Synthesis, Reactivity and Coordination Chemistry of Group 9 PBP Boryl Pincer Complexes: [(PBP)M(PMe₃)_n] (M = Co, Rh, Ir; n = 1, 2)

Abstract

The unsymmetrical diborane(4) derivative [(d(CH₂P(iPr)₂)abB)–Bpin] (1) proved to be a versatile PBP boryl pincer ligand precursor for Co(I) (2a, 4a), Rh(I) (2–3b) and Ir(I/III) (2–3c, 5–6c) complexes, in particular of the types [(d(CH₂P(iPr)₂)abB)M(PMe₃)₂] (2a–c) and [(d(CH₂P(iPr)₂)abB)M–PMe₃] (2b–c). Whilst similar complexes have been obtained before, for the first time, the coordination chemistry of a homologous series of PBP pincer complexes, in particular the interconversion of the five- and four-coordinate complexes 2a–c/3a–c, was studied in detail. For Co, instead of the mono phosphine complex 2a, the dinitrogen complex [(d(CH₂P(iPr)₂)abB)Co(N₂)(PMe₃)] (4a) is formed spontaneously upon PMe₃ abstraction from 2a in the presence of N₂. All complexes were comprehensively characterised spectroscopically in solution via multinuclear (VT-)NMR spectroscopy and structurally in the solid state through single-crystal X-ray diffraction. The unique properties of the PBP ligand with respect to its coordination chemical properties are addressed.

1. Introduction

Since their first report in 2009 by Nozaki, Yamashita and a co-worker, PBP pincer ligands with a diaminoboryl framework have been explored with respect to their coordination chemistry with various transition metals, in particular cobalt, rhodium and iridium, as well as with respect to potential applications in different catalytic and stoichiometric processes [1,2,3,4,5,6,7]. Whilst the majority of boryl pincer complexes are of this PBP diaminoboryl type, a number of boryl pincer complexes with other ligands frameworks, often quite unique ones, have also been reported [8,9,10,11,12,13,14]. Transition metal PBP diaminoboryl pincer complexes are fundamentally accessible through the oxidative addition of a hydridoborane ligand precursor, possibly followed by further modifications, a route already developed by Nozaki and Yamashita in their seminal work [1,2,3,4,5,6,7]. To overcome the inherent obstacles by this ‘B–H oxidative addition route’, we recently developed an unsymmetrical diborane(4), pinB–B(d(CH₂P(iPr)₂)ab) (1) (pin = (OCMe₂)₂, d(CH₂P(iPr)₂)ab = 1,2-(N(CH₂P(iPr)₂))₂C₆H₄), as a versatile PBP ligand precursor. This precursor provides direct access to PBP complexes through σ bond metathesis, as exemplified with the copper boryl complex [(d(CH₂P(iPr)₂)abB)Cu]₂, and, alternatively, oxidative addition, as exemplified with the platinum bis-boryl complexes cis-[(d(CH₂P(iPr)₂)abB)(iPr₃P)Pt–Bpin] and trans-[(d(CH₂P(iPr)₂)abB)Pt–Bpin] (Scheme 1) [15].

In the present work, we endeavoured to explore the use of pinB–B(d(CH₂P(iPr)₂)ab) (1) as a precursor for a series of group 9 PBP boryl pincer complexes and study their fundamental coordination chemistry. To facilitate the access to a range of PBP boryl pincer complexes, we chose three easily available group 9 metal complexes [(Me₃P)₄Co–Me], [(Me₃P)₃Rh–Cl] and [(cod)Ir–Cl]₂ as precursors [16,17,18].

2. Results

2.1. Cobalt Complexes

The reaction of 1 with [(Me₃P)₄Co–Me] results in the mono boryl complex [(d(CH₂P(iPr)₂)abB)Co–(PMe₃)₂] (2a) (Scheme 2), presumably via an oxidative addition/reductive elimination pathway [19,20,21]. The reaction delivers 2a after 24 h at 50 °C as dark orange crystals in a 66% isolated yield. A single crystal X-ray diffraction study on 2a revealed a five coordinate 18-valence electron Co(I) complex (Scheme 2). The complex 2a crystallises in an achiral non-centrosymmetric space group of the type Pca2₁ with four molecules in the unit cell (Z = 4, Z’ = 1) (Supplementary Materials) [22].

The coordination environment at the cobalt atom in 2a is best described as distorted trigonal bipyramidal with the boryl ligand and one PMe₃ ligand in the apical positions, and the angle between these positions deviates by 7° from linearity. Moreover, the strong σ donor properties of the boryl ligand result in an elongation of the Co1–P4 distance of the apical PMe₃ ligand, compared to distance Co1–P3 of the equatorial PMe₃ ligand by 0.03 Å.

The equatorial positions are occupied by the two pincer phosphine donors and a second PMe₃ ligand, resulting in a sum of angles in the equatorial plane [P1,P2,P3] of 348.14°, whereby the angle P1–Co1–P2, involving the two pincer phosphorus atoms, is slightly larger than the other angles. For the deviation of the sum of angles, from 360° accounts for the significant displacement of Co1 from the [P1,P2,P3] plane by 0.4372(3) Å towards the P4 atom. This distortion of the trigonal bipyramidal coordination environment at the cobalt atom is due to the restraints imposed by the five-ring chelates in 2a. Whilst the solid-state molecular structure of 2a does not exhibit any crystallographic symmetry, it is virtually C_s symmetric, with a mirror plane through the atoms [B1,P3,P4,Co1] (Figure S37).

An analogous unrestraint mono boryl complex [(PMe₃)₄Co–Bcat] (cat = 1,2-O₂C₆H₄) exhibits a slightly longer B–Co distance of 1.9545(4) Å and a slightly shorter trans-B P–Co distance of 2.1897(1) Å, together with a less pronounced displacement of the cobalt atom from the equatorial ligand plane [21]. The equatorial Co–P distance in [(PMe₃)₄Co–Bcat], however, is more equally distributed around 2.17 Å. The closely related square planar PBP complex [(d(CH₂P(tBu)₂)abB)Co–N₂], reported to undergo reversible H₂ activation by Peters and a co-worker, exhibits similar Co–B and Co–P distances of 1.946(1) Å and 2.1884(4)/2.1901(3) Å and also significant deviation of the P–Co–P angle from linearity of 156.26(1)° as a result of the five-ring chelation [3].

The ³¹P{¹H} NMR spectrum of 2a comprises three distinct signals, two signals of the two distinct PMe₃ ligands, one in the apical—trans boryl—position around 0 ppm and the second around –16 ppm for the equatorial PMe₃ ligand. The third signal around 83 ppm is assigned to the two equivalent PBP pincer ligand P(iPr)₂ groups (Figure 1(top) and Figure S4). Whilst these signals do not exhibit any fine structure at room temperature (Figure S4), at lower temperatures, the signals split in a doublet of doublets at 82.9 ppm (–69 °C) for the P(iPr)₂ groups, an apparent broadened quartet at 0.7 ppm (–69 °C) for the apical PMe₃ ligand and a triplet of doublets at –16.2 ppm (–69 °C) for the equatorial PMe₃ ligand (Figure 1(top) and Figure S4). This is in agreement with the mutual couplings within an A₂MN spin system. This agrees with a conformation of the complex in solution similar to the one found in the solid state.

However, the temperature-dependent broadening is indicative of dynamic processes present in solution. A ¹H-¹H NOESY NMR spectrum at room temperature gives a fitting picture. Distinct NOE contacts between the PMe₃ signals and the methine CHMe₂ signals allow for the assignment of the PMe₃ ligands to the apical and equatorial positions, respectively. Exchange signals are observed between the two PMe₃ ligands, but also between pairs of methyl groups of the two distinct isopropyl moieties and the methine protons of these groups (Figure 1 (bottom)). This is fundamentally in agreement with two possible exchange mechanisms: (i) via the dissociation of a PMe₃ ligand with a transient four coordinate 16-electron complex [(d(CH₂P(iPr)₂)abB)Co–PMe₃] and the re-association of a PMe₃ ligand; (ii) a concerted mechanism exchanging the PMe₃ ligand positions via a (distorted) square pyramidal intermediate is feasible. Note also that the NMR data do not suggest any appreciable dissociation of PMe₃ from 2a, contrary to the heavier rhodium homolog 2b (vide infra).

