Investigations into the reactivity of lithium indenyl with alpha diimines with chlorinated backbones and formation of related functional ligands and metal complexes

Proofs to Dr S. Pascu

Investigations into the reactivity of lithium indenyl with alpha diimines

with chlorinated backbones and formation of related functional ligands

and metal complexes

1

a,b

b

c,d

Chi-Tien Chen,* Mark E. Fischer, Caroline Windsor, Ino C. Vei, David G. Calatayud, Malcolm

b

b,c

L. H. Green and Sofia I. Pascu*

2 2

Reaction between lithium indenyl and a chlorine substituted alpha diimine of the form [{Cl(NPh) }C)] unexpectedly yielded the

corresponding NH rearranged derivative [PhN(H)C(C )] (1) rather than the predicted symmetrical α-diimine. This compound 1 was

9

H

6

2

1

13

1

characterised by H NMR, C{ H} and mass spectrometry, and additionally by X-ray diffraction. It was found that 1 was the first

indene-substituted and symmetric secondary amine which was also highly fluorescent in DMSO. The reactivity of 1 towards simple

inorganic and organometallic transition metals precursors based on the MX

methyl groups, has been investigated. Surprisingly, the reaction with [PtMe

2

fragments, where M = Group 10 metals and X = halides or

(COD)] led to the coupling reaction between the indenyl

2

groups incorporated at the C-C ligand backbone and a new ligand (2) was discovered, in an attempt to synthesise the metal-linked

diamine. Single crystal X-ray diffraction studies confirm this compound 2 to feature coupled indenyl residues and delocalised C-C bonds

in the solid state. Structural authentication by X-ray crystallography showed compound 2 to be a very rare example of flat and extended

aromatic organic molecule and mass spectrometry, IR and NMR spectroscopy were carried out to gain further insight into the solid state

2

and solution phase structures. Further experiments to synthesise analogues of [PhN(H)C(Ind)] aiming to shift a likely equilibrium in

favour the imine tautomer, by introducing bulky ortho substituents onto the benzene ring (R = Me, iPr) showed the presence of the imine

1

tautomer to be increasingly favoured in H NMR spectra, with an increase in the steric bulk of the ortho substituents. However, the

enamine tautomer is still observed to a minor extent even with isopropyl substituents and yields of these desired compounds were low on

steric grounds.

INTRODUCTION

The discovery of the α-diimines led to the preparation of the Brookhart-type Group 10 catalysts, which constituted a paradigm shift

th

in transition metals catalysis in the last decade of the 20 century [1-7]. Bis(imidoyl)chloride, a versatile ligand incorporating both

diimine and dichloride functionalities can be used as a starting material in the synthesis of diimine [8], heterocyclic [9, 10] or

oxalamidine [11, 12] ligands when reacted with the suitable alkyl group, dianions or amines [13]. The steric and electronic effects of

these versatile ligands are known to be programmable through the variations of the substituents on the nitrogen atoms [11] and to the

groups attached to the carbon atoms. The diimine-containing ligands have attracted interest, mainly driven by the discovery that the late

transition metal diimine complexes display efficient activity in catalysing the olefin polymerisation reaction [7, 14-24]. Furthermore, the

well-known metallocene compounds also display excellent catalytic ability in olefin polymerisation reactions [25]. Therefore, studies to

prepare ligands containing the two types of donor groups have been explored [26].

Tuning the electronic and structural properties of the ligand by changing its donor functionalities (in type and / or number), bite

angle, bulkiness and chirality, has marked effects on transition metal catalysis [2, 3, 27, 28]. We describe here the synthesis and

characterisation of a new class of ligands. These were designed such that they may held two pre-catalytic metallic centres in close

proximity to the rigid framework reminiscent of an ansa-metallocene-derivatised bulky diimine. Such system might combine the

different advantages of catalytic processes within early and late transition metal systems to generate new catalysts for coupling reactions,

of interest for both academic and industrial research [29-31].

In spite of some significant advances in the chemistry of transition metals chemistry over the past decade [7, 25, 32-34], including

the reports considered advances in ligand design for heterobimetallic chemistry [35-40], the quest for molecules and materials with

1

Address:

a

National Chung Hsing University, Department of Chemistry 250, Kuo Kuang Rd., Taichung 402, Taiwan R.O.C.

Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX2 6TT

Department of Chemistry, University of Bath, Claverton Down, BA2 7AY, E-mail: s.pascu@bath.ac.uk

Department of Electroceramics, Instituto de Ceramica y Vidrio CSIC, Kelsen 5, Campus de Cantoblanco, 28049 Madrid, Spain

b

c

d

1

catalytic properties thus far unexplored is ongoing, leading to new chemistry of relevance to enhancing our understanding of future

processes for sustainable chemistry applications.

We describe here the synthesis and characterisation of a new class of mixed ligands based on bis(imidoyl)chloride, and the

exploration of their chemistry. Some related design attempts have been made recently involving organic chemistry approaches and led to

the synthesis of a couple of examples of fluorescent molecules with antibacterial applications, but no catalytic applications have been

pursued thus far [41]. Here we also build upon our earlier forays into establishing the synthetic methodology and delineating the

challenges encountered at the synthesis of new organic ligands having the combined the ability of the donor centres to bind to early- or

late- transition metals [31-33, 42]. Since late TM-catalysts produce oligomers or highly-branched polyolefins [25] and early

metallocenes generate linear (non-branched) polyolefins [43], our ligand system designed as shown in Scheme 1, opens up new

prospects for the formation of functional ‘polymers of oligomers’ with properties as yet unobserved.

Scheme 1. Preparation of [2,6-R

2 6 3 9 6 2 2

C H -N(H)C(C H ) ] . Proposed equilibrium between the diimine and dienamine tautomers of

[

ArNC(Ind)] for compound 1 (R=H).

2

RESULTS AND DISCUSSIONS

Synthesis and characterisation of the [C

6 5 9 6 9 6 6 5

H NH-C(C H )-C(C H )-NHC H ], 1

Preparation of lithium indenyl from indene and nBuLi followed an adaptation of the literature methods [9] leading to a highly air-

and moisture-sensitive off-white powder, which was immediately used in the next reaction step. As such, two equivalents of lithium

indenyl in diethyl ether cooled to -78°C were added dropwise to a suspension of one equivalent of [PhN=C(Cl)]

8°C. After 18 hours, while allowing the reaction mixture to reach room temperature, the volatiles were removed and the bright-red

precipitate extracted into CH Cl giving rise to a bright-red powder. This was washed in diethyl ether at -78°C to give the desired

product compound 1 which was deemed analytically pure by elemental analysis and NMR spectroscopy. Interestingly, the compound

2

in diethyl ether at -

7

2

1

13

1

was identified by H and C{ H} NMR and X-ray crystallography as an α-dienamine of the form B (Scheme 1) rather than the expected

α-diimine compound. Similar procedures were adapted and employed for the analogous reactions between indenyl lithium and related

i

chlorine substituted alpha diimines of the form [PhN=C(Cl)]

2

, where R = Me and Pr, however no pure derivative of either type A or B

1

alone could be isolated alone thus far when such bulky substituents were employed. Instead, for R = Me or iPr, VT H NMR

experiments in d-toluene or CD Cl showed that, unlike the case when R = Ph, an inseparable mixture of the isomers A and B, as well as

the related, mixed, imine-enamine derivative (denoted C) were thus far identified as the species present simultaneously in solution for R

2

i

=

Me and Pr (vide infra).

To characterise compound 1, a series of spectroscopic methods were employed and the findings are discussed below. The

1

assignments, made by determination of the coupling constants and of the H- H COSY NMR spectra point out that, instead of the target

9 7 2

compound, the α-diimine form denoted 1A for the [PhNC(C H )] , the α-dienamine tautomer of the desired product Scheme 1B was

obtained.

1

For R=H (compound 1), the H NMR (C

6

D

6

) spectrum of the emerging product exhibits a singlet at δ 6.06, assignable to the protons

1

resonances localised at the two nitrogen centres as it does not couple into any other resonance by the H- H COSY spectroscopy

experiment. The assignment of this resonance as an indenyl C-bound proton would be inconsistent with the diimine structure of the

isomer A given in Scheme 1 (A) which would be expected to exhibit resonances with vicinal coupling. All other proton resonances are

assignable in a manner consistent with the rest of the α-dienamine structure.

2

1

8 2 2

Variable temperature H NMR experiments in either d -toluene, THF or CD Cl showed some slight broadening of the resonances,

but the isolation of the isomer 1A from the mixture could not be demonstrated, and it is currently believed that the equilibrium was

shifted firmly towards the formation of 1B during the reaction process.

1

3

1

6 6

The C{ H} NMR (C D ) spectrum showed 16 signals, in the range δ 144.2 to 118 ppm, consistent with the number of non-

2

equivalent carbon atoms in the compound. All carbons are sp hybridised and either secondary or tertiary and thus fall in a rather narrow

chemical shift range. The infrared spectroscopy of the product recorded in the solid state further supports the hypothesis that the

hydrogens are more likely bound to the nitrogen atoms rather than the indenyl groups. Thus, a sharp, rather intense single peak is

-

1

-1

observed at 3390 cm indicative of an N-H stretch, as well as some broader signals in the 1588 cm region of the spectrum, assignable

to a C=N or electron-delocalized N(H)C group.

