Mixed triorganobismuthines RAr₂Bi [Ar = C₆F₅, 2,4,6-(C₆F₅)₃C₆H₂] and hypervalent racemic Bi-chiral diorganobismuth(III) bromides RArBiBr (Ar = C₆F₅, Mes, Ph) with the ligand R = 2-(Me₂NCH₂)C₆H₄. Influences of the organic substituent

Abstract

Triorganobismuthines R(C₆F₅)₂Bi (1) and R[2,4,6-(C₆F₅)₃C₆H₂]₂Bi (2) [R = 2-(Me₂NCH₂)C₆H₄] were synthesized by reaction of RBiBr₂ with C₆F₅MgBr and 2,4,6-(C₆F₅)₃C₆H₂Li, respectively, in a 1 : 2 molar ratio. The Bi-chiral bromides R(C₆F₅)BiBr (3), R(Mes)BiBr (4), and R(Ph)BiBr (5) were obtained from RBiBr₂ and C₆F₅MgBr, MesMgBr or PhMgBr, or from PhBiBr₂ and RLi, in a 1 : 1 molar ratio. The molecular structures of 1–5 were determined by single-crystal X-ray diffraction and are discussed taking into account the chirality of the species. Supramolecular aspects in the solid state are presented. The solution behaviour of the title compounds, including dynamic aspects, are discussed on the basis of multinuclear (¹H, ¹³C, ¹⁹F) NMR spectroscopy.

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Introduction

Bismuth(III) salts and organobismuth(III) compounds continue to receive considerable attention regarding their use in a growing number of applications including reagents involved in various organic transformations, precursors for the development of materials, and catalysis,^1–6 but also fundamental aspects with respect to the M–bismuth (M = transition metals such as Cr, Mo, W, Mn, Fe, Pt, Cu, Ag, Au) interactions in complexes containing organobismuth(III) species as ligands.^7–13 From this point of view the chemistry of bismuth is still far from being exhausted. Owing it to several advantages (e.g. low toxicity, commercial availability of starting materials) the use of bismuth compounds in catalysis and particularly asymmetric catalysis is highly attractive.^2,14

Several different types of chirality can be recognized in heavy group 15 element organometallic compounds. The most often encountered are: (i) M-center chirality in organopnicogen compounds with three different substituents (e.g. R¹R²R³M, RR′MX; M = pnicogen, X = halide); (ii) chelate-induced metal chirality (e.g. R^C,EMR¹₂ or R^C,EMX₂, R^C,E = substituent capable of E→M intramolecular coordination, E = donor atom such as N, P, O, S, etc.); (iii) helical chirality (observed in the solid state for some (R^C,E)₃M compounds when all three substituents are chelating the metal centre); (iv) planar chirality arising from the non-planarity of the chelate ring formed by intramolecular coordination of R^C,E substituents to the metal; (v) substituent- or ligand-chiral compounds (e.g. in compounds containing chiral substituents or ligands).³

Although many reports discuss chiral organophosphorus(III), organoarsenic(III), and organoantimony(III) compounds, solid state structural investigations of chiral organobismuth(III) derivatives are rather scarce.^3,15–22 This has been accounted to a lack of appropriate synthetic protocols and to the greater instability of organobismuthines.²¹ However, since the first reports on chiral organobismuth compounds,^23,24 advances have been made e.g. several strategies that allowed the synthesis of organobismuthines bearing three different substituents were developed,^21,25–29 and the synthesis of optically pure Bi-chiral diastereomers has been achieved.^30,31

Murafuji et al. reported the only optically pure Bi-chiral triorganobismuthine characterized by single-crystal X-ray diffraction, obtained diastereoselectively as Bi-(R) and Bi-(S) diastereomers, namely R^Fc(4-MeC₆H₄)(4-ClC₆H₄)-Bi-(R) and R^Fc(4-MeC₆H₄)(4-ClC₆H₄)-Bi-(S), R^Fc = (S)-2-[(R)-1-(dimethylamino)ethyl]ferrocenyl.³⁰ Bismuth-chiral diorganobismuth(III) halides are better represented in the literature. Examples of compounds structurally characterized in the solid state include, but may not be limited to, R(4-MeC₆H₄)BiCl [R = 2-(Me₂NCH₂)C₆H₄],²⁷ R^Fc(4-ClC₆H₄)BiI (R = (R)-(+)-N,N-dimethyl-1-ferrocenylethylamine),³¹ 2,6-[(4-MeC₆H₄)(Cl)Bi]₂C₆H₃SO₂(t-Bu)³² R^C,NBi(Ph)Cl (R^C,N = [2-(CH=N-2,6-iPr₂C₆H₃)C₆H₄]),³³ R^C,O(4-MeC₆H₄)BiBr (R^C,O = 2-MeC(O)C₆H₄),²⁹ and others.^26,34

A significant number of organobismuth(III) compounds with intramolecular coordination-induced chirality at the Bi centre and planar chirality have been reported to date.^3,4

We report here on the structural characterisation of R(C₆F₅)₂Bi (1), R[2,4,6-(C₆F₅)₃C₆H₂]₂Bi (2), and racemic Bi-chiral diorganobismuth(III) bromides R(C₆F₅)BiBr (3), R(Mes)BiBr (4), and R(Ph)BiBr (5) [R = 2-(Me₂NCH₂)C₆H₄]. An improved method of synthesis for the previously reported compound 5²⁷ and its complete NMR characterisation is also reported in this work.

Results and discussion

Synthesis

The triorganobismuth(III) derivatives R(C₆F₅)₂Bi (1) and R[2,4,6-(C₆F₅)₃C₆H₂]₂Bi (2) were synthesized in the reaction of [2-(Me₂NCH₂)C₆H₄]BiBr₂ with C₆F₅MgBr or 2,4,6-(C₆F₅)₃C₆H₂Li, respectively, in a 1 : 2 molar ratio. The reaction of [2-(Me₂NCH₂)C₆H₄]BiBr₂ with one equivalent of the corresponding organomagnesium bromide afforded the chiral organobismuth(III) bromide, [2-(Me₂NCH₂)C₆H₄](Ar)BiBr [Ar = C₆F₅ (3), Mes (4)] (Scheme 1).

Scheme 1 Preparation of compounds 1–5.