The reaction of 2a with an equimolar amount of BAr₃ as a Lewis acid should lead to abstraction of a PMe₃ ligand and, after reorganisation, to the complex [(d(CH₂P(iPr)₂)abB)Co–PMe₃] (3a). However, whilst one PMe₃ ligand can indeed be abstracted by BPh₃, the complex 3a is not isolated. Instead, in a dinitrogen atmosphere, its dinitrogen adduct [(d(CH₂P(iPr)₂)abB)Co–(N₂)(PMe₃)] (4a) crystallises in minute amounts after several days at –40° C (Scheme 3).

The N₂ complex 4a crystallises in a space group of the type P2₁/c with two independent molecules in the asymmetric unit (Z = 8, Z’ = 2) (Supplementary Materials) [22]. Both molecules exhibit only a marginal geometric difference, and only one is discussed exemplarily (Figure S40).

As for 2a, the coordination geometry of 4a is best described as distorted trigonal bipyramidal with the boryl ligand and the N₂ ligand in the apical positions. The B–Co distance remains virtually unchanged by this substitution of the trans boryl ligand and is also identical to the distance found in the closely related four-coordinate PBP complex [(d(CH₂P(tBu)₂)abB)Co–N₂] reported by Peters et al. of 1.946(1) Å [3]. This indicates again that the B–Co distance is largely determined by the geometrical restraints of the five-ring chelates (vide infra). The close to linear B–Co–N, the angle deviates only by less than 7° from the value found in 2a and in Peter’s N₂ complex [3]. The equatorial ligands experience more substantial changes, although their sum of angles around the cobalt atom increases only slightly by 2° to 350.14°, and consistently, the deviation of the cobalt atom from the [P1,P2,P3] plane decreases by 0.05 Å. The angle between the pincer P atoms deviates significantly by 9°; hence, 4a is more distorted from an ideal trigonal bipyramidal geometry towards a square pyramidal arrangement than 2a. However, the reduced steric demand of the ligand in the apical position trans to the boryl ligand in 4a as compared to 2a leads to a relaxation of the B1–Co1–P3 angle by about 7°. The N–N distance in the N₂ ligand in 4a compares well with the distance of 1.119(2) Å found in Peter’s N₂ complex; the N–Co distance, however, is in 4a slightly—by 0.035 Å—enlarged [3].

As we failed to isolate 4a in any appreciable amounts, we resorted to its spectroscopic in situ characterisation (Figure 2). Performing the reaction of 2a with B(C₆F₅)₃ in toluene and monitoring this reaction via IR spectroscopy gives clear evidence of the immediate formation of an N₂ complex, based on the appearance of a strong IR band at 2061 cm⁻¹ if the reaction is conducted under an N₂ atmosphere, whereas only a minute signal is observed under an argon atmosphere, presumably due to the presence of adventitious N₂ (Scheme 3). This compares well to the N≡N stretching frequency of 2013 cm⁻¹ reported for the related complex [(d(CH₂P(tBu)₂)abB)Co–N₂] (vide supra) by Peters et al. [3].

Following the reaction of 2a with B(C₆F₅)₃ under an N₂ atmosphere via ³¹P and ¹¹11B NMR spectroscopy (Figure 2) gives a consistent picture: upon addition of the Lewis acid, the chemical shifts change from those of 2a (Figure 2). Whilst the ¹¹B{¹H} NMR signal shifts only by about 3 ppm, it gives evidence for the presence of the PBP boryl ligand. The changes in the ³¹P{¹H} NMR spectrum are more substantial. The two ³¹P NMR signals of the PMe₃ ligands in 2a change to a broad signal at −13 ppm and a second comparably narrow signal at −7 ppm without an appreciable fine structure. Upon cooling, however, the latter signal broadens, and its intensity reduces, whist the former signal changes into a well-developed triplet (−11.4 ppm, ²J_PP = 73 Hz) at −80 °C (Figure 2). The latter triplet corresponds to the doublet at higher chemical shifts (94.6 ppm, ²J_PP = 73 Hz). This is readily explained by the abstraction of one PMe₃ ligand to give the Lewis acid base adduct Me₃P–B(C₆F₅)₃ (δ_31P = −6.1 ppm, δ_11B = −14.7 ppm in CD₂Cl₂) and a PBP pincer cobalt complex bearing only one additional PMe₃ ligand Me₃P–B(C₆F₅)₃ is only sparingly soluble and to a large extend removed prior to the measurement. The remaining dissolved adduct, however, precipitates upon cooling resulting in a reduced ³¹P NMR signal at lower temperatures. The chemical shift of −11.4 ppm and the P–P coupling constant of around 80 Hz suggest that this PMe₃ ligand occupies an equatorial position in a trigonal bipyramidal complex, as it resembles the chemical shift, but in particular, the higher P_eq–P_PBP coupling constant found in 2a. In other words, the complex that is quantitatively formed is not the four-coordinate complex 3a but a five coordinate complex with a single PMe₃ ligand in an equatorial position—the nitrogen complex 4a.

Gas-phase DFT computations on the thermodynamics of the complexes 3a and 4a and their heavier homologues (vide infra) as central atoms indeed show that for cobalt as the central atom, the formation of a five-coordinate N₂ complex of the type [(d(CH₂P(iPr)₂)abB)M–(N₂)(PMe₃)] is strongly favoured over the four coordinate complex [(d(CH₂P(iPr)₂)abB)M–(PMe₃)] by ΔG₂₉₈ = −22.4 kJ mol⁻¹ (ΔE₀ = −70.6 kJ mol⁻¹), despite the entropic penalty occurring from the coordination of gaseous N₂. However, for the rhodium and iridium analogue, the coordination of an N₂ ligand to the latter four-coordinate complex is—in agreement with our observations (vide infra)—disfavoured by ΔG₂₉₈ = 51.3 kJ mol⁻¹ (ΔE₀ = 8.2 kJmol⁻¹) for rhodium and ΔG₂₉₈ = 51.4 kJ mol⁻¹ (ΔE₀ = 8.4 kJ mol⁻¹) for iridium (Supplementary Materials) [22]. The computed N≡N stretching frequency in 4a of 2170 cm⁻¹ is by about 100 cm⁻¹ off the experimental values, but within the expected range considering the harmonic nature of the computation and other approximations [22].

Due to an initial computation of the force constant between Co and N₂, the bonding in 4a is quite strong (Co–N: 2.33 N cm⁻¹), whilst the trans-B Co–P bond in 2a shows the expected kinetic lability (Co–P: 1.36 N cm⁻¹) of a spectator ligand. More importantly, the electronic coupling in 4a between the N–N bond and the Co–N coordination is pronounced (Co–N/N–N coupling force constant: −0.02 cm N⁻¹) and synergistic (negative sign), pointing to an effective back donation [23]. And indeed, the experimental N₂ IR wavenumber of 2061 cm⁻¹ is in line with a modest activation relative to free N₂ (~2330 cm⁻¹). Finally, the Co–B bond trans to the N₂ ligand seems to be very strong (Co–B: 2.28 N cm⁻¹), reducing the flexibility to access different coordination geometries [24].

2.2. Rhodium Complexes

The reaction of 1 with [(Me₃P)₃Rh–Cl] in the presence of KOtBu leads to the formation of a rhodium(I) boryl complex (Scheme 4). It may be speculated that the reaction proceeds via an intermediate rhodium alkoxido complex as discussed for the formation of the related complex [(dmabB)Rh(PMe₃)₃] (dmab = 1,2-(NMe)₂C₆H₄) [20]. However, the reaction of 1 with [(Me₃P)₃Rh–Cl] in the presence of KOtBu leads to the formation of an equilibrium mixture of the square planar complex [(d(CH₂P(iPr)₂)abB)Rh–PMe₃] (3b) and the five coordinate complex [(d(CH₂P(iPr)₂)abB)Rh(PMe₃)₂] (2b) (Scheme 4), whilst in the absence of KOtBu, no reaction is observed (Supplementary Materials) [22] (Figures S12 and S13). After recrystallisation from diethyl ether, the four coordinate complex 3b is obtained as bright orange crystals at a 70% yield, whereas crystallisation from n-pentane in the presence of an excess PMe₃ leads to the isolation of the five-coordinate complex 2b as crystalline material at a 43% yield. The spontaneous dissociation of one PMe₃ ligand from 2b to give 3b is not contradicting gas-phase DFT computational data (Table S10), suggesting an endothermic (15 kJ mol⁻¹) dissociation from 2b to 3b + PMe₃, but overall, an entropy driven exergonic process (−47 kJ mol⁻¹) (Supplementary Materials) [22].