1

Figure 1 (a) H NMR in C

6

D

6

(500 MHz) with assignments emerging from H- H COSY and HMBC experiments showing the

Cl are also given in Experimental section.

resonance assignments for compound 1, as isomer B. Detailed assignments for sample in CD

2

3

6 5 9 6 9 6 6 5

Figure 2. The molecular structure of [C H NH-C(C H )-C(C H )-NHC H ] and crystal packing (1B). Hydrogen atoms on the indenyl

and on the phenyl groups are omitted for clarity. The H attached at the N(1) and N(2) were localised in Fourier maps. Ellipsoids at 50%

of probability.

Dark-red crystals of compound 1B suitable for X-ray refinement were grown from a concentrated diethyl ether solution layered with

pentane at -25 ºC. A crystal of dimensions 0.30 x 0.40 x 0.80 mm was chosen under an inert atmosphere, covered with

perfluoropolyether oil, and mounted on the end of glass fibre. The data were collected at 293(2) K and indexed on a monoclinic cell,

consistent with the P21/c space group. The molecular structure is shown in Figure 2 and selected bond distances and angles are listed in

Table 1. Full bond distances and angles are listed in Table S1, ESI. The molecular structure is symmetric and confirms that the hydrogen

atoms are not placed at the expected carbon centres, but rather attached to the nitrogen atoms which are in a secondary amine form, and

overall the compound 1B can be regarded as a dienamine instead of the expected diimine compound. It is likely that the H’s migrated via

a 1,3-shift from the carbon atoms in the indenyl groups during the synthesis. The bond lengths N(1)-C(7) and N(2)-C(23) are found to be

of distances between the typical values for covalent single C-N and double C=N (1.47 Å and 1.27 Å respectively (1.3759(15) Å and

1

.3791(15) Å respectively) [10]. The bond lengths C(7)-C(8) and C(23)-C(24) are closer to the distances found in typical double C=C

bonds (1.3725(16) Å and 1.3652(17) Å respectively) than in single C-C bond (ca. 1.54 Å). The bond lengths of C(9)-C(10), 1.349(2) Å,

and C(25)-C(26), 1.3474(19) Å, are also closer to double C=C bonds than the rest of the carbon-carbon bonds of the indenyl groups. As

expected, the rest of the carbon-carbon distances in the six-carbon ring of the indenyl groups, 1.386(2) to 1.4208(17) Å are having

lengths which place them between single bond and double bond and suggest that there is a strong delocalisation over the relevant six

carbon atoms of this functionality. The bond angles around C(7), 117.14(10) to 122.26(11)º and C(23), 117.35(10) to 121.66(11)º all

2

reflect the sp character of the bridged carbons.

6 3 2 2

Figure 3. The known, related, ligands pTol-BIAN (diamine) [11] and [C H (i-Pr) ] -AIC] [12] showing a mixed imine-enamine form.

These distances can be compared with the diimine compound pTol-BIAN, was fully characterised by Elsevier et al. [11]. The α-

diimine structure has bond lengths of C(7)-N(1) and C(23)-N(2) of 1.267 Å, typical of C=N bonds, and bond lengths C(1)-C(21) and

C(2)-C(41) are 1.480 Å, more typical of C-C bonds. These contrast largely with the bond lengths found in compound 1B,

[

PhN(H)C(C

the equivalent [PhN(H)C(C

of the ligand. The formation of an α-dienamine ligand rather than of an α-diimine was therefore somewhat unexpected. However,

migration” of protons from the carbon to the nitrogen in such compounds has been reported previously [11, 12]. For example, the rigid

compound, [{C (i-Pr) -AIC] (Figure 3), was characterised spectroscopically not as the α-diimine but rather as the imine/enamine

9

H

6

)]

2

. The bond angles C(1)-N(1)-C(7) and N(1)-C(7)-C(23) of 121.7º and 120.4º, respectively, show deviation away from

9

H

6 2

)] values. Thus this comparison with a typical alpha-diimine further confirms the α-dienamine structure

“

6

H

3

2 2

}

tautomer [12]. It is thought that the enhanced stability of this form is due to the greater stability of the endocyclic double bond and the

formation of an intramolecular hydrogen bridge between the amine and imine N atoms.

In an attempt to shift the reaction shown in Scheme 1 towards the formation of the diimine isomer compound 1A, the as-formed

[

2 2

PhN(H)C(Ind)] was treated with [PtMe (COD)]. This reaction, shown in Scheme 2, was carried out in toluene between -78 ºC and

1

room temperature, as discussed in the Experimental section. H NMR spectroscopy of the reaction mixture indicated that no reaction had

occurred. The reaction mixture was therefore kept under reflux for one hour, yielding an orange-brown solid and a grey solid. The grey

solid was shown by elemental analysis and NMR spectroscopy to contain mainly platinum and decomposition material. H- and C{ H}

1

13

1

NMR spectroscopies of the orange-brown solid, together with X-ray crystal diffraction determination, showed formation of compound 2

1

with the formula [PhN(H)C]

2

(C18H

10). The presence of the NH proton is confirmed by a singlet in the H NMR spectrum (δ 5.68) which

4

1

couples to no other protons by the H- H COSY spectroscopy, whilst the IR spectroscopy was inconclusive, showing broad bands both

-

1

-1

in the 3500 cm as well as in the 1500 cm regions.

2 2

Scheme 2. Reaction between [PhN(H)C(Ind)] and PtMe (COD)

2 2

The reflux reaction of [PhN(H)C(Ind)] (1) in toluene for one hour in the absence of [Pt(Me) (COD)] did not result in formation of

compound 1. This suggests that the Pt(II) complex catalyses its formation, leading to Pt(0) and methane elimination. An analogous

carbon-carbon coupling has been reported in the state-of-the-art, for a substituted bis-indenyl system, mediated by a Ti(IV)/Ti(III)

catalyst system [44].

Figure 4 shows the molecular structure of the compound 2, viewed in several different selected views to illustrate its planarity.

Selected bond lengths and angles emerging from the single crystals X-ray diffraction are given in Table 1. For comparison, the bond

lengths and angles of the starting material 1B, [PhN(H)C(Ind)] are also included. The coupled 5-membered rings are co-planar and one

2

NH proton is lying above and the other below their plane. The N(1) - C(7) bond is notably longer than in the starting compound,

showing a decrease in the double bond character as formation of the diimine would result in loss of more aromatic stabilisation energy

for this compound than the starting compound. Its length is still intermediate in value between the double (typically 1.32 Å) and single

(

typically 1.51 Å) bond [10] but indicates that this compound is in an enamine form also seen in the reduced C(1)-N(1)-C(7) bond angle

of 123.14(16)º, with respect to that of the starting material 1B, 127.00(10)º.

5

9 6 9 6

Figure 4. The molecular structure of [PhNH-C(C H )-C(C H )-NHPh] (2) and crystal packing. Hydrogen atoms on the indenyl and on

the phenyl groups are omitted for clarity. The H attached at the N(1) and N(2) were localised in Fourier maps. Ellipsoids at 50% of

probability.

Table 1 Selected bond lengths (Å) and angles (º) for 1B and of the coupled derivative 2.

Product Compound 2

PhN(H)C]

Starting compound 1B

[PhN(H)C(Ind)]

2

[

2

(C18

H

10

)

Bond Lengths (Å)

N(1)-C(1)

1.401(2)

1.422(3)

N(1)-C(1)

1.4122(15)

1.3759(15)

N(1)-C(7)

C(7)-C(8)

C(8)-C(9)

1.396(3)

C(7)-C(8)

1.3725(16)

1.4609(16)

127.00(10)

117.14(10)

122.26(11)

1.405(3)

C(8)-C(9)

o

Bond Angles ( )

C(1)-N(1)-C(7)

123.14(16)

119.38(11)

121.19(17)

C(1)-N(1)-C(7)

N(1)-C(7)-C(7)#1

N(1)-C(7)-C(8)

N(1)-C(7)-C(7)#1

N(1)-C(7)-C(8)

The C(1)-N(1)-C(7) angle is somewhat greater than the 120º value expected for a trigonal planar geometry. The typical values for C-

C single bond of 1.54 Å and C=C double bond of 1.34 Å places the C(7)-C(8) and C(8)-C(9) bonds of intermediate bond length,

suggesting delocalisation of electron density across the molecule as would be expected for aromatic compounds. The differences in

length compared with the equivalent bonds in the starting material reflect the changes in formal single bond and double bond status. This

is seen in the N(1)-C(7)-C(7) #1 and N(1)-C(7)-C(8) bond angles of the two compounds, with a reduced angle for the former and

2

increased angle for the latter parameter in [PhN(H)C(Ind)] the result of the greater space required by the formal double bond character

of C(7)-C(8) of 1.

