Evaluating stereoelectronic properties of bulky dialkylterphenyl phosphine ligands

Evaluating Electronic and Steric Properties of Bulky Dialkylterphenyl Phosphine Ligands † Mario Marín,b Juan J. Moreno,*a María M. Alcaide,a Eleuterio Álvarez,a Joaquín López- Serrano,a Jesús Campos,a M. Carmen Nicasio,b Ernesto Carmona*a aInstituto de Investigaciones Químicas (IIQ), Departamento de Química Inorgánica and Centro de Innovación en Química Avanzada (ORFEO-CINQA), Consejo Superior de Investigaciones Científicas (CSIC) and Universidad de Sevilla, Avenida Américo Vespucio 49, 41092 Sevilla, Spain bDepartamento de Química Inorgánica, Universidad de Sevilla, Aptdo 1203, 41071 Sevilla, Spain Abstract The stereoelectronic properties of a series of sterically hindered phosphines containing a terphenyl substituent, PR2Ar’ (R = alkyl; Ar’ = C6H3-2,6-Ar2), have been evaluated by various methods. Their σ-donating capacity has been assessed on the basis of the carbon monoxide stretching frequencies in benchmark iridium [IrCl(CO)2(PR2Ar’)] and rhodium [Rh(acac)(CO)2(PR2Ar’)] (acac = acetylacetonate) complexes, as well as by measuring 31P-77Se scalar coupling constants (1JSeP) for the corresponding phosphine selenides (Se=PR2Ar’). In turn, the steric profile of terphenyl phosphines has been gauged by calculating Tolman Cone Angle (TCA), ligand shielding (G) and percent buried volume (%VBur) parameters. These calculations have been carried out from both X-ray diffraction and DFT-optimized structures. We have also examined several of the widely used biaryl phosphines for comparative purposes. Keywords Bulky phosphine; ligand parameterization; Tolman; selenides; buried volume; terphenyl Introduction † This paper is dedicated to the memory of Professor Pascual Royo, honoring his career and in recognition of his most valuable contribution to the development of Organometallic Chemistry in Spain. The stereoelectronic parameters associated with ancillary ligands play a crucial role in the control of the activity, selectivity and stability of transition metal catalysts. 1 In this regard, tertiary phosphines are key ligands in organometallic chemistry and homogeneous catalysis. 2 The facility with which their electronic and steric properties can be tuned in a precise manner, by changing the nature of the substituents attached to the P atom, has made of phosphines the most versatile ancillary ligands so far. 3 During the last two decades, several types of sterically demanding electron-rich phosphines have emerged as prominent supporting ligands in the context of C-C and C- heteroatom bond forming reactions. 4 Examples underlining this assertion include trialkyl phosphines like PtBu31c, 5 and heteroleptic phosphines 6 such as PAd2R developed by Beller, 7 ferrocenyl-derived phosphines introduced by Hartwig 8 and dialkylbiaryl phosphines put on stage by Buchwald. 9 The latter have become a large family of phosphines since their modular synthesis permits to customize the ligand for a particular application. 10 Currently, dialkylbiaryl phosphines are amongst the most effective ligands in Pd-catalyzed cross-coupling reactions. 11 Although the singular steric protection imparted by terphenyl fragments has been well documented, 12 it is surprising that dialkylterphenyl phosphines, bearing a bulkier terphenyl group instead of the biaryl fragment, have been scarcely studied. 