Both complexes 2b and 3b crystallise in monoclinic space groups of the type P2₁/n and P2₁/c, respectively, and contain one complex molecule in the asymmetric unit (Z = 4, Z’ = 1) (Supplementary Materials) [22]. The molecular structure of complex 2b is analogous to that of the cobalt homologue 2a (Figure S38). The rhodium ion is distorted trigonal bipyramidaly coordinated by the boryl pincer ligand and one PMe₃ ligand in the apical positions (Figure 3, right). The sum of angles in the equatorial plane [P1,P2,P3] comprising the pincer phosphorus atoms and one PMe₃ ligand is with 347° only insignificantly smaller than in 2a, whereby the angle P1–Co1–P2, involving the pincer phosphorus atoms, is by about 2° larger than in 2a. The displacement of Rh1 from the [P1,P2,P3] plane is by 0.05 Å larger than in 2a, an effect of the increased radius of the rhodium ion within the restraining pincer coordination environment.

Complex 3b is best described as a distorted square planar complex with a nearly linear B1–Rh1–P3 angle and a significantly (by 27°), from linearity, deviating P1–Rh1–P2 angle. However, this angle is significantly closer to linearity than the respective angle in the five-coordinate complex 2b (Figure 3, left). The change in the Rh···P/B distances between 2a and 3b is comparably small, despite the change in the coordination number. Most pronounced is a decrease in the pincer phosphorus atoms to rhodium distances in comparison to 2b by about 0.06 Å, which may be attributed to the less strained ligand conformation in the more planar 3b.

The equilibrium between 2b and 3b, as a fundamental aspect of their coordination chemistry, was further studied via NMR spectroscopy. NMR titration of 3b with increasing amounts of PMe₃ shows a highly dynamic behaviour in the ³¹P{¹H} NMR spectra at room temperature (Figure 4 and Figures S17–S19). Only one set of signals of the PBP ligand and the trans-B PMe₃ ligand is observed, respectively. Whilst the ³¹P NMR signal of the PBP ligand changes appreciably from 84 ppm to 75 ppm with increasing amounts of PMe₃ added, the signal of the trans-B PMe₃ ligand, in 2b, is only marginally influenced (−27.3 to −26.4 ppm). An additional signal is observed shifting from −37 ppm at low amounts of PMe₃ to −62 ppm after the addition of an excess of PMe₃. This is readily explained by a rapid exchange among 3b, 2b and free PMe₃ on the NMR time scale and consequently, the observation of an averaged chemical shift of the exchanging PMe₃ moieties throughout this process. In agreement with that, the spectrum observed for isolated 2b is very virtually identical to the spectrum of 3b after the addition of an equimolar amount of PMe₃.

At low temperatures, however, the exchange among 3b, 2b and free PMe₃ becomes slow on the NMR timescale, and well-resolved signals for 2b and free PMe₃ are observed (Figure 4,Figures S14 and S15). The ³¹P{¹H} NMR spectrum of 2b itself at −46 °C comprises three signals (A, M and N) of an A₂MNX spin system with the expected ³¹P-³¹P and ³¹P-¹⁰³Rh couplings (Figure S16, Table S3). Following the reaction of 3b with different amounts of PMe₃ via UV-Vis spectroscopy corroborates the rapid equilibrium between 3b and 2b being rather on the side of 3b and free PMe₃ (Figures S20 and S21).

In conclusion, it may be stated that the five-coordinate trigonal bipyramidal complex 2b, in contrast to the Co analogue, easily dissociates one PMe₃ ligand to give the distorted square planar complex 3b. The virtual indifference in the ³¹P NMR chemical shift (and line shape) of the apparently not-exchanging trans-B PMe₃ ligand around 27 ppm suggests that this exchange does not affect this ligand but involves only the equatorial PMe₃ ligand.

2.3. Iridium Complexes

Whilst for the formation of the cobalt and rhodium PBP pincer complexes 2a and 2b/3b, it may be arguable whether activation of the diborane precursor 1 proceeds via a σ bond metathesis or an oxidative addition/reductive elimination pathway, the reaction of 1 with the iridium(I) complex [Ir(cod)Cl]₂ (cod = 1,5-cyclooctadien) to give the bis-boryl complex [(d(CH₂P(iPr)₂)abB)Ir(Bpin)(Cl)] (5c) is obviously an oxidative addition reaction (Scheme 5). This five-coordinate complex reacts with excess PMe₃ to give the six-coordinate complex [(d(CH₂P(iPr)₂)abB)Ir(Bpin)(PMe₃)(Cl)] (6c).

Both complexes 5c and 6c crystallise in monoclinic space groups of the type P2₁/c. The solid-state structure of 5c contains one complex molecule in the asymmetric unit (Z = 4, Z’ = 1), whereas 6c comprises two independent molecules in the asymmetric unit (Z = 8, Z’ = 2). The Bpin moiety in 5c shows some positional disorder that is neglected in the further discussion; for 6c, however, one of the independent molecules shows severe disorder and is not considered for further geometrical analysis (Supplementary Materials) [22].

The trigonal bipyramidal geometry of 5c may be considered typical for a five-coordinate Ir bis-boryl complex with phosphine ligands (Figure 5). All five structurally characterised complexes of this type adopt a trigonal bipyramidal geometry with the two phosphine ligands in the axial positions (P–Ir–P angle 157–172°, for PXP pincer ligands P–Ir–P angle 157–162°) and small B···B distances and B–Ir–B angles in the ranges of 2.22–2.41 Å and 65.8–76.7°, respectively [25,26,27,28,29].

In 6c, the PMe₃ ligand adopts a position trans to the PBP pincer boryl ligand, whilst the chlorido ligand occupies a position trans to the Bpin ligand (Figure 5). As a result, 6c may best be described as a strongly distorted octahedral complex with the Bpin and chlorido ligand in the axial positions. Structurally, the extension of the coordination sphere to the distorted octahedral complex 6c is accompanied by some ligand reorganisation. The P1–Ir–P2 angle reduces upon coordination by about 3° to deviate more from linearity, whereas the B–Ir–B angle deviates in 6c by about 6° less from 90° than in 5c (in accordance with the B···B distance increasing from 5c to 6c by 0.25 Å). The d(CH₂P(iPr)₂)abB ligand backbone in 6c (mean plane [B1,N1,N2,C₆H₄]) includes an angle of 24.8(8)° with the equatorial plane of the complex (mean plane [P1,P2,P3,B1,Ir1]), 20° more than in the five-coordinate 5c. This is a result of the increased steric encumbrance induced by the extension of the coordination sphere in 6c. The B–Ir distance increases slightly upon PMe₃ coordination in 6c because of the presence of trans ligands. This is more significant for B1, which is trans to the stronger trans influencing ligand PMe₃ as opposed to the chlorido ligand for B2. The Cl–Ir distance increases accordingly, whereas the P1/P2–Ir1 distances remain virtually unaffected. Again, because of the strong trans influence of the boryl ligand, the P–Ir distance of the PMe₃ ligand is longer than those of the pincer phosphine atoms by about 0.06 Å [30].

The solution-state ¹H, ³¹P and ¹³C NMR spectroscopic data for 5c and 6c fulfil the expectations and can readily be explained by the solid-state structures. Surprising, however, are the ¹¹B NMR chemical shifts. For both complexes, two very distinct, somewhat broadened singlets at chemical shifts of 39.7 ppm (Δw_½ = 340 Hz) and 19.9 ppm (Δw_½ = 330 Hz) for 5c and of 48.8 ppm (Δw_½ = 460 Hz) and 26.6 ppm (Δw_½ = 450 Hz) for 6c are observed in THF-d₈ at room temperature. This chemical shift range is somewhat different from the ¹¹B NMR data for the reported Ir(III) boryl in a range of 29–35 ppm for five-coordinate and of 30–43 ppm for six-coordinate complexes, respectively [1,2,7,25,26,28,31].