Given that the organic compounds 1B and 2 both exhibited interesting structures in the solid state, which are incorporating fused

aromatic groups, as well as the fact that they are intensely colored (i.e. red and orange/brown, respectively) in solutions, the optical

characterization of the compounds was carried out. Two-photon fluorescence emission spectroscopy and lifetime measurements were

recorded using time-correlated single photon counting with an excitation wavelength of 910 nm and the emission measured at 360-700 nm

(Figure 5). Fluorescence lifetimes of both compounds 1B and 2 decay as two component systems, with minor components (τ2) in the order

of several nanoseconds (2.0 and 3.5 ns, respectively) and major components (τ1) in the order of several hundred picoseconds (0.3 and 0.3

ns, respectively, see experimental section)

6

Figure 5. Overlays of (a) Two-photon fluorescence emission spectroscopy and (b) time-correlated single photon counting: fluorescence

decay traces and corresponding fitted curves for the lifetime determinations (λex = 910 nm, Compounds 1B and 2, each in 10 mM conc.

in pure DMSO, 5.8 mW).

9 6 2 2

Reactivity of the ligand [PhN(H)C(C H )] towards MX fragments (M = Group 10 transition metal, X = halides or Me)

Once compound 1 was isolated and characterised, several different for further derivatisation were pursued aiming towards the

isolation of new, early-late heterobimetallic metal complexes. Since the labile protons are likely those bound to the nitrogens rather than

C-H, there are two possible reactions on deprotonation and subsequent reaction with a Zr(IV) complex precursor such as Zr(NMe) . It

4

was hypothesised that the negative charge may move to the indenyl ring and therefore allow Zr(IV) co-ordination to form the ansa-

metallocene. Zirconium(IV), in addition, is known to readily form chelating diamide complexes and the presence of an extended

conjugated system may well favour the formation of a α-diamide complex of zirconium. For example, the compound, [1,3-

C

3

H

6

{N(H)SiMe

the β-diamide complex [1,3-C

compound 1 with Zr(NMe in toluene, between -78 C and room temperature over several hours, only formation of a mixture of

3

}

2

], a known β-diamine, when refluxed in a toluene solution of Zr(NMe

2 4

) results in an deamination reaction to afford

3

H

6

{NSiMe Zr(NMe ] [14]. When such synthetic avenues were explored hereby, by reacting

3

}

2

2 2

)

o

2 4

)

1

compounds were obtained, and despite promising analytical data from H NMR the mass spectrometry was difficult to interpret, and

further studies are in progress to establish the identity of the emerging compounds.

9 6 2

The treatment of [PhN(H)C(C H )] with Group 10 transition metal precursors, either based on Ni(II) or Pd(II), first were performed

as the first functionalisation steps, in an attempt to minimise the likely isomers that emerge from the multiple metal derivatisation of the

bis-indenyls observed for Zr(IV) precursors. This approach followed the earlier observations that the known imine/enamine compound

[

{C

6

H

3

(i-Pr)

2

}

9

H

2

-AIC] resulted in the formation of an α-diimine complex on reaction with [NiBr

)] (1B) was therefore reacted in the similar way with [NiBr (DME)] as well as with [(PhCN)

)} NiBr ] or [{PhNC(C )} PdCl ] where it was envisaged that the protons bound to the nitrogen atoms

would be migrating to the indenyl groups. As such, one equivalent of [NiBr (DME)] was treated with one equivalent of

PhN(H)C(C )] in CH Cl and the reaction mixture stirred at room temperature for 18 hours. The volatiles were removed and the

solid washed in diethyl ether and pentane followed by recrystallisation from CH Cl and pentane to give the desired NiBr -chelated

product, denoted 3, (Scheme 3) upon drying in vacuo. The reaction between one equivalent of [(PhCN) PdCl ] and one equivalent of

PhN(H)C(C )] was carried out in the same manner, aiming to obtain the α-diimine form of compound 4, [{PhNC(C )} PdCl ].

2

(DME)] [12]. The ligand

PhN(H)C(C

6

2

PdCl ] to give the

2

corresponding [{PhNC(C

9

H

7

2

9

H

7

2

[

9

H

6

2

[

9

H

6

2

9

H

7

2

7

9 7 2 2 9 7 2 2

Scheme 3 Reaction pathways towards [{PhNC(C H )} NiBr ] (Compound 3) and [{PhNC(C H )} PdCl ] (Compound 4)

Characterisation of compound 3 by elemental analysis shows agreement with the calculated values for [{PhNC(C

H NMR was broad and un-interpretable, which suggests that this is likely a tetrahedral Ni(II) complex (high spin, d , paramagnetic

9

H

7

)}

2 2

NiBr ]. The

1

8

complexes). As the analogous DAB ligand, it is known to be a good π-acceptor and therefore induces a large field splitting that should

favour the square planar geometry. However, relatively bulky substituents, such as bromine, can remove this preference because groups

in a tetrahedral environment are further apart in space than in a square planar geometry. In fact, most [NiBr (DAB)] complexes are

2

tetrahedral due largely to this steric factor, and numerous examples have been reported [1]. Importantly for polymerisation abilities,

+

these complexes once activated by MAO, thus forming [NiMe(DAB)(solvent)] , present a square planar geometry, as the case for the

+

-

derivative [{(2,6-C

of 4, [{PhNC(C

properties of this compound, i.e. to induce paramagnetism. The H NMR spectrum (CD

6

H

3

(i-Pr)

2

)NC(Me)}Ni(Me)(OEt

2 6 5 4

)] [B(C F ) ] [1]. Thus it was expected a tetrahedral geometry for the diimine form

9

H

7

)}

2

NiBr

2

], as well as its isomer incorporating ligand in 1B form. This feature was expected to influence the magnetic

1

2 2

Cl ) shows a set of broad unresolved resonances

between δ 7.3 and 6.1 ppm possibly due to coupling to the paramagnetic centre thus confirming the expectation.

The solid state magnetic moment was determined from magnetic susceptibility studies on a SQUID instrument at a field strength of

8

1

000 G over a range of temperatures, shown in Figure 6. As expected for a pseudo-tetrahedral d complex with a T1 ground state, a large

variation was observed between 0 K, where µeff ≈1.7 BM, to ambient temperatures, where µeff ≈ 3.0 BM. This is because the

perturbation produced is much smaller than kT (where k =Boltzmann constant, T= Temperature in K). The sample contained no

ferromagnetic impurities. This provides further confirmation that the d Ni(II) centre is placed in a tetrahedral environment.

8

+

In the low resolution FAB mass spectrum signals, which exhibited the expected isotropic pattern, were assigned to:

2+

+

[

PhNC(C

9

H

7

)] , [{PhNC(C

9

H

7

2

)} Ni] and [{PhNC(C

9

H

7

2 9 7 2 2

)} NiBr] . The peak for the molecular ion, [{PhNC(C H )} NiBr ] was not

present, but that of a dimer with M/z = 1229.2. The fact that, in the FAB MS spectrum, a peak corresponding to a dimer with M/z =

+

1

229.2 is detected, and the monomeric ion [{PhNC(C

9

H

7

2

)} NiBr

2

] is absent, may point out that the solid state structure of these Ni(II)

complexes correspond to neutral dimers, in which 2 Cl atoms are bridging the two Ni(II) atoms (vide infra).The IR spectrum did not

-

1

confirm the complete loss of the N-H bonds: the diagnostic peak at 3356 cm was much broader than for the free ligand and there is a

-

1

-1

strong, broad band centred at 1594 cm (higher than that observed in the free ligand, at 1588 cm ), linked to a likely C=N bond rather

than a delocalised C-N(H): overall this analytical data is suggesting that although the reaction occurred, the presence of the NH

functionality may well still remain in the final composition, therefore the formation of a Brookhart type complex as the sole isomer

cannot be predicted. Given the good match of the elemental analysis, and the expected mass spectrometry fragments, it is likely that the

compound displays a number of isomers that could not be separated, and likely some of these are supported by the enamine ligand form

1B rather than 1A, analogous to those shown in Scheme 4 and 5 below for Pd(II) analogous systems.

By reacting [PhN(H)C(C

with few exceptions, the co-ordination of Pd(II) is square planar and a search in the CCDC showed that virtually all 4-coordinated DAB-

supported [Pd(II)] complexes are displaying this geometry. Thus it was expected that the target complex [{PhNC(C )} PdCl ] would

be of square planar geometry too. The elemental analysis of the product was consistent with the proposed structure in scheme above for

9 6 2 2 2

H )] with [(PhCN) PdCl ], formation of a Pd(II) complex of square planar geometry was expected, since,

9

H

7

2

+

4

, showing some close agreement with the calculated values. The low resolution FAB mass spectroscopy showed signals, which

2+

+

9 6 9 7 2

exhibited the expected isotropic pattern, that could be assigned to the following species: [PhN(H)C(C H )] , [{PhNC(C H )} Pd] ,

2

] and particularly, that of M+Na ion with correct

isotopic pattern, centred at 635.1/636.1/637.1. However, as before, the presence of a broad peak centred at 3324 cm in the IR spectrum

+

[

{PhNC(C

9

H

7

)}

2

PdCl] and included the molecular ion [{PhNC(C

9

H

7 2

)} PdCl

-

1

does not confirm the complete loss of the N-H bonds and the H NMR (CD

2

Cl

2

) including VT NMR showed a mixture of products.

