Molecular Bismuth Cations: Assessment of Soft Lewis Acidity.

Three-coordinate cationic bismuth compounds [Bi(diaryl)(EPMe3 )][SbF6 ] have been isolated and fully characterized (diaryl=[(C6 H4 )2 C2 H2 ]2- , E=S, Se). They represent rare examples of molecular complexes with Bi⋅⋅⋅EPR3 interactions (R=monoanionic substituent). The 31 P NMR chemical shift of EPMe3 has been found to be sensitive to the formation of LA⋅⋅⋅EPMe3 Lewis acid/base interactions (LA=Lewis acid). This corresponds to a modification of the Gutmann-Beckett method and reveals information about the hardness/softness of the Lewis acid under investigation. A series of organobismuth compounds, bismuth halides, and cationic bismuth species have been investigated with this approach and compared to traditional group 13 and cationic group 14 Lewis acids. Especially cationic bismuth species have been shown to be potent soft Lewis acids that may prefer Lewis pair formation with a soft (S/Se-based) rather than a hard (O/N-based) donor. Analytical techniques applied in this work include (heteronuclear) NMR spectroscopy, single-crystal X-ray diffraction analysis, and DFT calculations.

Introduction

pan class="Chemical">Bismuth(III) compn>ounds are frequently apn>plied as n>an class="Chemical">Lewis acids in stoichiometric and catalytic organic and organometallic transformations. Various types of reactions have been realized with this strategy. The long (and non‐exhaustive) list of examples includes pericyclic reactions (such as Diels–Alder reactions),[ , ] addition reactions (such as hydroamination, hydrosilylation, and carbo/amino‐bismuthation),[ , , , ] addition‐elimination sequences (such as aldol and Mannich reactions),[ , , , ] electrophilic aromatic substitution reactions (such as Friedel Crafts alkylations and acylations),[ , , , , ] S type reactions (such as dehydro‐halogenation reactions),[ , ] CH activation (such as the metalation of (C5H5)−, (C5H4Me)−, (NAr2)−), and small‐molecule activation (such as carbon monoxide insertion). The utilization of bismuth Lewis acids can lead to selectivities and activities that are difficult or impossible to obtain with other reagents.[ , , , ] In other cases bismuth compounds show advantages in terms of affordability, stability towards air and moisture, or functional group tolerance.[ , , ] In addition, prospects for catalyst recyclability have been demonstrated.[ , ]

While the abovementioned applications make upan class="Gene">se of n>an class="Chemical">Lewis acid/base interactions between bismuth compounds and organic substrates, the coordination of metal‐centered Lewis bases to Lewis acidic bismuth components has also been reported. This has been exploited for the design of bismuth‐containing Z‐type (donor/acceptor) ligands, in which the bismuth atom is responsible for the electron‐accepting character of the overall ligand.

We have recently suggested a scheme for the claspan class="Chemical">sification of n>an class="Chemical">bismuth Lewis acids. Distinctions are made between three types of compounds: class A) R2Bi‐X with an electronegative ligand X; class B) R2Bi⋅⋅⋅X’ with ligands X’ such as (O3SCF3)− or (AlCl4)−, which lead to a weak Bi⋅⋅⋅X’ interaction; and class C) cationic species [R2Bi(L)][WCA] without any directional bonding interactions between bismuth and the weakly coordinating counteranion (WCA), for which [SbF6]− or [B(C6F5)4]− are typical examples. While σ*(Bi‐X/X’)‐orbitals are responsible for the Lewis acidic character of class A and B compounds, an empty bismuth‐centered p‐orbital will accept electron density from Lewis basic bonding partners in the case of class C complexes. For all three classes of bismuth Lewis acids, the orbitals involved in the formation of interactions with Lewis bases are large and diffuse, which is why they may be expected to be soft Lewis acids. This is supported by the fact that interactions with typical soft Lewis bases such as arene moieties, stibanes, and telluroethers have been reported.[ , , , , ] It can be anticipated that the expectedly soft character of bismuth‐based Lewis acidity is integral to some of the observed properties and reactivity patterns of bismuth species. Examples include their relatively high tolerance towards (hard) oxygen‐based functional groups and their ability to efficiently coordinate and activate (soft) arene and olefin donor groups.[ , , , , , , , , ]

In contrast to the frequent application of pan class="Chemical">bismuth Lewis acids in synthen>an class="Chemical">sis and catalysis, efforts to quantify bismuth‐based Lewis acidity have only scarcely been reported.[ , , ] More specifically, there are no detailed studies available that deal with the quantification of the hard or soft character of bismuth Lewis acids.

Here we report Lewis acid/ban>an class="Gene">se pair formation between well‐defined bismuth compounds and soft Lewis bases EPMe3 (E=S, Se) and suggest an operationally simple method to establish trends in the softness of Lewis acidity.