13 This motivated us a few years ago to investigate the synthesis and the coordination properties of this kind of ligands. First, we focused on the preparation of dialkylterphenyl phosphines PR2Ar’ (Ar’ = terphenyl group) bearing methyl and ethyl substituents directly bound to the phosphorous atom and examined their coordination towards various late transition metals, namely Rh, Ir, Pt and Au. 14 These studies illustrated the ability of these phosphines to adopt different coordination modes, which not only involve the P atom but also weak MCarene interactions with one of the flanking aryl rings.13,14 Later, the work was extended to the synthesis of branched and cyclic R-substituted PR2Ar’ phosphines. Regarding ligand conformation, we concluded from these studies that dialkylterphenyl phosphines, PR2Ar’, may adopt one of three possible structures in the solid state, namely A, B and C in Figure 1, 15 with the prevailing one resulting from the minimization of steric and electronic repulsions between the phosphine substituents and the phosphorus lone pair. Figure 1. Solid-state conformations for dialkylterphenyl phosphines. We also targeted the analysis of the -donor ability of our entire series of dialkylterphenyl phosphines by gathering IR data for Ni(0)-CO-phosphine compounds,15 Ni(CO)3(PR2Ar’), following Tolman’s approach. 16 While the less sterically hindered phosphines (i.e. PMe2Ar’ and PEt2Ar’) formed the expected tricarbonyl adducts, those having the bulkier branched or cyclic alkyls groups, i.e. iPr, c-C5H9 and c-C6H11, yielded uncommon dicarbonyl nickel complexes Ni(CO)2(PR2Ar’) instead, precluding a comparable estimation of their electronic features. A similar situation has been described for certain bulky ligands, such as biaryl phosphines and sterically demanding N-heterocyclic carbenes. 17 Among the alternative approaches developed to overcome this problem, the method introduced by Crabtree, 18 a and later refined by Nolan,18b consisting in measuring the C-O stretching frequencies for dicarbonyl iridium complexes IrCl(CO)2(L), is found to be the most general. Herein, we report on the preparation and characterization of a series of IrCl(CO)2(PR2Ar’) and Rh(acac)(CO)(PR2Ar’) (where acac = acetylacetonate) compounds bearing the dialkylterphenyl phosphines outlined in Figure 2. Their infrared carbonyl stretching frequencies have been measured to quantify the electronic parameter of PR2Ar’ ligands. These data have been compared with those obtained from measuring 31P-77Se scalar coupling constants (1JSeP) of phosphine selenides (Se=PR2Ar’). To complement these studies, a thorough experimental and computational analysis of the steric parameters of this family of bulky phosphines has also been accomplished. Figure 2. Terphenyl phosphines used in this work. Results and discussion Dialkylterphenyl phosphine iridium(I) and rhodium(I) carbonyl complexes We previously reported the synthesis of iridium dicarbonyls IrCl(CO)2(PR2Ar’) with the PMe2-containing phosphines L9 and L11 (Fig.2) by a two-step procedure. First, IrCl(COD)(PMe2Ar’) (COD = 1,5-cyclooctadiene) complexes were isolated from the reaction of the dimer [IrCl(COD)]2 with ligands L9 and L11, and then converted into the dicarbonyls by treatment with carbon monoxide in dichloromethane at room temperature.14c The same methodology was applied to the synthesis of the iridium derivatives with phosphines L10 and L12. However, any attempts to isolate Ir-COD- phosphine adducts with sterically crowded ligands L3-L8 resulted fruitless. Thus, the preparation of these iridium dicarbonyl complexes, 1PR2Ar’, was accomplished in a one-pot synthesis, by reacting the corresponding phosphine with the Ir(I) dimer, in a 1:1 metal-to-ligand ratio, under an atmosphere of CO (Scheme 1). The reactions were carried out at 0º C to avoid the formation of a recurrent, unidentified byproduct common to all examined phosphines, which diminished the yield of the expected dicarbonyl