,

236.1 Hz, C2). MS (EI, 70 eV, 345 ^ꢁC): m/z (%) ¼ 376 (3), 363 (7). 361

(5), 261 (6), 260 (17) [M^þ-AgCl], 241 (11), 229 (16), 203 (27) [M^þ-

tBu-AgCl], 199 (10), 185 (12), 135 (100), 133 (2), 120 (12), 77 (18), 43

(23). Elemental analysis: calcd. for C₁₆H₂₆AgClN₃(403.72): C, 47.60,

H, 6.49, N, 10.41; found: C, 47.79, H, 6.42, N, 10.34.

¹J_FC¼ 321.2 Hz, CF₃SO^ꢀ₃), 127.16 (d, J_AgC¼ 5.4 Hz, C_q7a), 145.93

3

1

(CH5), 146.83 (d, J_AgC¼ 7.3 Hz, C_q3a), 192.15 (2 d, J_AgC¼ 189.4,

218.8 Hz, C-2). MS (EI, 70 eV, 345 ^ꢁC): m/z (%) ¼ 368 (3) [L¹⁰⁹Ag]^þ,

366 (3) [L¹⁰⁷Ag]^þ, 260 (100) [L þ H]^þ, 259 (17), 258 (12), 244 (23),

203 (78), 174 (22), 133 (65), 132 (39), 120 (30), 43 (49). Elemental

analysis: calcd. for C₃₃H₅₁AgF₃N₆O₃S (776.73): C 51.03, H 6.62;

found: C 51.06, H 6.68.

4.20. Preparation of bis(1,3-dineopentyl-imidazolino[4,5-b]pyridin-

4.17. Preparation of bis(1,3-dineopentyl-imidazolino[4,5-c]pyridin-

2-ylidene)palladium dichloride (8pb)

2-ylidene)silver triﬂate (3pc)

The silver complex 7pb (260 mg, 0.64 mmol) was dissolved in

DCM (15 mL), and Pd(CH₃CN)₂Cl₂(84 mg, 0.32 mmol) was added.

The mixture was stirred in the dark at room temperature for 12 h,

then ﬁltered to give a clear yellow solution. The solvent was partly

removed, and the saturated solution was stored at ꢀ20 ^ꢁC for 24 h

yielding 164 mg (73%) yellow crystals of 8pb ¹H NMR (CD₂Cl₂):

Silver triﬂate (218 mg, 0.84 mmol) was added to a THF solution

of carbene 1pc (440 mg, 1.70 mmol) at room temperature and

stirred overnight. Workup as described for 3pb gave 535 mg (81%)

colorless solid 3pc, soluble in DCM. XRD analysis (Cu K_a) of a small

crystal, obtained from DCM/diethyl ether, displayed considerable

disorder but conﬁrmed the structure qualitatively. ¹H NMR

d

¼ 1.26 (s, 36 H, 4 CMe₃), 4.905, 4.913 (2 s, each 4 H, NCH₂), 5.006,

(CD₂Cl₂):

d

¼ 1.12 (s, 18 H, CMe₃), 1.15 (s, 18 H, CMe₃), 4.36 (s, 4 H,

5.013 (2 s, each 4 H, NCH₂), 7.22 (dd, ³J ¼ 8.2, 4.8 Hz, 2 H, H6), 7.80

(dd, ³J ¼ 8.2, ⁴J ¼ 1.2 Hz, 2 H, H7), 8.34 (dd, ³J ¼ 4.8, ⁴J ¼ 1.0 Hz, 2 H,

NCH₂), 4.42 (s, 4 H, NCH₂), 7.63 (dd, ³J ¼ 5.7, ⁵J ¼ 0.8 Hz, 2 H, H7),

8.59 (d, ³J ¼ 5.7 Hz, 2 H, H6), 9.02 (br, 2 H, H4). ¹³C{¹H} NMR

H5). ¹³C{¹H} NMR (CD₂Cl₂):

d

¼ 29.83, 29.84, 29.84, 29.87 (3 s, 4

(CD₂Cl₂):