Compound 5 was previously synthesized by reaction of Ph₂BiCl with [2-(Me₂NCH₂)C₆H₄]Li followed by treatment of RPh₂Bi first with BF₃·Et₂O, then with NaCl. The subsequent halide exchange reaction between the resulting R(Ph)BiCl and NaBr gave 5 with an overall yield of 47%.²⁷ We prepared compound 5 by two other routes. The salt elimination reaction between [2-(Me₂NCH₂)C₆H₄]Li and phenylbismuth(III) dibromide in THF (Scheme 1) afforded 5 with a rather low yield (34%). The slow addition of a solution of PhMgBr to a suspension of [2-(Me₂NCH₂)C₆H₄]BiBr₂ in THF improved considerably the yield of 5 (67%).

Compounds 1–5 are colourless crystalline solids, soluble in common organic solvents. Compounds 2, 4 and 5 are air-stable for weeks, while 1 and 3 decompose very quickly when their solutions are exposed to traces of water, with formation of C₆F₅H and poorly soluble (in CDCl₃ or C₆D₆) by-products. In contrast with 1, compound 2 is air and moisture stable although it contains two electron-withdrawing substituents, i.e. 2,4,6-(C₆F₅)₃C₆H₂. The steric protection provided by the two m-terphenyl-like substituents increases the kinetic stability of 2.

Solid state structures

Single-crystal X-ray diffraction studies were carried out in order to establish the molecular structures of 1–5 which are depicted in Fig. 1, 2 and 4–6. Selected molecular parameters are given in Table 1.

Fig. 1 Molecular structure of isomer (A_Bi,pS_N)-1. Thermal ellipsoids are drawn at the 30% probability.

Fig. 2 Molecular structure of 2. Thermal ellipsoids are drawn at the 20% probability. Hydrogen atoms are omitted for clarity.

Fig. 3 Dimer associations between (A_Bi,pS_N) and (C_Bi,pR_N) isomers in the crystal of 1. Hydrogen atoms are omitted for clarity.

Fig. 4 Molecular structure of isomer (A_Bi1,pS_N1)-3a. Thermal ellipsoids are drawn at the 40% probability.

Fig. 5 Molecular structure of isomer (C_Bi,pR_N)-4. Thermal ellipsoids are drawn at the 40% probability.

Fig. 6 Molecular structure of isomer (A_Bi,pR_N)-5. Thermal ellipsoids are drawn at the 40% probability.

Table 1 Selected bond distances (Å) and angles (°) for compounds 1–5

		1	2	3		4	5
				3a	3b
a Non-bonding distance.
	a	2.725(6)	[3.334(16)]^a	2.563(9)	2.511(9)	2.570(5)	2.536(13)
	b	2.363(7)	2.309(11)	2.747(2)	2.807(2)	2.7702(9)	2.789(2)
	c	2.287(7)	2.338(12)	2.352(11)	2.277(11)	2.282(5)	2.241(16)
	d	2.230(7)	2.239(18)	2.250(13)	2.233(9)	2.251(5)	2.224(19)
	a–b	162.1(2)	—	161.3(2)	162.2(2)	165.39(10)	164.6(3)
	a–c	85.5(2)	—	84.5(4)	85.9(3)	87.21(16)	86.9(5)
	a–d	70.9(2)	—	72.5(4)	74.2(4)	72.46(17)	72.8(6)
	b–c	94.4(2)	108.8(4)	88.4(3)	87.0(3)	94.89(13)	89.9(4)
	b–d	91.6(3)	89.3(6)	91.0(4)	90.2(3)	92.94(14)	92.4(5)
	c–d	100.0(3)	104.4(6)	95.3(4)	95.2(4)	99.52(19)	94.0(6)

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The Bi–C bond distances are similar in the heteroleptic bismuthines 1 [Bi(1)–C(1) 2.230(7) Å, Bi(1)–C(10) 2.363(7) Å, Bi(1)–C(16) 2.287(7) Å] and 2 [Bi(1)–C(1) 2.24(2) Å, Bi(1)–C(10) 2.309(11) Å, Bi(1)–C(35) 2.339(12) Å]. However, there are important structural differences between these two compounds. In the molecule of 1 the nitrogen of the pendant arm is strongly coordinated to the metal [Bi(1)–N(1) 2.725(6) Å] trans to a C₆F₅ group [N(1)–Bi(1)–C(10) 162.1(2)°] (Fig. 1) [cf. sum of the van der Waals radii for the corresponding atoms, ∑r_vdW(Bi,N) 3.94 Å].³⁵ The Bi–N distance in 1 is longer than that observed in the related complex containing the Martin ligand [2-(Me₂NCH₂)C₆H₄]Bi[C₆H₄{C(CF₃)₂O}-2] [2.629(95) Å].³⁶

Although the pendant arm brings its nitrogen atom in a quite close proximity of the bismuth atom [Bi(1)–N(1) 3.33(2) Å], the NMe₂ group is twisted and its lone pair is not oriented towards the metal centre (an angle of 34.9° between the Bi–N vector and the normal to the plane defined by the three carbons attached to nitrogen) to allow a N→Bi interaction in the molecule of 2 (Fig. 2).

The coordination of the NMe₂ group effectively results in the formation of a hypervalent 10-Bi-4 [(C,N)BiC₂ core] species^37,38 in the case of 1, and the overall geometry at the bismuth centre becomes distorted pseudo-trigonal bipyramidal (“see-saw”), with the C(1) and C(16) atoms in equatorial positions. The C–Bi–C angles are in the range 91.6(3)–100.0(3)°, close to the C_Ph–Bi–C_Ph angles in triphenylbismuthine (92–96°).³⁹ The intramolecular N→Bi interaction results in chelate-induced Bi-chirality^3,40 (in the molecule of 1 the C₆F₅ groups are different, being placed in axial and equatorial positions, respectively) which can be described in terms of A_Bi and C_Bi isomers.⁴¹ The C₃BiN chelate cycle in 1 is not planar, the nitrogen atom being out of the plane defined by the C₃Bi unit. This gives rise to coordination-induced planar chirality, with the aromatic ring as the chiral plane and the N atom as the pilot atom (for planar chirality the enantiomers are given as pS_N and pR_N).⁴² The crystal of 1 contains a racemic mixture of isomers. Dimeric associations between (A_Bi,pS_N)-1 and (C_Bi,pR_N)-1 (Fig. 3) are formed through weak intermolecular Bi⋯F contacts [3.562(5) Å; cf. sum of the corresponding van der Waals radii, ∑r_vdw(Bi,F) 3.75 Å (ref. 35)] and π⋯π interactions [Ar_centroid{C(10)–C(15)}⋯Ar_centroid{C(10′)–C(15′)} distance 3.41 Å].⁴³ Such dimers are further connected through additional weak intermolecular F⋯H interactions involving an ortho-F of the axial C₆F₅ group from a neighbouring dimer [F(5)⋯H(3d) 2.54 Å; cf. sum of the corresponding van der Waals radii, ∑r_vdW(F,H) 2.55 Å (ref. 35)], resulting in a layer network (Fig. S7, ESI†). No interactions between parallel layers were observed.