Whilst complex 6c is stable under inert conditions, it reacts readily with an equimolar amount of KOtBu to give the Ir(I) PBP pincer complex 3c (Figure 6, Scheme 5). Monitoring this reaction via in situ NMR spectroscopy (Figure 6 and Figure S34–S36) shows an essentially clean conversion to 3c, as indicated by its characteristic signals around 80 ppm (doublet, J_P–P = 5 Hz) for the pincer phosphorus atoms and a broadened singlet for the PMe₃ ligand around –20 ppm (Figure S35 Supplementary Materials) [22]. Only minor amounts of a so far unidentified side product with a ³¹P NMR signal at 45 ppm (II) are observed. However, upon closer evaluation, two transient species are observed during this reaction. At one hand side, the five-coordinate complex 2c (vide infra) is formed in small amounts in the beginning but is later on fully consumed (Figure 6). On the other side, a species with a ³¹P{¹H} NMR singlet signal at 64.5 ppm (I) is observed. In agreement with this, the ¹¹B NMR data suggest the presence of a transient boryl intermediate at 40 ppm, whereas 3c itself exhibits a moderately broad ¹¹B{¹H} NMR signal around 56 ppm (Figure S36 Supplementary Materials) [22,32]. It may be assumed that the conversion of 6c to 3c proceeds via the initial coordination of a OtBu ligand followed by (possibly after some reorganisation) the reductive elimination to an Ir(I) PBP complex, 3c or a closely related species. The intermediate presence of 2c may be explained by the intermediate liberation of PMe₃ and its transient addition to 3c during this process. An in situ ³¹P{¹H} NMR spectrum of a mixture of 3c and excess PMe₃ corroborates the facile formation of 2c (Figure 6, top). Moreover, it must be emphasised that the system 2c/3c/PMe₃ exhibits much less dynamic behaviour than the homologous rhodium system 2b/3b/PMe₃ (vide supra). Contrary to the latter, even in the presence of excess PMe₃ at room temperature, a well-resolved, ³¹P{¹H} NMR spectrum (A₂MN spin system) with a narrow linewidth is observed, indicating only comparably slow exchange of a coordinated PMe₃ ligand with free PMe₃. Contrary to 2b, distinct signals for both PMe₃ ligands are observable for 2c at room temperature in the presence of free PMe₃ (Figure 6, top). One of these signals (around −70 ppm), however, sharpens upon only moderate cooling to an apparent quartet (Figure S27 Supplementary Materials) [22]. In agreement with that, in situ UV-Vis spectroscopic data of 3c in the presence of different amounts of PMe₃ indicate a rapid equilibration, rather on the side of 2c (Figures S30 and S31). The ¹¹B NMR shift of 2c of around 55 ppm is virtually unaffected by the change in the coordination number.

In conclusion, it may be stated that the five-coordinate trigonal bipyramidal complex 2c, similarly to the cobalt analogue 2a and opposed to the rhodium homologue, shows only little dynamic behaviour in solution and does not readily dissociate a PMe₃ ligand to give the distorted square planar complex 3c. However, gas-phase DFT computational data suggest similar thermodynamic data for the dissociation of PMe₃ from 2c (ΔE₀ = 16 kJ mol⁻¹, ΔG₂₉₈ = −48 kJ mol⁻¹) as for the rhodium analogue 2b (Supplementary Materials) [22].

The complexes 2c and 3c crystallise isostructurally with the homologous rhodium complexes in monoclinic space groups of the types P2₁/n and P2₁/c, respectively (Z = 4, Z’ = 1) (Supplementary Materials) [22]. As a consequence, the molecular structure of 2c (Figure 7, right) differs only marginally form the structure of the lighter homologue 2b and from the cobalt homologue 2a (Figure S38).

The sum of angles in the equatorial plane [P1,P2,P3] of the distorted trigonal bipyramidal complex 2c is, with 347.76°, only insignificantly different from that in 2a and 2b. The angle P1–Ir1–P2, involving the pincer phosphorus atoms, is larger than that in 2a by about 2° and, hence, virtually identical to that in 2b. The displacement of Ir1 from the [P1,P2,P3] plane is in the middle between the values for two lighter homologues, by 0.03 Å larger than in 2a and by 0.02 Å smaller than in 2b. Generally, the M–P distances, however, increase from 2a to 2b and 2c by about 0.12 Å, most significantly between the cobalt and the rhodium complex.

Overall, the PBP pincer ligand shows, within the series 2a, 2b 2c, a high ability to coordinate different metal ions. The high flexibility of this ligand is also illustrated by a comparison of the five-coordinate complexes 5c and 2c. For both complexes, a trigonal bipyramidal geometry is observed; however, whilst in 2c, the phosphorus atoms of the PBP pincer ligand occupy two equatorial positions and the boryl moiety is bound in an axial position, in 5c, two phosphorus atoms coordinate in the two axial positions and the boron atom in an equatorial position. This is illustrated by P–M–P angles included by the pincer phosphorus atoms decreasing by 30° from 5c to 2c.

The solid-state structure of the distorted square planar complex 3c is again very similar to that of its rhodium homologue 3b (Figure S39) with a nearly linear B1–Ir1–P3 angle and a P1–Ir1–P2 angle of 152.95(2)° deviating significantly from linearity. Noteworthy is the Ir1–B1 distance in 3c that is slightly (0.01 Å) longer, whereas the pincer P–M distances are identical, and the trans-B P–M distance is slightly shorter (0.02 Å) than the respective distance in the rhodium homologue 3b.

3. Discussion

A series of either group 9 PBP diaminoboryl pincer complexes was synthesised using the unsymmetrical diborane(4) 1 as a PBP pincer precursor and fully characterised. In an extension of our earlier work [15], this exemplifies again the versatility of this compound as a PBP pincer ligand precursor. The Co^I and Rh^I complexes [(d(CH₂P(iPr)₂)abB)Co–(PMe₃)₂] (2a) and [(d(CH₂P(iPr)₂)abB)Rh–(PMe₃)_n] (2b (n = 2), 3b (n = 1)), respectively, were obtained in a one-step reaction from the respective Co^I and Rh^I precursors (Scheme 2 and Scheme 4). Whilst an oxidative addition/reductive elimination pathway is, for both reactions, feasible, in the rhodium case, a σ bond metathesis pathway may be feasible, considering results based on a related non-pincer ligand [20]. The heavier Ir^I homologue, however, was obtained via the isolated intermediate Ir^III complexes [(d(CH₂P(iPr)₂)abB)Ir(Bpin)(Cl)] (5c) and [(d(CH₂P(iPr)₂)abB)Ir(Bpin)(PMe₃)(Cl)] (6c). Complex 5c is formed upon an oxidative addition reaction of the diborane(4) 1 with [Ir(cod)Cl]₂ (cod = 1,5-cyclooctadien) and subsequently reacts via PMe₃ addition to 6c. The coordination chemistry of the resulting homologous complexes [(d(CH₂P(iPr)₂)abB)M(PMe₃)₂] (2a–c) and [(d(CH₂P(iPr)₂)abB)M–PMe₃] (3b,c) was studied structurally in the solid state, as well as spectroscopically in solution. However, for M = Rh and Ir, both complexes are structurally very similar but differ in the dynamic behaviour and the relative accessibility of the four (3b,c) vs. the five (2b,c) coordinated complexes. For Co only the five-coordinate complex 2a is accessible, whereas Lewis acid-promoted PMe₃ abstraction under a dinitrogen atmosphere leads to the formation of the surprisingly stable N₂ complex [(d(CH₂P(iPr)₂)abB)Co–(N₂)(PMe₃)] (4a).

Having, with the unsymmetrical diborane(4) [(d(CH₂P(iPr)₂)abB)–Bpin] (1), a well accessible and versatile PBP ligand precursor that is capable of oxidative addition (Pt^II, Co^I, Rh^I (possibly), Ir^I) and σ bond metathesis (Cu^I and possibly Rh^I) reactions [15,20] will stimulate the further development of PBP pincer ligands. In conclusion, PBP diaminoboryl pincer ligands are a ligand class with remarkable ligand properties with respect to their high σ donor strength and weak π acceptor properties—leading to a strong trans effect and influence [30]—that provide stability for the inherently reactive B–M bond due to their pincer framework. Furthermore, PBP pincer ligands are tuneable based on the backbone and P atoms substituents, making them interesting for a broad range of applications from catalysis to the stabilisation of reactive intermediates.

4. Materials and Methods

4.1. General Considerations

pinB–B(d(CH₂P(iPr)₂)ab) (1), [(Me₃P)₄CoMe], [(Me₃P)₃RhCl] and [(cod)IrCl]₂ were prepared according to literature procedures [15,16,17,18,33]. All other compounds were commercially available and were used as received; their purity and identity were checked using appropriate spectroscopic methods. Unless otherwise noted, all solvents were dried using an MBraun solvent purification system, deoxygenated using the freeze-pump-thaw method and stored under purified nitrogen. Unless noted otherwise, 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 and Avance III 400 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; PhMe-d₈: ¹H NMR: 2.08 ppm, ¹³C NMR: 20.43 ppm; THF-d₈: ¹H NMR: 1.72 ppm, ¹³C NMR: 25.31 ppm) [34]. ¹¹B and ³¹P NMR chemical shifts are reported relative to pseudo external BF₃·Et₂O and 85% H₃PO_4(aq), respectively. ¹³C{¹H}, ¹¹B{¹H} and ³¹P{¹H} NMR spectra were recorded employing composite pulse ¹H decoupling. ¹¹B NMR spectra were processed applying a back linear prediction, in order to suppress the broad background signal due to the boron in the NMR tube and instrument. A Lorentz-type window function (LB = 10 Hz) was used, and the spectra were carefully evaluated to ensure that no genuinely broad signals of the sample were suppressed. Simulations were conducted with the TOPSPIN/DAISY program package (Bruker). 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 Braunschweig using an Elementar vario MICRO cube instrument. A Bruker Vertex 70 spectrometer was used for recording IR spectra. The IR spectra were recorded in PhMe solutions in a cuvette of an approximately 1 mm optical path length equipped with NaCl windows.

