Ligand properties of boryl ligands in bis-boryl rhodium(III) complexes: a case study

Abstract

The oxidative addition of five diborane(4) derivatives, symmetrical and unsymmetrical, to [Rh(PMe₃)₃Cl] was studied. Only for the more electron poor diboron derivatives, B₂cat₂, B₂pin₂ and catB–Bpin the resulting octahedral bis-boryl complexes [(PMe₃)₃Rh(boryl)₂Cl] were obtained, while for the more electron rich congeners only the equilibrium oxidative addition (catB–Bdmab) or no significant reaction (pinB–Bdmab) was observed (pin = (OCMe₂)₂, cat = 1,2-O₂C₆H₄, dmab = 1,2-(NMe)₂C₆H₄). By abstraction of the chlorido ligand with NaBArF (BArF = tetrakis-[3,5-bis-(trifluormethyl)-phenyl]-borat) in the presence of a neutral ligand (L = PMe₃, MeCN, MeNC) the corresponding cationic octahedral complexes [(PMe₃)₃Rh(boryl)₂L]⁺ were obtained. All isolated complexes were fully characterised including single crystal X-ray diffraction and heteronuclear, temperature dependent NMR spectroscopy. Whilst the complexes [(PMe₃)₃Rh(boryl)₂Cl] and [(PMe₃)₃Rh(boryl)₂L]⁺ show many similarities, their detailed structural and spectroscopic properties depend crucially on the properties of both boryl ligands.

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Introduction

Boryl complexes derived from diborane(4) derivatives, in particular B₂cat₂ (1a) and B₂pin₂ (1b) (pin = (OCMe₂)₂, cat = 1,2-O₂C₆H₄), are central as reactive intermediates for an ever growing number of transition metal catalysed borylation reactions.² Despite their broad application, the relevant boryl complexes are still the subject of ongoing studies.

Theoretically it is well established that the substitution pattern of a boron moiety influences both its properties as a ligand in transition metal complexes as well as the reactivity of the corresponding diborane(4) derivatives.³ However, experimental studies of series of analogous boryl complexes with distinct boryl ligands are scarce.^3a

For such a study we rationalized that bis-boryl complexes accessible by oxidative addition of a diborane(4) to low-valent transition metal complexes should be suitable. In particular, when using unsymmetrical diborane(4) derivatives, unsymmetrical bis-boryl complexes result that allow the direct comparison of two distinct boryl ligands within one complex. Platinum(II) bis-boryl complexes of the type [(Ph₃P)₂Pt((boryl)₂] appear suitable complexes, however, we have shown that the boryl ligands in these complexes are not independent,⁴ as is – and possibly more pronounced – the case for bis /tris-boryl complexes of cobalt or iridium.^5,6 On the synthetic side, however, the oxidative addition of diboranes(4) to rhodium(I) complexes is reported for different aryloxy as well as alkyloxy based diboranes(4) (Fig. 1).

Fig. 1 Examples of rhodium bis- and tris-boryl complexes accessible from diboranes(4) (cat* derivatives = 1,2-O₂-4-tBuC₆H₃, 1,2-O₂-3,5-tBu₂C₆H₂, 1,2-O₂-3-MeC₆H₃, 1,2-O₂-4-MeC₆H₃, 1,2-O₂-3-MeOC₆H₃, 1,2-S₂C₆H₄; tart = R,R-(OCH(CO₂Me))₂).¹

Given the plethora of reported – though, not necessarily structurally characterized – rhodium(III) bis-boryl complexes we endeavoured to employ the oxidative addition of diboranes(4) to a rhodium(I) complex to access a series of analogous bis-boryl complexes. In particular the report by Marder, Norman and co-worker on the reaction of [RhCl(PR₃)₃] with 1a to give 2b, inspired us to use the sterically non-demanding and electron-rich complex [RhCl(PMe₃)₃] for further studies.^1d Moreover, it could be envisaged that the complexes [(PMe₃)₃RhCl(boryl)₂] resulting from an oxidative addition of a diborane(4) can be converted to the cations [Rh(PMe₃)₄(boryl)₂]⁺ upon reaction with PMe₃ in the presence of a salt of a weakly coordinating anion. Both classes of complexes should provide ample structural as well as NMR-spectroscopic probes to characterize the donor properties of the boryl ligands.

Results and discussion

In situ NMR study on the reaction of [Rh(PMe₃)₃Cl] with diboranes(4): [(PMe₃)₃Rh(boryl)₂Cl]

For an initial assessment of the scope of the formation of bis-boryl complexes by reaction of [Rh(PMe₃)₃Cl] with different diboranes(4) (Scheme 1) an in situ NMR study was conducted with a series of five diborane(4) derivatives: the symmetrical diboranes(4) B₂cat₂ (1a) and B₂pin₂ (1b) as well as the unsymmetrical congeners catB–Bpin⁷ (1c), catB–Bdmab⁷ (1d) and pinB–Bdmab^4a (1e) (dmab = 1,2-(NMe)₂C₆H₄).

Scheme 1 Envisaged formation of rhodium(III) bis-boryl complexes by oxidative addition of diboranes(4) 1a–d as studied by NMR spectroscopy.

Following the reactions by in situ³¹P and ¹¹B NMR spectroscopy the progress of the reaction is indicated by a decrease of the ¹¹B NMR signal (or signals in the case of the unsymmetrical 1c–e) of the diboranes at, typically, below 38 ppm and the appearance of one or two signals above that, indicative for boryl complexes (Fig. 2).

Fig. 2 In situ ³¹P{¹H} and ¹¹B{¹H} NMR spectra of the reactions of [Rh(PMe₃)₃Cl] with the diboranes(4) 1a–e (121.5/96.3 MHz, rt); unassigned signals: unidentified species.⁸

The ¹¹B NMR data indicate that for 1a and 1c the diborane(4) is consumed rapidly, whereas for 1b a slower reaction is observed. The ³¹P NMR data give a fitting picture: for 1a–c after full conversion two signals are found, one doublet in the −10–0 ppm range and second one, a singlet, around −30 ppm. As expected for the formation of [(Me₃P)₃RhCl(boryl)₂] complexes. Both signals are however significantly broadened, as compared e.g. to the signals of the starting material [Rh(PMe₃)₃Cl] (Fig. 2), but also significantly shifted, indicating the formation of new complexes. The broad line shape of the ³¹P NMR signals is partly due to the quadrupolar nature of the ¹¹B/¹⁰B nuclei in the resulting boryl complexes, but also due to the dynamics present (vide infra). This is particular apparent in the reaction of 1b after 0.5 h, while unreacted 1b is present: the ³¹P NMR spectrum exhibit only two signals with chemical shifts between those for [Rh(PMe₃)₃Cl] and 2b, indicative for rabid exchange (vide infra).

The observations are different for the unsymmetrical diamino dialkoxy diboranes(4) 1d and 1e. Whilst for 1d the in situ NMR data indicate, according to the characteristic ¹¹B NMR chemical shift of just above 38 ppm, the formation of a boryl complex, in the ³¹P NMR spectrum only one very broad signal is unambiguously identified, presumably due to rapid exchange between a putative boryl complex and [Rh(PMe₃)₃Cl]. Whereas for 1e no consumption of the diborane(4) is evident, however, a broadening of the ³¹P NMR signals of [Rh(PMe₃)₃Cl] is stated.

This is well rationalized by the electronic properties of the diboranes(4) 1a–e. The electron poor B₂cat₂ (1a) is more prone to undergo oxidative addition to a metal than the comparably electron rich B₂pin₂ (1b), while the unsymmetrical tetraalkoxydiborane catB–Bpin (1c) exhibits an intermediate reactivity.

The unsymmetrical diamino dialkoxy diboranes(4), with their very electron rich diamino boron moiety, react more sluggishly (catB–Bdmab (1d)) if at all (pinB–Bdmab (1e)) (Fig. 2).⁸ This observation is in agreement with the general trend that the electron poor B₂cat₂ (1a) is more reactive in oxidative addition reactions than the more electron rich B₂pin₂ (1b) and diamino diborane(4) derivatives.^1,2,9

Isolation of [(PMe₃)₃Rh(boryl)₂Cl] (2a–c)

Based on the results of the in situ NMR experiments the isolation of bis-boryl complexes derived from 1a–d and [Rh(PMe₃)₃Cl] was explored, whilst the apparently much less reactive 1e was not included in further studies.

For 1a and 1c the bis-boryl tris-phosphine complexes [(Me₃P)₃RhCl(Bcat)₂] (2a) and [(Me₃P)₃RhCl(Bcat)(Bcat)] (2c) start to crystallise as colourless crystalline solids within minutes after combination of the solution of the starting materials (in THF/PhMe and pure PhMe, respectively) and were obtained in excellent 92% and 91% yield, respectively (Scheme 2).