The coordination geometry around the bismuth atom in 2 (Fig. 2) can be described as distorted pyramidal, with two enlarged C–Bi–C angles [C(1)–Bi(1)–C(35) 104.4(5)°, for C(10)–Bi(1)–C(35) 108.8(4)°] as a consequence of the steric pressure imposed by the polyfluorinated substituents.

There are no interactions between heavy atoms in the crystal of 2, but the molecules are connected through intermolecular C–F⋯π (Ar_centroid) interactions [C(24)–F(24)⋯Ar_centroid{C(47)–C(52)} 3.16 Å] to form chain polymers (Fig. S13 in ESI†), without further inter-chain contacts.

The chiral bromides R(Ar)BiBr [R = 2-(Me₂NCH₂)C₆H₄; Ar = C₆F₅ (3) (Fig. 4), Mes (4) (Fig. 5), Ph (5) (Fig. 6)] exhibit similar molecular structures in the solid state. The crystal of the bromide 3 contains two independent molecules (3a and 3b) which exhibit only small differences in the magnitude of the molecular parameters (Table 1).

Some common trends observed at the molecular level for compounds 3–5 are: (i) the Bi–Br bond lengths are shorter than in the related bromide [2-(Me₂NCH₂)C₆H₄]₂BiBr [Bi(1)–Br(1) 2.845(7) Å];⁴⁴ (ii) the primary coordination sphere consists of a trigonal pyramidal C₂BiBr core; (iii) a strong intramolecular N→Bi interaction [2.563(9) Å for (3a) and 2.511(9) Å for (3b); 2.570(5) Å for (4); 2.536(13) Å for (5); cf. sum of the corresponding covalent radii, ∑r_cov(Bi,N) 2.27 Å (ref. 35)] of similar magnitude as found in [2-(Me₂NCH₂)C₆H₄]₂BiBr [Bi(1)–N(1) 2.534(5) Å],⁴⁴ is placed trans to the Bi–Br bond; (iv) taking into account the N→Bi interaction the overall geometry around bismuth is distorted pseudo-trigonal bipyramidal [(C,N)CBiBr core; hypervalent 10-Bi-4 species^37,38], with the axial positions occupied by the nitrogen and bromine atoms (see Table 1 for the N–Bi–Br angles); (v) the resulting folded BiC₃N chelate ring induces planar chirality; (vi) the compounds crystallize as racemates: (A_Bi1,pS_N1)-3a/(C_Bi1,pR_N1)-3a and (A_Bi2,pS_N2)-3b/(C_Bi2,pR_N2)-3b isomers for 3, (A_Bi,pS_N)-4/(C_Bi,pR_N)-4 isomers for 4, and (A_Bi,pR_N)-5 and (C_Bi,pS_N)-5 isomers for 5.

The N–Bi distances found in compounds 3–5 are rather similar and do not reflect a clear trend depending on the electronic properties of the second organic substituent (with C₆F₅, Ph and Mes substituents as electron-deficient, electron-neutral, and electron-rich groups, respectively). The strength of the N→Bi interaction depends more on the number of organic ligands attached to the metal and parallels the increased Lewis acidity of the bismuth centre in the series R₃Bi < R₂BiX < RBiX₂ [e.g. N–Bi distances in [2-(Me₂NCH₂)C₆H₄](4-MeC₆H₄)₂Bi 2.902(4) Å,⁴⁵ [2-(Me₂NCH₂)C₆H₄](4-MeC₆H₄)BiCl 2.525(6) Å,²⁷ and [2-(Me₂NCH₂)C₆H₄]BiCl₂ 2.458(6) Å (ref. 44)].

The deviations of the bond angles at the bismuth centre from the ideal values are mainly due to the constraint imposed by the small bite of the C,N-chelating ligand and in the cases of 3 and 4 also due to the bulkier C₆F₅ and mesityl groups, respectively, as compared to the phenyl substituent in 5.

A closer investigation of the crystal packing of 3 revealed that intermolecular Bi⋯Br interactions generate a ribbon-like chain polymeric arrangement which consists of binuclear units of (A_Bi2,pS_N2)-3b/(C_Bi2,pR_N2)-3b isomers [Bi(2)⋯Br(2′) 3.608(2) Å] connected through bridging (A_Bi1,pS_N1)-3a and (C_Bi1,pR_N1)-3a isomers [Bi(1)⋯Br(2) 3.439(2) Å and Bi(2b)⋯Br(1) 3.893(2) Å; cf. the sum of the corresponding van der Waals radii, ∑r_vdW(Bi,Br) 4.35 Å (ref. 35)] (Fig. 7). This chain polymer is supported by additional intra-chain, weak intermolecular F⋯H and Br⋯H interactions (Fig. S20, ESI†). Further weak intermolecular F⋯H, C–H⋯π (Ar_centroid) and C–F⋯π (Ar_centroid) contacts (Fig. S21, ESI†) connect such chains in a 3D supramolecular architecture.

Fig. 7 Fragment of the chain polymeric arrangement with a skeleton based on intermolecular Bi⋯Br interactions between (A_Bi1,pS_N1)-3a/(C_Bi1,pR_N1)-3a and (A_Bi2,pS_N2)-3b/(C_Bi2,pR_N2)-3b isomers in the crystal of 3 (only atoms directly connected to the metal centres and carbons of the chelate rings are shown).

In the crystals of the bromides 4 and 5 there are no interactions between heavy atoms. Chains of either (A_Bi,pS_N)-4 or (C_Bi,pR_N)-4 (Fig. S26, ESI†) isomers are formed through Br⋯H and C–H⋯π (Ar_centroid) contacts. Alternating such parallel chains which develop along the b axis are connected through Br⋯H interactions [Br(1)⋯H(14b′′) 2.99 Å; cf. the sum of the corresponding van der Waals radii, ∑r_vdW(Br,H) 3.15 Å (ref. 35)] to give layers (Fig. S28, ESI†) which stack into a three-dimensional arrangement based on C–H_aryl⋯π (Ar_centroid) interactions [i.e. H⋯Ar_centroid contacts shorter than 3.1 Å and the angle γ between the normal to the aromatic ring and the line through the H atom and Ar_centroid smaller than 30°:⁴⁶ C(3)–H(3)⋯Ar_centroid{C(10d)–C(15d)} 2.78 Å, γ = 13.9°] (Fig. S30, ESI†).

