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

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.

In the present work, we endeavoured to explore the use of pinB-B(d(CH 2 P(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 [(Me 3 P) 4 Co-Me], [(Me 3 P) 3 Rh-Cl] and [(cod)Ir-Cl] 2 as precursors [16][17][18].

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 PMe 3 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 PMe 3 ligand, compared to distance Co1-P3 of the equatorial PMe 3 ligand by 0.03 Å.
The equatorial positions are occupied by the two pincer phosphine donors and a second PMe 3 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).
The 31 P{ 1 H} NMR spectrum of 2a comprises three distinct signals, two signals of the two distinct PMe 3 ligands, one in the apical-trans boryl-position around 0 ppm and the second around -16 ppm for the equatorial PMe 3 ligand.The third signal around 83 ppm is assigned to the two equivalent PBP pincer ligand P(iPr) 2 groups (Figures 1(top) and 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 PMe 3 ligand and a triplet of doublets at -16.2 ppm (-69 • C) for the equatorial PMe 3 ligand (Figures 1(top) and S4).This is in agreement with the mutual couplings within an A 2 MN 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 1 H-1 H NOESY NMR spectrum at room temperature gives a fitting picture.Distinct NOE contacts between the PMe 3 signals and the methine CHMe 2 signals allow for the assignment of the PMe 3 ligands to the apical and equatorial positions, respectively.Exchange signals are observed between the two PMe 3 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 PMe 3 ligand with a transient four coordinate 16-electron complex [(d(CH 2 P(iPr) 2 )abB)Co-PMe 3 ] and the re-association of a PMe 3 ligand; (ii) a concerted mechanism exchanging the PMe 3 ligand positions via a (distorted) square pyramidal intermediate is feasible.Note also that the NMR data do not suggest any appreciable dissociation of PMe 3 from 2a, contrary to the heavier rhodium homolog 2b (vide infra).
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 N 2 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(CH 2 P(tBu) 2 )abB)Co-N 2 ] 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 N 2 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 N 2 ligand in 4a compares well with the distance of 1.119(2) Å found in Peter's N 2 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(C 6 F 5 ) 3 in toluene and monitoring this reaction via IR spectroscopy gives clear evidence of the immediate formation of an N 2 complex, based on the appearance of a strong IR band at 2061 cm −1 if the reaction is conducted under an N 2 atmosphere, whereas only a minute signal is observed under an argon atmosphere, presumably due to the presence of adventitious N 2 (Scheme 3).This compares well to the N≡N stretching frequency of 2013 cm −1 reported for the related complex  Following the reaction of 2a with B(C6F5)3 under an N2 atmosphere via 31 P and 11 B 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 11 B{ 1 H} NMR signal shifts only by about 3 ppm, it gives evidence for the presence of the PBP boryl ligand.The changes in the 31 P{ 1 H} NMR spectrum are more substantial.The two 31 P 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, 2 JPP = 73 Hz) at −80 °C (Figure 2).The latter triplet corresponds to the doublet at higher chemical shifts (94.6 ppm, 2 JPP = 73 Hz).This is readily explained by the abstraction of one PMe3 ligand to give the Lewis acid base adduct Me3P-B(C6F5)3 (δ31P = −6.1 ppm, δ11B = −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 Following the reaction of 2a with B(C 6 F 5 ) 3 under an N 2 atmosphere via 31 P and 11 11B 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 11 B{ 1 H} NMR signal shifts only by about 3 ppm, it gives evidence for the presence of the PBP boryl ligand.The changes in the 31 P{ 1 H} NMR spectrum are more substantial.The two 31 P NMR signals of the PMe 3 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, 2 J PP = 73 Hz) at −80 • C (Figure 2).The latter triplet corresponds to the doublet at higher chemical shifts (94.6 ppm, 2 J PP = 73 Hz).This is readily explained by the abstraction of one PMe 3 ligand to give the Lewis acid base adduct Me 3 P-B(C 6 F 5 ) 3 (δ 31P = −6.1 ppm, δ 11B = −14.7 ppm in CD 2 Cl 2 ) and a PBP pincer cobalt complex bearing only one additional PMe 3 ligand Me 3 P-B(C 6 F 5 ) 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 31 P 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 PMe 3 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 PMe 3 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 N 2 complex of the type [(d(CH 2 P(iPr) 2 )abB)M-(N 2 )(PMe 3 )] is strongly favoured over the four coordinate complex [(d(CH 2 P(iPr) 2 )abB)M-(PMe 3 )] by ∆G 298 = −22.4kJ mol −1 (∆E 0 = −70.6 kJ mol −1 ), despite the entropic penalty occurring from the coordination of gaseous N 2 .However, for the rhodium and iridium analogue, the coordination of an N 2 ligand to the latter four-coordinate complex is-in agreement with our observations (vide infra)-disfavoured by ∆G 298 = 51.3kJ mol −1 (∆E 0 = 8.2 kJmol −1 ) for rhodium and ∆G 298 = 51.4kJ mol −1 (∆E 0 = 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 N 2 , 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 N 2 IR wavenumber of 2061 cm −1 is in line with a modest activation relative to free N 2 (~2330 cm −1 ).Finally, the Co-B bond trans to the N 2 ligand seems to be very strong (Co-B: 2.28 N cm −1 ), reducing the flexibility to access different coordination geometries [24].