products. This method was also applied for the synthesis of complexes 1L1 and 1L2 based on the less bulky ligands L1 and L2. Scheme 1. Synthesis of IrCl(CO)2(PR2Ar’) complexes. Complexes bearing the phosphines L1, L3, L6, L7, L10 and L12 were isolated and spectroscopically characterized, but all others were only analysed by IR spectroscopy immediately after generation. Their 31P{1H} NMR resonances span a broad range, mainly depending on the nature of the R substituents, and appear shifted by ca. 15-33 ppm toward higher frequencies relative to the free phosphine ligands. This observation can be taken as suggestive of monodentate P-coordination of the terphenyl phosphine, void of Ir···Carene interactions with a flanking aryl ring.15 The 1H NMR spectra of these species are in accordance with a symmetrical environment around the terphenyl moiety, most probably as a consequence of free rotation around the PCaryl bond on the NMR time scale. Taking as an example compound 1L1, the four tBu substituents on the side aryl rings give rise to only one resonance at 1.39 ppm and the two P-Me groups originate a doublet centred at 1.21 ppm (2JHP = 9.8 Hz). In the 13C{1H} NMR spectra of these dicarbonyl species, two different resonances are observed for the CO ligands, one at around 175-180 ppm (2JCP ≈ 115-125 Hz) and the other at around 165-170 ppm (2JCP ≈ 11-13 Hz), due to the CO molecule in trans and cis disposition to the phosphine ligand, respectively. The molecular structures of 1L1 and 1L3 were confirmed by X-ray diffraction studies (Figure 3). As expected, both compounds display a slightly distorted squareplanar geometry. The phosphine ligand in complex 1L1 adopts conformation A (Figure 1), in which the two phosphine P-Me groups occupy the same region of space with reference to a plane containing the central aryl ring of the terphenyl moiety. In the perspective shown in Figure 3, the two P-Me bonds are situated underneath this plane. Furthermore, the PMe2 terminus is symmetrically arranged relative to the terphenyl central ring, inasmuch as the two P-Cipso-Cortho angles are nearly identical at 120.5° and 121.0°. This conformation is alike that adopted by free PMe2ADtbp2, L1, and generally, it is the preferred geometry for the least sterically demanding phosphines.15 However, in the structures already described for 1L9 and 1L11, the phosphine ligands exhibit the geometry defined as B in Figure 1, where one of the P-Me bonds is almost coplanar with the terphenyl central ring, as a consequence of the increased steric demands brought about by the substituents at the 2 and 6 positions of the flanking rings.14c It is worth mentioning that, while in 1L9 and 1L11, the planar IrCl(CO)2 moiety is placed parallel to the neighbouring side ring, in 1·L1 one of the Ir-CO bonds (Ir1-C37) is nearly parallel to the P‒Cipso bond (Cipso-P-Ir-CCO torsion = −6.2(1)°). As a result, the Ir(CO)2Cl plane is approximately perpendicular to the central aryl ring. Given that the free phosphine L3 (Fig. 2), that contains branched isopropyl substituents and the terphenyl 2,6-C6H3-(3,5-C6H3-(CMe3)2), with 3,5-disubstituted lateral rings, adopts a structure of type C (Fig. 1), it might appear surprising that in the iridium complex 1·L3 (Fig. 3) the coordinated phosphine adopts the intermediate conformation B. In the latter arrangement, one of the P-C(H)Me2 bonds almost eclipses the central aryl ring, that is, the said P-C(H)Me2 bond and the two Cipso-Cortho bonds are practically co-planar. Despite somewhat stronger steric repulsions among the phosphine substituents in the B conformation in comparison with C,15 in the latter the phosphorus lone pair of electrons would point