Classification of the Electronic Properties of Chelating Ligands in cis

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Classification of the Electronic Properties of Chelating Ligands in cis[LL′Rh(CO)2] Complexes Yves Canac*,†,‡ and Christine Lepetit*,†,‡ †

CNRS, LCC (Laboratoire de Chimie de Coordination), 205, route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France Université de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France

‡

ABSTRACT: By analogy to the Tolman electronic parameter, a ligand electronic parameter, referred to as L2EP, is introduced here for estimating the donating ability of chelating ligands, featuring two coordinating extremities. It is based on the average of the computed infrared stretching frequencies of CO in a series of isostructural rhodium(I)-dicarbonyl complexes, that is linearly correlated to the number x of Nheterocyclic carbene coordinating ends (x = 0, 1, or 2). The L2EP values allow the design of an unified scale for the classification of the electron donation of chelating ligands, based on an ortho-phenylene bridge substituted by two coordinating extremities, which may have a different donating character. Strengths and limitations of the L2EP scale are illustrated for a large diversity of bidentate chelating ligands with coordinating ends ranging from extremely electron-rich phosphonium yldiides to extremely electron-poor amidiniophosphonites.

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Ni(CO)3L complexes.9 Inspired by the pioneer reports of Strohmeier10 and Cotton,11 the Tolman’s concept originally developed for the phosphine ligand relies on the use of CO as an “electronic probe” for estimating the donating properties of the ligand: the more electron-donating the ligand, the lower the CO stretching frequency, due to the π-back-donation from the filled metal d orbitals into the low-lying antibonding π*CO orbital. Noteworthy, the TEP allows for quantifying the global metal−ligand electronic exchange, but the estimation of the relative underlying σ- and π contributions requires theoretical analyses.2 The TEP values providing a convenient ranking of the donating ability of alkyl- and aryl-phosphines were extended later by Bartik et al. to a larger range of phosphorus ligands.12 In principle, TEP-like values and scales can be estimated from any metal−carbonyl complex for which enough experimental data are available. Linear correlations are then required to link up all sets of data and interconvert the different scales. This was examplified by the disclosure of the Crabtree’s scale based on dicarbonyl complexes of [MCl(CO)2L with M = Ir or Rh] type, where the average of the two infrared CO stretching frequencies is quoted, rather than only one single stretching frequency in the Ni system.13 Excellent correlations were generally obtained between experimental and calculated (vA1 IR (CO) data for a broad range of L-type ligands (Green classification),14 such as phosphines,15 but also for carbon representatives as NHC,16 abnormal NHC17 and divalent carbon(0) species.18 Noteworthy, few studies about electron-poor ligands with strong π-accepting ability have been reported to date. To explore further the electronic properties of various ligands for which the IR data of

INTRODUCTION Metal−ligand bonding in transition metal complexes is commonly described through the Dewar−Chatt−Duncanson model, in terms of σ-donation from an occupied molecular orbital (MO) of the ligand to a vacant MO of the metal (L → M), and back-bonding from an occupied MO of the metal to an empty orbital of the ligand (M→L).1 Following this model, theoretical methods for quantifying σ- and π-electronic contributions in a metal−ligand interaction have been developed.2 Various experimental approaches have been used to evaluate the electron-donating ability of ligands. The Lever’s electrochemical parameter (LEP) is derived from the redox potentials E0 values of a redox couple (e.g., RuII/III) of complexes bearing the ligands of interest.3 Nuclear magnetic resonance (NMR) has been shown to be a relevant technique for comparing the ligand donor ability in metal complexes.4 For example, 13C NMR chemical shifts have been used to compare the donating ability of bidentate ligands in a series of Nheterocyclic carbene (NHC) palladium complexes:4c the more electron-donating the coligand L, the more downfield chemical shift of the carbenic carbon atom. However, 13C NMR chemical shifts do not always match the electron density at the carbon center, as illustrated recently for alkylidene metal complexes.5 A detailed analysis of the three components of the shielding tensor was indeed required to elucidate the unexpected low downfield 13C NMR chemical shifts of such carbenoid species, featuring a carbon atom with a nucleophilic character.5 Experimental scales relying on UV−visible measurements of chromium carbonyl-6 or tetrathiomolybdate rhodium complexes7 have been also reported. However, the most widely employed strategy is related to the Tolman’s electronic parameter (TEP),8 based on the infrared (IR) stretching frequency of carbon monoxide of A1 symmetry (vA1 IR (CO)), in © XXXX American Chemical Society