X-ray Structure Determination. The single crystals were transferred into inert perfluoroether oil inside a nitrogen-filled glovebox and, outside the glovebox, rapidly mounted on top of a CryoLoop (Hampton Research) and placed on the diffractometer in the cold nitrogen gas stream of a Cryostream 800 cooling system (Oxford Cryosystems) [35]. The data were collected on a Rigaku Oxford Diffraction Synergy-S instrument using either mirror-focused MoKα or CuKα radiation (Rigaku PhotonJet microfocus sources). The reflections were indexed and integrated, and appropriate absorption corrections were applied as implemented in the CrysAlisPro software package [36]. The structures were solved employing the program SHELXT and refined anisotropically for all non-hydrogen atoms via full-matrix least squares based on all F² values using SHELXL software [37,38,39]. 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 OLEX², PLATON, Mercury and Diamond were used [40,41,42,43]. Unless noted otherwise non-C,H atoms are depicted as ellipsoids at the 50% probability level, whereas the carbon atom framework is depicted as a stick model (grey), and hydrogen atoms are omitted for clarity. Adapted numbering schemes may be used to improve the readability. Further crystallographic details can be found in the Supplementary Materials available.

4.2. Experimental Procedures and Analysis Data

4.2.1. [(d(CH₂P(iPr)₂)abB)Co(PMe₃)₂] (2a)

In a Schlenk-flask, d(CH₂P(iPr)₂)abB–Bpin (1) (100 mg, 0.198 mmol, 1 equiv.) and [(Me₃P)₄CoMe] (75 mg, 0.198 mmol, 1 equiv.) were dissolved in toluene (50 mL) and stirred for 24 h at 50 °C whilst a reduced pressure was applied for about 50% of the time (the pressure was normalised overnight). The solvent was completely removed in vacuo and the brown residue was dissolved in n-pentane and recrystallised at −40 °C. The resulting dark orange crystals were washed with cold n-pentane (1 mL) and dried in vacuo (77 mg, 0.131 mmol, 66%).

¹H NMR (PhMe-d₈, 400.4 MHz, rt) δ = 6.96–6.91 (m, 2 H, 3-HC_Ar), 6.74–6.79 (m, 2 H, 2-HC_Ar), 3.50 (d, ²J_H-H = 11.0 Hz, 2 H, CHH’), 3.47 (d, ²J_H-H = 11.0 Hz, ²J_H-P = 4 Hz, 2 H, CHH’), 1.98 (app. sept., ³J_H-H = 7.6 Hz, ³J_H-H = 7.0 Hz, 2 H, CH(CH₃)₂), 1.89 (m, ³J_H-H = 7.0 Hz, ³J_H-H = 7.4 Hz, J_H-P = 2.5, 4.0 Hz, 2 H, C’H(CH₃)₂), 1.29 (d, ²J_H-P = 5.0 Hz, 9 H, P_ap(CH₃)₃), 1.23 (app. q, ³J_H-H = 7.6 Hz, J_H-P = 6.8, 6.0 Hz, 6 H, CH(CH₃)(C’H₃)), 1.11 (m, ³J_H-H = 7.0 Hz, J_H-P = 5.7, 3.6 Hz, 6 H, CH(CH₃)(C’H₃)), 1.04 (d, ²J_H-P = 4.8 Hz, 9 H, P_eq(CH₃)₃), 0.98 (app. q, ³J_H-H = 7.0 Hz, J_H-P = 6.9, 6.5 Hz, 6 H, C’H(CH₃)(C’H₃)), 0.78 (app. q, ³J_H-H = 7.4 Hz, J_H-P = 6.5, 5.7 Hz, 6 H, C’H(CH₃)(C’H₃)). ¹³C{¹H} NMR (PhMe-d₈, 100.7 MHz, rt) δ = 140.6 (app. t, J_C-P = 6 Hz, 1-C_Ar), 117.2 (s, 3-HC_Ar), 106.5 (s, 2-HC_Ar), 44.5 (m, CHH’), 32.0 (app. dt, J_C-P = 19, 3 Hz, CH(CH₃)₂), 29.1 (app. t, J_C-P = 5 Hz, C’H(CH₃)₂), 27.8 (m, P_ap(CH₃)₃), 25.7 (app. dq, J_C-P = 15, 4 Hz, P_eq(CH₃)₃), 21.9 (s, CH(CH₃)(C’H₃)), 20.3 (s, CH(CH₃)(C’H₃)), 19.4 (s, C’H(CH₃)(C’H₃)), 18.9 (app. t, J_C-P = 3 Hz, C’H(CH₃)(C’H₃)). ³¹P{¹H} NMR (PhMe-d₈, 162.1 MHz, rt) δ = 83.0 (br. s, Δw_½ = 217 Hz, CH₂P(iPr)₂), −1.2 (br. s, Δw_½ = 132 Hz, P(CH₃)₃), −17.4 (br. s, Δw_½ = 245 Hz, P(CH₃)₃). ¹¹B{¹H} NMR (PhMe-d₈, 128.5 MHz, rt) δ 57.0 (br. s, Δw_½ = 360 Hz). ¹H NMR (PhMe-d₈, 400.4 MHz, −69 °C) δ = 7.20–7.14 (m, 2 H, C_Ar), 6.95–6.89 (m, 2 H, HC_Ar), 3.51–3.33 (m, 4 H, CHH’), 1.92 (br. s, 2 H, CH(CH₃)₂), 1.80 (br. app. sept., J_H-H = 7 Hz, 2 H, C’H(CH₃)₂), 1.22 (d, ²J_H-P = 5 Hz, 9 H, P(CH₃)₃), 1.20 (br. s, 6 H, CH(CH₃)(C’H₃)), 1.06 (d, ²J_H-P = 5 Hz, 9 H, P(CH₃)₃), 1.03 (br. s, 6 H, CH(CH₃)(C’H₃)), 0.99–0.90 (m, 6 H, C’H(CH₃)(C’H₃)), 0.75 (br. s, 6 H, C’H(CH₃)(C’H₃)). ³¹P{¹H} NMR (PhMe-d₈, 162.1 MHz, −69 °C) δ = 82.9 (dd, ²J_P-P = 80, 30 Hz, CH₂P(iPr)₂), 0.7 (app. q, ²J_P-P = 30, 28 Hz, P_ap(CH₃)₃), −16.2 (td, q, ²J_P-P = 80, 28 Hz, P_eq(CH₃)₃). ¹H NMR (C₆D₆, 300.1 MHz, rt) δ = 7.10–7.03 (m, 2 H, HC_Ar), 6.92–6.85 (m, 2 H, HC_Ar), 3.50 (m, 4 H, CHH’), 2.06–1.85 (m, 4 H, CH(CH₃)₂), 1.29 (d, ²J_H-P = 5.0 Hz, 9 H, P(CH₃)₃), 1.28–1.19 (m, 6 H, CH(CH₃)(C’H₃)), 1.14–1.07 (m, 6 H, CH(CH₃)(C’H₃)), 1.07 (d, ²J_H-P = 4.8 Hz, 9 H, P(CH₃)₃), 1.03–0.94 (m, 6 H, C’H(CH₃)(C’H₃)), 0.87–0.76 (m, 6 H, C’H(CH₃)(C’H₃)). ¹¹B{¹H} NMR (C₆D₆, 96.3 MHz, rt) δ 57.2 (br. s, Δw_½ = 460 Hz). ³¹P{¹H} NMR (C₆D₆, 121.5 MHz, rt) δ 82.9 (br. s, Δw_½ = 200 Hz, CH₂P(iPr)₂), −1.5 (br. s, Δw_½ = 135 Hz, P(CH₃)₃), −17.5 (br. s, Δw_½ = 240 Hz, P’(CH₃)₃). Anal. Calcd. for C₂₆H₅₄BCoN₂P₄ (2a): C, 53.08; H, 9.25; N, 4.76. Found: C, 52.84; H, 9.27; N, 5.13. m.p.: 160–163 °C.