Scheme 2 Synthesis of 2a–c, 3b and 2b·3b_n.

For 1b, however, the tris-phosphine complexes [(Me₃P)₃RhCl(Bpin)₂] (2b) does not crystallise from the reaction mixture. However, upon removal of the solvent in vacuo, extractive work-up and crystallization from n-pentane mixtures of 3b and 2b (2b·3b_n, up to 71% yield based on Rh) were obtained. Whereas extensive exposure to vacuum during the work-up led to the exclusive isolation of the bis-boryl bis-phosphine complex [(Me₃P)₂RhCl(Bpin)₂] (3b) (51% yield). However, 3b is readily converted to 2b by addition of a stoichiometric amount of PMe₃. An excess of PMe₃, however, facilitates, in agreement with earlier results on 2a, the decomposition of 2b/3b leading to the crystallisation of [Rh(PMe₃)₄]Cl and 1b.^1d

For the diborane 1d, already reacting sluggishly in the in situ NMR study, only [(Me₃P)₄RhH(Cl)][B(1,2-O₂C₆H₄)₂] and B₂dmab₂, possible decomposition products of an intermediate rhodium boryl complex, were obtained.^8,10

It should be emphasised that this formation of the symmetrical diborane B₂dmab₂ from the unsymmetrical 1d is the only instance were scrambling of unsymmetrical diboranes(4) was observed. In particular was no evidence found for any scrambling of the distinct boryl ligand in 2c, putatively resulting either in the formation of symmetrical bis-boryl complexes or symmetrical diboranes(4).

Synthesis of cationic rhodium(III) bis-boryl complexes [(PMe₃)₃Rh(L)(boryl)₂][BArF]

Whilst the series of boryl complexes 2a–c allows for a comparison of the boryl ligand donor properties amongst the three different complexes, a comparison of different boryl ligands within a single complex would be interesting. Replacing the chlorido ligand in 2a–c by an additional PMe₃ ligand leads to the cationic complexes 4a–c (Scheme 3). These complexes, are straight-forwardly obtained upon reaction of 2a–c with an equimolar amount of PMe₃ in the presence of Na[BArF] (BArF = tetrakis-[3,5-bis-(trifluoromethyl)phenyl]borate). Complex 4c allows now for a comparison of two different boryl ligands within a single complex.

Scheme 3 Synthesis of the cationic complex [(PMe₃)₄Rh(boryl)₂][BArF].

The replacement of the chlorido ligand in 2c by other neutral ligands was, in our hands, limited to MeCN and MeNC leading to the well-defined complexes [Rh(PMe₃)₃(Bcat)(Bpin)(MeCN)][BArF] (5c) and [Rh(PMe₃)₃(Bcat)(Bpin)(MeNC)][BArF] (6c).⁸ Attempts to use other ligands such as CO, PEt₃ or P(OMe)₃ furnished in our hands [(Me₃P)₃Rh(CO)₂][BArF] or [(Me₃P)₄Rh][BArF] as the only crystallographically identified species.

Structural characterisation: [(PMe₃)₃Rh(boryl)₂Cl]

Single crystals of sufficient quality for X-ray diffraction studies were obtained from 2a–c. Whereas, 2a crystallises in the monoclinic space-group type P2₁/n with one molecule in the asymmetric unit, 2b and 2c were obtained as the solvates 2b(PhMe) and 2c(THF)_1/2, respectively, the first in the monoclinic system in C2/c (Z = 8, Z′ = 1), the latter in an orthorhombic space-group Pbcn (Z = 8, Z′ = 1). In addition a molecular structure of 2b was also obtained from 2b co-crystallised with 3b (2b·3b), however, the geometrical molecular data of 2b differ not significantly and only the data from 2b are discussed.⁸

None of the complexes 2a–c exhibits crystallographic point symmetry in the solid state and share the same general structural motifs. However, subtle changes indicate the different ligand properties of the Bpin and Bcat ligands.

The complexes 2a–c exhibit six-fold coordinated rhodium atoms (Fig. 3) in a distorted octahedral geometry with the apical positions occupied by two PMe₃ ligands, whereas the equatorial plane is occupied by two cis boryl ligands, one additional PMe₃ ligand and the chlorido ligand. This geometry in the solid state resembles the geometry in solution, as indicated by NMR data (vide supra).

Fig. 3 Molecular structures of 2a–c from X-ray diffraction studies on 2a, 2b(PhMe) and 2c(THF)_1/2.^H.

The cis arrangement of the two boryl ligands in 2a–c is evidence for their strong σ-donor properties and, hence, trans influence of the boryl ligand disfavouring a mutual trans arrangement. In fact, only two complexes with trans boryl ligands are known so far, both exhibiting either geometric or electronic peculiarities.^5a,12

The ligand properties of the boryl ligands, in particular the trans influence is further crucial for the complex geometry (Table 1). The trans-B P–Rh distances (P2–Rh1) are by 0.09–0.14 Å longer as the, within 2a–c quite consistent, trans-P P–Rh distances (P1–Rh1 and P3–Rh1) of 2.32 Å. Moreover, in 2c the chlorido ligand, as weaker σ-donor ligand than PMe₃, occupies the position trans to the more trans influencing ligand Bpin.³Vice versa, the B–Rh distances trans to a PMe₃ ligand (B1–Rh1) are longer than those of the same boryl ligand trans to the chlorido ligand.

Table 1 Selected geometrical data of the molecular structures of the 6-coordinate rhodium boryl complexes of 2a–c and the complex cations in 4a–c

	2a	2b from	2c from	4a from	4b	4c from
		2b (PhMe)	2c (THF)1/2	4a (PhMe)		4c (THF)
Rh1–Cl [Å]	2.5315(3)	2.5539(2)	2.5686(7)
Rh1–P1 [Å]	2.3176(3)	2.3213(2)	2.3164(7)	2.3349(8)	2.3534(8)	2.3481(5)
Rh1–P2 [Å]	2.4171(3)	2.4660(2)	2.4241(6)	2.4524(8)	2.4325(8)	2.4694(5)
Rh1–P3 [Å]	2.3271(3)	2.3270(2)	2.3261(7)	2.3672(8)	2.3724(8)	2.3519(5)
Rh1–P4 [Å]				2.4411(7)	2.4483(8)	2.4243(5)
Rh1–B1 [Å]	2.058(1)	2.0796(6)	2.054(3)	2.068(3)	2.109(3)	2.092(2)
Rh1–B2 [Å]	2.005(1)	2.0353(6)	2.015(3)	2.057(3)	2.107(3)	2.082(2)
B1–Rh1–P2 [°]	173.47(4)	163.09(2)	175.51(8)	169.0(1)	161.4(1)	167.43(6)
B2–Rh1–Cl [°]	176.72(4)	176.44(2)	175.63(8)
B2–Rh1–P4 [°]				172.38(9)	171.98(9)	175.19(7)
P1–Rh1–P3 [°]	166.617(13)	163.446(6)	164.44(3)	158.66(3)	171.85(3)	163.31(2)
B1⋯B2 [Å]	2.903(2)	2.5678(9)	2.857(4)	2.888(5)	2.574(5)	2.786(3)
B1–Rh1–B2 [°]	91.19(6)	77.21(3)	89.2(1)	88.8(1)	75.3(1)	83.74(8)
τ B1 ^11 [°]	1.7(1)	83.3(1)	1.8(10)	31(1)	60(1)	53(1)
τ B2 ^11 [°]	69.5(1)	42.5(1)	61.3(20)	59.8(1)	49(2)	38.7(3)

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The B⋯B distance (in correlation with the B1–Rh1–B2 angle) is by more than 0.3 Å shorter for the bis-Bpin complex 2b than for the bis-Bcat complex 2a, whereas in the unsymmetrical Bpin/Bcat complex 2c an only slightly smaller value as in 2a is observed. The B⋯B distances in 2a–c are significantly longer than in diboranes(4) (1.7 Å), but also than in cobalt and iridium bis /tris-boryl complexes were B⋯B interactions have been evidenced by computational means (mer-[(Me₃P)₃Co(Bcat)₃] (2.1541(5) Å), [(R₃P)₃Co(Bcat)₂] (2.185–2.271 Å) and [((C₆H₄)(NPh)(NCH₂PPh₂)B)₂IrCl] (2.221 Å)).^5–7,13 However, the B⋯B distance in 2b is in the range also found for a series of platinum complexes [(Ph₃P)₂Pt(boryl)₂] (2.44–2.56 Å) where we proposed a certain amount of B⋯B interaction.⁴

Structural characterisation: [(PMe₃)₄Rh(boryl)₂]⁺

The cationic complexes 4a–c crystallise with one formula unit in the asymmetric unit as 4b (P2₁/n) or as the solvates 4a(PhMe) (P1̄) and 4c(THF) (P2₁/n), respectively. The complex cations are situated on general positions not exhibiting any higher point symmetry.