towards the proximal lateral ring, such that coordination to an IrCl(CO)2 fragment, with no foreseeable readily dissociable ligands, would create unsustainable steric hindrance between the IrCl(CO)2 moiety and the proximal 3,5-C6H3(CMe3)2 side ring. Hence, adoption of conformation B by the coordinated PiPr2ArDtbp2 ligand in complex 1·L3. Indeed, even under these premises, the iridium coordination plane deviates from planarity, with the CO ligand trans to phosphorus bending away from the neighbouring aromatic ring, forming a P1-Ir1-C42 bond angle of 170.1(2)°. Figure 3. Molecular structures of 1·L1 (left) and 1·L3 (right). Hydrogen atoms and minor disorder are omitted for clarity and thermal ellipsoids are set at 50% level probability. Selected bond distances (Å) and angles (°):1·L1: Ir1-P1 2.3599(6), Ir1-Cl1 2.3552(7), Ir1-C38 1.910(3), Ir1-C37 1.854(3), C37-O1 1.081(3), C38-O2 1.119(3), C37-Ir1-C38 92.85(12), C38-Ir1-Cl1 87.43(9), Cl1-Ir1-P1 87.48(2), P1-Ir1- C37 92.40(8), Ir1-P1-C1-C2 −87.50(17). 1·L3: Ir1-P1 2.387(1), Ir1-Cl1 2.349(2), Ir1-C42 1.884(5), Ir1- C41 1.824(7), C41-O1 1.145(9), C42-O2 1.135(7), C41-Ir1-C42 90.5(3), C42-Ir1-Cl1 88.8(2), Cl1-Ir1-P1 87.33(5), P1-Ir1-C41 93.9(2), Ir-P1-C1-C2 −60.6(4). The C‒O stretching frequencies recorded for complexes 1·PR2Ar’ are collected in Table 1. As expected, due to the cis disposition of the CO ligands in these molecules, two CO stretching bands were observed in their IR spectra. The TEP of the ligands can be determined from these values using the correlations developed by Crabtree18a and Nolan.18b The results of these calculations are shown in Table 1, which also includes, for comparison purposes, the TEP values of the less sterically hindered terphenyl phosphines determined from Ni(CO)3(PR2Ar’)15 and those gathered for commercially available PMe2Ph18a and the biaryl phosphine XPhos.17c It is clear that TEP values obtained from the IrCl(CO)2(L) complexes of terphenyl phosphines L1, L2, L5, L9- L12, as well as XPhos, differ from those observed for Ni(CO)3(L) by as much as 12 cm−1 (for L1). Furthermore, variation among the values for similar phosphines is relatively large (see, for example, the five dimethyl-substituted PMe2Ar’ phosphines, or dicyclohexyl-substituted L5, L8 and XPhos). Thus, comparison of the IR data for different families of complexes should be made with caution. Table 1. IR wavenumbers (cm−1) for the C‒O stretching vibrations in some IrCl(CO)2(L) complexes in CH2Cl2 solution. Ligand Complex sym asym TEP (Ir)a TEP (Ni) L1 IrCl(CO)2(PMe2ArDtbp2) 2063 1984 2051.1 2063 L2 IrCl(CO)2(PEt2ArDtbp2) 2061 1983 2049.8 2061 L3 IrCl(CO)2(PiPr2ArDtbp2) 2060 1980 2048.2 - L4 IrCl(CO)2(PCyp2ArDtbp2) 2058 1978 2046.5 - L5 IrCl(CO)2(PCy2ArDtbp2) 2056 1980 2046.5 2060 L6 IrCl(CO)2(PiPr2ArXyl2) 2064 1982 2050.7 - L7 IrCl(CO)2(PCyp2ArXyl2) 2063 1979 2049.0 - L8 IrCl(CO)2(PCy2ArXyl2) 2062 1980 2049.0 - L9 IrCl(CO)2(PMe2ArXyl2) 2068 1987 2054.5 2063.8 L10 IrCl(CO)2(PMe2ArMes2) 2066 1986 2053.2 2063 L11 IrCl(CO)2(PMe2ArDipp2) 2067 1987 2054.1 2062.9 L12 IrCl(CO)2(PMe2ArTripp2) 2065 1986 2052.8 2062 XPhos IrCl(CO)2(PCy2ArTripp)c 2070 1985 2053.3 2059 PMe2Ph IrCl(CO)2(PMe2Ph)d 2084 1999 2066.4 2065.3e aCalculated using the correlation found in ref. 18. bFrom ref. 15. cFrom ref. 17c. dFrom ref. 18. eFrom ref. 16a. For the sake of completeness, complexes Rh(acac)(CO)(PR2Ar’), 2·PR2Ar’ bearing the dialkyl terphenyl phosphines L1-L12 and the biaryl phosphine XPhos, were also prepared following the synthetic pathway illustrated in Scheme 2. As above, a few representative examples, namely 2·L1, 2·L2, 2·L5 and 2·L7, were isolated and spectroscopically characterized, whereas others were