4.2.2. [(d(CH₂P(iPr)₂)abB)Co(N₂)(PMe₃)] (4a)

Single crystals of 4a: In a nitrogen-filled glovebox, 2a (10 mg, 17 µmol, 1 equiv.) and triphenylborane (4.1 mg, 17 µmol, 1 equiv.) were dissolved in C₆D₆ (0.7 mL). After 3 d at room temperature, the solution was layered with n-pentane. Colourless crystals of Me₃P–BPh₃ separated. The supernatant solution was decanted, and the solvent was removed in vacuo. The residue was dissolved in toluene (0.5 mL), and the solution was layered with n-pentane and cooled to −40 °C. Colourless crystals formed overnight, from which the supernatant solution was decanted and cooled again to −40 °C. A few orange single crystals of 4a suitable for x-ray diffraction were obtained from this solution. In situ IR characterisation of 4a was as follows: in a nitrogen-filled glovebox, 2a (10 mg, 17 µmol, 1 equiv.) and tris(pentafluorophenyl)borane (8.7 mg, 17 µmol, 1 equiv.) were dissolved in toluene (0.4 mL) and transferred into an IR cuvette. An IR spectrum of this solution was recorded. The reaction under an Ar atmosphere was conducted analogously in an Ar-filled glovebox. In situ NMR characterisation of 4a was performed as follows: in a nitrogen-filled glovebox, 2a (16.1 mg, 27 µmol, 1 equiv.) and tris(pentafluorophenyl)borane (14 mg, 27 µmol, 1 equiv.) were dissolved in toluene-d₈ and filtered through a small pad of celite. NMR spectra of this solution were recorded.

¹H NMR (PhMe-d₈, 400.4 MHz, rt) δ = 6.88 (br. s, 2 H, HC_Ar), 6.66 (br. s, 2 H, HC_Ar), 3.50 (br. s, 2 H, CH₂), 3.34 (br. s, 2 H, CH₂), 2.11 (overlapping with the residual solvent signal, CH(CH₃)₂), 1.48–0.65 (P(CH₃)₃) and CH(CH₃)₂). ³¹P{¹H} NMR (PhMe-d₈, 162.1 MHz, rt) δ = 93.5 (br. s, Δw_½ = 211 Hz, CH₂P(iPr)₂), −13.4 (br. s, Co–P(CH₃)₃). ¹¹B{¹H} NMR (PhMe-d₈, 128.5 MHz, rt) δ 54.3 (br. s, Δw_½ = 630 Hz). ¹H NMR (PhMe-d₈, 400.4 MHz, −69 °C) δ = 6.77 (br. s, 2 H, HC_Ar), 3.35 (br. s, 2 H, CH₂), 3.13 (br. d, 2 H, CH₂), 1.95 (br. s, 4 H, CH(CH₃)₂), 1.40 (br. s, 6 H, CH(CH₃)₂), 1.19 (br. s, 6 H, CH(CH₃)₂), 1.04 (br. s, 15 H, CH(CH₃)₂) and P(CH₃)₃), 0.80 (br. s, 6 H, CH(CH₃)₂). ³¹P{¹H} NMR (PhMe-d₈, 162.1 MHz, −80 °C) δ = 94.6 (d, ²J_P-P = 74 Hz, CH₂P(iPr)₂), −11.4 (t, ²J_P-P = 74 Hz, Co–P(CH₃)₃).

4.2.3. [(d(CH₂P(iPr)₂)abB)Rh(PMe₃)₂] (2b)

The reaction was performed as described for 3b on a 55 μmol scale (vide infra). After filtration, an excess of PMe₃ (30 µL, 22 mg, 0.3 mmol, 5.5 equiv.) was added, and the resulting yellow solution was cooled to −40 °C. After 48 h, bright yellow crystals suitable for X-ray crystallography had formed. The supernatant solution was decanted, and the crystals were dried in vacuo (15 mg, 24 μmol, 43%). NMR spectra of the isolated material show an equilibrium among 2b, 3b and free PMe₃ (Figures S17–S19). NMR spectra of 2b were recorded from a solution of 3b (15 mg, 28 μmol) in THF-d₈ (0.7 mL) after the addition of PMe₃ (3.7 µL, 2.7 mg, 37 μmol, 1.3 equiv.).

¹H NMR (THF-d₈, 400.4 MHz, −46° C) δ 6.58–6.47 (m, 4 H, 2,3-HC_Ar), 3.61–3.42 (m, 4 H, CH₂), 2.12 (app. sept., J = 6.9 Hz, 2 H, CH(CH₃)₂), 1.64 (br. s, 2 H, Δw_½ = 25 Hz, CH(CH₃)₂), 1.43 (d, ²J_H-P = 4.8 Hz, 9 H, P(CH₃)₃), 1.34–1.21 (m, 12 H, CH(CH₃)₂), 1.11 (d, ²J_H-P = 4.8 Hz, 9 H, P’(CH₃)₃), 1.03 (app. q, J = 5.6 Hz, 6 H, CH(CH₃)₂), 0.94 (d, ²J_H-P = 2 Hz, 2.9 H, free P(CH₃)₃), 0.7 (app. q, J = 7.1 Hz, 6 H, CH(CH₃)₂). ¹¹B{¹H} NMR (THF-d₈, 128.5 MHz, rt) δ 55.4 (s, Δw_½ = 365 Hz). ³¹P{¹H} NMR (THF-d₈, 162.1 MHz, rt) δ 76.6 (br. d, ¹J_P-Rh = 153 Hz, Δw_½ = 150 Hz, CH₂P(iPr)₂), −26.4 (br. d, ¹J_P-Rh = 97 Hz, Δw_½ = 105 Hz, P(CH₃)₃), −37 (br. s, Δw_½ = 1000 Hz, P(CH₃)₃), −54 (br. s, Δw_½ = 1600 Hz, free P(CH₃)₃). ³¹P{¹H} NMR (THF-d₈, 162.1 MHz, −46 °C) δ 75.5 (ddd, ¹J_P-Rh = 157 Hz, ²J_P-P = 38, 103 Hz, CH₂P(iPr)₂), −25.1 (app. dq, ¹J_P-Rh = 105 Hz, ²J_P-P = 43, 38 Hz, P_ap(CH₃)₃), −32.3 (dtd, ¹J_P-Rh = 157 Hz, ²J_P-P = 103, 43 Hz, P_eq(CH₃)₃). Anal. Calcd. for C₂₆H₅₄BN₂P₄Rh (2b): C, 49.39; H, 8.61; N, 4.30. Found: C, 48.91; H, 8.56; N, 4.47.

4.2.4. [(d(CH₂P(iPr)₂)abB)Rh(PMe₃)] (3b)

In a nitrogen-filled glovebox, d(CH₂P(iPr)₂)abB–Bpin (1) (41 mg, 82 μmol, 1 equiv.) and [Rh(PMe₃)₃Cl] (30 mg, 82 μmol, 1 equiv.) were combined and dissolved in toluene (10 mL). A solution of KOtBu (9 mg, 82 μmol, 1 equiv.) in THF (2 mL) was added, and the bright orange solution was stirred for 5 min at room temperature. The solvent was removed in vacuo. The residue was extracted with n-pentane (2 × 3.5 mL) and filtered through a pad of celite. The solvent was removed in vacuo. The orange residue was recrystallised from diethyl ether (3 mL) at −40 °C to give bright orange crystals of [(d(CH₂P(iPr)₂)abB)Rh(PMe₃)] (3b) (30 mg, 56 μmol, 70%).