Structurally, the complex cations resemble the parent chlorido complexes 2a–c (Table 1, Fig. 4), with the chlorido ligand exchanged for an additional PMe₃ ligand.

Fig. 4 Molecular structures of the cations in 4a–c from X-ray diffraction studies on 4a(PhMe), 4b and 4c(THF).⁸

These complexes allow now a direct comparison of the geometries of the two trans-B–Rh–P entities under virtually identical conditions. The B–Rh distances are slightly shorter for the bis-Bcat complex 4a than for the complexes 4b and 4c. In the unsymmetrical complex 4c both B–Rh distances are identical within error and rather on the long side, in between the distance found for 4a and 4b. This suggests that the B–Rh distances are not very characteristic for a certain boryl ligand.⁴

A look on the P–Rh distances reveals that, as for 2a–c, the trans-B P–Rh distances are by about 0.1 Å larger than the trans-P P–Rh distances (vide supra). The trans-B P–Rh distances among the complexes 4a–c, however, do not vary significantly with the individual type of the trans-boryl ligand; although the trans-Bpin P–Rh distance in 4c is longer than the trans-Bcat P–Rh distance. This is in contrast to the observation of a comparable long P–Rh distance trans to the Bpin ligand in 2b compared to 2a,c. Again suggesting, that the picture of an virtually exclusive influence of the trans ligand on the P–Rh is not comprehensive.

The B⋯B distance in 4a–c are in the same range as for 2a–c with a by >0.2 Å significantly shorter B⋯B distance for the bis-Bpin complex 4b than for 4a,c.

NMR spectroscopic characterisation

The characterisation of 2a–c by ¹H, ¹¹B and ³¹P NMR spectroscopy shows a set of common characteristic features. The complexes 2a and 2b comprising two boryl ligands of the same type at ambient temperature, exhibit in the ¹H NMR spectrum at rt only one set of signals for the two structurally distinct boryl moieties (Fig. S1c.1 and 3† ⁸). In agreement with that, for the bis-Bpin complex 2b only one ¹¹B NMR signal is detected at 37.4 ppm (Δw_1/2 = 240 Hz), whereas two distinct signals are observed for the bis-Bcat complex 2a at 43.2 ppm (Δw_1/2 = 550 Hz) and 47.1 ppm (Δw_1/2 = 620 Hz). The ¹H NMR boryl ligand signals of 2a,b, however, split at lower temperatures, −2 °C for 2a and −44 °C for 2b, into individual signals for each boryl ligand (Fig. S1c.1 and 3† ⁸).

The ³¹P NMR data also share many characteristics, for 2a–c at room temperature two signals are observed, one broadened doublet (due to ³¹P–¹⁰³Rh coupling) at −7.2 ± 2.4 ppm for the trans-P phosphine ligands and one very broad singlet at lower chemical shifts for the trans-B phosphine ligand (2a −29.0 ppm (Δw_1/2 = 190 Hz), 2b −39.2 ppm (Δw_1/2 = 135 Hz) and 2c –31.2 ppm (Δw_1/2 = 280 Hz)) (Fig. S1c.2, 4 and 12† ⁸). At lower temperatures (10 °C for 2a,c and −44 °C for 2b) the first signal becomes a doublet of doublets due to the now resolved ³¹P–³¹P coupling. The latter signal becomes a more or less well resolved doublet of triplets only at much lower temperatures (ca. −60 °C). Whilst the trans-P phosphine signals only shifts marginally with temperature (1.5 ppm), the trans-B phosphine signals exhibit more distinct shifts towards higher chemical shift upon cooling. However, for the bis-Bpin complex 2b a significantly wider shift range (−39.2 to −30.2 ppm, 10 ppm), is observed, as for 2a (−29.0 to −25.5 ppm, 3.5 ppm) and 2c (−31.2 to −28.0 ppm, 3.2 ppm).

In summary it is concluded that the complexes 2a–c are dynamic in solution at room temperature, whilst at lower temperature their solution state structures resemble their solid state structures.

Exemplarily the behaviour of 2b towards excess PMe₃ was studied by ³¹P NMR spectroscopy (Fig. S1b.16†). It was found that excess PMe₃ leads to a sharpening of the signal of the mutual trans-P phosphine ligands at −4.8 ppm but only a minute shift of this signal. Whereas the trans-B PMe₃ signal shifts more pronounced from −39 ppm to −59 ppm, whilst the linewidth is not significantly affected; it may be emphasised that no additional signal of free PMe₃ is detected. These data suggest that the trans-P PMe₃ ligands do not undergo rapid exchange with free PMe₃, the slight narrowing of these signals rather suggest reduced intramolecular dynamics involving these phosphines in the presence of free PMe₃. In contrast, the trans-B ligand undergoes rapid exchange with free PMe₃ leading to a broad, unfeatured singlet with a chemical shift, depending on the amount of PMe₃ present.⁸

An (additional) intramolecular dynamic of predominantly the trans-B PMe₃ ligand is suggested by the observation of only one set of boryl ligand signals for 2a,b at room temperature and it splitting upon cooling, whereas no change is observed for the inherently unsymmetrical complex 2c.

Also the more pronounced temperature dependence of the trans-B phosphine ligand ³¹P NMR shift in 2b agrees with this interpretation; the stronger trans effect/influence of the Bpin ligand leads to weaker bonding and more labile trans-B phosphine ligands.^3b This fits also to the observation of a five coordinate bis phosphine complex, 3b, only for the Bpin ligand (vide infra).

The NMR data of the five coordinate complex 3b show little temperature dependence. The ³¹P NMR spectrum shows independently of the temperature a doublet due to ¹⁰³Rh–³¹P coupling, that narrows slightly upon cooling, whereas the ¹H NMR signals exhibit some broadening upon cooling. This is in agreement with an absence of dynamic processes for 3b, in contrast to the highly dynamic behaviour of 2b. More interesting are the NMR data of the co-crystallised mixture 2b·3b. This exhibits at room temperature two broadened ³¹P NMR signals, one singlet around −37 ppm and a doublet around 0 ppm (Fig. 5).

Fig. 5 Variable temperature ³¹P{¹H} NMR spectra of 2b·3b, 2b and 3b (162.1 MHz, THF-d₈, * refers to an unassigned impurity).

At −90 °C the latter signal has split into a doublet at 4.9 ppm, indicative for 3b and a doublet of doublets at −5.9 ppm indicative for 2b. The second signal shifts to −30.2 ppm, as reported for 2b. It must be concluded that at room temperature 2b and 3b are in rapid exchange by dissociation/association of the trans-B PMe₃ ligand. The ratio 2b : 3b is estimated from the averaged chemical shift in 2b·3b and the individual chemical shifts for 2b and 3b at room temperature to 0.39. This is in good agreement with the ratio 2b : 3b as suggested by integration of the low temperature ³¹P{¹H} and ¹H NMR data of 0.38 and 0.42 and the room temperature ¹H NMR data of 0.43.⁸ And is also in agreement with the elemental analysis for this sample.

The ³¹P NMR spectra of the cationic complexes 4a–c exhibit the expected two signals for the trans-P and trans-B PMe₃ ligands as two doublet of triplets for 4a–b, and a doublet of doublets of doublets and doublet of triplets of doublet for the unsymmetrical 4c, respectively. However, those complexes exhibit only marginal signal shifts with temperature (<2.5 ppm) but some narrowing upon cooling, suggestive for only little dynamics (Fig. S1d.1, 4 and 6† ⁸). The NMR data of 5c and 6c are very similar and resemble those of 2c (Fig. S1d.8 and 10† ⁸).

The ¹¹B NMR data of 4a–c, 5c and 6c are however, more insightful. For 4a–c distinct different ¹¹B NMR shifts are observed for the symmetrical bis-Bcat (4a, 44.8 ppm) and bis-Bpin (4b, 39.7 ppm) complexes and consequently for the unsymmetrical complex 4c the two distinct signals are assigned to the Bcat (44.8 ppm) and the Bpin (40.0 ppm) moieties. For the MeCN and MeNC complexes 5c and 6c¹¹B NMR signals at 46.4/46.3 ppm are assigned to the trans-P Bcat ligand. To the Bpin ligand the signals at 36.9 ppm for the MeCN complex (5c) and at 40.7 ppm for the MeNC complex (6c) are assigned. This agrees also with a shift of 38.3 ppm for the Bpin ligand in 2ctrans to the chlorido ligand. These data may suggest a minor influence of the ligands trans to a boryl ligand on its ¹¹B NMR chemical shift. Moreover, this agrees with the observation of two distinct NMR signals for the distinct Bcat ligands in 2a of 43.2 ppm (trans-Cl) and 47.1 ppm (trans-PMe₃).