d

¼ 28.78 (CMe₃), 28.82 (CMe₃), 34.07 (CMe₃), 34.26

CMe₃), 34.16, 34.18, 34.60, 34.61 (4 s, CMe₃), 57.87, 57.90 (2 s, 3-

NCH₂), 60.03 (br, 2 1-NCH₂), 118.14 (br, 2 CH6), 120.17, 120.21 (2 s,

2 CH7), 127.93 (2 C_q7a), 143.64 (2 CH5), 147.81 (2 C_q3a), 185.61

(2C2). Calcd (%) for C₃₂H₅₁Cl₂N₆Pd: 695.26 (100), 697.26 (98). MS

(FAB pos., NBA): m/z (%) ¼ 697 (70), 695 (70), 662 (60), 660 (100),

658 (95), 626 (60), 624 (95), 622 (45).

(CMe₃), 60.71 (3-NCH₂), 61.21 (1-NCH₂), 107.95 (CH7), 121.24 (q,

¹J_FC¼ 321.4 Hz, CF₃SO^ꢀ₃), 132.24 (d, J_AgC¼ 6.3 Hz, C_q3a), 136.13

1

(CH6), 140.07 (d, J_AgC¼ 6.3 Hz, C_q7a), 143.98 (CH4), 193.94 (2d,

¹J_AgC¼ 190.5, 219.7 Hz, C2). MS (EI, 70 eV, 345 ^ꢁC): m/z (%) ¼ 368 (5)

[L¹⁰⁹Ag]^þ, 366 (5) [L¹⁰⁷Ag]^þ, 260 (94) [L þ H]^þ, 259 (25), 258 (12),

203 (100), 174 (25), 133 (74), 132 (42), 120 (37), 43 (60). Elemental

analysis: calcd. for C₃₃H₅₁AgF₃N₆O₃S (776.73): C 51.03, H 6.62;

found: C 51.14, H 7.08.

4.21. Preparation of (1,3-dineopentyl-imidazolino[4,5-c]pyridin-2-

ylidene)palladium chloride (9pc)

4.18. Preparation of (1,3-dineopentyl-imidazolino[4,5-b]pyridin-2-

ylidene)silver chloride (7pb)

Pd(CH₃CN)₂Cl₂(193 mg, 0.74 mmol) was added to the silver

complex 7pc (300 mg, 0.74 mmol), dissolved in DCM (15 mL). The

mixture was stirred in the dark at room temperature for 12 h,

then ﬁltered to give a clear yellow solution. The solvent was

partly removed and the saturated solution was stored at ꢀ20 ^ꢁC

for 24 h. Rapid ﬁltration and drying of the precipitate under

vacuum gave 250 mg (77%) 9pc as yellow powder. The product

was insoluble in CD₂Cl₂and in d₈-THF and sparingly soluble in

moist DMSO‑d₆. The latter solution displayed NMR signals of two

closely related complexes (ca. 4:6) with most of their signals

broad and superimposed. ¹H NMR (DMSO‑d₆, ref. solv. 2.50):

Ag₂O (392 mg, 1.71 mmol) and freshly activated molecular sieve

(3 Å) was added to a suspension of 2pb_Cl(500 mg, 1.69 mmol) in

DCM (20 mL). The suspension was stirred for 24 h at room tem-

perature and then ﬁltered. Concentration of the ﬁltrate under

vacuum (to ca. 4 mL) and addition of hexane (5 mL) led to a white

precipitate. This was separated and recrystallized from DCM to give

570 mg (84%) of 7pb. Selected bond lengths and angles are

compiled in Table 1, crystal data in Table 5. ¹H NMR (CD₂Cl₂):