No C–H⋯π (Ar_centroid) or π–π interactions are observed in the crystal of 5. However, several weak intermolecular Br⋯H interactions result in a 3D architecture based on parallel, alternating chain polymers built from (A_Bi,pR_N)-5 and (C_Bi,pS_N)-5 isomers, respectively (Fig. S34–40, ESI†).

Solution behaviour

All compounds 1–5 were investigated by multinuclear NMR spectroscopy in solution. The assignment of the resonance signals in the ¹H and ¹³C NMR spectra was based on the H–H (COSY) and H–C (HSQC, HMBC) correlation spectra. The numbering scheme used to designate the ¹H and ¹³C NMR resonance signals is presented in Table 2. The numbering of the hydrogen atoms follows that of the carbon atom to which they are attached.

Table 2 Numbering scheme for the NMR assignment for compounds 1–5

	Compound	L	R^X	R^Y
	1	C6F5	F	F
	2	2,4,6-(C6F5)3C6H2	C6F5	H
	3	Br	F	F
	4	Br	CH3	H
	5	Br	H	H

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The room temperature ¹H NMR spectra of 1 and 2 in solution exhibit two sharp singlet resonances for the NMe₂ and CH₂ protons, which is consistent either with the absence of intramolecular N→Bi coordination or a fast coordination–dissociation process between nitrogen and bismuth, therefore yielding averaged NMR resonances. The most downfield shifted doublet resonances correspond to the aromatic protons H6 (δ 8.34 ppm for 1 and 7.69 ppm for 2). The ¹H NMR spectrum of 1 at −60 °C (in CDCl₃) exhibited sharp resonances for the methyl and methylene protons similar to those observed in the spectrum measured at room temperature, which is consistent with a similar dynamic behaviour of the compound even at this low temperature.

In the case of the bromides 3–5 the ¹H NMR spectra recorded at room temperature in CDCl₃ show two singlet signals for the dimethylamino group and an AB spin system for the methylene protons, indicating that in solution the pendant arm is coordinated to the chiral bismuth centre through the nitrogen atom. These spectral features have been reported before for the phenyl-substituted compound 5 and related bismuthines.²⁷ The resonance of the H6 proton is further shifted downfield in the series of compounds 3–5, i.e. δ 9.20 ppm (3) > 9.05 ppm (4) > 9.03 ppm (5). For compound 4 two singlet resonances for the methyl protons from the ortho and para positions of the mesityl moiety and one singlet resonance for the aromatic protons of the mesityl group were observed, consistent with a free rotation of the mesityl group around the Bi–C bond.

The dynamic behaviour of the bromides 3–5 was studied by ¹H NMR experiments at higher temperature. The resonance signals corresponding to the protons of the methyl groups of 3 reached coalescence at 30 °C in C₆D₆ (ΔG^‡ = 16.5 kcal mol⁻¹). The AB system characteristic for the diastereotopic methylene protons broadened significantly at 70 °C but did not coalesce. For compound 4, coalescence of the NMe₂ proton resonances was observed at 48 °C (ΔG^‡ = 16.6 kcal mol⁻¹) in the ¹H NMR spectrum (in C₆D₆), while the AB system corresponding to the methylene protons remained unchanged even at 60 °C. The coalescence temperatures of the NMe₂ proton resonances of 5 and of the related chloride [2-(Me₂NCH₂)C₆H₄](Ph)BiCl were determined by Suzuki et al. to be 65 °C and 70 °C, respectively, in toluene-d₈.²⁷ We observed for 5 the coalescence of the NMe₂ proton resonances at 62 °C (ΔG^‡ = 16.5 kcal mol⁻¹) in C₆D₆. The behaviour of the ¹H NMR resonances of the NMe₂ groups in 3, 4 and 5 indicate that the dissociation of the N→Bi interaction takes place followed by a vertex inversion at the N atom and subsequent restoration of the N→Bi coordinative bond. However, an inversion at the Bi centre (indicated by the coalescence of the methylene proton resonances) requires more energy to take place.

The ¹³C NMR spectra of 1 and 2 exhibit two sharp singlet resonances, i.e. one for the NMe₂ group and one for the methylene carbon. The ipso carbon atoms of the C₆F₅ groups of 1 give rise to a broad triplet at δ 125.40 ppm. The other ¹³C resonances of the C₆F₅ groups are observed as doublets of multiplets with ¹J_FC coupling constants of ca. 250 Hz. The fine splitting of these signals caused by further couplings with fluorine nuclei at two, three, and four bonds distance considerably diminish their relative intensity. A similar pattern was observed in the ¹³C NMR spectrum of 3 for the resonance signals corresponding to the C₆F₅ group attached to the bismuth atom. In the ¹³C NMR spectrum of 2, two different sets of resonance signals can be distinguished which correspond to the non-equivalent pentafluorophenyl groups belonging to the 2,4,6-(C₆F₅)₃C₆H₂ substituent (i.e. two C₆F₅ groups in positions 2 and 6, and one C₆F₅ group in position 4). The ¹³C NMR spectrum of compound 4 exhibits in the aliphatic region two singlet resonances (δ 21.37 ppm for p-CH₃ and 27.32 ppm for o-CH₃) for the carbon atoms of the ortho and para methyl groups from the mesityl moiety. Two sharp singlet resonances corresponding to the NMe₂ groups and one sharp signal for the CH₂ group were assigned to the pendant arm of the [2-(Me₂NCH₂)C₆H₄] moiety. The same pattern was observed for the [2-(Me₂NCH₂)C₆H₄] group in the ¹³C NMR spectra of 3 and 5.

In the ¹⁹F NMR spectrum of 1, three resonances for the two equivalent C₆F₅ groups bound to bismuth were observed at δ −118.14, −153.82, and −160.45 ppm for the ortho, para and meta fluorine atoms, respectively. The three resonances corresponding to the ortho, meta and para fluorine atoms of 3 were observed at chemical shifts close to those of 1.

The ¹⁹F NMR spectrum of 2 exhibits three resonance signals for the ortho fluorine atoms and three resonances for the meta fluorine atoms of the C₆F₅ moieties. This indicates that the rotation of the C₆F₅ groups in positions 2 and 6 is hindered, and thus one half of a C₆F₅ group becomes non-equivalent with the other half. The resonance signals corresponding to the para fluorine atoms of all C₆F₅ groups overlap at δ −153.2 ppm.