Conclusions

The rhodium(III) bis-boryl complexes of the type [(PMe₃)₃Rh(boryl)₂Cl] and [(PMe₃)₃Rh(boryl)₂L]⁺ (boryl = Bpin, Bcat; L = Cl, PMe₃, MeCN, MeNC) share their general structural, NMR spectroscopic and chemical properties. Structurally the complexes comprise distorted octahedrons with two phosphine ligands in the apical positions (trans-P phosphine) and the boryl ligands, one phosphine (trans-B phosphine) and L in the equatorial plane. The boryl ligands are arranged cis to each other, as expected from them being the strongest σ-donating ligands. The individual geometrical properties, however, depend on the nature of the boryl ligands. In the unsymmetrical bis-boryl complexes 2c, 4c, 5c and 6c the ligand L (L = Cl, PMe₃, MeCN, MeNC) is always trans to the boryl ligand with the strongest σ-donor properties and trans influence, the Bpin ligand.³ The bis-Bpin complex [(PMe₃)₃Rh(Bpin)₂Cl] (2b) loses easily the trans-Bpin phosphine ligand resulting in the five coordinate [(PMe₃)₂Rh(Bpin)₂Cl] (3b). According to the heteronuclear NMR data in solution, essentially the similar structures are realised as in the solid state, however, at room temperature all NMR spectra are dynamic to a certain degree. In summary unsymmetrical diboron(4) derivatives have proven useful precursors to otherwise inaccessible unsymmetrical bis-boryl complexes that allow a detailed insight into the coordination properties of boryl ligands.

Experimental

General considerations

PinB–Bcat (1c),⁷ catB–Bdmab (1d),⁷ pinB–Bdmab (1e),^4a [Rh(PMe₃)₃Cl],^14a–c [Rh(PMe₃)₄Cl],^14a–c [Rh(PPh₃)₃Cl],^14d PMe₃^14e and MeNC^14f were synthesized using literature procedures. All other chemicals were commercially available and were used as received; their purity and identity were checked by appropriate methods. Unless noted otherwise, all solvents were dried using MBraun solvent purification systems, deoxygenated using the freeze–pump–thaw method and stored under purified nitrogen. All manipulations were performed using standard Schlenk techniques under an atmosphere of purified nitrogen or in a nitrogen filled glove box (MBraun). NMR spectra were recorded on Bruker Avance II 300, Avance III HD 300, Avance III 400 or Avance III HD 500 spectrometers. NMR tubes equipped with screw caps (WILMAD) were used and the solvents were dried over potassium/benzophenone and degassed. Chemical shifts (δ) are given in ppm, using the (residual) resonance signal of the solvents for calibration (C₆D₆: ¹H NMR: 7.16 ppm, ¹³C NMR: 128.06 ppm; THF-d₈: ¹H NMR: 1.72 ppm, ¹³C NMR: 25.31 ppm).¹⁵¹¹B, ¹⁹F and ³¹P NMR chemical shifts are reported relative to pseudo external BF₃·Et₂O, CFCl₃ and 85% phosphoric acid, respectively. ¹³C, ¹¹B and ¹⁹F NMR spectra were recorded employing composite pulse ¹H decoupling unless noted otherwise. If necessary 2D NMR techniques were employed to assign the individual signals (¹H–¹H NOESY (1 s mixing time), ¹H–¹H COSY, ¹H–¹³C HSQC and ¹H–¹³C HMBC). ¹¹B NMR spectra were processed applying a back linear prediction in order to suppress the broad background signal due to the borosilicate glass in the NMR tube and instrument and a Lorentz type window function (LB = 10 Hz); the spectra were carefully evaluated to ensure that no genuinely broad signals of the sample were suppressed. Melting points were determined in flame sealed capillaries under nitrogen using a Büchi 535 apparatus and are not corrected. Elemental analyses were performed at the Institut für Anorganische und Analytische Chemie of the Technische Universität Carolo-Wilhelmina zu Braunschweig using an Elementar vario MICRO cube instrument. GC/MS data are measured on a Shimadzu GCMS-QP2010SE in positive EI mode (70 eV, 60–700 or 60–1000 m/z, inject temperature 250 °C, split rate 100 : 1, interface temperature 280 °C, column ZB-5MS GUARDIAN, 30 m × 0.25 mm, 0.25 μm thickness, helium carrier gas, temperature program: 3 min 50 °C, heating rate 12 °C min⁻¹, end temperature 300 °C for 8 min or 6 min 40 °C, heating rate 12 °C min⁻¹, end temperature 300 °C for 38 min).

X-ray structure determinations. The single crystals were transferred into inert perfluoroether oil inside a nitrogen-filled glovebox and, outside the glovebox, rapidly mounted on top of a human hair or Hampton loop and placed in the cold nitrogen gas stream on the diffractometer.^16a The data were either collected on a Rigaku Oxford Diffraction Synergy-S instrument, using mirror-focused CuK_α radiation, a Rigaku Oxford Diffraction Synergy-S instrument, using mirror-focused MoK_α radiation or an Oxford Diffraction Xcalibur EOS instrument using graphite monochromated MoK_α radiation. The reflections were indexed, integrated and absorption corrections were applied as implemented in the CrysAlisPro software package.^16b The structures were solved employing the program SHELXT and refined anisotropically for all non-hydrogen atoms by full-matrix least squares on all F² using SHELXL software.^16c,d Generally hydrogen atoms were refined employing a riding model; methyl groups were treated as rigid bodies and were allowed to rotate about the E–CH₃ bond. During refinement and analysis of the crystallographic data the programs WinGX, Mercury, Diamond, PLATON, OLEX² and DSR were used.^16e–k Unless noted otherwise, the shown ellipsoids represent the 50% probability level and hydrogen atoms are omitted for clarity. Adapted numbering schemes may be used do improve the readability.

[Rh(PMe₃)₃(Bcat)₂Cl] (2a)^1d. [Rh(PMe₃)₃Cl] (100 mg, 273 μmol, 1 eq.) in toluene (2 mL) and 1a (75 mg, 315 μmol, 1.15 eq.) in THF (1 mL) were combined at room temperature. Crystallization of a colourless solid started after approx. 2 minutes. After one night at room temperature, the precipitate was separated from the supernatant solution, washed with n-pentane (3 × 4 mL) and dried in vacuo to give 2b as a colourless microcrystalline solid (151 mg, 250 μmol, 92%). Single crystals suitable of 2a for X-ray diffraction analysis were obtained within a few days from a solution in benzene layered with n-pentane at rt.

¹ H NMR (500.3 MHz, THF-d₈, rt): δ 1.37 (9 H, d, J = 6.5 Hz, trans-B-PMe₃), 1.49 (18 H, app. dt, J = 1.0, 3.2 Hz, trans-P-PMe₃), 6.88 (4 H, br. s, Δw_1/2 = 18.2 Hz, CH_cat), 7.04–7.09 (4 H, m, CH_cat). ¹³C{¹H} NMR (125.8 MHz, THF-d₈, rt): δ 18.4 (d, J = 18 Hz, trans-B-PMe₃), 20.1 (app. t, J = 17 Hz, trans-P-PMe₃), 111.6 (br. d, J = 60 Hz, Δw_1/2 = 36 Hz, CH_cat), 121.8 (CH_cat), 150.6 (C_cat). ¹¹B{¹H} NMR (160.5 MHz, THF-d₈, rt): δ 43.2 (s, Δw_1/2 = 550 Hz), 47.1 (s, Δw_1/2 = 620 Hz). ³¹P{¹H} NMR (162.1 MHz, THF-d₈, rt): δ −29.0 (br. s, Δw_1/2 = 190 Hz, trans-B-PMe₃), −7.9 (d, J_P–Rh = 102 Hz, Δw_1/2 = 22 Hz, trans-P-PMe₃). ¹H NMR (300.1 MHz, C₆D₆, rt): δ 1.08 (9 H, d, J = 6.5 Hz, trans-B-PMe₃), 1.38 (18 H, br. dt, J = 1.0, 3.2 Hz, trans-P P(CH₃)₃), 6.79 (4 H, br. s, Δw_1/2 = 15 Hz, CH_cat), 7.07 (4 H, br. s, Δw_1/2 = 14 Hz, CH_cat). ¹¹B{¹H} NMR (96.3 MHz, C₆D₆, rt): δ 45.1 (s, Δw_1/2 = 858 Hz). ³¹P{¹H} NMR (121.5 MHz, C₆D₆, rt): δ −29.9 (br. s, Δw_1/2 = 167 Hz, trans-B-PMe₃), −8.7 (d, J = 105 Hz, trans-P-PMe₃). ¹H NMR (400.4 MHz, THF-d₈, −103 °C): δ 1.35 (9 H, d, J = 6.7 Hz, trans-B-PMe₃), 1.45 (18 H, br. s (sh), trans-P-PMe₃), 6.88–6.93 (2 H, m, CH_cat), 6.93–6.99 (2 H, m, CH_cat), 7.14–7.21 (4 H, m, CH_cat). ³¹P{¹H} NMR (162.1 MHz, THF-d₈, −103 °C): δ −25.6 (dt, Δw_1/2 = 19 Hz, ¹J_P–Rh = 71 Hz, ²J_P–P = 31 Hz, trans-B-PMe₃), −6.4 (dd, Δw_1/2 = 3 Hz, ¹J_P–Rh = 100 Hz, ²J_P–P = 31 Hz, trans-P-PMe₃). m.p.: 188–191 °C (decomp.). Anal. Calcd for C₂₁H₃₅B₂ClO₄P₃Rh C, 41.73; H, 5.84. Found: C, 41.81; H, 5.99.