analysed solely by means of IR spectroscopy directly from the reaction mixture. Scheme 2. Synthesis of Rh(acac)(CO)(PR2Ar’) complexes. The 31P{1H} NMR spectra of these rhodium carbonyl compounds exhibit doublets (1JPRh between 166-184 Hz) with δ values shifted ca. 54-68 ppm to higher frequency with respect to the uncoordinated phosphines. These ∆δ values are significantly larger than those found for the iridium complexes discussed above, an observation for which no reasonable explanation can be given with data presently available. Likewise, their 1H NMR spectra are fairly simple with only one set of resonances for the side aryl rings of the phosphine, consistent with a fast rotation of the phosphine in the NMR timescale at 25 ºC. The carbonyl ligand resonance, which appears at a chemical shift of around 190-192 ppm in the 13C{1H} NMR spectra, also shows the coupling with the two magnetically active nuclei (1JCRh ≈ 77 Hz, 2JCP ≈ 24 Hz). In the solid state structure (Fig. 4), the molecules of 2·L1 show a disposition similar to the iridium analogue 1·L1, that is, with the phosphine adopting conformation of type A and the Rh(acac)(CO) fragment contained in the symmetry plane perpendicular to the central aryl ring of the terphenyl moiety. Figure 4. Molecular structure of Rh(acac)(CO)(PMe2ArDtbp2), 2·L1. Hydrogen atoms and minor disorder are omitted for clarity and thermal ellipsoids are set at 50% level probability. Selected bond distances (Å) and angles (°): Rh1-P1 2.2374(11), Rh1-O1 2.051(3), Rh1-O2 2.068(3), Rh1-C25 1.797(5), C25-O3 1.162(7), P1-Rh1-O1 92.22(9), O1-Rh1-O2 88.65(14), O2-Rh1-C25 91.90(19), C25-Rh1-P1 87.23(16), Rh1-P1-C1-C2 −94.0(3). Finally, C‒O stretching frequencies recorded for these complexes span the range 1954-1963 cm−1 (see Supplementary Data, Table S1). As in the case of the iridium complexes discussed above, IR data do not correlate well with the TEPs observed in the Ni(CO)3(L) complexes.15 However, there is a moderately good correlation (Figure S1) between the CO values in IrCl(CO)2(L) and Rh(acac)(CO)(L) complexes of the phosphines described here. This permits using either of the metal-phosphine systems studied, namely IrCl(CO)2(PR2Ar’) or Rh(acac)(CO)(PR2Ar’), to compare the donor capacity of the terphenyl phosphine ligands. Dialkylterphenyl Phosphine Selenides To complete our studies on the evaluation of the σ-donor capacity of dialkylterphenyl phosphines we prepared their corresponding selenides Se=PR2Ar’ and extend this classification to four groups, with the larger cone angle values corresponding to phosphines with C6H3-3,5-tBu2 aryl rings on their terphenyl moiety, followed by PiPr2 and PMe2 terphenyl phosphines and finally, PCy2(Biaryl) with the smaller values (Figure S4). Figure 6. % Buried Volume for dialkylterphenyl and dicyclohexylbiaryl phosphines in 1·Ln complexes. Blue triangles denote values calculated from DFT-optimized geometries, orange circles and green squares are for values obtained from crystallographic data of dialkylterphenyl phosphine and dialkylbiaryl phosphine complexes, respectively. Taking into account crystallographic data only, Figure 7 reveals that the terphenyl phosphines discussed in this work appear to exert their steric bulk closer to the metal center in their square planar IrCl(CO)2·L complexes than their PCy2(Biaryl) counterparts. This is suggested by the better correlation of the TCAs with larger %VBur found for the former in comparison with the biaryl phosphines of comparable TCAs. This may be in part due to the presence of smaller R substituents at the phosphorous atom of the terphenyl phosphines relative to the bulky cyclohexyl groups of their biaryl counterparts. 