Experimental

General measurements and analysis instrumentation

The melting points are not corrected. The ¹H, ¹³C, and ¹⁹F NMR spectra were recorded at room temperature on Bruker Avance III 400 or Bruker Avance III 600 instruments. Low-temperature NMR spectra were recorded on the Bruker Avance III 400 apparatus. The ¹H chemical shifts are reported in δ units (ppm) relative to the residual peak of the deuterated solvent (CHCl₃, 7.26 ppm; C₆D₆, 7.16 ppm). The ¹³C chemical shifts are reported in δ units (ppm) relative to the peak of the deuterated solvent (CDCl₃, 77.16 ppm; C₆D₆, 128.06 ppm).⁴⁷ The ¹H and ¹³C resonances were assigned using H–H and H–C correlation NMR experiments (COSY, HSQC, HMBC). The ¹⁹F chemical shifts are quoted in δ units (ppm) relative to CFCl₃ as the external standard. The NMR spectra were processed using the MestReNova software.⁴⁸ Free energy of activation values were calculated using the Gutowsky–Holm equations: Kc [s⁻¹] = πΔν and ΔG^‡ [kcal mol⁻¹] = 4.58T_c10⁻³ (10.32 + log T_c/K_c).⁴⁹ The EI mass spectra were recorded on a Finnigan MAT 8200 instrument, while HRMS APCI(+) spectra were recorded on a Thermo Scientific Orbitrap XL mass spectrometer equipped with a standard ESI/APCI source. Data analysis and calculations of the theoretical isotopic patterns were carried out with the Xcalibur software package.⁵⁰

Crystal structure determination

Single crystals were obtained at −28 °C from concentrated solutions in hexane (for 1) or diethyl ether (for 3). Very slow evaporation of heptane from a concentrated solution of 2 gave single crystals of the compound, while slow evaporation of a CHCl₃ solution gave crystals of 4. Suitable crystals of 5 were obtained by hexane diffusion into a solution of the compound in toluene (toluene–hexane: 1/5, v/v). The details of the crystal structure determination and refinement for compounds 1–5 are given in Table S1 (see ESI†). Data were collected on Siemens P4 (1, 3, and 4) and Bruker SMART APEX (2 and 5) diffractometers, using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). For this purpose the crystals were attached with Kel-F oil (1, 3, 4) to a glass fiber or with Paratone oil (2, 5) to a cryo-loop. The crystals of 1, 3, and 4 were cooled under a nitrogen stream, while data for 2 and 5 was collected at room temperature. The structures were refined with anisotropic thermal parameters. All hydrogen atoms were refined with a riding model and a mutual isotropic thermal parameter. For structure solving and refinement the software package SHELX-97 and SHELXL-2014 were used.^51,52 The drawings were created with the Diamond program.⁵³ The CCDC reference numbers are 833208 (1), 1430429 (2), 833207 (3), 833209 (4) and 833206 (5).

Materials and methods

All reactions were carried out under argon using standard Schlenk techniques. Solvents were dried by standard procedures and were freshly distilled under argon prior to use. Bismuth(III) trichloride, N,N-dimethylbenzylamine, bromopentafluorobenzene, 2-bromo-1,3,5-trimethylbenzene and butyllithium (15% in n-hexane) were purchased from commercial suppliers and used without further purification. The other starting materials were prepared according to literature methods: [2-(Me₂NCH₂)C₆H₄]Li,⁵⁴ [2-(Me₂NCH₂)C₆H₄]BiBr₂,⁵⁵ 2,4,6-(C₆F₅)₃C₆H₂Br.⁵⁶ The concentration of the organomagnesium derivatives used for the synthesis of 1 and 3–5 was determined by iodimetric titration as described by Krasovskiy and Knochel.⁵⁷

Synthesis of [2-(Me₂NCH₂)C₆H₄](C₆F₅)₂Bi·(1). A solution of 0.33 M C₆F₅MgBr (6.6 mL, 2.2 mmol) in Et₂O was added dropwise to a suspension of [2-(Me₂NCH₂)C₆H₄]BiBr₂ (0.503 g, 1 mmol) in Et₂O (20 mL) at room temperature. The reaction mixture was stirred for approximately 2 h under argon, then the solvent was evaporated at reduced pressure. To the residue dry hexane (30 mL) and dry CH₂Cl₂ (10 mL) were added. The suspension was filtered over a pad of celite under argon and the solvent evaporated to dryness. To remove completely the residual magnesium salts, the solid product was again dissolved in CH₂Cl₂ and the filtration process was repeated. The solvent was then evaporated from the clear filtrate and the solid was dried in vacuum to give 1 as a white solid (0.63 g, 93%), m.p. 103 °C. ¹H NMR (CDCl₃, 400.13 MHz, r.t.): δ 2.23 (s, 6H, H-8 and H-9), 3.69 (s, 2H, H-7), 7.35 (dd, 1H, ³J_H,H = 7.3 Hz, H-5), 7.40 (dd, 1H, ³J_H,H = 7.3 Hz, H-4), 7.46 (d, 1H, ³J_H,H = 7.4 Hz, H-3), 8.34 (d, 1H, ³J_H,H = 7.3 Hz, H-6). ¹³C{¹H} NMR (CDCl₃, 100.61 MHz, r.t.): δ 45.11 (s, C-8 and C-9), 67.22 (s, C-7), 125.14 (t, ²J_FC = 49 Hz, C-10), 128.86 (s, C-4), 129.58 (s, C-3), 130.76 (s, C-5), 137.73 (dm, ¹J_FC = 257 Hz, C₆F₅), 141.32 (dm, ¹J_FC = 252 Hz, C₆F₅), 142.71 (s, C-6), 146.13 (s, C-2), 147.83 (dm, ¹J_FC = 234 Hz, C₆F₅), 160.64 (s, C-1). ¹⁹F{¹H} NMR (CDCl₃, 376.46 MHz, r.t.): δ (−117.90)–(−118.35) (m, 4F, ortho-F, C₆F₅), −153.86 (t, 2F, ³J_FF = 20 Hz, para-F, C₆F₅), (−160.20)–(−160.79) (m, 4F, meta-F, C₆F₅). MS (EI, 70 eV, 109 °C): m/z, (relative intensity, %) 510 (100) [M⁺ − C₆F₅], 376 (5) [C₆F₅Bi⁺ + H], 134 (26) [R⁺] [R = 2-(Me₂NCH₂)C₆H₄].