[Rh(PMe₃)₂(Bpin)₂Cl] (3b). [Rh(PMe₃)₃Cl] (126 mg, 344 μmol, 1 eq.) and 1b (100 mg, 393 μmol, 1.15 eq.) were mixed in toluene (5 mL) at room temperature. After 48 h all volatiles were removed in vacuo for several hours. The residue was extracted with n-pentane (2 × 10 mL) and filtered over Celite. After concentration in vacuo (3 mL) the solution was stored at −40 °C to deposit the product as a fine pale yellow to colourless solid. After 16 h, the mother liquor was decanted, the solid washed with cold n-pentane (1.5 mL, −40 °C) and dried in vacuo. The mother liquor was concentrated to approx. 1.5 mL and stored at −40 °C to give more product. Combined yield: 96 mg, 176 μmol, 51%. Single crystals suitable for X-ray diffraction were obtained upon recrystallization from n-pentane at −40 °C.

¹ H NMR (300.1 MHz, C₆D₆, rt): δ 1.08 (24 H, s, C(CH₃)₂), 1.54 (18 H, br. dt, J_H–P = 0.8, 3.5 Hz, P(CH₃)₃). ¹³C{¹H} NMR (75.5 MHz, C₆D₆, rt): δ 15.5 (app. dt, J = 1, 15 Hz, P(CH₃)₃), 25.3 (C(CH₃)₂), 82.0 (C(CH₃)₂). ¹¹B{¹H} NMR (96.3 MHz, C₆D₆, rt): δ 37.4 (s, Δw_1/2 = 240 Hz). ³¹P{¹H} NMR (121.5 MHz, C₆D₆, rt): δ 2.4 (d, J = 116 Hz, P(CH₃)₃). ¹H NMR (400.4 MHz, THF-d₈, rt): δ 1.18 (24 H, s, C(CH₃)₂), 1.44 (18 H, br. td, J_H–P = 1.0, 3.7 Hz, P(CH₃)₃). ³¹P{¹H} NMR (162.1 MHz, THF-d₈, rt): δ 2.8 (d, J = 119 Hz, P(CH₃)₃). ¹H NMR (400.4 MHz, THF-d₈, −90 °C): δ 1.17 (24 H, s, C(CH₃)₂), 1.42 (18 H, s, P(CH₃)₃). ³¹P{¹H} NMR (162.1 MHz, THF-d₈, −90 °C): δ 4.9 (d, J = 117 Hz, P(CH₃)₃). m.p.: 142–158 °C (decomp.). Anal. Calcd for C₁₈H₄₂B₂ClO₄P₂Rh C, 39.71; H, 7.78. Found: C, 39.82; H, 7.94.

[Rh(PMe₃)₃(Bpin)₂Cl]·[Rh(PMe₃)₂(Bpin)₂Cl] (2b·3b). As described above for 3b, but the exposure to vacuum is kept to approx. 30 min. The crystallization from n-pentane at −40 °C results in the co-crystals with the composition [Rh(PMe₃)₃(Bpin)₂Cl]·[Rh(PMe₃)₂(Bpin)₂Cl] (2b·3b); note that all single crystals studied from this material had the composition 2b·3b. However, after brief drying in vacuo the exact ratio of 2b to 3b varies in a range n(2b) : n(3b) = 0.35–1 depending on the evacuation.

Anal. Calcd for C₃₉H₉₃B₄Cl₂O₈P₅Rh₂ (2b·3b) C 40.21, H 8.05; for C₅₇H₁₃₅B₆Cl₃O₁₂P₇Rh₃ (2b·(3b)₂) C 40.05, H 7.96; for C₇₅H₁₇₇B₈Cl₄O₁₆P₉Rh₄ (2b·(3b)₃) C 39.97, H 7.92. Found C 40.03, H 8.01. Yield: 138 mg, 61 μmol of (2b·(3b)₃) (245 μmol Rh), 71% with respect to Rh.

[Rh(PMe₃)₃(Bpin)₂Cl] (2b). 3b (40 mg, 73.5 μmol, 1 eq.) was dissolved in n-pentane (3 mL) and PMe₃ (7.6 μL, 5.6 mg, 73.5 μmol, 1 eq.) was added. After approx. 15 min all volatiles are removed in vacuo for period of 10 min at rt to give 2b quantitatively. Single crystals of 2b(PhMe) suitable for X-ray diffraction were obtained upon crystallization from a solution in PhMe layered with n-pentane at −40 °C. Alternatively, the co-crystalized mixture 2b·3b and to a 3b equimolar amount of PMe₃ can be used. However, the 2b obtained is often contaminated with small amounts of 3b.

¹ H NMR (400.4 MHz, THF-d₈, rt): δ 1.18 (24 H, s, C(CH₃)₂), 1.26 (9 H, br. d, J_H–P = 3.2 Hz, P(CH₃)₃), 1.52 (18 H, td, J = 0.9, 3.6 Hz, P(CH₃)₃). ¹¹B{¹H} NMR (96.3 MHz, THF-d₈, rt): δ 37.4 (s, Δw_1/2 = 240 Hz). ³¹P{¹H} NMR (162.1 MHz, THF-d₈, rt): δ −39.2 (br. s, Δw_1/2 = 135 Hz, trans-B–P(CH₃)₃), −5.0 (br. d, J_P–Rh = 113 Hz, Δw_1/2 = 28 Hz, trans-P-P(CH₃)₃). ¹H NMR (300.1 MHz, C₆D₆, rt): δ 1.12 (24 H, s, C(CH₃)₂), 1.17 (9 H, br. d, J_H–P = 4.4 Hz, P(CH₃)₃), 1.50 (18 H, br. t, J = 3.5 Hz, P(CH₃)₃). ¹³C{¹H} NMR (75.5 MHz, C₆D₆, rt): δ 18.4 (d, J_C–P = 9.4 Hz, Δw_1/2 = 12 Hz, trans-B–P(CH₃)₃), 19.4 (t, J = 16 Hz, trans-P–P(CH₃)₃), 25.9 (C(CH₃)₂), 81.5 (C(CH₃)₂). ¹¹B{¹H} NMR (96.3 MHz, C₆D₆, rt): δ 39.2 (s, Δw_1/2 = 275 Hz). ³¹P{¹H} NMR (121.5 MHz, C₆D₆, rt): δ −37.9 (s, Δw_1/2 = 62 Hz, trans-B–P(CH₃)₃), −4.8 (d, J_P–Rh = 112 Hz, Δw_1/2 = 15 Hz, P(CH₃)₃).

¹ H NMR (400.4 MHz, THF-d₈, −62 °C): δ 1.15 (12 H, s, C(CH₃)₂), 1.18 (12 H, s, C(CH₃)₂), 1.32 (9 H, d, J = 6.1 Hz, P(CH₃)₃), 1.52 (18 H, br. t, J = 3.2 Hz, P(CH₃)₃). ³¹P{¹H} NMR (202.5 MHz, THF-d₈, −93 °C): δ −30.2 (dt, Δw_1/2 = 21 Hz, ¹J_P–Rh = 68 Hz, ²J_P–P = 33 Hz, trans-B-PMe₃), −5.9 (dd, Δw_1/2 = 3 Hz, ¹J_P–Rh = 112 Hz, ²J_P–P = 33 Hz, trans-P-PMe₃). m.p.: 125–133 °C (decomp.). Anal. Calcd for C₂₁H₅₁B₂ClO₄P₃Rh C, 40.65; H, 8.28. Found: C, 40.40; H, 8.27.

[Rh(PMe₃)₃(Bcat)(Bpin)Cl] (2c). From [Rh(PMe₃)₃Cl]: [Rh(PMe₃)₃Cl] (218 mg, 594 μmol, 1 eq.) and 1c (168 mg, 683 μmol, 1.15 eq.) were dissolved in toluene (10 mL). Crystallization of a colourless solid started after approx. 5 minutes and was facilitated by storage at −40 °C. After one night, the mother liquor was decanted, concentrated in vacuo (5 mL), layered with n-pentane (5 mL) and stored at −40 °C. This process was repeated until no further product deposited. The obtained solid were washed with n-pentane (2 × 2 mL) and dried in vacuo to give 1c as a colourless microcrystalline solid (330 mg, 539 μmol, 91%).