Received: October 27, 2016

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DOI: 10.1021/acs.inorgchem.6b02610 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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RESULTS AND DISCUSSION Classification of Global Electron-Donation Using Calculated Infrared CO Stretching Frequencies. The design of chelating ligands derived from a N-bonded Nmethylimidazolylidene (that will be referred to as NHC hereafter) and a P-bonded diphenylmethylphosphonium ylide (that will be referred to as phosphonium ylide hereafter) moieties, linked via an ortho-phenylene bridge32 raised the fundamental question of the ranking of NHC and phosphonium ylides in the existing classification of the electrondonating strength of monodentate ligands. This concern was paradoxically clarified by the preparation of isostructural chelating bis-NHC (1), NHC-phosphonium ylide (2), and bis-phosphonium ylide (3) Rh(CO)2 complexes (Scheme 1).

metal carbonyl complexes are not experimentally available, density functional theory (DFT) calculations have been widely performed. The relevance of the TEP concept as well as other experimental observables as Lever’s electrochemical parameters (LEP)3 or Hammett constants were also validated through the determination of a computed electronic parameter (CEP).19 In order to overcome the infrared mode−mode coupling flaws (for example, between M−C and C−O stretching modes) and the missing relationship between the TEP and the metal− ligand bond strength, Cremer et al. introduced recently the metal−ligand electronic parameter (MLEP) based on the metal−ligand local stretching force constant.20 The strength of the Ni−L bond was thus quantified in a broad range of L− Ni(CO)3 complexes, wherein carbene and cationic ligands were shown to be the most strongly bonded to the nickel center.20 Recently, Bertrand et al. reported that the 31P NMR chemical shifts of carbene-phosphinidene adducts do not correlate with the TEP of the corresponding carbenes.21 This was rationalized by the fact that the 31P NMR chemical shifts are indicative of the π-accepting properties of the phosphorus ligand, while the TEP is related to the overall donating character of the ligand. Hopefully, the σ-donation of the carbene donor moiety could be indirectly estimated from the knowledge of both parameters. The electronic properties of a given ligand can be also determined through the calculation of the molecular electrostatic potential (MESP).8 Suresh et al. have indeed demonstrated that the minimum value of MESP, located at the lone pair region of the ligand, correlates well with the corresponding TEP values. This method was applied to both phosphine22 and NHC23 ligands, without modeling the metal center. Electronic properties of chelating ligands have been so far less studied than those of monodentate ligands, as summarized hereafter. The Tolman’s scale based on the Ni carbonyl complex was adapted by Crabtree et al. by considering cis[Mo(CO)4L2] complexes where L2 can represent alternatively a bidendate phosphine or two monodentate phosphines.24 TEP values of bidendate NHCs25 and secondary phosphine oxides26 were extrapolated from the calculated vA1 IR (CO) value in the corresponding cis-[Mo(CO)4L2] complexes. Lovitt et al. extended the original Tolman’s approach to bidentate phosphines, through an empirical equation, where in addition to the nature of the P-substituents, the number of carbon atoms linking the two P-coordinating ends was also taken into account in the contribution to the TEP.27 Despite these few reports, the ranking of the donor ability of a chelating ligand featuring two coordinating extremities still remains a challenge, as the direct comparison of infrared CO stretching frequencies in metal carbonyl complexes featuring different geometry, oxidation state, or total charge may be unreliable or questionable. Facing these difficulties, an experimental scale related to cis-[LL′Rh(CO)2] complexes where LL′ represents a cis-chelating ligand with the two coordinating ends of the same nature or not was recently disclosed and applied for the classification of strongly donating carbon ligands, namely, NHCs and phosphonium ylides.28 DFT calculations of the CO stretching frequencies in the related Rh(I) complexes were undertaken at the B3PW91/6-31G**/ LANL2DZ*(Rh), indicating a good agreement with the experimental values. This approach is now generalized into a computational scale that can be applied to a broader range of chelating ligands exhibiting two coordinating extremities, ranging from extremely electron-rich phosphonium ylide29 or yldiides30 to extremely electron-poor amidiniophosphines.31

Scheme 1. Representation of Rh(CO)2 Complexes 1−3 built from an ortho-Phenylene Bridge Substituted by NHC and Phosphonium Ylide Donor Extremities