8

1

3

1

Multiple sets of resonances between 7.7 and 6.1 ppm were observed, together with peaks in the range δ 5.0 to 2.5 ppm. The C { H}

NMR (CD Cl ) spectrum exhibits more peaks than the expected 16 due to the number of inequivalent carbons in the structure given in

2

1

13

1

Scheme 3. No signals due to starting material appear in either spectra. Analysis of the H NMR spectrum, in conjunction with C{ H},

H- H COSY and the H- C HSQC NMR, led to some resonances outside the aromatic region being assigned to protons of several

1

13

3

likely isomers for 4 shown in Scheme 4. A triplet at δ 4.55 ppm is assigned to H

a

, coupling to inequivalent H

and H , coupling with each other and H

2 2 2

50 Hz). The H- C HSQC NMR (CD Cl ) spectrum confirmed the presence of the CH fragment. However, resonances observed that

b c

and H ( Jab, Jac = 7 Hz).

2

A pair of a doublet of doublets are observed at δ 2.77 and δ 2.59 ppm corresponding to H

b

c

a

( Jbc

1

13

=

could not be assigned, in particular a doublet at δ 4.94 ppm and a doublet of doublets at δ 3.18 ppm, led to the following hypothesis.

Temperature / K

9 7 2 2

Figure 6. Variable temperature plot of µeff for [{PhNC(C H )} NiBr ] at 1000 G

The behaviour of acidic protons, such as N-H, is far from predictable in solution, especially in the presence of halides, i.e. from

(PhCN) PdCl ]. Although it has been expected that the N-H protons will migrate to the indenyl ring to give the proposed structure for 4,

a number of side reactions are also possible and Scheme 4 outlines the some of the possibilities backed by the spectroscopic data and

[

2

-

1

literature publications so far. Interestingly, the IR spectroscopy in solid state showed only broad resonances in the 3424 cm region, and

-

1

an intense, double band at 1578.0 cm , which may well account for the formation of the desired diimine compound

{PhN=C(C )} PdCl ]. Di-enamine and diamide complexes of Pd(II) are well known [45] and the possibility of formation of

[

9

H

7

2

oligomers [46], such as the examples given below in Scheme 5, cannot be discounted. It can be seen that all these products mentioned

are consistent with both analytical purity and mass spectrum species observed for Pd complex 4 (Scheme 4), and, for this compound,

1

intense resonances in the H NMR spectrum are assignable to a symmetric species such as the desired α-diimine compound. In view of

the IR spectrum, which shows the presence on N-H bonds the possibility that an impurity in the NMR such as a complex supported by a

coupled ligand, e.g. a coupled complexed related to form 1A may well be formed additionally to the expected products: this should not

be discounted as Pd(II) is known to catalyse the activated C-C bond coupling reactions.

9

H

N

Ph

[(PhCN)PdCl2]

o/n

CH2Cl2, RT

Ph

N

Ph

N

Ph

N

Ph

H

N

Cl

H

Cl

H

Pd

H

N

Ph

N

Ph

N

Ph

N

Ph

Di-enamine Isomer

for Complex 4

Alpha-diimine Isomers

for Complex 4

2 2

Scheme 4: Possible monomeric isomers formed as products of the reaction between [(PhCN) PdCl ] and Ligand 1B and

+

corresponding FAB m/z ion for [M+Na] .

Scheme 5: Possible oligomers for compound 3 and 4: diimine based (type A) and di-enamine based (type B). A further set of isomers

supported by mixed imine-enamine ligands (type C) could also be envisaged, and are not shown, for simplicity.

We were intrigued by the possible structures for (3) and (4) in the absence of single crystals of suitable quality for an X-ray

diffraction study. A detailed search in CCDC database (2016) revealed that there are several example of Ni which exhibit dimeric forms,

10

with two bromine or two chlorine centres bridging the two Ni(II) atoms [47-52] whereby in these dimers the metal centres are penta- or

hexa- coordinated. However, the crystal structures where Ni dimers are held together with 2 Br or Cl atoms bridging the metal centres

and simultaneously tetracoordinated nickel(II) have not been found thus far. In these common examples, five and/or six coordination

positions are occupied by bromide atoms [50-54] or solvent molecules, e.g. DMF or water molecules [47-49] (Scheme 6). This may

point out that the solid state structure of our Ni(II) complexes may well correspond to neutral dimers, in which 2 Br atoms are bridging

the two Ni(II) atoms, and in which the Nickel ions are pentacoordinated, with other bromide atom in the fifth coordination position, such

as [{PhNC(C

9 7 2 2 2

H )} NiBr ] , in analogy with the complexes previously reported and from which the most closely related species are (IV)-

(

VII) in Scheme 6 shown below [50-54]. This is in agreement with the fact that, in the FAB MS spectrum, a peak corresponding to a

+

9 7 2 2

dimer with M/z = 1229.2 is detected, and the monomeric ion [{PhNC(C H )} NiBr ] is absent.

Scheme 6. Examples of related dimeric Ni(II) complexes found in the CCDC: (I) Di-µ-chlorido-bis-[(2,2′-bi-pyridine-

2

0

2

κ N,N′)chlorido(N,N-di-methyl-formamide-κ )nickel(II)] [47]; (II) Di-µ-bromo-bis-[bis-(1,2-diamino-ethane-κ N,N)nickel(II)]

3

dibromide [48], (III) Di-µ-bromo-bis-{aqua-[(5-bromo-1H-imidazol-4-ylmethyl-ene)(2-pyridylmeth-yl)amine-κ N,N',N'']nickel(II)}

dibromide [49], (IV) Di-µ-bromo-bis[N,N’-(2,6-diisopropylphenyl)imino]acenaphthene}bromonickel [50], (V) Di-µ-bromo-bis[N,N’-

(

6-tert-butylphenyl)imino]acenaphthene}bromonickel [51], (VI) Di-µ-bromo-bis[N,N’-(4-tert-butyl-diphenylsilyl-2,6-

diisopropylphenyl)imino]acenaphthene}bromonickel [52], and (VII) Di-µ-bromo-(2,6-bis(1-methylethyl)-N-[(6-bromo-2-pyridinyl)-

methylene]phenylamine)bromonickel [53].

Polymerisation tests for 3 and 4

The metal complexes as-isolated from the reactions shown in Scheme 3 were tested with co-catalyst MAO (methylaluminoxane) for

catalytic activity towards the polymerisation of ethene as described in Experimental section. The activity of the MAO was initially tested

6

-1

using a known ethene polymerisation catalyst, Cp

using as benchmark the well-known Cp ZrCl and MAO as the precatalyst and activator in situ system, which was tested in the same

autoclave. The MAO was thus considered active as this value is sufficiently close to the reported activity of 3.57 x 10 g PE mol M h .

The toluene solution of [{PhNC(C )} NiBr ] in the presence of excess MAO was tested over one hour in the conditions given below.

2 2

ZrCl , with known activity [55]. An activity of 1.411 x 10 g PE mol M h was found

2

6

-1

9

H

7

2

Formation of solid, insoluble polymer was not observed. Deactivation of the catalyst, with an ethanol/HCl mixture, afforded a toluene

layer of oily appearance. A 10 mL sample taken and passed through a pipette containing silica to remove the metal, denoted sample (i).

11

The remainder was reduced on the rotary evaporator to ≈ 15mL and then passed through a pipette containing silica to remove the metal

to generate the fraction denoted sample ii, which was concentrated under the reduced pressure. These samples were analysed using gas

chromatography and the results are given in Table 2. As expected, the species observed in sample (i) with retention times and therefore

boiling points below that of toluene, do not appear in sample (ii), which was the more concentrated GC sample. The species with

retention times of 1.93, 3.75 and 9.35 minutes observed in sample (i) and 9.58 minutes in sample (ii) could infer the presence of a

species’ generated under the catalysis conditions, i.e. ethylene oligomers.

Table 2. Gas Chromatography results * = toluene ** = ethanol. Toluene with retention times of around 6 minutes can be seen in all

samples as the most dominant compound as this was the solvent media used. Ethanol, as part of the quenching solution, is expected and

is observed to have a retention time of around 3 minutes.