From [Rh(PMe₃)₄]Cl: [Rh(PMe₃)₄]Cl (120 mg, 271 μmol, 1.04 eq.) and 1c (64 mg, 260 μmol, 1 eq.) were combined in THF (8 mL). The suspension was treated with ultrasound in an ice-cooled water bath for 1.5 h. The, by then, nearly clear mixture was filtered and all volatiles were evaporated. The solid was taken up in THF (1 mL), filtered over a plug of glass wool and stored at −40 °C. After one night, the mother liquor is decanted, the solid washed with n-pentane (2 × 2 mL) and dried in vacuo to give 2c as a colourless microcrystalline solid (64 mg, 104 μmol, 40%).

Single crystals of 2c(THF)_1/2 suitable for X-ray diffraction were obtained upon crystallization from a solution in THF rt.

¹ H NMR (400.4 MHz, THF-d₈, rt): δ 1.27 (12 H, s, C(CH₃)₂), 1.41 (9 H, unresolved (overlapping), trans-B-PMe₃), 1.42 (18 H, app. dt (overlapping), J = 0.8, 3.4 Hz, trans-P-PMe₃), 6.81–6.88 (2 H, m, CH_cat), 7.01–7.08 (2 H, m, CH_cat). ¹³C{¹H} NMR (75.5 MHz, THF-d₈, rt): δ 18.1–18.8 (br. m, trans-B-PMe₃), 19.8–20.6 (br. m, trans-P-PMe₃), 25.9 (C(CH₃)₂), 82.5 (C(CH₃)₂), 111.5 (CH_cat), 121.5 (CH_cat), 150.8 (C_cat). ¹¹B{¹H} NMR (96.3 MHz, THF-d₈, rt): δ 38.3 (s, Δw_1/2 = 540 Hz), 46.7 (s, Δw_1/2 = 710 Hz). ³¹P{¹H} NMR (162.1 MHz, THF-d₈, rt): δ −31.2 (br. s, Δw_1/2 = 280 Hz, trans-B-PMe₃), −9.3 (br. d, J_P–Rh = 104 Hz, Δw_1/2 = 84 Hz, trans-P-PMe₃). ¹H NMR (300.1 MHz, C₆D₆, rt): δ 1.15 (12 H, s, C(CH₃)₂), 1.20 (9 H, br. d, J = 6.4 Hz, trans-B-PMe₃), 1.37 (18 H, br. t, J = 3.4 Hz, trans-P-PMe₃), 6.81–6.88 (2 H, m, CH_cat), 7.16–7.22 (overlapping with solvent signal, 2 H, m, CH_cat). ¹¹B{¹H} NMR (96.3 MHz, C₆D₆, rt): δ 37.7 (s, Δw_1/2 = 370 Hz), 46.7 (s, Δw_1/2 = 399 Hz). ³¹P{¹H} NMR (121.5 MHz, C₆D₆, rt): δ −30.8 (br. s, Δw_1/2 = 250 Hz, trans-B-PMe₃), −9.0 (br. d, J = 111 Hz, trans-P-PMe₃). ¹H NMR (400.4 MHz, THF-d₈, −50 °C): δ 1.27 (12 H, s, C(CH₃)₂), 1.38 (18 H, t, J = 3.4 Hz, trans-P-PMe₃), 1.40 (9 H, d, J = 6.5 Hz, trans-B-PMe₃), 6.85–6.91 (2 H, m, CH_cat), 7.06–7.12 (2 H, m, CH_cat). ³¹P{¹H} NMR (162.1 MHz, THF-d₈, −90 °C): δ −28.0 (dt, J_P–P = 36 Hz, J_P–Rh = 73 Hz, trans-B-PMe₃), −7.9 (dd, J_P–P = 36 Hz, J_P–Rh = 105 Hz, trans-P-PMe₃). m.p.: 189–193 °C (decomp.). Anal. Calcd for C₂₁H₄₃B₂ClO₄P₃Rh C, 41.18; H, 7.08. Found: C, 41.35; H, 7.07.

[(PMe₃)₄Rh(Bcat)₂][BArF] (4a). Solutions of 2a (50 mg, 83 μmol, 1 eq.) in THF (1 mL) and Na[BArF] (74 mg, 83 μmol, 1 eq.) in Toluene (1 mL) were mixed at room temperature. PMe₃ (6.9 mg, 9.4 μL, 91 μmol, 1.1 eq.) was added after 5 min and the turbid solution was filtered over Celite. The filtrate was layered with n-pentane (4 mL). A microcrystalline material was obtained after 16 h at −40 °C. The mother liquor was decanted, the solid washed with n-pentane (2 × 2 mL) and dried in vacuo. The obtained solid was dissolved in THF (1.5 mL), layered with n-pentane (4 mL) and stored at −40 °C to give a microcrystalline solid. The mother liquor is decanted, the solid washed with n-pentane and dried in vacuo to give 4a as a colourless solid (77 mg, 51 μmol, 61%). Single crystals of 4a(PhMe) suitable for X-ray diffraction were obtained upon crystallization from a solution in PhMe at −40 °C.

¹ H NMR (500 MHz, THF-d₈, rt): δ 1.53 (18 H, app. t, J = 3.4 Hz, PMe₃), 1.57–1.60 (18 H, m, PMe₃), 7.03–7.05 (4 H, m, 3,4-CH_cat), 7.23–7.25 (4 H, m, 2,5-CH_cat), 7.57 (4 H, br. s, Δw_1/2 = 5 Hz, p-CH_BArF), 7.76–7.79 (8 H, m, o-CH_BArF). ¹³C{¹H} NMR (125.8 MHz, THF-d₈, rt): δ 21.4–21.6 (m, PMe₃), 22.7 (tt, J_C–P = 4, 17 Hz, PMe₃), 112.3 (2,5-CH_cat), 118.2 (sept., J_C–F = 3 Hz, p-CH_BArF), 123.2 (3,4-CH_cat), 125.5 (q, J_C–F = 272 Hz, CF₃), 129.9 (qq, J_C–F = 3, 31 Hz, m-C(CF₃)_BArF), 135.6 (o-CH_BArF), 149.6 (1,6-C_cat), 162.8 (q, J_C–B = 50 Hz, ipso-C_BArF). ¹¹B{¹H} NMR (160.5 MHz, THF-d₈, rt): δ −6.0 (s, Δw_1/2 = 2 Hz, B(C₈H₃F₆)₄), 44.8 (br. s, Δw_1/2 = 380 Hz). ¹⁹F{¹H} NMR (282.5 MHz, THF-d₈, rt): δ −62.5 (s, BArF). ³¹P{¹H} NMR (202.5 MHz, THF-d₈, rt): δ −33.6 (br. s, Δw_1/2 = 185 Hz, trans-B-PMe₃), −14.0 (dt, J_P–P = 28 Hz, J_P–Rh = 94 Hz, trans-P-PMe₃). ¹H NMR (400.4 MHz, THF-d₈, −76 °C): δ 1.52 (18 H, br. s, Δw_1/2 = 9 Hz, PMe₃), 1.59 (18 H, br. s, Δw_1/2 = 9 Hz, PMe₃), 7.04–7.10 (4 H, m, 3,4-CH_cat), 7.29–7.35 (4 H, m, 2,5-CH_cat), 7.74 (4 H, s, p-CH_BArF), 7.88 (8 H, br. s, Δw_1/2 = 10 Hz, o-CH_BArF). ³¹P{¹H} NMR (162.1 MHz, THF-d₈, −76 °C): −31.1 (dt, Δw_1/2 = 22 Hz, ¹J_P–Rh = 68 Hz, ²J_P–P = 28 Hz, trans-B-PMe₃), −12.6 (dt, Δw_1/2 = 3 Hz, ¹J_P–Rh = 93 Hz, ²J_P–P = 28 Hz, trans-P-PMe₃). m.p.: decomp. >175 °C. Anal. Calcd for C₅₆H₅₆B₃F₂₄O₄P₄Rh C, 44.60; H, 3.74. Found: C, 44.56; H, 4.07.

[(PMe₃)₄Rh(Bpin)₂][BArF] (4b). [Rh(PMe₃)₃Cl] (50 mg, 136 μmol, 1 eq.) and 1b (40 mg, 158 μmol, 1.16 eq.) were combined in THF (2 mL). After 24 h at rt Na[BArF] (121 mg, 137 μmol, 1 eq.) is added followed by PMe₃ (approx. 3 min) (17 μL, 12 mg, 164 μmol, 1.2 eq.). The turbid solution was filtered over Celite and layered with n-pentane (5 mL). After 16 h at −40 °C a colourless solid had deposited. The mother liquor was decanted, the solid washed with n-pentane (2 × 2 mL) and dried in vacuo to give 4b (103 mg, 66 μmol, 49%). Single crystals of 4b suitable for X-ray diffraction were obtained upon crystallization from a solution in THF layered with n-pentane at −40 °C.