Synthesis of [2-(Me₂NCH₂)C₆H₄][2,4,6-(C₆F₅)₃C₆H₂]₂Bi (2). A suspension of [2-(Me₂NCH₂)C₆H₄]BiBr₂ (0.503 g, 1 mmol) in toluene (10 mL) was added to 2,4,6-(C₆F₅)₃C₆H₂Li (2 mmol) in toluene (20 mL) at 0 °C and the stirring was continued for 2 h at this temperature and then for 12 h at room temperature. The reaction mixture was filtered over a pad of celite. The solvent was removed by rotary evaporation. Purification by flash chromatography with hexane using aluminium oxide as the stationary phase afforded 2 as a pale yellow solid (1.05 g, 70%), m.p. 95–97 °C. ¹H NMR (CDCl₃, 600.13 MHz, r.t.): δ 2.00 (s, 6H, H-8 and H-9), 3.17 (s, 2H, H-7), 7.06 (dd, 1H, ³J_H,H = 7.3 Hz, H-5), 7.17 (dd, 1H, ³J_H,H = 7.3 Hz, H-4), 7.21 (d, 1H, ³J_H,H = 7.2 Hz, H-3), 7.40 (s, 4H, H-12 and H-14), 7.69 (d, 1H, ³J_H,H = 7.3 Hz, H-6). ¹³C{¹H} NMR (CDCl₃, 150.90 MHz, r.t.): δ 45.19 (s, C-8 and C-9), 67.12 (s, C-7), 113.57 (t, ²J_FC = 15.6 Hz, ipso-C, 4-C₆F₅), 116.87 [dd, ²J_FC = 17.6 Hz, ipso-C, 2,6-(C₆F₅)₂], 127.33 (s, C-13 or C-15), 127.62 (s, C-4), 130.97 (s, C-5), 131.22 (s, C-3), 134.80 (s, C-12 and C-14), 136.41 (s, C-15 or C-13), 137.72 (dm, ¹J_FC = 253.7 Hz, C₆F₅), 138.17 (dm, ¹J_FC = 251.1 Hz, C₆F₅), 141.12 (dm, ¹J_FC = 261.3 Hz, C₆F₅), 141.39 (dm, ¹J_FC = 256.1 Hz, C₆F₅), 141.84 (s, C-6), 143.75 (dm, ¹J_FC = 245.3 Hz, C₆F₅), 144.27 (dm, ¹J_FC = 243.1 Hz, C₆F₅), 144.79 (s, C-2), 166.93 (s, C-1), 169.56 (s, C-10). ¹⁹F{¹H} NMR (CDCl₃, 564.69 MHz, r.t.): δ −138.86 [m, 4F, ortho-F, 2,6-(C₆F₅)₂], −139.71 [m, 4F, ortho-F, 2,6-(C₆F₅)₂], −142.78 (m, 4F, ortho-F, 4-C₆F₅), −153.30 [m, 6F, para-F, 2,4,6-(C₆F₅)₃], −160.77 [m, 4F, meta-F, 2,6-(C₆F₅)₂], −160.95 [m, 4F, meta-F, 2,6-(C₆F₅)₂], −161.21 (m, 4F, meta-F, 4-C₆F₅). MS (EI, 70 eV, 200 °C): m/z, (relative intensity, %) 1359 (0.5) [M⁺ − R], 918 (100) [M⁺ − 2,4,6-(C₆F₅)₃C₆H₂], 343 (12) [RBi⁺] [R = 2-(Me₂NCH₂)C₆H₄]. HRMS (APCI+): [M⁺ + H] calcd for C₅₇H₁₆BiF₃₀N 1494.06803. Found: 1494.07070.

Synthesis of [2-(Me₂NCH₂)C₆H₄](C₆F₅)BiBr (3). A solution of 0.33 M C₆F₅MgBr (3 mL, 1 mmol) in Et₂O was added dropwise, under argon, to a suspension of [2-(Me₂NCH₂)C₆H₄]BiBr₂ (0.503 g, 1.0 mmol) in Et₂O (20 mL), cooled to −80 °C. The reaction mixture was stirred at −80 °C for 1 h, then warmed to room temperature overnight. The solvent was evaporated at reduced pressure, and then CH₂Cl₂ (20 mL) was added to the residue. The suspension was filtered over a pad of celite under argon. The solvent was removed in vacuum from the clear solution and the resulting off-white solid was washed with dry hexane (2 × 10 mL), under argon. The solid was dried in vacuum to obtain 3 as an off-white solid (0.49 g, 83%), m.p. 190 °C. The isolated product contains traces of 1 (1 : 0.03 as calculated from the integral ratio in the ¹⁹F NMR spectrum) and another minor unidentified impurity. Recrystallization by diffusion of hexane into a solution of 3 in CH₂Cl₂ did not improve significantly the purity of 3. ¹H NMR (CDCl₃, 400.13 MHz, r.t.): δ 2.52 (s, 3H, H-8), 2.58 (s, 3H, H-9), AB spin system with A at 3.79 ppm and B at 4.19 ppm (2H, ²J_HH = 14.2 Hz, H-7), 7.50 (ddd, 1H, ³J_HH = 7.5 Hz, ⁴J_HH = 1.3 Hz, H-4), 7.60–7.67 (m, 2H, H-3 and H-5), 9.20 (d, 1H, ³J_HH = 7.6 Hz, H-6). ¹H NMR (C₆D₆, 400.13 MHz, r.t.): δ 1.53 (s, 3H, H-8 or H-9), 1.54 (s, 3H, H-9 or H-8), AB spin system with A at 2.90 ppm and B at 3.12 ppm (2H, ²J_HH = 14.2 Hz, H-7), 7.10–7.21 (m, 2H, resonance signals for H-3 and H-4 overlapped by the residual C₆D₅H resonance signal), 7.39 (ddd, 1H, ³J_HH = 7.4 Hz, ⁴J_HH = 1.4 Hz, H-5), 9.65 (d, 1H, ³J_HH = 7.5 Hz, H-6). ¹H NMR (C₆D₆, 400.13 MHz, 30 °C): δ 1.55 (s, br, 6H, H-8 and H-9), AB spin system with A at 2.92 ppm and B at 3.13 ppm (2H, ²J_HH = 14.2 Hz, H-7), 7.10–7.21 (m, 2H, resonance signals for H-3 and H-4 overlapped by the residual C₆D₅H resonance signal), 7.39 (ddd, 1H, ³J_HH = 7.4 Hz, ⁴J_HH = 1.4 Hz, H-5), 9.64 (d, 1H, ³J_HH = 7.6 Hz, H-6). ¹³C{¹H} NMR (CDCl₃, 100.61 MHz, r.t.): δ 45.60 (s, C-9), 46.98 (s, C-8), 69.84 (s, C-7), 129.00 (s, C-3), 129.14 (s, C-4), 131.56 (s, C-5), 134.94 (t, br, ²J_FC = 43 Hz, C-10) 138.39 (dm, ¹J_FC = 257 Hz, C₆F₅), 142.13 (dm, ¹J_FC = 254 Hz, C₆F₅), 142.40 (s, C-6), 147.68 (dm, ¹J_FC = 238 Hz, C₆F₅), 148.00 (s, C-2), 174.59 (s, C-1). ¹⁹F{¹H} NMR (CDCl₃, 376.46 MHz, r.t.): δ (−116.61)–(−116.94) (m, 2F, ortho-F, C₆F₅), −150.87 (t, 1F, ³J_FF = 19.8 Hz, para-F, C₆F₅), (−158.00)–(−158.44) (m, 2F, meta-F, C₆F₅). MS (EI, 70 eV, 200 °C): m/z, (relative intensity, %) 589 (0.2) [M⁺], 510 (100) [M⁺ − Br], 422 (30) [M⁺ − C₆F₅].