Sample Number

Retention

Times

Concentration (%)

(

minutes)

Dry toluene

Ethanol

GC Sample (i)

5.55

3.03

1.93

99.420

100.00

0.037

2

.75

.83**

0.844

0.151

3

6

9

.05*

.35

6.08

.58

97.987

0.021

99.855

0.145

GC Sample (ii)

9

A highly-cited pioneering paper by Brookhart and Svejda [15] describes the ethene oligomerisation properties of a number of α-

diimine nickel(II) catalysts. It has been reported that when α-diimines without bulky ortho substituents at the aryl groups are used, the

resulting Ni(II)-based systems produce ethylene oligomers. Ethene polymerisation [(DAB)NiX

placed in axial sites to the plane of the aryl ring which effectively retard the rate of chain transfer relative to chain propagation. Without

these, in such complexes as [{(C (p-Me))NC(Me)} NiBr ], chain transfer via β-elimination is not retarded, resulting not in polymers

but shorter oligomers. Thus [{PhNC(C )} NiBr ], without bulky aryl ortho substituents, could be expected not to give polymers but

2

] catalysts have bulky ortho substituents

6

H

4

2

9

H

7

2

lower mass oligomers, which in general are soluble in toluene. The preliminary results from the gas chromatography experiment suggest

that this may indeed be the case but further experiments would be required, such as gas chromatography mass spectroscopy, to fully

characterise the activity and products of such systems. The toluene solution of proposed [{PhNC(C H )} PdCl ] with MAO was

9 7 2 2

likewise tested yielding a small amount of black solid, likely palladium metal because of its colour and appearance. The Pd(II) species

isolated previously is only sparingly soluble in toluene. The toluene layer on quenching was analysed in the gas chromatograph showing

only a peak due to toluene with retention time of 6.02 minutes. The inferred lack of activity from this preliminary experiment may be

due to the deficiency of [{PhNC(C H )} PdCl ] in the portion tested, through decomposition or the presence of a mixture of Pd(II)

9 7 2 2

species (see earlier discussion), but may also reflect the fact that palladium analogues of Brookhart-type nickel catalysts are invariably

less active catalysts [56].

2 2 6 3 2 6 3

Towards design and synthesis of bulky analogues [ArNC(Ind)] , Ar = 2,6- Me C H , (5) and 2,6- iPr C H , (6)

As the di-enamine form (1B) of the compound [PhN(H)C(Ind)]

2

had been shown to be the preferred isomer, rather than the diimine

[

PhNC(Ind)] , the synthesis of analogues of this ligand incorporating bulky substituents in the ortho positions of the aryl ring was also

2

attempted, aiming to probe the steric effect on the choice of di-enamine vs diimine formation. It was expected that the imine form of the

compound would be favoured with increase bulkiness of the amine substituent, which in turn would lead to favouring the co-ordination

to metal to form α-diimine complexes. In addition, these bulky substituents are known to favour polyolefin formation rather than

oligomers in ethylene polymerisation catalysis, due to their steric protection of the axial sites modulating the rate of β-elimination of the

growing chain. Two equivalents of lithium indenide were added to one equivalent of [ArN=C(Cl)]

THF (Ar = 2,6- PrC H

6 3

2

in Et

). In both cases, the reaction mixture was stirred for 60 hours starting at -78 ºC, and allowing these to reach the

2 6 3

room temperature, and in both cases the reactions proceeded with yields no higher than ca 20%. For the case where Ar = 2,6-Me C H ,

2 2 6 3

O (Ar = 2,6-Me C H ) or in

i

1

2 2

the H NMR (CD Cl ) spectroscopy showed three sets of resonances assignable to the isomers of types A, B and C of the desired

product and whoch suggested the formation of the bis-indenyl-diimine product alongside enamine-containing species, with the proposed

1

structures shown in Scheme 6. These were found to be in a ratio of approximately 1:2:1 by integration in H NMR spectrum. There were

strong similarities between this NMR spectrum and that of the literature data concerning a recently reported substituted alpha-diimine

[

2 2 6 3

ArN=C)(Fluorenyl)] , Ar = 2,6- Me C H [57]. A variable temperature experiment, performed between room temperature and -40 ºC

1

did not indicate any change of the appearance of the H NMR spectra with temperature, so did not support the theory that these three

species are in exchange within this temperature range.

12

6 3 2 2

Scheme 7 Reaction between {(2,6-C H Me )N=C(Cl)] and LiInd to give compound 5 as a mixture of isomers of types A-C

1

The following H NMR data was used in support of the hypothesis for the presence of the proposed species shown in Scheme 7:

2

d

2

•

For species 5A, a doublet at δ 3.62 ( JHH = 1.5 Hz) was assigned to H , the double doublet at δ 5.85 ( JHH= 6 Hz, JHH = 1.5 Hz)

e

2

f

was assigned to H and the doublet at δ 5.98 ( JHH = 6 Hz) was assigned to H .

f'

2

e'

2

For species 5B, H was found as a doublet at δ 6.40 ( JHH = 6 Hz) and a doublet for H at δ 6.90 ( JHH = 6 Hz). The NH proton

was assigned to a singlet at δ 6.85.

For species 5C, a doublet assigned to the H protons was found at δ 3.40 ( JHH = 2 Hz) and a triplet from H at δ 5.88 ( JHH

.5 Hz).

f''

2

e''

2

=

1

For 5, the triplet resonances at higher field in the H NMR spectrum (δ 2.39, δ 2.27, δ 2.02) were assigned to the methyl protons of

units in species 5A, 5B, and 5C respectively. The aromatic protons of the (2,6-Me ) group of the isomer 5A were found at

2

Me C

6

H

3

2 6 3

C H

2

δ 6.45 (a doublet with JHH=7.5Hz for the meta protons) and at δ 6.53 (a triplet with JHH=7.5Hz for the para hydrogen). The remaining

aromatic protons of the species 5A, 5B, and 5C are found between δ 7.80 and δ 6.60. Separation of the products could not be achieved

due to their similar solubilities in most organic solvents. The presence of the two diimine isomers, 5A and 5C, indicates that the possible

equilibrium of the type shown in Scheme 1 is shifted towards the formation of the imine isomer for bulky isomers, e.g. when R = Me.

However, the presence of some traces of species 5B in solution still suggests that the use of bulky ortho-substituents does not completely

prevent the tautomerisation of the imine to the enamine form during the reaction process, as previously seen for [PhNC(Ind)]

2

(

compound 1), discussed above.

The analogous reaction was carried out in order to synthesise the isopropyl-substitued, bulkier analogue, [(2,6-iPr

2

C

6

H

3

)N=C(Ind)]

2

,

1

denoted Compound 6, as shown in Scheme 8. On removal of solvent from the reaction mixture, a dark green solid was obtained. The H

NMR spectrum of the green solid shows that this is a mixture of the desired product, 6A, together with the dienamine isomer 6B, in the

1

ratio 3:1 by H NMR integration. The H NMR spectrum was assigned using a H- H COSY experiment. An NOE difference experiment

assisted the assignment, showing resonances corresponding to isomers of types 6A and 6B.

13

2 6 3 2

Scheme 8 Reaction between [(2,6-iPr C H )N=C(Cl)] and LiInd to give compound 6 as a mixture of isomers

The following resonances which support the hypothesis for the formation of the two species of types A and B were observed in the

H NMR spectra of compound 6 and correspond to the labelled, diagnostic, protons shown in Scheme 8:

1

k

3

j

3

•

For species A, H was seen as a doublet at δ 3.60 ( JHH=2.2Hz), H was observed as a doublet of doublets at δ 7.18 ( JHH=2.0

3

i

3

Hz, JHH=5.0Hz), and H as a doublet at δ 7.32 ( JHH=5.0Hz)

j

3

i

For species B, H ' was seen as a doublet at δ 6.40 ( JHH=5Hz), a broad peak assigned to H ’ was seen at δ 6.20 (thought to be

broad due to exchange with the NH proton) and the NH resonance observed at δ 6.45 as a singlet which is not seen to be

1

coupled with other resonances in the H- H COSY spectrum

The resonances assigned to the isopropyl CH groups were observed at δ 3.20 and δ 2.90 for species A and B respectively, in the ratio

:1. Methyl resonances were observed as doublets at δ 1.20 and δ 0.80 for species A and B respectively. Other resonances were seen in

3

the aromatic region, between δ 8.10 and δ 6.90, and are assignable to the indenyl groups.

The reactions described above indicate that the substitution of the chlorine atoms on the diimine backbone with indenyl groups

occurs. However, in contrast to the phenyl analogue (R = H, compound 1, isomer 1B), yields are poor, and purification was difficult due

to the similar solubility of the isomers obtained. The additional steric hindrance provided by the substituents on the aryl rings is

presumably sufficiently large to prevent the tautomerisation reaction from occurring in high yield. However, it can be seen that, as the

steric bulk of the ortho-substituents increases, so does the yield of the desired diimine adduct in the reaction mixture: with diisopropyl as

the ortho-substituent, the diimine is the predominant product, with only minor impurities of the dienamine apparent, however the

separation of the isomers remains a challenged to be addressed in further experiments. With the appreciation of the fact that the bulky

compounds 5 and 6 were present as an inseparable mixture of isomers, their reactivity towards a wide range of group 10 metal salts were

2 2

tested under analogous consitions applied for the phenyl analogue. Reactions with [NiBr (DME)], [PdMeCl(COD)], [PdBr ] and

[

PtCl (COD)] in toluene, at room temperature or under reflux, all resulted in the isolation of a mixture of metallated species, as

2

1

evidenced by mass spectrometry and, for the [Pd] and [Pt] species by N NMR spectroscopy. The tautomeric equilibrium expected for

the free ligands did not seem shifted sufficiently towards formation of one single metal complex either monomeric or dimeric in nature,

and our investigations into this rather complex group of molecules are ongoing.