d

¼ 1.08 (s, 9 H, CMe₃), 1.09 (s, 9 H, CMe₃), 4.29, 4.41 (2 s, 4 H, NCH₂),

d

¼ 1.19 (br s, 0.4 ꢂ 9 H, CMe₃), 1.30 (br s, 0.6 ꢂ 9 H, CMe₃), 4.94

7.36 (dd, ³J ¼ 8.2, 4.8 Hz, 1 H, H6), 7.88 (dd, ³J ¼ 8.2, ⁴J ¼ 1.3 Hz 1 H,

(superimposed br, NCH₂), 5.05, 5.15 (superimp. br m, NCH₂), 8.06,

8.13 (2 vbr unresolved d, H7), ca. 8.6, 8.69 (2 vbr unres. m, H6),

9.23 (vbr, ca. 0.5 H, H4), 9.35, 9.45 (vbr, 0.5 H, H4); 5.75 (s, 0.25

H7), 8.47 (dd, ³J ¼ 4.8, ⁴J ¼ 1.3 Hz, 1 H, H5). ¹³C{¹H} NMR (CD₂Cl₂):

d

¼ 28.84 (CMe₃), 28.88 (CMe₃), 33.94, 34.28 (2 CMe₃), 58.47 (1-

NCH₂), 61.37 (3-NCH₂), 119.30 (CH6), 120.88 (CH7), 127.03 (C_q7a),

CH₂Cl₂). ¹³C{¹H} NMR (DMSO‑d₆, ref. solv. 39.5):

d

¼ 28.95 (CMe₃),

3

145.16 (CH5), 146.97 (d, J_AgC¼ 6.2 Hz, C_q3a), 193.70 (2 d,

29.16 (st, CMe₃), 29.40 (CMe₃), 33.43 (CMe₃), 33.66 (CMe₃), 33.90

(CMe₃), 34.02 (CMe₃), 58.0e59.2 (br, NCH₂), 108.5 (br, CH7),

131.45, 131.56, 136.67, 137.23, 140.5 (br), 143.62, 143.93, 188.7 (br,

C2); 54.87 (CH₂Cl₂). MS (EI, 70 eV, 370 ^ꢁC): m/z (%) ¼ 401 (4)

[LPdCl]^þ, 243 (27), 242 (23), 178 (70), 169 (49), 113 (36), 105 (35),

43 (100). Elemental analysis: calcd. for C₁₆H₂₅N₃PdCl₂(436.72):

C, 44.00, H, 5.77; found: C, 43.02 (incomplete C combustion), H

6.05.

¹J_AgC¼ 270.6, 234.7 Hz, C-2). MS (EI, 70 eV, 200 ^ꢁC): m/z (%) ¼ 363

(0.2), 261 (8), 260 (43) [M^þ-AgCl], 259 (15), 203 (70), [M^þ-tBu-

AgCl], 174 (19), 135 (21), 133 (59), 132 (35), 120 (27), 43 (100).

Elemental analysis: calcd. for C₁₆H₂₆AgClN₃(403.72): C, 47.60, H,

6.49, N, 10.41; found: C, 47.59, H, 6.50, N, 10.57.

4.19. Preparation of (1,3-dineopentyl-imidazolino[4,5-c]pyridin-2-

ylidene)silver chloride (7pc)

4.22. Preparation of (h

³-allyl)(1,3-dineopentyl-benzimidazolin-2-

Complex 7pc was prepared in analogy to 7pb from Ag₂O

(392 mg, 1.71 mmol) and 2pc_Cl(500 mg, 1.69 mmol). Recrystalliza-

tion from DCM/hexane gave 520 mg (76%) colorless crystals.

Selected bond lengths and angles are compiled in Table 1, crystal

ylidene)palladium chloride (5b)

Allylpalladium chloride dimer (53 mg, 0.145 mmol) was added

to a solution of 7b (115 mg, 0.286 mmol) in DCM (5 mL) and stirred

for 4 h at room temperature. Then the mixture was ﬁltered; the

residue was washed with the same solvent and the ﬁltrate

data in Table 5. ¹H NMR (CD₂Cl₂):

d

¼ 1.09 (s, 9 H, CMe₃), 1.11 (s, 9 H,

CMe₃), 4.31, 4.38 (2 s, 4 H, NCH₂), 7.50 (dd, ³J ¼ 5.7, ⁵J ¼ 0.9 Hz, 1 H,

H7), 8.51 (d, ³J ¼ 5.7 Hz 1 H, H6), 8.93 (d, ⁵J ¼ 0.8 Hz, 1 H, H4). ¹³C

concentrated to give 62 mg (ca. 98%) 5b. ¹H NMR (CD₃OD):