Rhodium Complexes
The reaction of 1 with [(Me 3 P) 3 Rh-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(PMe 3 ) 3 ] (dmab = 1,2-(NMe) 2 C 6 H 4 ) [20].However, the reaction of 1 with [(Me 3 P) 3 Rh-Cl] in the presence of KOtBu leads to the formation of an equilibrium mixture of the square planar complex [(d(CH 2 P(iPr) 2 )abB)Rh-PMe 3 ] (3b) and the five coordinate complex [(d(CH 2 P(iPr) 2 )abB)Rh(PMe 3 ) 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 PMe 3 leads to the isolation of the five-coordinate complex 2b as crystalline material at a 43% yield.The spontaneous dissociation of one PMe 3 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 + PMe 3 , but overall, an entropy driven exergonic process (−47 kJ mol −1 ) (Supplementary Materials) [22].
ter 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.Both complexes 2b and 3b crystallise in monoclinic space groups of the type P2 1 /n and P2 1 /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 PMe 3 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 PMe 3 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 31   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 PMe 3 shows a highly dynamic behaviour in the 31  An additional signal is observed shifting from −37 ppm at low amounts of PMe 3 to −62 ppm after the addition of an excess of PMe 3 .This is readily explained by a rapid exchange among 3b, 2b and free PMe 3 on the NMR time scale and consequently, the observation of an averaged chemical shift of the exchanging PMe 3 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 PMe 3 .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 31 P 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.
In conclusion, it may be stated that the five-coordinate trigonal bipyramidal complex 2b, in contrast to the Co analogue, easily dissociates one PMe 3 ligand to give the distorted square planar complex 3b.The virtual indifference in the 31 P NMR chemical shift (and line shape) of the apparently not-exchanging trans-B PMe 3 ligand around 27 ppm suggests that this exchange does not affect this ligand but involves only the equatorial PMe 3 ligand.

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(CH 2 P(iPr) 2 )abB)Ir(Bpin)(Cl)] (5c) is obviously an oxidative addition reaction (Scheme 5).This five-coordinate complex reacts with excess PMe 3 to give the six-coordinate complex [(d(CH 2 P(iPr) 2 )abB)Ir(Bpin)(PMe 3 )(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 2.22-2.41Å and 65.8-76.7°,respectively [25][26][27][28][29].Both complexes 5c and 6c crystallise in monoclinic space groups of the type P2 1 /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].
In 6c, the PMe 3 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(CH 2 P(iPr) 2 )abB ligand backbone in 6c (mean plane [B1,N1,N2,C 6 H 4 ]) 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 PMe 3 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 PMe 3 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 PMe 3 ligand is longer than those of the pincer phosphine atoms by about 0.06 Å [30].[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 2.22-2.41Å and 65.8-76.7°,respectively [25][26][27][28][29].The solution-state 1 H, 31 P and 13 C NMR spectroscopic data for 5c and 6c fulfil the expectations and can readily be explained by the solid-state structures.Surprising, however, are the 11  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 (Figures 6 and 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 PMe 3 ligand around -20 ppm (Figure S35 Supplementary Materials) [22].Only minor amounts of a so far unidentified side product with a 31 P 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 31 P{ 1 H} NMR singlet signal at 64.5 ppm (I) is observed.In agreement with this, the 11 B NMR data suggest the presence of a transient boryl intermediate at 40 ppm, whereas 3c itself exhibits a moderately broad 11 B{ 1 H} 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 PMe 3 and its transient addition to 3c during this process.An in situ 31 P{ 1 H} NMR spectrum of a mixture of 3c and excess PMe 3 corroborates the facile formation of 2c (Figure 6, top).Moreover, it must be emphasised that the system 2c/3c/PMe 3 exhibits much less dynamic behaviour than the homologous rhodium system 2b/3b/PMe 3 (vide supra).Contrary to the latter, even in the presence of excess PMe 3 at room temperature, a well-resolved, 31 P{ 1 H} NMR spectrum (A 2 MN spin system) with a narrow linewidth is observed, indicating only comparably slow exchange of a coordinated PMe 3 ligand with free PMe 3 .Contrary to 2b, distinct signals for both PMe 3 ligands are observable for 2c at room temperature in the presence of free PMe 3 (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 PMe 3 indicate a rapid equilibration, rather on the side of 2c (Figures S30 and S31).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).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 PMe 3 ligand to give the distorted square planar complex 3c.However, gas-phase DFT computational data suggest similar thermodynamic data for the dissociation of PMe 3 from 2c (∆E 0 = 16 kJ mol −1 , ∆G 298 = −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 P2 1 /n and P2 1 /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.
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).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.
Having, with the unsymmetrical diborane(4) [(d(CH 2 P(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.