Synthesis of [2-(Me₂NCH₂)C₆H₄](Mes)BiBr (4). A solution of 0.1 M MesMgBr (10 mL, 1 mmol) in THF was added dropwise to a suspension of [2-(Me₂NCH₂)C₆H₄]BiBr₂ (0.503 g, 1.0 mmol) in THF (30 mL), cooled to −80 °C. The reaction mixture was stirred at −78 °C for 1 h, then warmed to room temperature overnight. Approximately 3 mL of dry dioxane was added and the resulting suspension was filtered over a pad of celite. The solvents were evaporated on a rotary evaporator and the resulting oil was triturated with pentane (15 mL). The solid was washed with pentane (3 × 15 mL), then toluene (50 mL) was added and the suspension was filtered over a pad of celite. The solvent removed in vacuum to give 4 was obtained as an off-white crystalline solid (0.23 g, 41%), m.p. 186–187 °C. ¹H NMR (CDCl₃, 400.13 MHz, r.t.): δ 2.21 (s, 3H, H-17), 2.31 (s, 3H, H-8), 2.33 (s, 6H, H-16 and H-18), 2.46 (s, 3H, H-9), AB spin system with A at 3.68 ppm and B at 3.80 ppm (2H, ²J_HH = 14.0 Hz, H-7), 7.10 (s, 2H, H-12 and H-14), 7.40 (dd, 1H, ³J_HH = 7.0 Hz, H-4), 7.46–7.55 (m, 2H, H-3 and H-5), 9.05 (d, 1H, ³J_HH = 8.0 Hz, H-6). ¹H NMR (C₆D₆, 400.13 MHz, r.t.): δ 1.57 (s, 3H, H-8), 1.61 (s, 3H, H-9), 2.06 (s, 3H, H-17), 2.29 (s, 6H, H-16 and H-18), AB spin system with A at 3.14 ppm and B at 2.93 ppm (2H, ²J_HH = 13.8 Hz, H-7), 6.92 (s, 2H, H-12 and H-14), 7.08 (d, 1H, ³J_HH = 7.4 Hz, H-3), 7.17 (m, 1H, resonance signal for H-4 overlapped by the residual C₆D₅H resonance signal), 7.34 (dd, 1H, ³J_HH = 7.4 Hz, H-5), 9.58 (dd, 1H, ³J_HH = 7.5 Hz, ⁴J_HH = 1.3 Hz, H-6). ¹H NMR (C₆D₆, 400.13 MHz, 48 °C): δ 1.65 (s, 6H, H-8 and H-9), 2.06 (s, 3H, H-17), 2.29 (s, 6H, H-16 and H-18), AB spin system with A at 3.20 ppm and B at 2.99 ppm (2H, ²J_HH = 13.9 Hz, H-7), 6.93 (s, 2H, H-12 and H-14), 7.09 (d, 1H, ³J_HH = 7.5 Hz, H-3), 7.17 (m, 1H, resonance signal for H-4 overlapped by the residual C₆D₅H resonance signal), 7.34 (dd, 1H, ³J_HH = 7.4 Hz, H-5), 9.53 (dd, 1H, ³J_HH = 7.6 Hz, ⁴J_HH = 1.5 Hz, H-6). ¹³C{¹H} NMR (CDCl₃, 100.61 MHz, r.t.): δ 21.39 (s, C-17), 27.36 (s, C-16 and C-18), 45.63 (s, C-9), 46.85 (s, C-8), 70.09 (s, C-7), 128.20 (s, C-4), 128.53 (s, C-3), 130.69 (s, C-12 and C-14), 131.14 (s, C-5), 138.91 (s, C-13), 140.96 (s, C-6), 146.56 (s, br, C11 and C15), 147.43 (s, C-2), 174.57 (s, C-1), 175.47 (s, C-10). MS (EI, 70 eV, 164 °C): m/z, (relative intensity, %) 462 (16) [M⁺ − Br], 422 (100) [M⁺ − Mes], 343 (90) [RBi⁺] [R = 2-(Me₂NCH₂)C₆H₄]. HRMS (APCI+): [M⁺ − Br] calcd for C₁₈H₂₃BiN 462.16288. Found: 462.16460.

Synthesis of [2-(Me₂NCH₂)C₆H₄](Ph)BiBr (5). Method A. A solution of 0.093 M PhMgBr (10.8 mL, 1 mmol) in THF was added dropwise to a suspension of [2-(Me₂NCH₂)C₆H₄]BiBr₂ (0.503 g, 1.0 mmol) in THF (30 mL), cooled to −78 °C. The reaction mixture was stirred for 1 h at this temperature, then left overnight to reach room temperature. Dry dioxane (3 mL) was added and the resulting suspension was filtered over a pad of celite. The oil that remained after complete removal of the solvents on a rotary evaporator was triturated with pentane (15 mL). The resulted solid was washed with pentane (3 × 15 mL), then treated with toluene (50 mL) and the suspension was filtered over a pad of celite. The solvent was removed in vacuum and the solid was dried at reduced pressure to afford 5 as an off-white crystalline solid (0.33 g, 67%).