d

¼ 1.10

{¹H} and DEPT-135 NMR (CD₂Cl₂):

d

¼ 28.82 (2 CMe₃), 34.09 (CMe₃),

(18 H, 2 CMe₃), 2.67 (d br, ³J ¼ 12.1 Hz, ca. ²/₃H, H_trans-a), 3.30 (d,

56

F. Ullah et al. / Journal of Organometallic Chemistry 890 (2019) 43e57

³J ¼ 12.1 Hz, ca. ¹/₃H, H_trans-a), 3.34 (d, superimposed by solvent,

³J z 13 Hz, H_rans-c), 3.68 (d br, ³J z Hz, ca. ²/₃H, H_{cis -a}), 3.68 (d br,

³J ¼ 6.8 Hz, ca. ¹/₃H, H_{cis -a}), 4.23 (dd, ³J ¼ 7.6, J ¼ 1.0 Hz, 1 H, H_cis-c),

4.0e4.9 (superimposed vbr s, 2 NCH₂), 5.49 (m, 1 H, CH_b), 7.31

(m_AA’, 2 H, H-4/7), 7.64 (m_BB’, 2 H, H-5/6). ¹³C NMR (CDCl₃):

signals of unconverted 2b_PF6and the complexes 10b and 11b.

4.25. Procedures for Suzuki-Miyaura coupling of p-bromotoluene

with PhB(OH)₂

d

¼ 29.82 (2 CMe₃), 35.02 (2 CMe₃), 52.06 (CH₂a), 59.84 (2 NCH₂),

a) Procedure with in-situ catalyst generation. A Schlenk ﬂask

equipped with magnetic stirrer was charged with Pd(OAc)₂(7 mg,

30 mmol), the respective annulated imidazolium salt (30 or 60 mmol,

73.38 (CH₂c), 116.40 (CHb), 113.37 (CH4, CH7), 123.69 (CH5, CH6),

136.79 (C_q3a, C_q7a), 194.44 (C2). Elemental analysis: Calculated for

C

₂₀H₃₁ClN₂Pd (441.35): C 54.43, H 7.08; found: C 54.12, H 6.85.

cf. Table 4) and dry K₃PO₄(425 mg, 2 mmol), Cs₂CO₃(652 mg,

2 mmol) or KH (80 mg, 2 mmol). After repeated evacuation and

reﬁlling with argon, 1,4-dioxane (5 mL) was added and the mixture

heated with stirring for 30 min at 80 ^ꢁC. Then the ﬂask was cooled

to r.t., a solution of p-bromotoluene (171 mg,1 mmol) and PhB(OH)₂

(183 mg, 1.5 mmol) in 1,4-dioxane (5 mL) was added, and heating

with stirring continued for 16 h at 80 ^ꢁC. After cooling to r.t., the

reaction was quenched with aq. NH₄Cl, the product extracted with

3 x 10 mL diethyl ether, the extract concentrated under slightly

reduced pressure and analyzed by GC against nonane or tridecane

(1 mmol) as internal standards. The results are compiled in Table 4.

b) Procedure with complexes as precatalyst. Aliquots of a solution of

4.23. Preparation of trans-Bis(1,3-dineopentyl-benzimidazolin-2-

yliden)palladium(II) chloride (8b)

a) (2,5-Norbornadiene)palladium chloride (167 mg, 0.62 mmol)

and two equivalents of 7b (498.2 mg, 1.24 mmol) were dissolved in

DCM (10 mL) and stirred overnight at room temperature. The

mixture was then ﬁltered, the precipitate washed with DCM (2 x

5 mL) and the solvent and volatile components removed from the

ﬁltrate under vacuum to give 422 mg (98%) pale yellow solid. ¹H

NMR (CD₂Cl₂):

d

¼ 1.26 (s, 36 H, CMe₃), 4.91 (s, 8 H, NCH₂), 7.25 (m,

4 H, H5/H6), 7.51 (m, 4 H, H4/H7). ¹³C{¹H}-NMR (CD₂Cl₂):

d

¼ 29.99

5 pb, 5pc, 5b, 5ph or 8b corresponding to 30 mmol (3 mol%) in 1,4-

(CMe₃), 34.62 (CMe₃), 59.18 (NCH₂), 112.70 (CH4/7), 122.42 (CH5/6),

135.82 (C_q3a/7a), 182.57 (C2).