Results and Discussion

Cationic pan class="Chemical">bismuth compounds show an enhanced n>an class="Chemical">Lewis acidity when compared with their neutral parent compounds. Cationic diaryl bismuth compounds are the most prominent subgroup of this family of compounds. They usually bind two equivalents of a Lewis base through their empty bismuth‐centered 6p‐orbital. We aimed at diaryl bismuth cations that preferentially bind one equivalent of a donor in order to allow direct comparison with archetypical examples of Lewis acids based on group 13 elements. Recently reported cationic bismepines such as compound 1 appeared to be promising candidates, because the olefin bridge in the ligand backbone reduces the flexibility of the aryl groups and provides moderate steric shielding of the bismuth center (Scheme 1). While two equivalents of Lewis bases with a low steric profile (such as thf) interact with the bismuth atom in 1, sterically more demanding Lewis bases should make the generation of Lewis acid/base adducts with a 1:1 stoichiometry possible. In order to test for the potential of the Lewis acidic bismuth center in 1 to bind soft donors by substitution of the hard thf ligands, compound 1 was reacted with one equivalent of the phosphane chalcogenides EPMe3 (E=S, Se; Scheme 1). Indeed, compounds 2‐SPMe and 2‐SePMe could be isolated in excellent yields of 87–91 % as pale yellow solids. Adducts of bismuth compounds and phosphane sulfides or selenides are extremely rare: compounds [Bi(SC6F5)3(SPPh3)] and [BiX3(SeP(4‐F‐C6H4)3)] (X=Cl, Br) have been reported. NMR spectroscopic data is not available for the former compound and only 31P NMR chemical shifts (without 1 J PSe coupling constants) are given for the latter two species, indicating a minor up‐field shift of up to 3.7 ppm with respect to the free phosphane selenide, SeP(4‐F‐C6H4)3. For compounds 2‐SPMe and 2‐SePMe, the 1H NMR spectra display resonances typical of a benzo group, plus a singlet in the olefinic region and a doublet for the methyl groups bound to phosphorus. In 13C NMR spectra, the resonances of the ipso‐carbon atoms (2‐SPMe: 176.9 ppm; 2‐SePMe: 162.9 ppm) are significantly up‐field‐shifted compared to 1 (δ=193.9 ppm) and close to those of the corresponding chlorobismepine Bi((C6H4)2C2H2)Cl (δ=172.9 ppm), suggesting considerable Bi⋅⋅⋅S/Se bonding interactions in solution. This is further supported by large down‐field shifts of the 31P and 77Se NMR spectroscopic resonances of these compounds when compared to the free phosphane chalcogenides (31P NMR: 2‐SPMe: 44.2 ppm; 2‐SePMe: 21.1 ppm; SPMe3: 29.2 ppm; SePMe3: 7.8 ppm; 77Se NMR: 2‐SePMe: −44.2 ppm; SePMe3: −234.9 ppm). A significant decrease of the 1 J PSe coupling constant from 689 Hz in free SePMe3 to 488 Hz in the bismuth complex 2‐SePMe indicates weakening of the P−Se bond[ , ] and is—to the best of our knowledge—the largest decrease reported for metal complexes of SePMe3.[ , ]

Scheme 1

Reaction of cationic bismepine 1 with soft donors EPMe3 to give compounds 2‐EPMe through thf elimination (E=S, Se).

Reaction of cationic bismepine 1 with soft donors En>an class="Chemical">PMe3 to give compounds 2‐EPMe through thf elimination (E=S, Se).

pan class="Chemical">Single‐n>an class="Chemical">crystal X‐ray analyses were carried out for compounds 2‐SPMe and 2‐SePMe, revealing an isostructural relationship (monoclinic space group P21/c with Z=4 in both cases; Figure 1). The complexes crystallize as separated ion pairs (i.e., without strong directional bonding interactions between cation and anion). The bismepine ligands adopt bent conformations with angles of 88.7° and 76.5° between the mean planes of the benzo groups, as recently reported for bismepines with three‐coordinate bismuth atoms. The bismuth atoms are found in pyramidal coordination geometries with bond angles around Bi1 ranging from 85.1–90.6° and 87.8–92.1° for 2‐SPMe and 2‐SePMe, respectively. A coordination number of three is extremely unusual for diorganobismuth cations, which commonly adopt coordination numbers of four with the empty 6p‐orbital of bismuth being involved in bonding interactions with two ligands.[ , , , , , , , ] The Bi1−S1/Se1 bond length of 2.61 Å and 2.72 Å are 31–33 % below the sum of the van der Waals radii (S, 1.80 Å; Se, 1.90 Å; Bi, 2.07 Å). They are much shorter than the corresponding bonds in the literature‐known adducts [Bi(SC6F5)3(SPPh3)] (Bi‐SPPh3, 3.01 Å) and [BiX3(SeP(4‐F‐C6H4)3)] (X=Cl, Br; Bi‐Se, 3.35–3.37 Å). In fact, they are in the range of values reported for regular covalent Bi−S/Se bonds in compounds of type Bi(aryl)2(EPh) (E=S: 2.54–2.63 Å; E=Se: 2.70–2.73 Å).[ , , ] In agreement with these findings, the P1−S1/Se1 bonds in 2‐SPMe (2.03 Å) and 2‐SePMe (2.19 Å) are significantly elongated as compared to those in SPMe3 (1.97 Å, Supporting Information) and SePMe3 (2.12 Å, Supporting Information). This effect is more pronounced in 2‐SPMe than in literature‐known metal complexes with terminal SPMe3 ligands that have been crystallographically characterized (M=Cr, Fe, Cu, In); 2‐SePMe is the first complex of the SePMe3 ligand that has been crystallographically characterized so that no direct comparison is possible.

Figure 1

Molecular structures of [Bi(C6H4)2C2H2(EPMe3)][SbF6] in the solid state: a) E=S: 2‐SPMe; b) E=Se: 2‐SePMe). Displacement ellipsoids are drawn at the 50 % probability level. Hydrogen atoms and a lattice‐bound CH2Cl2 molecule in the structure of 2‐SPMe are omitted for clarity. Selected bond lengths (Å) and bond angles (°): 2‐SPMe: Bi1−C1, 2.248(6); Bi1−C14, 2.246(5); Bi1−S1, 2.6105(16); P1−S1, 2.026(2); C1−Bi1−C14, 85.1(2); C1−Bi1−S1, 90.59(15); C14−Bi1−S1, 88.81(14); Bi1−S1−P1, 106.12(8). 2‐SePMe: Bi1−C1, 2.237(4); Bi1−C14, 2.240(4); Bi1−Se1, 2.7222(4); P1−Se1, 2.1889(11); C1−Bi1−C14, 87.77(13); C1−Bi1−Se1, 91.24(10); C14−Bi1−Se1, 92.08(10); Bi1−Se1−P1, 100.31(3).

Molecular structures of [n>an class="CellLine">Bi(C6H4)2C2H2(EPMe3)][SbF6] in the solid state: a) E=S: 2‐SPMe; b) E=Se: 2‐SePMe). Displacement ellipsoids are drawn at the 50 % probability level. Hydrogen atoms and a lattice‐bound CH2Cl2 molecule in the structure of 2‐SPMe are omitted for clarity. Selected bond lengths (Å) and bond angles (°): 2‐SPMe: Bi1−C1, 2.248(6); Bi1−C14, 2.246(5); Bi1−S1, 2.6105(16); P1−S1, 2.026(2); C1−Bi1−C14, 85.1(2); C1−Bi1−S1, 90.59(15); C14−Bi1−S1, 88.81(14); Bi1−S1−P1, 106.12(8). 2‐SePMe: Bi1−C1, 2.237(4); Bi1−C14, 2.240(4); Bi1−Se1, 2.7222(4); P1−Se1, 2.1889(11); C1−Bi1−C14, 87.77(13); C1−Bi1−Se1, 91.24(10); C14−Bi1−Se1, 92.08(10); Bi1−Se1−P1, 100.31(3).