EXPERIMENTAL SECTION

All manipulations were carried out under an atmosphere of dinitrogen using standard Schlenk-line or drybox techniques. The

nitrogen was purified by passage over 4Å molecular sieves and BASF R-311 catalyst. Solvents and solutions were transferred using a

positive pressure of inert gas through stainless steel cannulae. Filtrations were generally performed using modified stainless steel

cannulae fitted with glass fibre filter paper. All glassware and cannulae were dried overnight before use.

14

All preparations and manipulations were carried out under an inert atmosphere of dried nitrogen using either standard Schlenk

techniques or a dry box. Liquids were transferred through stainless steel cannulae (0.5mm-2mm diameter) by excess nitrogen pressure.

Unless otherwise stated, filtration was achieved using such cannulae modified to take a glass-fibre filter at one end. Celite 545

filtration aid (Koch-Light) was pre-dried at 80°C before use. All glassware and cannulae were dried in an oven at 140 ºC.

All solvents were thoroughly deoxygenated before use by repeated evacuation followed by admission of nitrogen. Solvents were

pre-dried over molecular sieves and refluxed over the appropriate drying agent (sodium for toluene and tetrahydrofuran,

sodium/potassium alloy for diethyl ether and n-pentane, calcium hydride for dichloromethane) under an atmosphere of dinitrogen and

collected by distillation. NMR solvents were dried over calcium hydride (dichloromethane, chloroform) at room temperature or molten

potassium (benzene, toluene), distilled under vacuum and stored under dinitrogen in Young's ampoules.

1

13

1

31

1

H, C{ H} and P{ H} NMR spectra were recorded either on Varian Unity 500 or Varian Mercury 300 spectrometers and

referenced internally to the residual solvent peak relative to tetramethylsilane. The spectra were referenced internally to the residual

solvent peak relative to SiMe4 (δ = 0.0). All chemical shifts are quoted in δ(ppm) and coupling constants are given in Hertz (Hz). Infra-

-

1

red spectra were recorded on a 6020 Galaxy Series FT-IR or a Perkin Elmer 1600 Series FT-IR instrument in the range 4000-400 cm .

-

1

Samples were prepared in the glovebox as pellets in caesium iodide, and data are quoted in wavenumbers (ν, cm ). Elementary analyses

data were obtained from the microanalysis department of the Inorganic Chemistry Laboratory. Mass spectra were obtained using the

EPSRC facility of the University of Swansea. Gas chromatography was performed on a Phillips-Pye Unicam P4550 Gas Chromatograph

(

FID). The chromatograph has a 25 m silica tube capillary column with a polar BP20 polyethylene glycol packing and helium was used

as the carrier gas. SQUID analysis was performed on a Quantum Design MPMS-5 Magnetometer. Ferromagnetic contributions to the

results were tested for and excluded and corrections for diamagnetic factors in the magnetic susceptibility were made with reference to

the literature [58].

Two-photon excitation experiments were performed at the Rutherford Appleton Laboratory following the methodology described by

Botchway et al. 2008 [59]. A mode locked Mira titanium sapphire laser (Coherent Lasers Ltd, USA), generating 180 fs pulses at 75

MHz and emitting light at a wavelength of 710-970 nm was used for the 2-photon excitation. The laser was pumped by a solid state

continuous wave 532 nm laser (Verdi V18, Coherent Laser Ltd), with the oscillator fundamental output of 915 ± 2 nm. The laser beam

was focused to a diffraction limited spot through a water immersion ultraviolet corrected objective (Nikon VC x60, NA1.2) and

specimens illuminated at the microscope stage of a modified Nikon TE2000-U with UV transmitting optics. The focused laser spot was

raster scanned using an XY galvanometer (GSI Lumonics). Fluorescence emission was collected and passed through a coloured glass

(BG39) filter and detected by fast microchannel plate photomultiplier tube used as the detector (R3809-U, Hamamatsu, Japan). These

were linked via a Time-Correlated Single Photon Counting (TCSPC) PC module SPC830. Lifetime calculations were obtained using

SPCImage analysis software (Becker and Hickl, Germany) or Edinburgh Instruments F900 TCSPC analysis software.

Single crystal X-ray diffraction.

Data collection

In each case, a single crystal was selected under an inert atmosphere, encased in perfluoro-polyether oil, and mounted on the end of a

glass fibre. Data was first collected on an Enraf-Nonius Mach-3 Serial diffractometer using monochromatic Cu K

α

radiation at λ =

1

.54184Å. Data frames were processed using the RC93 [60] program. The data collections for compounds 1B and 2 were performed

again, using an Enraf-Nonius DIP2000 image plate diffractometer with graphite monochromated Mo Kalpha radiation (λ = 0.71069 Å).

In each case the same structure was obtained, and the best data sets are reported. Corrections for Lorentz and polarisation effects were

performed. For structure solution and full-matrix least-squares refinement the WINGX-v2014 suite of programs was used [61]. All non-

hydrogen atoms were refined with anisotropic displacement parameters. C-H hydrogen atom were placed onto calculated positions and

refined riding on their parent atom. All hetero atom hydrogen atoms have been located in the difference Fourier map and were refined

freely. The program MERCURY [62] was employed for the graphics used hereby. Full experimental details are included in the ESI.

CCDC 1474229 contains the supplementary crystallographic data for compound 1B and CCDC 1474223 contains the supplementary

crystallographic data for compound 2.

Precursors synthesis:

The intermediate Ph

Also, the precursors (2,6-Me

prepared according to literature [65].

2

PC(=O)-C(=O)PPh

2

[63] and PhN=C(Cl)C(Cl)=NPh [64] were prepared via a modification of the literature.

i

2

C

6

H

3

)N=C(Cl)C(Cl)=N(2,6-Me C H ) [57] and (2,6- Pr C H )N=C(Cl)-C(Cl)=N(2,6-iPr C H ) were

2 6 3 2 6 3 2 6 3

Alternative method for the preparation of [PhN=C(Cl)]

Approximately 100 mL toluene was transferred to a 3-necked round-bottomed flask fitted with a condenser containing PCl

.24 mol) and [PhN(H)C(O)] (25g, 0.104mol). The mixture was refluxed for 3 hours affording a yellow solution, which was cooled to

room temperature and filtered to remove insoluble material.

2

5

(50g,

0

2

15

The solvent was removed under reduced pressure until a large amount of bright yellow solid was formed. The suspension was then

filtered to afford a bright yellow solid as the product, [PhN=C(Cl)] . The yellow filtrate was reduced in vacuo enabling more product to

2

be isolated from the resulting suspension. This procedure was repeated again giving overall three portions of [PhN=C(Cl)]

2

; 20.2g, total

1

yield = 69.6 %. H NMR (CD

2

Cl

2

)

6 5 6 5 6 5

δ 7.48 (t, 2H, m-C H ) , δ 7.30 (t, 1H, p-C H ), δ 7.12 (d, 2H, o-C H ) ppm. Elemental Analysis:

Found C, 59.95; H, 3.43; N, 9.84; Cl, 24.35%. Calc. C, 60.67; H, 3.64; N, 10.11; Cl, 25.58%. Selected IR data (nujol): V(C=N): 1664

-

1

cm

Synthesis of Compound 1:

9 6 2 2

Scheme 9: Preparation of [PhN(H)C(C H ) ] , isomer 1B

Method 1

Step1: Preparation of lithium indenide

Indene (25mL, 0.2143mol) was transferred into a large Schlenk vessel and mixed with approximately 500 mL petroleum ether (40 -

6

0°C). The solution was bubbled through with nitrogen to remove any air from the indene used. 75.2 mL of nBuLi (2.5M, 0.188mol)

was added dropwise over 30 minutes and the solution was stirred for a further 18 hours keeping at room temperature. The white

suspension was filtered and washed with 2 x 20mL petroleum ether (40-60°C) giving the off-white lithium indenide on drying in vacuo,

and was used for the next step without further purification.

Step 2: Preparation of the [C

A stirring yellow suspension of [C

a pale-yellow suspension of LiC (6.0g, 49.1mmol) in diethylether (30mL). The reaction mixture was warmed to the room

2

temperature and stirred overnight. After 21 h. of stirring, the solvent was removed and the residue was extracted with CH Cl (30 mL) to

6

H

5

NH-C(C

9 6 9 6 6 5

H )-C(C H )-NHC H ]

6

H

5

N=C(Cl)-C(Cl)=NC

6 5

H ] (2.81g, 24.6mmol) in diethylether (50 mL) at –78 ºC was treated with

9 7

H

2

afford a dark-red solid, which was washed with ether (20mL) followed by pentane (10mL) to afford a red solid. Yield: 4.48g, 60.08%.

Method 2

The compound [PhN=C(Cl)]

Likewise, lithium indenide (4.88g, 0.04mol) was dissolved in 100 mL Et

PhN=C(Cl)] solution vessel. The colour initially went a dark red, but over 18 hours of stirring while allowing to reach room

temperature, became a bright red suspension.

2

(5.54 g, 0.02 mol) was weighed out into a Schlenk vessel and dissolved in 100 mL Et

2

O at -78 ºC.

2

O, cooled to -78 ºC and added dropwise over 30 minutes to the

[

2

The Et

2

O was removed under reduced pressure and the bright red residue extracted into 100 mL CH

2 2

Cl . The solution was filtered

leaving a residue of LiCl, which was re-washed in 25 mL CH

2

Cl , to afford a filtrate containing the product.