37 Figure 7. Tolman Cone Angle (o) vs %VBur for dialkylterphenyl and dialkylbiaryl phosphines in 1·Ln complexes. Orange circles and green squares are for values obtained from crystallographic data of dialkylterphenyl phosphine and dialkylbiaryl phosphine complexes, respectively. Experimental section General considerations All preparations and manipulations were carried out under an atmosphere of dry oxygen-free dinitrogen by means of standard Schlenk or glovebox techniques. Solvents were rigorously dried and degassed before use. Ar’MgBr were prepared by following the synthesis reported by Power 38 for the related ArXyl2 compounds without adding I2 in the last step of the preparation. Ligands L1-L1214c,15 and complexes Ni(cod)2 39 and [Ir(cod)]2(μ-Cl)2 40 were synthesized by following previously reported procedures. PCl3 was distilled prior to use and kept under a nitrogen atmosphere. Other chemicals were purchased from commercial sources and used as received. Solution NMR spectra were recorded on Bruker Avance DPX-300, DRX-400 and DRX-500, and 400 Ascend/R spectrometers. The 1H and 13C resonances of the solvent were used as the internal standard and the chemical shifts are reported relative to TMS while 31P was referenced to external H3PO4. Infrared spectra were recorded on a Bruker Vector 22 Microanalyses were performed by the Microanalytical Service of the Instituto de Investigaciones Químicas (IIQ). X-ray diffraction data were collected on Bruker Nonius X8 APEX-II. CCDC 1912865 (1-L1), 1912866 (2-L3), 1912867 (2-L1) 1912868 (Se=PMe2ArDtbp2) and 1912869 (Se=PiPr2ArXyl2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre. Synthesis of representative metal complexes IrCl(CO)2(PMe2ArDtbp2), 1·L1. CH2Cl2 (3 mL) was added to a Schlenk tube charged with [Ir(μ-Cl)(cod)]2 (0.0337, 0.050 mmol) and PMe2ArDtbp2 (0.0570 g, 0.105 mmol, 5% excess). After stirring for 30 min, the vessel was cooled to 0 °C and charged with CO (0.9 bar). After the cool bath was removed, the solution was left to evolve from the initial red into yellow (ca. 1 h). The volatiles were evaporated under vacuum and the resulting residue was washed with pentane at −30 °C, rendering an intensely colored yellow solid. Yield: 0.0677 g, 85%. Samples suitable for X-ray diffraction were obtained by slow evaporation from a pentane solution at room temperature. IR (CH2Cl2): νCO = 2063, 1984 cm-1. Anal. Calcd. for C38H51ClIrO2P: C, 57.16; H, 6.44. Found: C, 56.81; H, 6.84. 1H NMR (300 MHz, C6D6, 298 K): δ 7.59 (t, 4JHH = 1.8 Hz, 2 H, p-Dtbp), 7.54 (d, 4JHH = 1.8 Hz, 4 H, o-Dtbp), ca. 7.16 (m-C6H3), 7.04-6.99 (m, 1 H, p-C6H3), 1.39 (s, 36 H, CH3 tBu), 1.21 (d, 2JHP = 9.8 Hz, 6 H, CH3 Me). 13C{1H} NMR (100 MHz, C6D6, 298 K): δ 179.1 (d, 2JCP = 126 Hz, CO trans P), 170.2 (d, 2JCP = 13 Hz, CO cis P), 151.2 (s, m-Dtbp), 149.20 (d, 2JCP = 9 Hz, o-C6H3), 142.28 (d, 3JCP = 4 Hz, ipso-Dtbp), 131.4 (d, 3JCP = 8 Hz, m-C6H3), 130.0 (d, 4JCP = 2 Hz, p-C6H3), ca. 129 (ipso-C6H3), 125.3 (s, o-Dtbp), 122.4 (s, p-Dtbp), 35.2 (s, C(CH3)3 tBu), 31.7 (s, CH3 tBu), 17.13 (d, 2JCP = 38 Hz, CH3 Me). 31P{1H} NMR (202 MHz, C6D6, 298 K): δ -3.5. Rh(acac)(CO)(PMe2ArDtbp2), 2·L1. THF (2 mL) was added to a Schlenk tube charged with Rh(acac)(CO)2 (0.0129 g, 0.050 mmol) and PMe2ArDtbp2 (0.0271 g, 0.053 mmol). The mixture was stirred for 90 minutes and then taken to dryness. The resulting residue was washed with pentane, extracted in CH2Cl2 and filtered to a weighed vial. Removal of volatiles, first under a N2 flow and then under vacuum, rendered the sought product as a yellow solid. Yield: 0.0297 g, 80%. The samples thus obtained are suitable for most purposes, but contain 5% of phosphine according to 1H NMR; analytically pure samples can be obtained by recrystallisation from a hexane/CH2Cl2 mixture at −30 °C. Samples suitable for X-ray diffraction studies can be obtained by slow evaporation from a pentane solution. IR (CH2Cl2): νCO = 1961 cm-1. Anal. Calcd. for C42H58O3PRh: C, 67.73; H, 7.85: Found: C, 67.99; H, 7.47. 