General Considerations
pinB-B(d(CH 2 P(iPr) 2 )ab) (1), [(Me 3 P) 4 CoMe], [(Me 3 P) 3 RhCl] 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 (C 6 D 6 : 1 H NMR: 7.16 ppm, 13 C NMR: 128.06 ppm; PhMed 8 : 1 H NMR: 2.08 ppm, 13 C NMR: 20.43 ppm; THF-d 8 : 1 H NMR: 1.72 ppm, 13 C NMR: 25.31 ppm) [34].11 B and 31 P NMR chemical shifts are reported relative to pseudo external BF 3 •Et 2 O and 85% H 3 PO 4(aq) , respectively.13 C{ 1 H}, 11 B{ 1 H} and 31 P{ 1 H} NMR spectra were recorded employing composite pulse 1 H decoupling. 11 B 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 F 2 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-CH 3 bond.During refine-ment and analysis of the crystallographic data, the programs OLEX 2 , 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.In a Schlenk-flask, d(CH 2 P(iPr) 2 )abB-Bpin (1) (100 mg, 0.198 mmol, 1 equiv.)and [(Me 3 P) 4 CoMe] (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%).

Figure 1 .
Figure 1. 31 P{ 1 H} NMR spectrum of 2a at -69 °C (top), and a section of the 1 H-1 H NOESY NMR spectrum of 2a (bottom), selected exchange (blue) and NOE (red) correlations are depicted (PhMe-d8, 400.4/162.1 MHz, rt).However, the temperature-dependent broadening is indicative of dynamic processes present in solution.A 1 H-1 H 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 (Figure1, 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).
P{ 1 H} NMR spectra at room temperature (Figures 4 and S17-S19).Only one set of signals of the PBP ligand and the trans-B PMe3 ligand is observed, respectively.Whilst the 31 P 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 virtu-
P{ 1 H} NMR spectra at room temperature (Figures 4 and S17-S19).Only one set of signals of the PBP ligand and the trans-B PMe 3 ligand is observed, respectively.Whilst the 31 P NMR signal of the PBP ligand changes appreciably from 84 ppm to 75 ppm with increasing amounts of PMe 3 added, the signal of the trans-B PMe 3 ligand, in 2b, is only marginally influenced (−27.3 to −26.4 ppm).

Figure 4 .
Figure 4.In situ 31 P{ 1 H} NMR spectra of the reaction of 3b with different amounts of PMe3 (121.6 MHz, C6D6, rt), isolated 2b and 3b with 1.3 equiv.PMe3 at −46 °C (162.1 MHz, THF-d8).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 (Figures 4, S14 and S15).The 31 P{ 1 H} NMR spectrum of 2b itself at −46 °C comprises three signals (A, M and N) of an A2MNX spin system with the expected 31 P-31 P and 31 P-103 Rh couplings (Figure S16, TableS3).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 (FiguresS20 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 31 P 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.

Figure 4 .
Figure 4.In situ 31 P{ 1 H} NMR spectra of the reaction of 3b with different amounts of PMe 3 (121.6MHz, C 6 D 6 , rt), isolated 2b and 3b with 1.3 equiv.PMe 3 at −46 • C (162.1 MHz, THF-d 8 ).At low temperatures, however, the exchange among 3b, 2b and free PMe 3 becomes slow on the NMR timescale, and well-resolved signals for 2b and free PMe 3 are observed (Figures 4, S14 and S15).The 31 P{ 1 H} NMR spectrum of 2b itself at −46 • C comprises three signals (A, M and N) of an A 2 MNX spin system with the expected 31 P-31 P and 31 P-103 Rh couplings (Figure S16, TableS3).Following the reaction of 3b with different amounts of PMe 3 via UV-Vis spectroscopy corroborates the rapid equilibrium between 3b and 2b being rather on the side of 3b and free PMe 3 (FiguresS20 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 PMe 3 ligand to give the distorted square planar complex 3b.The virtual indifference in the 31 P NMR chemical shift (and line shape) of the apparently not-exchanging trans-B PMe 3 ligand around 27 ppm suggests that this exchange does not affect this ligand but involves only the equatorial PMe 3 ligand.
The 11 B NMR shift of 2c of around 55 ppm is virtually unaffected by the change in the coordination number.Molecules 2023, 28, x FOR PEER REVIEW 12 of 21 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 11 B NMR shift of 2c of around 55 ppm is virtually unaffected by the change in the coordination number.