2

Solvent was removed from the filtrate leaving a bright red powder. The powder was then washed in approximately 100 mL Et

8 ºC, giving a suspension. This was filtered giving the desired product, a bright red powder as residue. Solvent was removed from the

filtrate, which still contained some of the desired product. The bright red solid was washed in 50 mL Et O at -78°C and filtered, giving

more product as the residue; 3.97g, total yield = 45.5 %. Elemental Analysis: Found (%): C, 87.12; H, 5.91; N, 6.22. Calculated (%): C,

2

O at -

7

2

8

1

8.04; H, 5.54; N, 6.42. Selected IR data (CsI pellet(cm-1)): 3396(NH), 3056m, 1576s, 1489s, 1446s, 1392s, 1350s, 1302s, 1266s,

+

196s, 1152m, 1110m, 1072m, 1026m, 934m, 900m, 866m, 750s, 726s, 690s, 524m, 448s, 414s. Mass spectrometry TOF EI : found

+

1

[

M] : 436.2083. H NMR, C

6 6

D , 500 MHz: 6.07(s, 2H, NH), 6.47(m, 4H, l-Ph), 6.59(d, 2H, J= 5.5Hz, c or d-Ind), 6.66(t, 2H, J= 7.5Hz,

n-Ph), 6.69(d, 2H, J= 5.5Hz, c or d-Ind), 6.78(m, 4H, m-Ph), 7.00(m, 2H, h or g-Ind), 7.13(m, 2H, h or g-Ind), 7.28(d, 2H, J= 7.0, f or i-

1

3

1

Ind), 7.60(d, 2H, J= 7.0, f or i-Ind). C{ H} NMR, C

6 6

D , 125 MHz: 118.5(s, l-Ph), 121.9(s, f or i-Ph), 122.6(s, f or i-Ph), 122.7(s, f or i

+

n-Ph), 124.8(s, g or h-Ph), 127.0(s, g or h-Ph), 127.1(s, c-In), 129.1(s, m-Ph), 130.0(s, d-In), 141.2 (s, k-Ph), 131.0, 134.4, 138.3,

1

6

7

44.2 (4 quaternary carbon, a,b,e,j), H NMR, CD

.96(t, 2H, J= 7.4Hz, n-Ph), 6.72 (d, 2H, J= 5.4Hz, d-Ind), 7.13(m, 4H, m-Ph), 7.18(m, 2H, h-Ind), 7.25(m, 2H, g-Ind), 7.43(d, 2H, J=

.5, f-Ind), 7.82(d, 2H, J= 7.5, i-Ind). C{ H} NMR, CD Cl , 125 MHz: 118.78(s, l-Ph), 121.72 (s, f -Ph), 122.33(s, i-Ph), 122.95 (s, n-

2 2

Cl , 500 MHz: 6.52(s, 2H, NH), 6.67(m, 4H, l-Ph), 6.67 (d, 2H, J= 5.4Hz, c-Ind),

1

3

1

Ph), 124.50 (s, h-Ph), 126.02 (s, d-In), 126.75(s, g-Ph), 129.23(s, m-Ph), 129.95 (s, b, quaternary), 130.01(s, c-In), 134.13 (s, j-In),

16

3

4

3

1

38.68 (s, a, quaternary), 140.87 (s, k-Ph), 143.86 (s, e-In). Coupling constants (Hz): JHm-Hn =7.4, JHl-Hn = 1.1, JHm-Hl = 8.4, JHc-Hd=5.4

4

5

3

4

3

4

5

Hz, JHf-Hd=0.6 , JHi-Hd =0.6 Hz, JHi-Hh =7.6, JHg-Hi=1.1, JHg-Hh = 7.5, JHf-Hg=7.5, JHf-Hh = 1.1, JHf-Hi = 0.7.

Preparation of Compound 2

2 2

Reaction between [PhN(H)C(Ind)] (Compound 1B) and [PtMe (COD)] leading to isolation

of Compound 2 proceeded as follows.

A cooled (-78°C) solution of [PhN(H)C(Ind)]

toluene (50 mL) was added dropwise to a cooled solution (-78°C) of [PdMe

2

(0-1009, 0.2 mmol) dissolved in

(COD)]

2

precursor, (0.076g, 0.2 mmol) in toluene (50mL). The orange-red reaction mixture

was allowed to return to room temperature with stirring overnight, after which there

was no observable colour change. The reaction mixture was refluxed for 3 hours. A

platinum mirror was observed after 1 hour. Filtration of the resultant mixture yielded an

orange solution, which gave an oily orange solid on removal of volatiles under reduced pressure, and a back precipitate likely platinum

particulates. Recrystallisation in pentane (20mL) afforded an oily orange solid from the filtrate and the desired product

1

2

Compound 1 as an orange-brown solid. Yield= 0.045g, 35.4%. H NMR, CD

2

Cl

2

, 500 MHz: 7.60 (d, 1H, H

c

or H

, J = 8.0 Hz) 7.26 (s, 1H, H ), 6.63 (t,

, 125 Mhz: 143 (C

). Elemental Analysis C32

f

, J =

2

7

2

.5 Hz), 7.54 (d, 1H, H

c

or H

f

, J = 7.5Hz), 7.46 (t, 1H, H

d

or H

e

, J = 8.0Hz), 7.36 (t, 1H, H

d

or H

e

b

2

13

H, H

), 138 (C

Found (Calculated) C 87.13 (88.45), H 5.31 (5.10), N 5.25 (6.45). Mass spectrometry TOF EI : found [M] : 434.1793 (calculated

accurate mass for C32 : 434.1783)

h

, J = 7.5Hz), 6.41 (d, 2H, H

g

, J = 7.5Hz), 6.30 (t, 1H, H

i

, J = 7.5Hz), 5.68 (s, 1H, H

a

). C NMR, CD

2

Cl

2

f

or C

c

f

or C ), 136 (C or C

c

d

e

), 130 (C

d

or C

e

), 127 (C

b

), 129 (C ), 122 (Ci), 121 (C ), 110 (C

a

g

h

22 2

H N %

+

24 2

H N

Preparation of [{PhNC(C

Approximately 200 mL CH

PhN(H)C(C )] (2.087 g, 0.0047 mol) The solution was stirred at room temperature overnight. After 18 hours of stirring, the volatiles

were removed to afford an oily brown solid which was washed with 100mL (50 mL in portions) Et O to give a light brown powder. This

powder was redissolved in approximately 25 mL CH Cl and transferred to a Schlenk vessel containing approximately 150 mL pentane.

A precipitate was formed upon addition and the solution was filtered giving as light/brown beige powder as product. Yield: 1.17 g,

9

H

7 2

)} NiBr

2

], Compound 3

2

Cl

2

was added to a round-bottomed flask containing [NiBr

2

(DME)] (1.34 g, 0.0043 mol) and

[

9 6 2

H

2

4

2

1.5%. Elemental Analysis Found (%): C, 58.01; H, 3.85; N, 4.16; Br, 24.78; Ni, 8.33. Calculated (%): C, 58.67; H, 3.69; N, 4.28; Br,

+

4.40; Ni, 8.99. Mass spectrometry TOF EI : found [M] M/z (intensity%): 436(68) [Ligand] , 494/496 (39) [M-2Br] , 573/575 (72)

+

[

M-Br]

Reaction between [(PhCN)

A similar procedure as described in above for compound 3 was followed for the synthesis of 4. The complex [(PhCN)

g, 0.0015 mol) was reacted with [PhN(H)C(C )] (0.726 g, 0.00163 mol) to afford a dark orange-brown powder as the product

denoted compound 4, with the proposed general formula [{PhNC(C )} PdCl ], likely a mixture of diamine/di-enamine isomers, with

the most significant of the H NMR resonances given in the main text. Reaction yield: 0.52g, 56.5%. Elemental Analysis: Found C,

2 2 9 6 2

PdCl ] and [PhN(H)C(C H )] , Compound 4

2

PdCl ] (0.58

H

9 6 2

9

H

7

2

1

+

6

0.52; H, 4.06; N, 4.08; Cl, 10.02; Pd,15.07%. Calc. C, 58.67; H, 3.69; N, 4.28; Cl, 11.55; Pd, 17.33%. Mass spectrometry TOF EI M/z

+

(

intensity%): found [M] : 436(100) [Ligand] , 540/542 (27) [M-2Cl] , 577/579 (28) [M-Cl] 613 (15) [M] .

Polymerisation tests

Ethene was purified by passage through 4 Å molecular sieve. Polymerisations were carried out in a Fischer-Porter reactor equipped

with a magnetic stirrer under conditions similar to those employed by Kaminsky et al. [55] Toluene (200 mL) was transferred into a

Fischer-Porter bottle, which was then connected to the ethene supply and filled with the gas. Methylaluminoxane (MAO) in toluene (9.4

mL) was then added, the ethene pressure increased to 2 bar and the mixture stirred at room temperature until saturated with ethene.