¹ H NMR (300.3 MHz, THF-d₈, rt): δ 1.28 (24 H, s, OCMe₂), 1.47 (18 H, br. s, Δw_1/2 = 6.5 Hz, PMe₃), 1.65 (18 H, app. t, J = 3.4 Hz, PMe₃), 7.57 (4 H, br. s, Δw_1/2 = 4.5 Hz, p-CH_BArF), 7.76–7.80 (8 H, m, o-CH_BArF). ¹³C{¹H} NMR (75.5 MHz, THF-d₈, rt): δ 21.7–22.6 (m, PMe₃), 23.3–24.0 (m, PMe₃), 26.6 (OC(CH₃)₂), 83.8 (OC(CH₃)₂), 118.2 (sept., J_C–F = 4 Hz, p-CH_BArF), 125.7 (q, J_C–F = 274 Hz, CF₃), 130.0 (qq, J_C–F = 3, 31 Hz, m-C(CF₃)_BArF), 135.6 (o-CH_BArF), 149.6 (1,6-C_cat), 163 (q, J_C–B = 50 Hz, ipso-C_BArF). ¹¹B{¹H} NMR (96.3 MHz, THF-d₈, rt): δ −6.0 (s, Δw_1/2 = 2 Hz, B(C₈H₃F₆)₄), 39.7 (br. s, Δw_1/2 = 300 Hz). ¹⁹F{¹H} NMR (282.5 MHz, THF-d₈, rt): δ −62.4 (s, BArF). ³¹P{¹H} NMR (121.5 MHz, THF-d₈, rt): δ −36.7 (br. s, Δw_1/2 = 140 Hz, trans-B-PMe₃), −14.0 (dt, J_P–P = 23 Hz, J_P–Rh = 104 Hz, trans-P-PMe₃).¹H NMR (400 MHz, THF-d₈, −70 °C): δ 1.26 (24 H, s, OCMe₂), 1.48 (18 H, app. t, J = 2.9 Hz, PMe₃), 1.65 (18 H, app. t, J = 3.2 Hz, PMe₃), 7.70 (4 H, br. s, Δw_1/2 = 4.5 Hz, p-CH_BArF), 7.78 (8 H, br. s, o-CH_BArF). ³¹P{¹H} NMR (162.1 MHz, THF-d₈, −70 °C): −34.9 (dt, Δw_1/2 = 35 Hz, ¹J_P–Rh = 64 Hz, ²J_P–P = 30 Hz, trans-B-PMe₃), −13.9 (dt, Δw_1/2 = 4 Hz, ¹J_P–Rh = 101 Hz, ²J_P–P = 30 Hz, trans-P-PMe₃). m.p.: decomp. >169 °C. Anal. Calcd for C₅₆H₇₂B₃F₂₄O₄P₄Rh C, 44.12; H, 4.76. Found: C, 44.05; H, 4.79.

[(PMe₃)₄Rh(Bcat)(Bpin)][BArF] (4c). 2c (50 mg, 82 μmol, 1 eq.) and Na[BArF] (73 mg, 82 μmol, 1 eq.) were combined in THF (2 mL). PMe₃ (6.8 mg, 9.3 μL, 90 μmol, 1.1 eq.) was added after 5 min. The turbid mixture was filtered through a plug of Celite, the filtrate layered with n-pentane (4 mL) and stored at −40 °C. After 16 h crystalline material had deposited and the mother liquor was decanted, the solid washed with n-pentane (2 × 2 mL) and dried in vacuo to give 4c as a colourless microcrystalline solid (71 mg, 47 mmol, 57%). Single crystals of 4c(THF) suitable for X-ray diffraction were obtained upon crystallization from a solution in THF layered with n-pentane at −40 °C.

¹ H NMR (400.4 MHz, THF-d₈, rt): δ 1.33 (12 H, s, OC(CH₃)₂), 1.52 (9 H, overlapping, trans-B-PMe₃), 1.55 (9 H, d, J_H–P = 6.0 Hz, trans-B-PMe₃), 1.59 (18 H, t, J_H–P = 3.1 Hz, trans-P-PMe₃), 6.97–7.01 (2 H, m, CH_cat), 7.13–7.18 (2 H, m, CH_cat), 7.56 (4 H, br. s, Δw_1/2 = 4.6 Hz, p-CH_BArF), 7.76–7.80 (8 H, m, o-CH_BArF). ¹¹B{¹H} NMR (96 MHz, THF-d₈, rt): δ −6.0 (s, Δw_1/2 = 10 Hz, B(C₈H₃F₆)₄), 40.0 (br. s, Δw_1/2 = 450 Hz), 44.8 (br. s, Δw_1/2 = 450 Hz). ¹⁹F{¹H} NMR (282.5 MHz, THF-d₈, rt): δ −62.4 (s, BArF). ³¹P{¹H} NMR (162.1 MHz, THF-d₈, rt): δ −36.7 (br. s, Δw_1/2 = 205 Hz, trans-B-PMe₃), −35.1 (br. s, Δw_1/2 = 215 Hz, trans-B-PMe₃), −14.4 (dt, J_P–P = 25 Hz, J_Rh–P = 98 Hz, trans-P-PMe₃). ¹H NMR (400.4 MHz, THF-d₈, −50 °C): δ 1.30 (12 H, s, OC(CH₃)₂), 1.52 (9 H, d, J_H–P = 6.3 Hz, trans-B-PMe₃), 1.56 (9 H, d, J_H–P = 6.5 Hz, trans-B-PMe₃), 1.59 (18 H, app. t, J_H–P = 3.0 Hz, trans-P-PMe₃), 6.98–7.02 (2 H, m, 3,4-CH_cat), 7.18–7.23 (2 H, m, 2,5-CH_cat), 7.68 (4 H, br. s, Δw_1/2 = 4.5 Hz, p-CH_BArF), 7.83–7.89 (8 H, m, o-CH_BArF).

¹³ C{ ¹ H} NMR (100.7 MHz, THF-d₈, −50 °C): δ 20.5 (d, J_C–P = 20 Hz, trans-B-PMe₃), 22.0 (d, J_C–P = 19 Hz, trans-B-PMe₃), 22.5 (tt, J_C–P = 4, 18 Hz, trans-P-PMe₃), 26.3 (OC(CH₃)₂), 83.80/83.83 (overlapping, OC(CH₃)₂), 112.0 (3,4-CH_cat), 118.4 (br. s., Δw_1/2 = 13 Hz, p-CH_BArF), 122.8 (2,5-CH_cat), 125.4 (q, J_C–F = 273 Hz, CF₃), 129.9 (q, Δw_1/2 = 10 Hz, J_C–F = 31 Hz, m-C(CF₃)_BArF), 135.4 (o-CH_BArF), 149.6 (1,6-C_cat), 162.9 (q, J_C–B = 50 Hz, ipso-C_BArF). ¹¹B{¹H} NMR (128.5 MHz, THF-d₈, −50 °C): δ −6.5 (s, BArF), no boryl signal observed. ¹⁹F{¹H} NMR (376.7 MHz, THF-d₈, −50 °C): δ −62.9 (s, BArF). ³¹P{¹H} NMR (202.5 MHz, THF-d₈, −50 °C): −34.7 (br., Δw_1/2 = 35 Hz, ¹J_P–Rh = 59 Hz, ²J_{P–B(−33.1 ppm)} = 31 Hz, ²J_{P–P(−12.9 ppm)} = 33 Hz, trans-B-PMe₃), −33.1 (br. dtd, Δw_1/2 = 20 Hz, ¹J_P–Rh = 67 Hz, ²J_{P–B(−34.7 ppm)} = 31 Hz, ²J_{P–P(−12.9 ppm)} = 24 Hz, trans-B-PMe₃), −12.9 (ddd, Δw_1/2 = 5 Hz, ¹J_P–Rh = 97 Hz, ²J_{P–P(−34.7 ppm)} = 33 Hz, ²J_{P–P(−33.1 ppm)} = 24 Hz, trans-P-PMe₃). m.p.: decomp. >169 °C. Anal. Calcd for C₅₆H₆₄B₃F₂₄O₄P₄Rh (4c), 44.36; H, 4.25; for C₆₀H₇₂B₃F₂₄O₅P₄Rh (4c·THF), 45.37; H, 4.57. Found: C, 45.54; H, 5.12. The presence of one equivalent of co-crystallised THF is in agreement with the crystal structure being (4c·THF).