The spectroscopic and structurpan class="Chemical">al ann>an class="Chemical">alyses of 2‐EPMe revealed considerable bonding interactions between the Lewis acidic bismuth atoms in these compounds and the soft Lewis bases EPMe3 (E=S, Se). With its large, diffuse, and polarizable atomic orbitals, bismuth can indeed be expected to generate a soft Lewis acidity in its molecular complexes. Differences in the hardness/softness of Lewis acids have previously been discussed for small series of compounds such as B(C6F5)(OC6F5)3− (n=0–3), where the choice of the method for quantification can strongly affect the outcome of the measurement (e.g., OPEt3 as a donor in the Gutmann–Beckett (GB)[ , , ] method vs. crotonaldehyde as a donor in Childs method). To the best of our knowledge, however, attempts to assess the softness of a broader range of bismuth Lewis acids have not been reported to date. We suggest here a modification of the GB method in order to compare the softness of Lewis acids in an operationally simple approach. In the original GB method, a sample containing OPEt3 and a (potential) Lewis acid is analyzed by 31P NMR spectroscopy. An acceptor number (AN) can be calculated according to Equation 1:

where AN=0 corresponds to OPEt3 in n>an class="Chemical">hexane and AN=100 corresponds to OPEt3⋅SbCl5 in dichloroethane. We have exchanged the hard, oxygen‐based donor in the original GB method for the softer donors SPMe3 and SePMe3. In analogy with the original GB method, acceptor numbers AN (SPMe3) and AN (SePMe3) may be determined according to Equations 2 and 3:

where AN=0 corresponds to Epan class="Chemical">PMe3 in n>an class="Chemical">CH2Cl2 and AN=100 corresponds to EPMe3 with one equiv GaI3 in CH2Cl2 (E=S, Se). GaI3 was chosen as a reference point out of a range of potential candidates, because it is a simple, readily available, strong and soft Lewis acid that reacts with EPMe3 in simple and predictable Lewis pair formations in a 1:1 ratio (SbCl5, SbF5, and Bi(OTf)3, for instance, had to be ruled out due to side reactions; see Supporting Information).

With pan class="Chemical">SPMe3 as a Lewis ban>an class="Gene">se, aryl bismuth compounds and halobismepines showed no or only minor interactions according to acceptor numbers of AN (SPMe3)=0–12 (Table 1, entries 2–6). BiCl3, BiBr3, and BiI3 showed moderate to minor interactions with AN (SPMe3)=26, 17, and 13, respectively (entries 7–9). For compounds of type BiR2X and BiX3, the acceptor numbers increase with increasing electronegativity of the halide X. Stronger interactions were detected for the bismepine triflate Bi(diaryl)(OTf) (3) and Bi(OTf)3 (AN (SPMe3)=44–52; entries 10, 11; diaryl=[(C6H4)2C2H2]2−). Very high acceptor numbers of AN (SPMe3)=85–96 were obtained for the bismepine cation 1, its corresponding SPMe3 adduct 2‐SPMe, and the diphenyl bismuth cation [BiPh2(thf)2][SbF6] (4), all of which bear [SbF6]− counteranions (entries 12–14). The slightly lower acceptor number of 4 is ascribed to the freely accessible bismuth‐centered empty 6p‐orbital in the complex fragment [BiPh2]+, which allows binding of two equivalents of a Lewis base rather than only one as in 2‐SPMe. This is further supported by a single‐crystal X‐ray analysis of [BiPh2(SPMe3)2][SbF6], obtained from reaction of 4 with one equiv SPMe3 (Supporting Information).

Table 1

Investigations of potential Lewis acids with the modified Gutmann–Beckett method, using SPMe3 as a donor.

Entry	Compound	δ 31P [ppm][a]	AN (SPMe3)[b]
1	SPMe3	29.2	0 (by definition)
2	BiPh3	30.2	6
3	Bi2(diaryl)3 [a]	29.2	0
4	Bi(diaryl)Cl[a]	31.0	12
5	Bi(diaryl)Br[a]	30.3	7
6	Bi(diaryl)I[a]	29.2	0
7	BiCl3	33.2	26
8	BiBr3	31.8	17
9	BiI3	31.2	13
10	Bi(OTf)3	37.3	52
11	Bi(diaryl)(OTf) (3)[c,d]	36.1	44
12	1	44.0	95
13	2‐SPMe3	44.2[e]	96
14	[BiPh2(thf)2][SbF6] (4)	42.5	85
15	Me3SiOTf	29.3	1
16	B(C6F5)3	35.8	42
17	AlCl3	41.3	78
18	GaI3	44.8	100 (by definition)
19	B(C6F5)3(thf)	29.4	1
20	AlCl3(thf)	29.7	3
21	GaI3(thf)	43.4	91
22	GaI3(py)[f]	30.2	6
23	2‐SPMe3 + py[f]	40.8	74
24	2‐SPMe3 + 2 py[f]	35.8	42

[a] If not otherwise noted, CD2Cl2 solutions of equimolar amounts of the potential Lewis acid and SPMe3 were investigated at 23 °C (for details see experimental part). [b] determined according to equation 2 (see text). [c] diaryl=[(C6H4)2C2H2]2− as in compound 1; that is, [Bi(diaryl)]+ corresponds to a cationic dibenzobismepine complex fragment. [d] small amounts of THF were added to fully solubilize 3 (CD2Cl2/THF=25:1, v/v). [e] obtained from NMR spectroscopic analysis of isolated 2‐SPMe in CD2Cl2. [f] py=pyridine.

Investigations of potential n>an class="Chemical">Lewis acids with the modified Gutmann–Beckett method, using SPMe3 as a donor.