-

6

Meanwhile, 0.6 mL of MAO was added to 10 mL of the catalyst/ toluene solution (6.25 x 10 mol of transition metal in total) and stirred

for 15 minutes pre-activation. The catalyst-MAO mixture was added quickly to the Fischer-Porter bottle, and the mixture stirred

vigorously at room temperature under 2 bar pressure of ethene for 1 hour. The reaction was quenched by venting the ethene and pouring

the contents of the bottle into a conical flask containing 250 mL of 20% (by volume) solution of concentrated HCl in ethanol. After

stirring for 18 hours, the solution was filtered over a sintered glass funnel on a Buchner flask connected to a water aspirator, and

analysed by GC.

2 2 6 3 2 6 3

Approaches towards the synthesis of bulky analogues [ArNC(Ind)] , Ar = 2,6- Me C H , (5) and 2,6- iPr C H , (6)

A pale yellow solution of lithium indenide (1.470g, 12 mmol) in Et

solution of precursor [ArNC(Cl)] , Ar = 2,6- Me (2.005g, 6 mmol) in Et

2

O (30 mL) was added dropwise to a yellow

O (50 mL) at -78 ºC. The reaction mixture was

2

2 6 3

C H

allowed to return to room temperature under stirring, giving a deep orange suspension after 17 hours which turns dark

17

2

orangy-green after a further 60 hours. Removal of volatiles, washing with Et 0 (20 m1) followed by pentane (20m1) affords a bright

orange solution and a brown-green solid. Filtration and removal of the solvent in vacuo gives a bright orange solid, 5. (Yield: 1.129

g). Toluene (20 mL) was added to the remaining solid, yielding small amounts of a green-brown insoluble solid and a dark

1

orange solution. Removal of the solvent in vacuo gives 0.362g of dark orange solid whose H NMR spectrum (CD-2C12)

shows it to be a mixture of starting materials and the desired product (5) as a mixture of isomers.

The preparation of the diisopropyl analogue, 6, was attempted via a similar method using excess lithium indenide

(

2 2 6 3

0.465g, 38 mmol) and the [ArNC(Cl)] , Ar = 2,6- iPr C H diimine (0.800g, 18 mmol) using THF as the solvent. After 60

hours of stirring, a dark green solution was obtained which yielded a solid of the same colour on removal of the solvent.

1

Upon washing with pentane (20 mL), filtration and removal of solvent yields yellow oil (shown by the H NMR spectrum

to be unreacted [ArNC(Cl)] starting material). Washing the remaining solid fraction with toluene (20 mL) afforded a green

solution and some insoluble solid. The mixture was filtered, and solvent removed in vacuo to give a green solid (0.273g) which

was identified by NMR as the desired compound 6, as a mixture of isomers.

Two Photon Fluorescence spectroscopy

The solution measurements of 1 and 2 were carried out in DMSO solutions with concentration of 10 mM. Fluorescence emission

lifetime components data were processed using SPCImage analysis software (Becker and Hickl, Germany) or Edinburgh Instruments F900

2

TCSPC analysis software. The data profile includes the goodness of fit of the decay curves as χ , and the lifetime of each component and

2

their weighting of each component are given in Table 2. The χ value of ca 1.0 means an optimal single exponential fit of this measurement.

2

If the χ value is more than 1.3 there is an incomplete single exponential fit of this measurement, and this was the case here. High χ values

>1.5) mean either significant noise within the TCSPC setup (electronics and/or excitation source) or more than one component decay

profile. Here, for both Compounds 1 and 2 fluorescence lifetimes were found to decay as two component systems, with minor components

) in the order of several nanoseconds and major components (τ ) in the order of several hundred picoseconds (Table 3). The TSPC

(

τ

2

1

spectrum of 2 displays a similar decay behavior to that of 1 and in both cases evidence of some aggregation (in line with the X-ray structural

2

observations, Figures 2 and 4) may account for the extremely short lifetime components present in the decays spectra and the χ of 1.43 and

1.49 respectively, in indicating a reasonable fit for this multi-exponential model which also accounts for the aggregation features, consistent

with the crystallography observations, regarding possibility of aromatic stacking in the solid state as well as the possibility of H-bonding

interactions in solution with DMSO.

Table 3. Lifetime decay constants for compounds 1 and 2 point decay recorded in pure DMSO solutions (10 mM)

2

Compounds

τ

1

/ ns

A

1

%

τ

2

/ ns

A

2

%

χ

1

0.3

87

63

2.0

3.5

13

37

1.43

1.49

2

CONCLUSIONS

We report here our attempts to synthesize a new family of bis(indene) substituted alpha-diimines of the type [(C

from the reaction of chlorine substituted alpha-diimines, (ClC=N-aryl) , with 2 equiv. of lithium indenyl. The ultimate goal was the use

of [(C )C=N-aryl] as ligand precursors to heterobimetallic complexes containing in the same molecule two moieties: (a) a Zr ansa-

9 7 2

H )C=N-aryl] ,

2

9

H

7

2

metallocene part, and, (b) an alpha-diimine ligand coordinated to a group10 metal (Ni, Pd, Pt) counterpart; upon activation, this

bimetallic system could work as catalyst for olefin oligomerisation and polymerisation or copolymerization.

However, the chemistry revealed to be quite complex since dienamine tautomers [(C

R=Me (5), iPr (6)) were obtained instead of the desired [(C )C=N-aryl] . In fact, the reactions of the dienamine tautomers (where

aryl= C ) with metal compounds, such as [Zr(NMe ], [NiBr (DME)], [PdCl (PhCN) ] or [PtMe (COD)], gave rise either to

9 6 2 6 5 2 6 3

H )C-N(H)-aryl] (aryl=C H (1), 2,6-R C H ;

9

H

7

2

6

H

5

2

)

4

2

uncharacterised products (Zr), rather complicated mixtures of metal complexes (Ni and Pd) or a fluorescent pi-extended ring fused

organic diamine resulting from an intramolecular ring coupling of the two indenyl fragments in compound 1 (Pt).

We were fascinated by the fact that the reaction between lithium indenyl and chlorine substituted alpha diimides of the form

[

{Cl(NR)

derivatives quantitatively, rather than the expected symmetrical alpha-diimines and proceeded to investigate the spectroscopic properties

of the resulting compound and its reactivity. The enamine form, B of compound [PhN(H)C(Ind)] was found to be the most kinetically

2 2 6 3 6 3

}C)] (R = Ph, 2,6-MeC H , 2,6-iPrC H ) yielded serendipitously the corresponding di-enamines as the C-N(H) rearranged

2

favoured compound. Such products were characterised by NMR and mass spectrometry, and, in case of R=Ph, additionally by X-ray

diffraction. The compound 1 was further reacted with the organometallic and coordination compound precursors incorporating transition

metals such as Ni(II) and Pd(II) precursors, which gave rise to new complexes featuring both the enamine and imine bonding motifs.

Interestingly, compound 1, as well as its coupled reaction product 2, both show some interesting fluorescence emission properties in wet

DMSO conditions, and their fluorescence emission lifetimes were determined.

18

In summary, a new and highly conjugated flat aromatic derivative with two 5-carbon rings fused in an alternating manner with three

6

-carbon member rings, 2, has been synthesised and structurally characterised both in the solid state and in solution. NMR spectroscopy,

as well as 2-photon fluorescence spectroscopy in solution, provided further evidence for the function and structural forms of 1 and of the

corresponding coupled derivative 2. Single crystal X-ray diffraction studies confirm that 2 is a new flat aromatic organic compound

which features coupled indenyl residues and delocalised C-C bonds in the solid state. This strategy set out initially to give new

heterobimetallic compounds leads to new organic aromatics on a facile and clean lab scale procedure, with potential fluorescence

emissive potentials.

APPENDIX A. SUPPLEMENTARY DATA

CCDC 1474229 contains the supplementary crystallographic data for compound 1B and CCDC 1474223 contains the supplementary

crystallographic data for compound 2. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or

from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:

deposit@ccdc.cam.ac.uk.

ACKNOWLEDGEMENTS

SIP thanks the ERC Consolidator grant ‘O2Sense’ for funding and Balliol College Oxford for a Dervorguilla Scholarship, also the Royal

Society for funding and the EPSRC Natonal Mass spectrometry service at Swansea as well as Colin Sparrow (Oxford) for mass

spectrometry support. CTChen would like to thank the Ministry of Science and Technology of the Republic of China for financial

support (grant number MOST 104-2113-M-005-014).

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[

21

Reaction between lithium indenyl and a chlorine substituted alpha diimine of the form [{Cl(NPh)

rearranged derivative, rather than the symmetrical α-diimine. The reactivity of 1 towards simple inorganic and organometallic transition

metals precursors based on the MX synthons, was investigated. In the case of the reaction with [PtMe (COD)] the coupling reaction

between the indenyl groups incorporated at the C-C ligand backbone was observed.

2 2

}C)] yielded the corresponding NH

2

22

Article Doi

Investigations into the reactivity of lithium indenyl with alpha diimines with chlorinated backbones and formation of related functional ligands and metal complexes

DOI: 10.1016/j.poly.2016.09.021

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Full text of DOI:10.1016/j.poly.2016.09.021

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