[Rh(PMe₃)₃(NCMe)(Bcat)(Bpin)][BArF] (5c). 2c (42 mg, 69 μmol, 1 eq.) and NaBArF (61 mg, 69 μmol, 1 eq.) were dissolved in a mixture of toluene (2 mL) and MeCN (20 μL, 0.38 mmol, 5.5 eq.). After 5 min at rt the turbid mixture was filtered through a plug of Celite. After a few minutes, precipitation of a colourless solid started which is promoted by layering the mixture with n-pentane (4 mL) and storage at −40 °C. After five days the mother liquor was decanted, the solid is washed with n-pentane (2 × 2 mL) and dried in vacuo to give [Rh(PMe₃)₃(NCMe)(Bcat)(Bpin)][BArF] (5c) as a microcrystalline solid (69 mg, 47 mmol, 68%). Single crystals of 5c suitable for X-ray diffraction were obtained from the reaction mixture in PhMe at rt.

¹ H NMR (500.3 MHz, THF-d₈, rt): δ 1.32 (12 H, s, C(CH₃)₂), 1.44 (18 H, app. t, J = 3.6 Hz, trans-P-PMe₃), 1.48 (9 H, d, J_H–P = 6.5 Hz, trans-B-PMe₃), 2.40 (3 H, br. s, CH₃CN), 6.96–7.00 (2 H, m, CH_cat), 7.10–7.15 (2 H, m, CH_cat), 7.57 (4 H, s, p-CH_BArF), 7.76–7.80 (8 H, m, o-CH_BArF). ¹³C{¹H} NMR (125.8 MHz, THF-d₈, rt): δ 1.47 (CH₃CN), 18.2 (dt, J = 1, 20 Hz, trans-B-PMe₃), 19.6 (tdd, J = 1, 4, 17 MHz, trans-P-PMe₃), 25.8 (C(CH₃)₂), 83.7 (C(CH₃)₂), 112.1 (CH_cat), 118.2 (sept., J_C–F = 4 Hz, p-CH_BArF), 122.6 (CH_cat), 125.5 (q, J_C–F = 272 Hz, CF₃), 130.0 (qq, J_C–F = 3, 32 Hz, m-C(CF₃)), 135.6 (o-CH_BArF), 150.0 (C_cat), 162.8 (q, J_C–B = 50 Hz, ipso-C_BArF). The quaternary C of the MeCN ligand was not detected. ¹¹B{¹H} NMR (160.5 MHz, THF-d₈, rt): δ −6.0 (s, Δw_1/2 = 11 Hz, B(C₈H₃F₆)₄), 36.9 (br. s, Δw_1/2 = 318 Hz), 46.4 (br. s, Δw_1/2 = 288 Hz). ¹⁹F{¹H} NMR (282.5 MHz, THF-d₈, rt): δ −62.4 (s, B(C₈H₃F₆)₄). ³¹P{¹H} NMR (202.5 MHz, THF-d₈, rt): δ −31.0 (br. s, Δw_1/2 = 192 Hz, trans-B-PMe₃), −10.8 (dd, J_P–P = 33 Hz, J_Rh–P = 105 Hz, trans-P–P(CH₃)₃). ¹H NMR (400 MHz, THF-d₈, −80 °C): δ 1.29 (12 H, s, C(CH₃)₂), 1.42 (18 H, s, trans-P-PMe₃), 1.49 (9 H, d, J_H–P = 6.9 Hz, trans-B-PMe₃), 2.52 (3 H, s, CH₃CN), 6.99–7.04 (2 H, m, CH_cat), 7.15–7.22 (2 H, br. m, CH_cat), 7.78 (4 H, s, p-CH_BArF), 7.91 (8 H, br. s, o-CH_BArF). ³¹P{¹H} NMR (162.1 MHz, THF-d₈, −80 °C): δ −29.8 (dt, Δw_1/2 = 21 Hz, J_P–P = 34 Hz, J_Rh–P = 74 Hz, trans-B-PMe₃), −10.1 (dd, Δw_1/2 = 5 Hz, J_P–P = 34 Hz, J_Rh–P = 104 Hz, trans-P-PMe₃). m.p.: >158 °C (decomp.). Anal. Calcd for C₅₅H₅₈B₃F₂₄NO₄P₃Rh C, 44.60; H, 3.95; N, 0.95. Found: C, 44.17; H, 4.24; N, 1.29.

[Rh(PMe₃)₃(CNMe)(Bcat)(Bpin)][BArF] (6c). 5c (50 mg, 34 μmol, 1 eq.) was dissolved in THF (2 mL) and MeNC (10 μL, 168 mmol, 5 eq.) was added. The mixture was layered with n-pentane (4 mL) and stored at −40 °C. After 16 h a crystalline material had deposited, the mother liquor was decanted, the crystalline residue washed with n-pentane (2 × 2 mL) and dried under reduced pressure. The washing solution was added to the mother liquor, from which additional crystalline material deposited within few days at −40 °C. The solids are dried in vacuo to give [Rh(PMe₃)₃(CNMe)(Bcat)(Bpin)][BArF] (6c) as a crystalline material (34 mg, 23 μmol, 68%). Single crystals of 6c suitable for X-ray diffraction were obtained from THF solution layered with n-pentane at −40 °C.

¹ H NMR (500.3 MHz, THF-d₈, rt): δ 1.34 (12 H, s, C(CH₃)₂), 1.48 (18 H, app. t, J = 3.7 Hz, trans-P-PMe₃), 1.52 (9 H, d, J_H–P = 6.8 Hz, trans-B-PMe₃), 3.57 (3 H, CH₃NC, overlapping with solvent signal), 6.97–7.01 (2 H, m, CH_cat), 7.11–7.15 (2 H, m, CH_cat), 7.57 (4 H, s, p-CH_BArF), 7.76–7.80 (8 H, m, o-CH_BArF). ¹³C{¹H} NMR (125.8 MHz, THF-d₈, rt): δ 19.9 (dt, J = 2, 22 Hz, trans-B-PMe₃), 21.3 (tdd, J = 1, 4, 18 MHz, trans-P-PMe₃), 25.9 (C(CH₃)₂), 29.0 (br. s, Δw_1/2 = 13 Hz, CH₃CN), 83.5 (C(CH₃)₂), 112.1 (CH_cat), 118.2 (sept., J_C–F = 4 Hz, p-CH_BArF), 122.7 (CH_cat), 125.5 (q, J_C–F = 272 Hz, CF₃), 130.0 (qq, J_C–F = 3, 32 Hz, m-C(CF₃)), 135.6 (o-CH_BArF), 145.3 (br., CH₃NC), 150.1 (two signals, C_cat), 162.8 (q, J_C–B = 50 Hz, ipso-C_BArF). ¹¹B{¹H} NMR (160.5 MHz, THF-d₈, rt): δ −6.0 (s, Δw_1/2 = 3 Hz, B(C₈H₃F₆)₄), 40.7 (br. s, Δw_1/2 = 280 Hz), 46.3 (br. s, Δw_1/2 = 390 Hz). ¹⁹F{¹H} NMR (282.5 MHz, THF-d₈, rt): δ −62.4 (s, B(C₈H₃F₆)₄). ³¹P{¹H} NMR (202.5 MHz, THF-d₈, rt): δ −31.5 (br. s, Δw_1/2 = 170 Hz, trans-B-PMe₃), −11.5 (dd, J_P–P = 32 Hz, J_Rh–P = 99 Hz, trans-P-P(CH₃)₃). ¹H NMR (400 MHz, THF-d₈, −80 °C): δ 1.31 (12 H, s, C(CH₃)₂), 1.46 (18 H, s, trans-P-PMe₃), 1.53 (9 H, d, J_H–P = 7.1 Hz, trans-B-PMe₃), 3.64 (3 H, s, CH₃CN), 6.98–7.05 (2 H, m, CH_cat), 7.16–7.22 (2 H, m, CH_cat), 7.77 (4 H, s, p-CH_BArF), 7.89 (8 H, br. s, o-CH_BArF).³¹P{¹H} NMR (162.1 MHz, THF-d₈, −80 °C): δ −30.3 (dt, J_P–P = 33 Hz, J_Rh–P = 73 Hz, Δw_1/2 = 19 Hz, trans-B-PMe₃), −11.0 (dd, J_P–P = 33 Hz, J_Rh–P = 97 Hz, Δw_1/2 = 5 Hz, trans-P-PMe₃). Anal. Calcd for C₅₅H₅₈B₃F₂₄NO₄P₃Rh C, 44.60; H, 3.95; N, 0.95. Found: C, 44.63; H, 3.59; N, 1.24. m.p.: >148 °C (decomp.).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

C. K. and W. D. gratefully acknowledge support by a Research Grant (KL 2243/5-1) of the Deutsche Forschungsgemeinschaft (DFG). The authors thank AllyChem Co. Ltd for a generous gift of diboron reagents.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional NMR spectroscopic and crystallographic data. CCDC 2129858–2129867, 2129870 and 2129871. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1dt04276a

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