δ pan class="Chemical">31P [ppm][a]

AN (pan class="Chemical">SPMe3)[b]

pan class="Chemical">SPMe3

pan class="Chemical">Bipan class="Chemical">Ph3

pan class="Chemical">Bi2(diaryl)3 [a]

pan class="Chemical">Bi(diaryl)Cl[a]

pan class="Chemical">Bi(diaryl)Br[a]

pan class="Chemical">Bi(diaryl)I[a]

pan class="Chemical">BiCl3

pan class="Chemical">BiBr3

pan class="Chemical">BiI3

pan class="Chemical">Bi(OTf)3

pan class="Chemical">Bi(diaryl)(OTf) (3)[c,d]

2‐SPMe

44.2

pan class="Chemical">[BiPh2(thf)2][SbF6] (4)

pan class="Chemical">Me3SiOTf

pan class="Chemical">B(C6F5)3

pan class="Chemical">AlCl3

pan class="Chemical">GaI3

pan class="Chemical">B(C6F5)3(pan class="Chemical">thf)

pan class="Chemical">AlCl3(pan class="Chemical">thf)

pan class="Chemical">GaI3(pan class="Chemical">thf)

pan class="Chemical">GaI3(py)[f]

2‐pan class="Chemical">SPMe3 + py[f]

2‐pan class="Chemical">SPMe3 + 2 py[f]

[a] If not otherwipan class="Gene">se noted, n>an class="Chemical">CD2Cl2 solutions of equimolar amounts of the potential Lewis acid and SPMe3 were investigated at 23 °C (for details see experimental part). [b] determined according to equation 2 (see text). [c] diaryl=[(C6H4)2C2H2]2− as in compound 1; that is, [Bi(diaryl)]+ corresponds to a cationic dibenzobismepine complex fragment. [d] small amounts of THF were added to fully solubilize 3 (CD2Cl2/THF=25:1, v/v). [e] obtained from NMR spectroscopic analysis of isolated 2‐SPMe in CD2Cl2. [f] py=pyridine.

In order to allow a disn>an class="Chemical">cussion of Bi‐SPMe3 interactions in the broader context of Lewis acidity, Me3SiOTf, B(C6F5)3, and AlCl3 were chosen as typical examples of frequently applied, (relatively) hard Lewis acids and investigated with the modified GB method (for further examples see Supporting Information). It should be noted that for cationic Me3SiOTf an extraordinarily high acceptor number towards OPEt3 of AN (OPEt3)=116 was determined (Supporting Information). In contrast, an acceptor number of only AN (SPMe3)=1 was found for the softer donor SPMe3 (entry 15), demonstrating that the exceptional Lewis acidity of cationic bismuth species towards soft donors is not only due to their ionic character, but also to the shape, energy, and accessibility of their LUMO. Acceptor numbers AN (SPMe3) of 42 and 78 were obtained for the boron and the aluminum compound, respectively (entries 16, 17), which are significantly lower than those of the cationic bismuth species 1, 2‐SPMe, and 4 (entries 12–14). It should be noted that compound 1 contains two equivalents of thf and that the addition of 1 equiv SPMe3 to a solution of 1 gave an acceptor number which is virtually identical to that of isolated 2‐SPMe (entries 12, 13). In other words, the softer donor SPMe3 can efficiently displace two equivalents of the harder donor thf from the coordination sphere of the bismuth atom in 1. In order to evaluate the preference of the harder Lewis acids B(C6F5)3 and AlCl3 for either thf or SPMe3, samples containing one equivalent of the Lewis acid, one equivalent of thf, and one equivalent of SPMe3 were investigated. Acceptor numbers AN (SPMe3) of 1 and 3 were obtained (entries 19, 20), demonstrating that the softer donor SPMe3 cannot compete with the harder donor thf for the relatively hard binding sites in B(C6F5)3 and AlCl3.

The prepan class="Chemical">ference to n>an class="Chemical">bind a soft donor SPMe3 or a hard donor such as pyridine was also compared for compound 2‐SPMe and GaI3. While GaI3 showed only a minor acceptor number of 6 in the presence of one equivalent of pyridine, significant acceptor numbers of 74 and 42 were obtained for compound 2‐SPMe in the presence of one and two equivalents of pyridine, respectively (entries 22–24). Only with a large (≫20‐fold) excess of pyridine, values of AN (SPMe3) <10 were obtained for compound 2‐SPMe (for titration experiments see Supporting Information).

With pan class="Chemical">SePMe3 as a Lewis ban>an class="Gene">se, the trends were similar to those observed for SPMe3. Aryl bismuth compounds, halobismepines, and in this case even all bismuth halides BiX3 (X=Cl–I) showed acceptor numbers suggesting weak or even negligible Lewis acid/base interactions (AN (SePMe3)=0–14; Table 2, entries 2–9). For the bismuth triflates Bi(OTf)3 and 3, acceptor numbers of AN (SePMe3)=46 and 61 demonstrate significant Bi⋅⋅⋅SePMe3 bonding (entries 10, 11). For Bi(OTf)3, extended reaction times led to the appearance of an additional resonance in the 31P NMR spectrum, which is not due to a simple 1:1 adduct and is tentatively ascribed to the formation of [Bi(SePMe3)6][OTf]3 (Supporting Information). High acceptor numbers of 65–76 were obtained for cationic bismuth compounds 1, 2‐SePMe, and 4, indicating considerable interactions with SePMe3 (entries 12–14).

Table 2

Investigations of potential Lewis acids with the modified Gutmann–Beckett method, using SePMe3 as a donor.

Entry	Compound	δ 31P [ppm][a]	AN (SePMe3)[b]
1	SePMe3	7.8	0 (by definition)
2	BiPh3	8.9	6
3	Bi2(diaryl)3 [c]	7.8	0
4	Bi(diaryl)Cl[c]	9.0	7
5	Bi(diaryl)Br[c]	8.4	3
6	Bi(diaryl)I[c]	8.0	1
7	BiCl3	10.3	14
8	BiBr3	9.7	11
9	BiI3	9.5	10
10	Bi(OTf)3	15.8[d]	46
11	Bi(diaryl)(OTf) (3)[d,e]	18.5	61
12	1	19.2	65
13	2‐SePMe3	21.1[f]	76
14	[BiPh2(thf)2][SbF6] (4)	20.5	73
15	Me3SiOTf	7.9	1
16	B(C6F5)3	13.0	30
17	AlCl3	20.5	73
18	GaI3	25.3	100 (by definition)
19	B(C6F5)3(thf)	7.9	1
20	AlCl3(thf)	8.0	1
21	GaI3(thf)	24.2	94
22	GaI3(py)[g]	9.0	7
23	2‐SePMe3+py[g]	16.5	50
24	2‐SePMe3+2 py[g]	15.3	43