¹H NMR (THF-d₈, 400.4 MHz, rt) δ 6.72–6.67 (m, 2 H, HC_Ar), 6.67–6.62 (m, 2 H, HC_Ar), 3.64 (app. t, J = 2, 2 Hz, 4 H, NCH₂P), 2.18 (app. sept., J = 7, 6, 1.5, 1.5 Hz, 4 H, CH(CH₃)₂), 1.39 (dd, ²J_H-P = 4.3, ³J_H-Rh = 0.6 Hz, 9 H, P(CH₃)₃), 1.19 (app. q, J = 7.0, 7.4, 7.4 Hz, 12 H, CH(CH₃)₂), 1.09 (app. q, J = 6.0, 6.3, 6.3 Hz, 12 H, CH(CH₃)₂). ¹³C{¹H} NMR (THF-d₈, 101.7 MHz, rt) δ 140.7 (app. td, J_C-P = 9 Hz, J_C-Rh = 1.5 Hz, 1-C_Ar), 117.3 (s, HC_Ar), 104.9 (s, HC_Ar), 43.9 (m, CH₂), 28.6 (app. t, J_C-P = 8.5 Hz, CH(CH₃)(C’H₃)), 23.0 (app. dtd, J_C-P = 13, 3 Hz, ²J_C-Rh = 1 Hz, P(CH₃)₃), 20.8 (app. t, J_C-P = 5 Hz, CH(CH₃)(C’H₃)), 20.8 (br. s, CH(CH₃)(C’H₃)). ¹¹B{¹H} NMR (THF-d₈, 128.5 MHz, rt) δ 52.4 (s, Δw_½ = 400 Hz). ³¹P{¹H} NMR (THF-d₈, 162.1 MHz, rt) δ 84.1 (dd, ¹J_P-Rh = 173 Hz, ²J_P-P = 17 Hz, CH₂P(iPr)₂), −27.1 (br. d, ¹J_P-Rh = 111 Hz, Δw_½ = 90 Hz, P(CH₃)₃). ³¹P{¹H} NMR (THF-d₈, 162.1 MHz, −102 °C) δ 83.8 (dd, ¹J_P-Rh = 171 Hz, ²J_P-P = 17 Hz, CH₂P(iPr)₂), −25.4 (dt, ¹J_P-Rh = 113 Hz, ²J_P-P = 17 Hz, P(CH₃)₃). Anal. Calcd. for C₂₃H₄₅BN₂P₃Rh (3b): C, 49.66; H, 8.15; N, 5.04. Found: C, 49.43; H, 8.11; N, 5.38. m.p.: 199–200 °C.

4.2.5. [(d(CH₂P(iPr)₂)abB)Ir(PMe₃)₂] (2c))

In a nitrogen-filled glovebox, 3c (30 mg, 46 μmol, 1 equiv.) was dissolved in THF (5 mL), and PMe₃ (23.6 µL, 17.7 mg, 0.23 mmol, 5 equiv.) was added. The solvent was removed under in vacuo conditions. The light yellow residue was recrystallised from diethyl ether (2 mL) at −40 °C to give light yellow crystals of [(d(CH₂P(iPr)₂)abB)Ir(PMe₃)₂] (2c) (7 mg, 9.7 µmol, 21%).

¹H NMR (PhMe-d₈, 400.4 MHz, rt) δ 6.95–6.89 (m, 2 H, 3-HC_Ar), 6.83–6.77 (m, 2 H, 2-HC_Ar), 3.53–3.39 (m, 4 H, CH₂), 1.96 (app. br. sept., J = 7 Hz, 2 H, CH(CH₃)₂), 1.63 (br. s, 2 H, Δw_½ = 25 Hz, C’H(CH₃)₂), 1.51 (d, ²J_H-P = 5.9 Hz, 9 H, P(CH₃)₃), 1.24 (d, ²J_H-P = 6.3 Hz, 9 H, P’(CH₃)₃), 1.20–1.08 (m, 12 H, CH(CH₃)₂), 0.89 (app. q, J = 6.9 Hz, 6 H, C’H(CH₃)(C’H₃)), 0.64 (app. q, J = 7.1 Hz, 6 H, C’H(CH₃)(C’H₃)). ¹¹B{¹H} NMR (PhMe-d₈, 128.5 MHz, rt) δ 55.2 (s, Δw_½ = 475 Hz). ³¹P{¹H} NMR (PhMe-d₈, 162.1 MHz, −35 °C) δ 50.7 (dd, ²J_P-P = 27, 111 Hz, CH₂P(iPr)₂), −65.7 (td, ²J_P-P = 27, 111 Hz, P’(CH₃)₃), −69.9 (app br. q, ²J_P-P = 27, 27 Hz, P(CH₃)₃). ³¹P{¹H} NMR (PhMe-d₈, 162.1 MHz, rt) δ 50.7 (dd, ²J_P-P = 27, 112 Hz, CH₂P(iPr)₂), −65.6 (td, ²J_P-P = 27, 112 Hz, P’(CH₃)₃), −69.9 (br. s, Δw_½ = 95 Hz, P(CH₃)₃). ³¹P{¹H} NMR (THF-d₈, 121.5 MHz, rt) δ 50.6 (dd, ²J_P-P = 28, 111 Hz, CH₂P(iPr)₂), −66.4 (td, ²J_P-P = 27, 111 Hz, P’(CH₃)₃), −70.0 (br. s, Δw_½ = 100 Hz, P(CH₃)₃). ¹³C{¹H} NMR (PhMe-d₈, 100.7 MHz, rt) δ = 141.2 (app. t, J_C-P = 5 Hz, 1-C_Ar), 117.3 (s, 3-HC_Ar), 107.5 (s, 2-HC_Ar), 47.7 (app. td, app. t, J_C-P = 19, 10 Hz, CH₂), 30.3 (overlapping m, C’H(CH₃)₂ and P(CH₃)₃), 28.9 (app. t, J_C-P = 11 Hz, CH(CH₃)₂), 27.9 (br. d, J_C-P = 19 Hz, P’(CH₃)₃), 21.2 (s, CH(CH₃)₂), 19.8 (s, CH(CH₃)₂), 19.7 (s, C’H(CH₃)(C’H₃)), 18.8 (s, C’H(CH₃)(C’H₃)). Anal. Calcd. for C₂₆H₅₄BN₂P₄Ir (2c): C, 43.27; H, 7.54; N, 3.88. Found: C, 42.79; H, 7.27; N, 4.02.

4.2.6. [(d(CH₂P(iPr)₂)abB)Ir(PMe₃)] (3c)

In a Schlenk-flask, 5c (50 mg, 68 μmol, 1 equiv.) was dissolved in THF (5 mL). Trimethylphosphine (34.7 µL, 26 mg, 0.342 mmol, 5 equiv.) was added, and the solution was stirred for 5 min at room temperature. The solvent was removed in vacuo. The colourless residue was dissolved in THF (5 mL), and a solution of KOtBu (7.6 mg, 68 μmol, 1 equiv.) in THF (1 mL) was added. The resulting red–green solution was stirred for 2 h at room temperature. The solvent was removed in vacuo, and the residue was extracted with n-pentane (2 × 5 mL). The extract was filtered through a pad of celite and stored at −40 °C. After 24 h, dark red crystals with a greenish hue had separated. The supernatant solution was decanted, and the residue was washed with cold n-pentane (2 × 1 mL) and dried in vacuo (22 mg, 34 μmol, 50%).

¹H NMR (PhMe-d₈, 400.4 MHz, rt) δ 7.08–7.03 (m, 2 H, 3-HC_Ar), 6.99–6.93 (m, 2 H, 2-HC_Ar), 3.63 (app. t, J_H-P = 2.1, 2.1 Hz, 4 H, CH₂), 2.04 (app. sept. t, ³J_H-H = 7.0, 7.0 Hz, J_H-P = 2.2, 2.2 Hz, 4 H, CH(CH₃)₂), 1.42 (d, ²J_H-P = 5.7 Hz, 9 H, PMe₃), 1.05 (app. q, ³J_H-H = 7.0 Hz, J_H-P = 7.9, 7.9 Hz, 12 H, CH(CH₃)(C’H₃)), 0.97 (app. q, ³J_H-H = 7.0 Hz, J_H-P = 6.1, 6.1 Hz, 12 H, CH(CH₃)(C’H₃)). ¹H NMR (THF-d₈, 300.1 MHz, rt) δ 6.73–6.66 (m, 2 H, HC_Ar), 6.66–6.58 (m, 2 H, HC_Ar), 3.74 (app. t, J_H-P = 2.0, 2.0 Hz, 4 H, CH₂), 2.32 (app. sept. t, ³J_H-H = 7.0, 7.0 Hz, J_H-P = 2.2, 2.2 Hz, 4 H, CH(CH₃)₂), 1.57 (d, ²J_H-P = 5.7 Hz, 9 H, PMe₃), 1.17 (app. q, ³J_H-H = 7.0 Hz, J_H-P = 7.9, 7.9 Hz, 12 H, CH(CH₃)(C’H₃)), 1.11 (app. q, ³J_H-H = 7.0 Hz, J_H-P = 6.1, 6.1 Hz, 12 H, CH(CH₃)(C’H₃)). ¹³C{¹H} NMR (PhMe-d₈, 100.7 MHz, rt) δ 140.1 (app. t, J_C-P = 8 Hz, C_Ar), 117.7 (s, 3-HC_Ar), 108.6 (s, 2-HC_Ar), 44.4 (app. td, J_C-P = 21, 12 Hz, NCH₂P), 28.6 (app. t, J_C-P = 12 Hz, CH(CH₃)₂), 24.8 (app. dt, J_C-P = 24, 2 Hz, PMe₃), 20.2 (app. dt, J_C-P = 4 Hz, CH(CH₃)(C’H₃)), 19.3 (br. s, CH(CH₃)(C’H₃)). ¹¹B{¹H} NMR (PhMe-d₈, 128.5 MHz, rt) δ 57.5 (s, Δw_½ = 430 Hz). ¹¹B{¹H} NMR (THF-d₈, 96.3 MHz, rt) δ 55.3 (s, Δw_½ = 380 Hz). ³¹P{¹H} NMR (PhMe-d₈, 162.1 MHz, rt) δ 81.5 (d, J_P–P = 5 Hz, CH₂P(iPr)₂), −18.6 (br s, P(CH₃)₃). ³¹P{¹H} NMR (THF-d₈, 121.5 MHz, rt) δ 80.0 (d, J_P–P = 5 Hz, CH₂P(iPr)₂), −20.1 (br s, P(CH₃)₃). Anal. Calcd. for C₂₃H₄₅BIrN₂P₃ (3c): C, 42.79; H, 7.03; N, 4.34. Found: C, 43.22; H, 7.25; N, 4.59. m.p.: 204–206 °C.