Method B. A suspension of [2-(Me₂NCH₂)C₆H₄]Li (1 g, 7.09 mmol) in THF (50 mL) was added dropwise to a suspension of PhBiBr₂ (3.16 g, 7.09 mmol) in THF (25 mL), at −78 °C. The reaction mixture was stirred at this temperature for 2 h, then allowed to warm to room temperature and stirred for another 72 h. The solvent was removed under vacuum and the remaining solid was treated with hot toluene. The suspension was filtered and the solvent removed under vacuum to give 5 as a white solid (1.20 g, 34%), m.p. 158–160 °C (lit.²⁷ 163–165 °C). ¹H NMR (CDCl₃, 400.13 MHz, r.t.): δ 2.30 (s, 3H, H-8), 2.54 (s, 3H, H-9), AB spin system with A at 3.58 ppm and B at 3.88 ppm (2H, ²J_HH = 14.0 Hz, H-7), 7.33 (tt, 1H, ³J_HH = 7.0 Hz, ⁴J_HH = 3.0 Hz, H-13), 7.45–7.56 (m, 4H, H-3, H-4, H-12 and H-14), 7.59 (ddd, 1H, ³J_HH = 7.0 Hz, ⁴J_HH = 3.0 Hz, H-5), 8.10 (dd, 2H, ³J_HH = 6.0 Hz, ⁴J_HH = 3.0 Hz, H-11 and H-15), 9.03 (d, 1H, ³J_HH = 7.2 Hz, H-6). ¹H NMR (C₆D₆, 400.13 MHz, r.t.): δ 1.54 (s, 3H, H-8), 1.68 (s, 3H, H-9), AB spin system with A at 3.03 ppm and B at 2.96 ppm (2H, ²J_HH = 14.0 Hz, H-7), 7.05 (t, 1H, ³J_HH = 7.5 Hz, H-13), 7.10 (d, 1H, ³J_HH = 7.5 Hz, H-3), 7.18–7.26 (m, 3H, H-4, H-12 and H-14), 7.41 (dd, 1H, ³J_HH = 7.4 Hz, H-5), 8.11 (d, 2H, ³J_HH = 7.4 Hz, H-11 and H-15), 9.57 (d, 1H, ³J_HH = 7.4 Hz, H-6). ¹H NMR (C₆D₆, 400.13 MHz, 62 °C): δ 1.66 (s, br, 6H, H-8 and H-9), 3.07 (br, 2H, H-7), 7.06 (t, 1H, ³J_HH = 7.4 Hz, H-13), 7.11 (d, 1H, ³J_HH = 7.5 Hz, H-3), 7.18–7.26 (m, 3H, H-4, H-12 and H-14), 7.41 (ddd, 1H, ³J_HH = 7.4 Hz, ⁴J_HH = 1.3 Hz, H-5), 8.07 (d, 2H, ³J_HH = 6.4 Hz, H-11 and H-15), 9.49 (d, 1H, ³J_HH = 7.5 Hz, H-6). ¹³C{¹H} NMR (CDCl₃, 100.61 MHz, r.t.): δ 45.85 (s, C-9), 46.63 (s, C-8), 69.18 (s, C-7), 128.56 (s, C-4), 128.81 (s, C-13), 129.23 (s, C-3), 131.29 (s, C-5), 131.49 (s, C-12 and C-14), 137.58 (s, C11 and C15), 140.72 (s, C-6), 147.51 (s, C-2), 174.16 (s, C-10), 177.93 (s, C-1). MS (EI, 70 eV, 200 °C): m/z, (relative intensity, %) 499 (1) [M⁺], 422 (90) [M⁺ − C₆H₅], 420 (100) [M⁺ − Br], 343 (54) [RBi]⁺, 286 (5) [PhBi⁺] [R = 2-(Me₂NCH₂)C₆H₄]. HRMS (APCI+): [M⁺ − Br] calcd for C₁₅H₁₇BiN 420.11749. Found: 420.11593.

Conclusions

In contrast to 1, for which the nitrogen of the pendant arm is coordinated intramolecularly to the metal centre in the solid state, the bulkiness of the polyfluorinated substituents in the molecule of 2 prevents the formation of a similar coordinative interaction in the solid state or in solution. Variable temperature ¹H NMR spectra of the heteroleptic triorganobismuthine 1 indicate either a lack of coordination of nitrogen to bismuth or the existence of a fast coordination–dissociation process in solution even at −60 °C. This behaviour may be caused by the fact that the two C₆F₅ substituents do not increase effectively the Lewis acidity of the metal centre.

The chiral bromides 3–5 exhibit in the solid state a strong intramolecular N→Bi coordination established trans to the Bi–Br bond. The N–Bi bond distances in 3–5, although smaller than in 1, are similar, indicating that the second organic substituent (C₆F₅, Ph and Mes, respectively) does not influence considerably the length of the coordinative bond. The room temperature ¹H NMR spectra of 3–5 exhibit an AB system for methylene protons and two resonances for the NMe₂ groups, consistent with the coordination of nitrogen to bismuth in solution.

The heteroleptic triorganobismuthine 1 and the bromides 3–5 crystallize as racemates, i.e. 1 : 1 mixtures of (A_Bi,pS_N) and (C_Bi,pR_N) isomers for 1, 3 and 4, and (A_Bi,pR_N) and (C_Bi,pS_N) isomers for 5. Chain polymeric and layered structures, as well as 3D supramolecular architectures, based on different type of interactions [e.g. Bi⋯F, Bi⋯Br, F⋯H, Br⋯H, C–H⋯π (Ar_centroid), C–F⋯π (Ar_centroid) or π⋯π], are evidenced in the solid state.

Acknowledgements

Financial support from the National Research Council of Romania (CNCS, Research Project No. PN-II-ID-PCE-2011-3-0933) is greatly appreciated. We also thank Deutsche Forschungsgemeinschaft and DAAD for financial support and Universität Bremen for providing research facilities during the research stays of M.G.N. and C.S. The support provided by Alexandra Pop and Richard A. Varga (National Center for X-Ray Diffraction, Babeş-Bolyai University, Cluj-Napoca, Romania) for the solid state structure determinations is highly acknowledged.

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Footnotes

† Electronic supplementary information (ESI) available: NMR spectra; X-ray crystallographic data in CIF format for 1–5; figures representing the optical isomers as well as the supramolecular architectures in the crystals of these compounds. CCDC 833206–833209 and 1430429. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt05074j

‡ These co-authors have contributed equally to the work.

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