dioxane were added to a mixture of K₂CO₃(277 mg, 2 mmol) or

K₃PO₄(425 mg, 2 mmol) with p-bromotoluene (171 mg, 1 mmol)

and PhB(OH)₂(183 mg, 1.5 mmol) in 1,4-dioxane (5 mL), heated for

2 or 16 h at 80 ^ꢁC, and worked up and analyzed as described above;

results see Table 4.

c) GC analysis. Tridecane was added after quenching, nonane

after concentration of the ethereal extract. Sensitivity factors rela-

tive to the internal standard were determined for p-bromotoluene,

p-methyldiphenyl, diphenyl, benzene and toluene and repeated for

each measuring series.

b) ¹H and ¹³C NMR spectra of the product, obtained with a 1:1 M

ratio of [Pd(NBD)Cl₂] and 7b (each 0.12 mmol) under the same

conditions, displayed identical NMR spectra. HRMS (ESI in MeOH):

C

₃₄H₅₂ClN₄Pd (694.13), strongest peaks calcd. for [MꢀCl]^þ

657.2921, 659.2912; found: 657.2892, 659.2903; isotopic pattern in

accordance with calcd. pattern.

4.24. Detection of cis- and trans-bis(1,3-dineopentyl-

benzimidazolin-2-ylidene)palladium(II) acetates (10b and 11b)

4.26. Crystal structure analysis of 4pb, 4pc [9b], 3pb, 7pb, 7pc and

2b_PF6·H₂O

a) Reaction of palladium(II) acetate (108.9 mg, 0.49 mmol) with

7b (389.7 mg, 0.97 mmol) in DCM (10 mL) and workup as described

for 8b provided a solid mixture of 10b and 11b, slightly contami-

nated by two other Pd complexes of this ligand (integral ratio of tBu

¹H NMR signals ca. 5:3:1:1). Attempts to obtain single crystals or to

Data were recorded on a Bruker SMART 1000 CCD (4pb, 4pc,

7pb) or an Oxford Diffraction Xcalibur S diffractometer (3pb), both

using Mo K

Nova O diffractometer (7pc, 2b_PF6$H₂O) using Cu K

¼ 1.54184 Å). Absorption corrections were applied by face

a

radiation (

l

¼ 0.71073 Å), or an Oxford Diffraction

separate the complexes failed. HRMS (ESI in MeCN): L₂Pd(OAc)₂

C

-

a

radiation

₃₈H₅₈N₄O₄Pd (741.31), calcd. for strongest peak of [M ꢀ OAc]^þ

(l

681.3360; found: 681.3356; good accordance of the isotopic pattern

indexing (10pc) or multi-scans. Structures were reﬁned aniso-

tropically on F²using the program SHELXL-97 [29]. Hydrogen

atoms of coordinated double bonds, carbenium H, NH and OH

groups were reﬁned freely but with C-H or O-H distances restrained

equal (where applicable); methyl groups were reﬁned as rigid

groups allowed to rotate but not tip; other H were included using a

riding model. Special details: For compound 4pb, the atoms C24,

C27 and C28 of the COD ring are disordered over two positions, but

the minor position is occupied only to the extent of 8%.

with calc. values; LPd^þpeaks were also observed. 10b (cis) - ¹H

NMR (CD₂Cl₂):

d

¼ 1.26 (s, 4 CMe₃), 1.8e1.9 (vbr, MeCOO), 4.86, 4.94

(2 d, ²J ¼ 14.4 Hz, 4 NCH_AH_B); 7.26 (superimposed m, H-4/7), 7.53

(superimposed m, H-5/6). ¹³C{¹H} NMR (CD₂Cl₂):