[a] If not otherwise noted, CD2Cl2 solutions of equimolar amounts of the potential Lewis acid and SPMe3 were investigated at 23 °C (for details see experimental part). [b] determined according to equation 3 (see text). [c] diaryl=[(C6H4)2C2H2]2− as in compound 1; that is, [Bi(diaryl)]+ corresponds to a cationic dibenzobismepine complex fragment. [d] An additional resonance is detected after extended reaction times (for discussion see text and Supporting Information). [e] small amounts of THF were added to fully solubilize 3 (CD2Cl2/THF=25:1, v/v). [f] obtained from NMR spectroscopic analysis of isolated 2‐SePMe in CD2Cl2. [g] py=pyridine.

Investigations of potential n>an class="Chemical">Lewis acids with the modified Gutmann–Beckett method, using SePMe3 as a donor.

δ pan class="Chemical">31P [ppm][a]

AN (Span class="Chemical">ePMe3)[b]

pan class="Chemical">SePMe3

pan class="Chemical">Bipan class="Chemical">Ph3

pan class="Chemical">Bi2(diaryl)3 [c]

pan class="Chemical">Bi(diaryl)Cl[c]

pan class="Chemical">Bi(diaryl)Br[c]

pan class="Chemical">Bi(diaryl)I[c]

pan class="Chemical">BiCl3

pan class="Chemical">BiBr3

pan class="Chemical">BiI3

pan class="Chemical">Bi(OTf)3

pan class="Chemical">Bi(diaryl)(OTf) (3)[d,e]

2‐pan class="Gene">SePMe

21.1

pan class="Chemical">[BiPh2(thf)2][SbF6] (4)

pan class="Chemical">Me3SiOTf

pan class="Chemical">B(C6F5)3

pan class="Chemical">AlCl3

pan class="Chemical">GaI3

pan class="Chemical">B(C6F5)3(pan class="Chemical">thf)

pan class="Chemical">AlCl3(pan class="Chemical">thf)

pan class="Chemical">GaI3(pan class="Chemical">thf)

pan class="Chemical">GaI3(py)[g]

2‐pan class="Chemical">SePMe3+py[g]

2‐pan class="Chemical">SePMe3+2 py[g]

[a] If not otherwipan class="Gene">se noted, n>an class="Chemical">CD2Cl2 solutions of equimolar amounts of the potential Lewis acid and SPMe3 were investigated at 23 °C (for details see experimental part). [b] determined according to equation 3 (see text). [c] diaryl=[(C6H4)2C2H2]2− as in compound 1; that is, [Bi(diaryl)]+ corresponds to a cationic dibenzobismepine complex fragment. [d] An additional resonance is detected after extended reaction times (for discussion see text and Supporting Information). [e] small amounts of THF were added to fully solubilize 3 (CD2Cl2/THF=25:1, v/v). [f] obtained from NMR spectroscopic analysis of isolated 2‐SePMe in CD2Cl2. [g] py=pyridine.

Anpan class="Chemical">alyn>an class="Chemical">sis of Me3SiOTf, B(C6F5)3, and AlCl3 with the modified GB method revealed that the Si species shows a negligible acceptor number, while the B and the Al compound show moderate to high acceptor numbers towards the soft donor SePMe3 (entries 15–17). But in contrast to the bismuth cation 1 (entries 12, 13), the presence of one equivalent of the hard donor thf leads to negligible B/Al⋅⋅⋅SePMe3 interactions, as judged from acceptor numbers of only 1 in both cases (entries 19, 20). Similar results were obtained for competition experiments with GaI3 and 2‐SePMe as Lewis acids and pyridine and SePMe3 as Lewis bases. GaI3 showed a minor acceptor number of 7 in the presence of pyridine, while acceptor numbers of 50 and 43 indicate significant Bi⋅⋅⋅SePMe3 interactions for 2‐SPMe in the presence of one and two equivalents of pyridine, respectively (entries 22–24). A large (≫20‐fold) excess of pyridine was necessary to lower the acceptor number AN (SePMe3) to an insignificant value of 2 for compound 2‐SePMe (for titration experiments see Supporting Information).

Major findings from our studies of pan class="Chemical">bismuth‐centered n>an class="Chemical">Lewis acidity based on the GB method (previous work and Supporting Information) and modified versions thereof (this work) are summarized in Figure 2. As a trend it is apparent that bismuth Lewis acids of class A with a σ*(Bi‐X) acceptor orbital (cf. introduction) show moderate Lewis acidities towards the strong and hard donor OPEt3, but only relatively low Lewis acidities towards weaker and softer donors SPMe3 and SePMe3. In contrast, bismuth Lewis acids of class C with an empty 6p(Bi) acceptor orbital show a considerable Lewis acidity towards all three types of donors. It is especially remarkable that (in contrast to Me3SiOTf, B(C6F5)3, and AlCl3) cationic bismuth compounds of class C maintain an extraordinary Lewis acidity towards the soft donors SPMe3 and SePMe3 even in the presence of a hard donor such as thf and pyridine.

Figure 2

Comparison of acceptor numbers for selected Lewis acids obtained from the Gutmann–Beckett method (top, previous results and this work) and modified versions (middle and bottom, this work). “Organobismuth cations” refers to class C compounds (cf. introduction and ref. [12]) without strong directional cation⋅⋅⋅anion interactions.

Comparison of acceptor numbers for selected n>an class="Chemical">Lewis acids obtained from the Gutmann–Beckett method (top, previous results and this work) and modified versions (middle and bottom, this work). “Organobismuth cations” refers to class C compounds (cf. introduction and ref. [12]) without strong directional cation⋅⋅⋅anion interactions.