4.2.7. [(d(CH₂P(iPr)₂)abB)IrCl(Bpin)] (5c)

In a Schlenk-flask, 1 (100 mg, 0.198 mmol, 1 equiv.) and [Ir(cod)Cl]₂ (66.5 mg, 99 μmol, 1 equiv. Ir) were combined in n-pentane (50 mL). The yellow suspension was stirred at room temperature overnight before all volatiles were removed in vacuo. The bright yellow residue was recrystallised from n-pentane (20 mL) at −40 °C to give 5c as bright yellow crystals (107 mg, 0.146 mmol, 74%).

¹H NMR (THF-d₈, 400.4 MHz, rt) δ 6.77–6.71 (m, 2 H, 2-HC_Ar), 6.69–6.64 (m, 2 H, 3-HC_Ar), 3.87 (app. dt, ²J_H-H = 11.6 Hz, J_H-P = 2.0, 2.0 Hz, 2 H, CHH’), 3.77 (app. dt, ²J_H-H = 11.3 Hz, J_H-P = 3.0, 3.0 Hz, 2 H, CHH’), 3.02 (m, ³J_H-H = 7.2, 7.1 Hz, J_H-P = 3.4, 2.2 Hz, 2 H, CH(CH₃)₂), 2.99 (m, ³J_H-H = 7.6, 7.3 Hz, J_H-P = 4.8, 5.8 Hz, 2 H, C’H(CH₃)₂), 1.47 (m, ³J_H-H = 7.6 Hz, J_H-P = 7.9, 9.0 Hz, 6 H, C’H(CH₃)(C’H₃)), 1.45 (app. q, ³J_H-H = 7.2 Hz, J_H-P = 7.4, 7.4 Hz, 6 H, CH(CH₃)(C’H₃)), 1.39 (app. q, ³J_H-H = 7.3 Hz, J_H-P = 6.5, 6.5 Hz, 6 H, C’H(CH₃)(C’H₃)), 1.14 (app. q, ³J_H-H = 7.1 Hz, J_H-P = 7.0, 7.0 Hz, 6 H, CH(CH₃)(C’H₃)), 0.81 (s, 12 H, OC(CH₃)₂). ¹³C{¹H} NMR (THF-d₈, 100.7 MHz, rt) δ 140.8 (app. t, J_C-P = 7 Hz, C_Ar), 118.3 (s, 3-HC_Ar), 108.2 (s, 2-HC_Ar), 83.2 (s, OC(CH₃)₂), 45.7 (app. t, J_C-P = 22 Hz, NCH₂P), 29.8 (app. t, J_C-P = 12 Hz, CH(CH₃)₂), 28.9 (app. t, J_C-P = 12 Hz, C’H(CH₃)₂), 24.8 (s, OC(CH₃)₂), 20.4 (app. t, J_C-P = 3 Hz, C’H(CH₃)(C’H₃)), 19.7 (s, CH(CH₃)(C’H₃)), 18.7(s, CH(CH₃)(C’H₃)), 18.2 (s, C’H(CH₃)(C’H₃)). ¹¹B{¹H} NMR (THF-d₈, 128.5 MHz, rt) δ 39.7 (s, Δw_½ = 340 Hz), 19.9 (s, Δw_½ = 330 Hz). ³¹P{¹H} NMR (THF-d₈, 162.1 MHz, rt) δ 66.4 (s). Anal. Calcd. for C₂₆H₄₈B₂N₂O₂P₂IrCl (5c): C, 42.67; H, 6.61; N, 3.83. Found: C, 42.48; H, 6.41; N, 4.06. m.p.: 232–234 °C.

4.2.8. [(d(CH₂P(iPr)₂)abB)IrCl(Bpin)(PMe₃)] (6c)

In a nitrogen-filled glovebox, 5c (15 mg, 20 μmol, 1 equiv.) was dissolved in n-pentane (5 mL), and trimethylphosphine (10.4 µL, 7.8 mg, 0.102 mmol, 5 equiv.) was added. The solvent was removed after 5 min at room temperature to give 6c as a colourless solid. Single crystalline 6c was obtained from the above mixture upon crystallisation at −40 °C (6 mg, 7 μmol, 37%).

¹H NMR (THF-d₈, 400.4 MHz, rt) δ 6.74–6.69 (m, 2 H, 2-HC_Ar), 6.67–6.62 (m, 2 H, 3-HC_Ar), 3.88 (app. dt, ²J_H-H = 11.0 Hz, J_H-P = 2.2, 2.2 Hz, 2 H, CHH’), 3.64 (app. dt, ²J_H-H = 11.0 Hz, J_H-P = 2.2, 2.2Hz, 2 H, CHH’), 3.06 (m, ³J_H-H = 7.1, 7.1 Hz, J_H-P = 3.5, 3.5 Hz, 2 H, CH(CH₃)₂), 2.61 (m, ³J_H-H = 7.2, 7.2 Hz, J_H-P = 3.7, 3.7 Hz, 2 H, C’H(CH₃)₂), 1.68 (d, ²J_H-P = 7.2 Hz, 9 H, PMe₃), 1.42 (app. q, ³J_H-H = 7.1 Hz, J_H-P = 7.0, 7.0 Hz, 6 H, CH(CH₃)(C’H₃)), 1.41 (app. q, ³J_H-H = 7.1 Hz, J_H-P = 7.0, 7.0 Hz, 6 H, CH(CH₃)(C’H₃)), 1.38 (app. q, ³J_H-H = 7.2 Hz, J_H-P = 7.0, 7.0 Hz, 6 H, C’H(CH₃)(C’H₃)), 1.31 (app. q, ³J_H-H = 7.2 Hz, J_H-P = 7.1, 7.1 Hz, 6 H, C’H(CH₃)(C’H₃)), 0.66 (s, 12 H, OC(CH₃)₂). ¹³C{¹H} NMR (THF-d₈, 100.7 MHz, rt) δ 142.4 (app. td, J_C-P = 8, 2 Hz, C_Ar), 117.9 (s, 3-HC_Ar), 108.5 (s, 2-HC_Ar), 81.7 (s, OC(CH₃)₂), 46.3 (app. td, J_C-P = 20, 7 Hz, NCH₂P), 31.1 (app. t, J_C-P = 14 Hz, CH(CH₃)₂), 27.6 (app. td, J_C-P = 11, 2 Hz, C’H(CH₃)₂), 25.7 (s, OC(CH₃)₂), 20.5 (d, ²J_C-P = 24 Hz, PMe₃), 21.1 (s, C’H(CH₃)(C’H₃)), 20.0 (s, C’H(CH₃)(C’H₃)), 19.6 (s, CH(CH₃)(C’H₃)), 19.0 (s, CH(CH₃)(C’H₃)). ¹¹B{¹H} NMR (THF-d₈, 128.5 MHz, rt) δ 48.8 (s, Δw_½ = 460 Hz), 26.6 (s, Δw_½ = 450 Hz). ³¹P{¹H} NMR (THF-d₈, 162.1 MHz, rt) δ 41.4 (d, J_P–P = 12 Hz, CH₂P(iPr)₂), −51.9 (br. s, Δw_½ = 70 Hz, PMe₃). Anal. Calcd. for C₂₉H₅₇B₂N₂O₂P₃IrCl (6c): C, 43.11; H, 7.11; N, 3.47. Found: C, 42.83; H, 7.03; N, 3.58. m.p.: 182–184 °C (decomp).