d

¼ 22.84

(CH₃CO), 29.50 (CMe₃), 34.37 (CMe₃), 59.20 (NCH₂), 112.93 (CH4/7),

122.70 (CH5/6), 135.50 (C_q3a/7a), 175.73 (MeCOO), 181.58 (C2). 11b

(trans) - ¹H NMR (CD₂Cl₂):

d

¼ 1.25 (s, 4 CMe₃), 1.47 (s, MeCOO,

assignment uncertain), 4.95 (br s, NCH₂), CH₃COO and aryl-H sig-

nals superimposed. ¹³C{¹H} NMR (CD₂Cl₂):

d

¼ 22.89 (MeCOO),

It is important to assign correctly the C and N atoms in the

pyridinic rings. We are reasonably conﬁdent that our assignments

are correct: H atoms of the CH groups are all found in difference

syntheses, whereas there is no residual electron density near the N

atoms; ellipsoids are of normal dimensions; pyridinic bond lengths

at N are signiﬁcantly and consistently shorter; in some cases the CH

group acts as a weak H bond donor and the N atom as an acceptor,

29.78 (CMe₃), 34.04 (CMe₃), 59.25 (NCH₂), 112.40 (CH4/7), 122.26

(C-5/6), 135.43 (C_q3a/7a), 175.80 (MeCOO), 183.75 (C2). Side prod-

ucts (anion unidentiﬁed) - ¹H NMR (CD₂Cl₂):

d

¼ 1.28, 1.29 (2 s,

CMe₃), 4.84, 5.05 (2 d, ²J ¼ 14.4 Hz, NCH_AH_B), 4.91, 4.99 (2 d,

²J ¼ 14.7 Hz, NCH_AH_B); aryl-H signals superimposed. ¹³C{¹H} NMR

(CD₂Cl₂):

d

¼ 29.92, 30.08 (CMe₃), 34.41, 34.57, 34.64 (CMe₃), 59.04,

59.39 (NCH₂), 112.59, 113.02 (CH-4/7), 122.33, 122.79 (CH-5/6),

135.15, 135.61 (C_q-3a/7a), 180.92, 182.71 (C2).

and an H atom added artiﬁcially at N makes unusually short H^…

H

contacts. However, the assignments of light atoms in heavy-atom

structures is always challenging, and a limited amount of disorder

can never be completely ruled out by crystallographic methods

alone.

For crystal data see Table 5. Complete crystallographic data have

been deposited with the Cambridge Crystallographic Data Centre as

supplementary publication no. CCDC 617669 (4pb), 617670 (4pc),

617671 (7pb), 1880308 (3pb), 1880310 (7pc), 1080311 (2b_PF6$H₂O).

b) Formation under conditions of in-situ catalyst generation: A

mixture of 2b_PF6(161.7 mg, 0.4 mmol), Pd(OAc)₂(44.9 mg,

0.2 mmol) and K₃PO₄(191.6 mg, 0.9 mmol) in 1,4-dioxane (10 mL)

was heated for 30 min at 80 ^ꢁC, ﬁltered and the residue washed

with dioxane (2 x 5 mL). After removal of the solvent from the dark

red ﬁltrate the residue was dissolved in CD₂Cl₂for NMR analysis.

The ¹H and ¹³C NMR spectra displayed roughly equally intense

F. Ullah et al. / Journal of Organometallic Chemistry 890 (2019) 43e57

57

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We thank Dr. O. Kühl for the catalytic tests of the (NHC)Pd(Allyl)

Cl complexes, Dr. M. K. Kindermann, B. Witt and W. Heiden for NMR

and LRMS spectra, and Dr. Karch, Umicore, for the gift of Pd(0)

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€

(Universitat Paderborn), Dr. C. Fischer (Leibniz-Institut für Katalyse

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Chem. Eur J. 12 (2006) 3143e3154;

€

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Sommer-Udvarnoki (Georg-August-Universitat Gottingen).

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€

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Appendix A. Supplementary data

Supplementary data to this article can be found online at

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Article Doi

Influence of pyrido-annulation on N,N′-dineopentyl-imidazolin-2-ylidene and associated transition metal complexes; comparison with benzo-, naphtho- and quinoxalino-annulation

DOI: 10.1016/j.jorganchem.2019.04.001

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