DFT cpan class="Chemical">aln>an class="Chemical">culations and natural bond orbital (NBO) analyses were performed in order to characterize the Bi−S/Se interactions in compounds 2‐EPMe in more detail (E=S, Se; for details see experimental and Supporting Information). The bismuth Lewis acidic component in these complexes is the (so far non‐isolable) low‐valent bismepine cation [Bi((C6H4)2C2H2)][SbF6]. A frontier orbital analysis of this species was carried out, with its conformation fixed to that found in the optimized structure of the adduct 2‐SePMe. The LUMO of [Bi((C6H4)2C2H2)][SbF6] is best described as an empty bismuth 6p‐orbital with only minor contributions from other atomic orbitals (Figure 3 a). The bent conformation of the bismepine core results in steric protection of one lobe of the LUMO by the olefinic functional group of the ligand backbone. This should favor adduct formation in a 1:1 stoichiometry with donors that have at least a moderate steric load, as experimentally observed for 2‐SPMe and 2‐SePMe. Theoretical analysis of 2‐SPMe and 2‐SePMe indicate that the Bi−S/Se interactions can be interpreted as regular covalent bonds, the NBOs of which are mainly composed of bismuth 6p atomic orbitals and 3/4p S/Se atomic orbitals and strongly polarized towards the chalcogen (S: 79.9 %; Se: 78.0 %). The molecular orbitals with significant Bi−S/Se bonding contributions are the HOMO−5 (only for 2‐SePMe) as well as the HOMO−6 and HOMO−7 (for 2‐SPMe and 2‐SePMe), which are mainly composed through linear combinations of NBOs associated with Bi−S/Se (5–25 %) and Bi−C bonds (8–29 %) as well as Bi (10 %) and S/Se lone pairs (7–32 %) (Figure 3 b and Supporting Information). In agreement with these results, natural resonance theory (NRT) revealed exclusively resonance structures featuring R2Bi‐(S/Se)‐P+Me3 structural motifs (with mesomeric effects being relevant only within the bismepine core and the (SbF6)− anion; resonance structure of the cations shown in Figure 3 c). According to NRT, the Bi−S/Se single bonds are generated through 40 %/44 % covalent and 60 %/56 % ionic contributions. This is in agreement with large Wiberg bond indices of 0.61 (Bi−S) and 0.65 (Bi−Se) for these bonds.

Figure 3

a) LUMO of [Bi(C6H4)2C2H2][SbF6] with its conformation fixed to that found in the optimized structure of 2‐SePMe. b) Selected molecular orbitals (top) and NBOs (bottom) of 2‐SePMe with isovalues of 0.04. c) Resonance structure of the 2‐EPMe according to NRT (E=S, Se; [SbF6]− omitted for clarity; only one (out of many) resonance structures of the bismepine core is depicted (see text)).

a) LUMO of [n>an class="CellLine">Bi(C6H4)2C2H2][SbF6] with its conformation fixed to that found in the optimized structure of 2‐SePMe. b) Selected molecular orbitals (top) and NBOs (bottom) of 2‐SePMe with isovalues of 0.04. c) Resonance structure of the 2‐EPMe according to NRT (E=S, Se; [SbF6]− omitted for clarity; only one (out of many) resonance structures of the bismepine core is depicted (see text)).

Thermodynamic parameters of adduct formations between Epan class="Chemical">PMe3 and n>an class="Chemical">Lewis acids based on bismuth and group 13 elements (B–Ga) were determined (E=O, S, Se; Table 3 (ΔG values) and Supporting Information (ΔH values and additional examples)). Starting from coordinatively unsaturated Lewis acids, the adduct formations are strongly exergonic (ΔG=−20 to −45 kcal mol−1) for all compounds but B(C6F5)3, which shows a mildly exergonic reaction with OPMe3 and endergonic reactions with S/SePMe3 (entries 1–6, for ΔH values see Supporting Information). Most importantly, reactions of all the group 13 compounds become less exergonic (or even endergonic) upon changing the Lewis base from OPMe3 to S/SePMe3 (entries 1–5). The opposite behavior is observed for the cationic bismuth species (entry 6), underlining its soft character according to the HSAB principle.

Table 3

Calculated free reaction enthalpy of Lewis pair formation with varying Lewis acids and donors EPMe3 (E=O, S, Se) in the presence of 0–2 equivalents of thf.

Entry	[LA]	n	ΔG [kcal mol−1]
			E=O	E=S	E=Se
1	B(C6F5)3	0	−0.7	+16.7	+18.3
2	AlCl3	0	−41.8	−23.0	−22.0
3	GaCl3	0	−39.6	−25.0	−24.4
4	GaBr3	0	−37.7	−24.3	−23.8
5	GaI3	0	−32.7	−19.5	−19.6
6	[Bi(diaryl)][SbF6][a]	0	−38.6	−43.6	−44.8
7	B(C6F5)3	1	+8.8	+26.3	+27.9
8	AlCl3	1	−10.8	+8.1	+9.1
9	GaCl3	1	−10.4	+4.2	+4.8
10	GaBr3	1	−10.9	+2.5	+3.0
11	GaI3	1	−10.3	+2.9	+2.8
12	[Bi(diaryl)][SbF6][a]	1	−12.4	−17.4	−18.6
13	[Bi(diaryl)][SbF6][a]	2	+0.3	−4.7	−5.9

[a] diaryl=[(C6H4)2C2H2]2−; that is, [LA]=[Bi(diaryl)][SbF6] with two thf ligands (n=2) corresponds to compound 1.

Caln>an class="Chemical">culated free reaction enthalpy of Lewis pair formation with varying Lewis acids and donors EPMe3 (E=O, S, Se) in the presence of 0–2 equivalents of thf.

ΔG [kcpan class="Chemical">al mol−1]

E=pan class="Gene">Se

pan class="Chemical">B(C6F5)3

pan class="Chemical">AlCl3

pan class="Chemical">GaI3

pan class="Chemical">[Bi(diaryl)][SbF6][a]

pan class="Chemical">B(C6F5)3

pan class="Chemical">AlCl3

pan class="Chemical">GaI3

pan class="Chemical">[Bi(diaryl)][SbF6][a]

pan class="Chemical">[Bi(diaryl)][SbF6][a]

[a] diaryl=[(C6H4)2C2H2]2−; that is, [LA]=n>an class="Chemical">[Bi(diaryl)][SbF6] with two thf ligands (n=2) corresponds to compound 1.