1H NMR (300 MHz, CDCl3, 298 K): δ 7.56 (br, 4 H, o-Dtbp), 7.50-7.45 (m, 3 H, p-C6H3, p-Dtbp), 7.30 (dd, 3JHH = 7.4 Hz, 4JHP = 3.0 Hz, 2 H, m-C6H3), 5.47 (s, 1 H, CH acac), 2.06 (s, 3 H, CH3 acac), 1.54 (s, 3 H, CH3 acac), 1.36 (s, 36 H, CH3 tBu), 0.77 (dd, 2JHP = 9.1 Hz, 3JHRh = 1.9 Hz, 6 H, P–CH3). 13C{1H} NMR (100 MHz, CDCl3, 298 K): δ 189.7 (dd, 1JCRh = 78 Hz, 2JCP = 27 Hz, CO), 187.8 (s, CO acac), 184.8 (s, CO acac), 151.0 (s, m-Dtbp), 147.9 (d, 2JCP = 7 Hz, o-C6H3), 142.5 (d, 3JCP = 3 Hz, ipso-Dtbp), 130.7 (d, 1JCP = 41 Hz, ipso-C6H3), 130.5 (d, 3JCP = 7 Hz, m-C6H3), 128.6 (d, 4JCP = 2 Hz, p-C6H3), 124.7 (s, m-Dtbp), 121.7 (s, p- Dtbp), 100.5 (d, 4JCP = 2 Hz, CH acac), 35.1 (s, C tBu), 31.7 (s, CH3 tBu), 27.8 (d, 4JCP = 5 Hz, CH3 acac), 27.3 (s, CH3 acac), 20.4 (dd, 1JCP = 38 Hz, 3JCRh = 2 Hz, P–CH3). 31P{1H} NMR (162 MHz, CD2Cl2, 298 K): δ 28.4 (d, 1JPRh = 167 Hz). Conclusions We have evaluated the stereoelectronic properties of 12 sterically demanding dialkylterphenyl phosphines, PR2Ar’, employing various approaches. The ligands analyzed herein have permitted interrogating the effects of using methyl or larger branched and cyclic R groups at the phosphorus center (series L1-L5 and L6-L9) and those derived from increasing the size of distant substituents (series L9-L12) or modifying their positions (L1 vs. L9-L12) at the terphenyl fragment. Tolman Electronic Parameters (TEP) have been calculated from infrared data on CO stretching frequencies of IrCl(CO)2(PR2Ar’) and Rh(acac)(CO)2(PR2Ar’) complexes. Although a fair correlation was found for νCO data derived from iridium and rhodium complexes, those results differ somewhat from the ones obtained from Ni(CO)3(PR2Ar’) species. This difference could be attributed to the different nature of the complexes utilized for the parameterization. On the other hand, 31P-77Se scalar coupling constants correlate better with TEP(Ni) parameters, although the method fails when series of ligands that create very different front-strain are confronted. On this basis, we conclude that the selection of a single method to grade ligand σ-donor capacity should be handled with caution, and advise employing a combination of the different available parameters to obtain truly reliable information. The steric properties of terphenyl phosphines have also been examined by means of Tolman Cone Angles (TCAs), ligand shielding (G) and percent buried volume (%VBur) calculations from experimental X-ray diffraction data and DFT-optimized geometries. These results point out that: (i) the steric protection provided by terphenyl phosphines as monodentated κ1-P ligands is considerably superior to that conferred by the widely employed biaryl counterparts; (ii) substituting the methyl groups in PMe2Ar’ by other hydrocarbyl fragments has the most prominent effect on all calculated steric descriptors; (iii) in IrCl(CO)2(PR2Ar’) complexes, terphenyl phosphines seem to exert their steric demands in closer proximity to the metal center than their PCy2(biaryl) counterparts. Acknowledgements We thank MICIU (Grants CTQ2013-42501-P, CTQ2016-75193P, CTQ2014- 52769-C3-3-R and CTQ2017-82893-C2-2-R) for financial support. J .J .M., M. M. G. and M. M. A. thank the Universidad de Sevilla (V Plan Propio de Investigación) and MICIU for research fellowships. 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