In order to evaluate trends in the an>an class="Chemical">bility of donors EPMe3 to displace one or two thf ligands from the coordination sphere of coordinatively saturated Lewis acids (from a thermodynamic point of view), reactions with compounds [LA]‐(thf) were investigated ([LA]=Lewis acid; n=1–2). With the exception of B(C6F5)3 (entry 7), exergonic reactions are observed for the group 13 compounds with OPMe3 (ΔG=−10 to −11 kcal mol−1, n=1; entries 8–11), while those with bismuth compounds are clearly exergonic for n=1 and marginally endergonic for n=2 (ΔG=−12.4 and +0.3 kcal mol−1; entries 12,13). Ligand substitutions with S/SePMe3 at the hard aluminum center are endergonic by 8 to 9 kcal mol−1 (entry 8). For the softer gallium compounds, substitution of thf by S/SePMe3 is only slightly endergonic (ΔG=+3 to +5 kcal mol−1, n=1; entries 9–11). For the bismuth cations, these reactions are clearly exergonic for n=1 (ΔG=−17 to −19 kcal mol−1, entry 12) and still slightly exergonic for n=2 (ΔG=−5 to −6 kcal mol−1, entry 13). These trends are in good agreement with experimental results obtained from the modified GB method.

Conclusions

The apan class="Chemical">bility of n>an class="Chemical">bismuth(III) compounds to form Lewis acid/base adducts with the soft donors EPMe3 has been investigated (E=S, Se). Cationic bismuth compounds [BiR2(EPMe3)][SbF6] featuring rare Bi⋅⋅⋅EPR3 interactions were isolated and fully characterized. The bismuth atoms in these compounds show coordination numbers of three, which is extremely unusual for cationic bismuth species without directional Bi⋅⋅⋅counteranion interactions. Detailed experimental and theoretical analyses revealed significant Bi⋅⋅⋅EPR3 bonding with strong covalent contributions that persists in the solid state and in solution. The 31P NMR chemical shift of EPMe3 in the presence of a compound “LA” may be used as an easily accessible experimental parameter to investigate the Lewis acidity of LA. In specific, we suggest the utilization of soft donors such as EPMe3 in order to assess the hardness/softness of a Lewis acid. This is equivalent to an extension of the Gutmann–Beckett method. We have investigated bismuth compounds of type BiR3, BiR2X, BiX3, and [BiR2]+ with this approach, delivering experimental evidence for their soft Lewis acidity. Especially cationic bismuth species [BiR2]+ that interact with the donor through an empty p‐orbital (not through a σ*‐orbital) are potent soft Lewis acids. In contrast to well‐established, relatively hard Lewis acids such as Me3SiOTf, B(C6F5)3, and AlCl3 and the softer Lewis acid GaI3, they can still efficiently activate soft donors in the presence of hard donors such as thf and pyridine (cf. Figure 2). Future research efforts will be directed towards the exploitation of these findings in the activation of substrates with soft donor functionalities for stoichiometric and catalytic transformations.

Experimental Section

General considerations

pan class="Chemical">All air‐ and moisture‐n>an class="Gene">sensitive manipulations were carried out using standard vacuum line Schlenk techniques or in gloveboxes containing an atmosphere of purified argon. Solvents were degassed and purified according to standard laboratory procedures. NMR spectra were recorded on Bruker instruments operating at 400 or 500 MHz with respect to 1H. 1H and 13C NMR chemical shifts are reported relative to SiMe4 using the residual 1H and 13C chemical shifts of the solvent as a secondary standard. 31P and 77Se NMR chemical shifts are reported relative to H3PO4 (85 % aqueous solution) and SeMe2 (plus 5 % C6D6) as external standards. NMR spectra were recorded at ambient temperature (typically 23 °C), if not otherwise noted. Elemental analyses were performed on a Leco or a Carlo Erba instrument. Single‐crystals suitable for X‐ray diffraction were coated with polyisobutylene or perfluorinated polyether oil in a glovebox, transferred to a nylon loop and then transferred to the goniometer of a diffractometer equipped with a molybdenum X‐ray tube (λ=0.71073 Å). The structures were solved using intrinsic phasing methods (SHELXT) completed by Fourier synthesis and refined by full‐matrix least‐squares procedures.

Deposition Numbers 1961401, 1961402, 1961405, 1961406, 1961407, and 1994719, 1994720, 1994721 contain the supn>plementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.

DFT cpan class="Chemical">aln>an class="Chemical">culations were performed with the Gaussian program using the 6‐31G(d,p) [H, C, N, O, F], 6–311G(d,p) [Al, P, S], and the LANL2DZ [Ga, In, Se, Sb, Bi, Br, I] basis set and the B3LYP functional. The D3 version of Grimme's dispersion model with the original D3 damping function was applied. Frequency analyses of the reported structures showed no imaginary frequencies for ground states. Thermodynamic parameters were calculated at a temperature of 298.15 K and a pressure of 1.00 atm. NBO analyses were performed using the program version NBO 7.

The following labeling Scheme has been used for the asn>an class="Chemical">signment of atoms to resonances detected in NMR spectroscopic experiments:

[Bi((C: To a solution of n>an class="Chemical">[Bi((C6H4)2C2H2)(thf)2][SbF6] (1) (30.0 mg, 39.1 μmol) in CH2Cl2 (0.7 mL) was added SPMe3 (4.2 mg, 39.1 μmol) at ambient temperature. The light yellow solution was layered with n‐pentane (0.7 mL) and stored at −30 °C. A pale yellow solid had precipitated after 1 d, was isolated by filtration, and dried in vacuo. Yield: 26.0 mg, 35.6 μmol, 91 %. 1H NMR (400 MHz, CD2Cl2): δ=2.21 (d, 9 H, 2 J PH=13.3 Hz, PMe3), 7.09 (s, 2 H, H‐7, H‐8), 7.43 (t, 2 H, 3 J HH=7.7 Hz, C6H4), 7.71 (m, 4 H, C6H4), 8.41 (m, 2 H, C6H4)  ppm. 1H NMR (500 MHz, [D8]THF): δ=1.94 (d, 12 H, 2 J PH=13.7 Hz, PMe3), 6.75 (s, 2 H, H‐7, H‐8), 7.40 (ddd, 2 H, 4 J HH=1.1 Hz, 3 J HH=7.4 Hz, 3 J HH=7.7 Hz, H‐4, H‐11) 7.59 (ddd, 2 H, 4 J HH=1.3 Hz, 3 J HH=7.4 Hz, 3 J HH=7.4 Hz, H‐3, C‐12), 7.84 (dd, 2 H, 4 J HH=1.1 Hz, 3 J HH=7.7 Hz, H‐5, H‐10), 8.18 (dd, 2 H, 4 J HH=1.3 Hz, 3 J HH=7.4 Hz, H‐2, H‐13) ppm. 13C NMR (126 MHz, [D8]THF): δ=19.15 (d, 1 J CP=53.8 Hz, PMe3), 129.76 (s, H‐4, H‐11), 130.15 (s, C‐3, C‐12), 132.95 (s, C‐7, C‐8), 135.91 (s, C‐5, C‐10), 136.95 (s, C‐2, C‐13), 146.21 (s, C‐6, C‐9), 176.86 (br, C‐1, C‐14, detected via 13C, 1H HMBC experiments) ppm. 31P NMR (202 MHz, CD2Cl2): δ=44.2 (s) ppm. 31P NMR (202 MHz, [D8]THF): δ=41.4 (s) ppm.

Elemental ann>an class="Chemical">alysis: Anal. calc. for: [C17H19BiPSSbF6] (731.10 g mol−1): C 27.93, H 2.62, S 4.39; found: C 28.19, H 2.58, S 4.36.

[Bi((C: To a solution of n>an class="Chemical">[Bi((C6H4)2C2H2)(thf)2][SbF6] (1) (20.0 mg, 26.1 μmol) in CH2Cl2 (0.5 mL) was added SePMe3 (4.2 mg, 26.1 μmol) at ambient temperature. The light yellow solution was layered with n‐pentane (0.7 mL) and stored at −30 °C. A pale yellow solid had precipitated after 1 d, was isolated by filtration, and dried in vacuo. Yield: 18.0 mg, 23.1 μmol, 87 %. 1H NMR (500 MHz, CD2Cl2): δ=2.23 (d, 9 H, 2 J PH=13.4 Hz, PMe3), 7.01 (s, 2 H, H‐7, H‐8), 7.43 (ddd, 2 H, 4 J HH=1.2 Hz, 3 J HH=7.6 Hz, 3 J HH=7.7 Hz, H‐4, H‐11) 7.63 (ddd, 2 H, 4 J HH=1.3 Hz, 3 J HH=7.5 Hz, 3 J HH=7.5 Hz, H‐3, C‐12), 7.72 (dd, 2 H, 4 J HH=1.2 Hz, 3 J HH=7.6 Hz, H‐5, H‐10), 8.43 (dd, 2 H, 4 J HH=1.2 Hz, 3 J HH=7.5 Hz, H‐2, H‐13) ppm. 13C NMR (126 MHz, CD2Cl2): δ=19.48 (d, 1 J CP=47.8 Hz, PMe3), 129.61 (s, H‐4, H‐11), 132.24 (s, C‐3, C‐12), 133.49 (s, C‐7, C‐8), 133.81 (s, C‐5, C‐10), 136.77 (s, C‐2, C‐13), 143.49 (s, C‐6, C‐9), 162.89 (br, C‐1, C‐14) ppm. 31P NMR (202 MHz, CD2Cl2): δ=21.1 (s, 1 J PSe=488.2 Hz) ppm. 31P NMR (202 MHz, THF): δ=19.1 (s) ppm. 77Se NMR (100 MHz, CD2Cl2): δ=−44.2 (br, detected via 1H, 77Se HMBC experiments) ppm.

Elemental ann>an class="Chemical">alysis: Anal. calc. for: [C17H19BiPSeSbF6] (778.01 g mol−1): C 26.24, H 2.46; found: C 25.88, H 2.48.

[Bin>an class="Chemical">Ph: To a solution of diphenylbismuth chloride (50.0 mg, 0.13 mmol) in THF (1 mL) was added a solution of AgSbF6 (43.1 mg, 0.13 mmol) in THF (0.5 mL). The colorless suspension was filtered. The filtrate was layered with n‐pentane (1.5 mL) and stored at −30 °C. The product was obtained after 2 d by filtration, and dried in vacuo. Yield: 83 mg, 0.11 mmol, 86 %. 1H NMR (500 MHz, CD2Cl2): δ=1.82 (m, 4 H, β‐thf), 3.70 (m, 4 H, α‐thf), 7.56–7.69 (m, 2 H, p‐C6H5), 8.02 (dd, 4 H, 3 J HH=6.6, 3 J HH=6.6 Hz, o‐C6H5), 8.47 (d, 4 H, 3 J HH=6.9 Hz, m‐C6H5)  ppm. 13C NMR (126 MHz, CD2Cl2): δ=26.03 (s, β‐thf), 71.12 (s, α‐thf), 130.86 (s, p‐C6H5), 133.63 (s, o‐C6H5), 137.64 (s, m‐C6H5) 198.46 (s, ipso‐C6H5)  ppm.

Elemental ann>an class="Chemical">alysis: Anal. calc. for: [C12H10BiSbF6](OC4H8)2 (743.16 g mol−1): C 32.32, H 3.53; found: C 32.42, H 3.64.

General procedure for modified Gutmann–Beckett method

If not otherwipan class="Gene">se noted, equimolar amounts of the potentin>an class="Chemical">al Lewis acid and the Lewis base EPMe3 were dissolved in dichloromethane (E=S, Se). In competition experiments, the required amount of another Lewis base was added (the sequence of addition was not relevant; see Supporting Information). One of the following three different methods was used for the determination of accurate 31P NMR chemical shifts: i) the use of CD2Cl2 as the solvent, so that locking and shimming was possible; ii) the use of CH2Cl2 as the solvent along with a capillary containing deuterated acetone, so that locking and shimming was possible; iii) the use of CH2Cl2 as the solvent along with a capillary containing an 85 % aqueous solution of H3PO4 as a reference. The three methods gave identical results, when applied to identical samples. For details see Supporting Information.

Conflict of interest

The authors declare no conflict of interest.

As a pan class="Gene">service to our authors and readers, this journn>an class="Chemical">al provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supplementary

Click here for additionpan class="Chemical">al data file.