The global ligand−metal electronic exchange could be estimated from the average of both experimental CO infrared stretching frequencies vav IR (CO)), indicating the following increasing donating character: bis-NHC (1) < NHCphosphonium ylide (2) < bis-phosphonium ylide (3) (Scheme 1), suggesting straightforwardly that a phosphonium ylide behaves as a stronger donor ligand than does a diaminocarbene.28 The experimental classification was then further confirmed by DFT calculations and the good agreement between calculated IR CO stretching frequencies and the three available experimental values (Figure 1).28 A constant shift of about 106 cm−1 was calculated at the B3PW91/6-31G** level (Figure 1a, left), similar to that reported (90 cm−1) between the experimental TEP and the computed CEP at the same level of calculation.19 The shift was reduced to 9−13 cm−1 at the PBE/6-31G** level of calculation (Figure 1a, left). The good performance of the PBE functional for the calculation of IR CO stretching frequencies was also reported by Nolan et al.13b Both the B3PW91 and the PBE functionals are therefore able to account for the experimental variation of stretching frequencies depending on the number of NHC extremities. Although B3PW91 performs slighty better than PBE, as indicated by the slope and the linear correlation coefficient R values (Figure 1b), the PBE calculation level of lower computational cost was selected for the studies of the whole series of ligands considered hereafter. In the series of complexes 1−3 (Schemes 1 and 2, [η2-oC6H4AxB2‑xRh(CO)2][TfO] with A = NHC; B = phosphonium ylide; x = 0, 1 or 2), the linear correlation of vav IR(CO) with the number of NHC extremities (x) is noticeable. 28 The corresponding slopes are indeed very similar for both experimental and calculated values (Figure 1b, right). This suggests that the two donor extremities act in an independent way and are therefore linked via an insulating bridge. This is further demonstrated by considering the NHC-phosphonium ylide complex (2a) where the o-phenylene bridge is removed (Scheme 2). The calculated vav IR(CO) of complexes 2 (ca. B


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Inorganic Chemistry

Figure 1. (a) Experimental (in black) and calculated average IR stretching frequencies of CO (vav IR(CO)) in Rh(CO)2 complexes 1−3, depending on the calculation level. (b) Variation of experimental (in black) and calculated average IR stretching frequencies of CO in Rh(CO)2 complexes 1−3, depending of the number of NHC extremities (x = 0, 1, or 2). B3PW91/6-31G**/LANL2DZ*(Rh) level of calculation (in red) and PBE/6-31G**/ LANL2DZ*(Rh) level of calculation (in blue).

Scheme 2. Bis-NHC Rh(I)-Dicarbonyl Complex 1 and Related Complexes 2 and 3 Featuring One or Two Phosphonium Ylide Extremities

2146.2 cm−1) and 2a (ca. 2144.0 cm−1) are indeed very similar at the B3PW91 level of calculation. The meta-phenylene-bisphosphonium ylide RhCO2 complex 3b was recently prepared −1 is very close to and the experimental vav IR(CO) = 2018.0 cm av that of the ortho-isomer 3a (vIR(CO)) = 2017.5 cm−1).33 These findings also favor an insulating o-phenylene bridge (Scheme 2). Design of the Ligand Electronic Parameter (L2EP) by Analogy to the Tolman Electronic Parameter (TEP). The computation of infrared signatures performed in the previous section for complexes 1−3 was extended to other Rh(I) dicarbonyl complexes of [η2-o-C6H4AxB2‑xRh(CO)2]q type (x = 0, 1, or 2, −1 ≤ q ≤ +3) containing various electron-rich and/ or electron-poor extremities. They are built either from two identical coordinating ends (Figure 2, series a and d) or by associating the electron-rich NHC (A) with another donor end (B) which can be either electron-rich (Figure 2, series b) or electron-poor (Figure 2, series c). In the carbon series, the vav IR(CO) values of complexes 4 and 7 are lower than that of 1, suggesting that abnormal NHC is more donating than the parent NHC ligand (Figure 2a,b). Unsurprisingly, both anionic bis-imidazolyl and bis-diphenylphosphonium yldiide ligands of complexes 5 and 6, respectively, are shown to be more strongly donating than the neutral bis-diphenylphosphonium ylide ligand of complex 3 (Figure 2a). In the bis-phosphonium yldiide series, only the bridged bis-diphenylphosphonium yldiide complex 6 (A, B = (P+Ph2C2−)2CH2) could be isolated as a minimum on the potential energy surface. In the phosphorus series, chelating Rh(CO)2 complexes derived from neutral phosphines (18) and phosphonites (16) but also from cationic counterparts, namely, amidiniophosphines (17) and amidiniophosphonites (15) were taken into account (Figure 2d). On the basis of vav IR(CO) values, bisamidiniophosphonites34 are the less electron-donating ligands

of the series (Figure 2c,d). With respect to complex 16, the lower vav IR(CO) value of complex 15 suggests that the cationic bis-amidiniophosphonite is even more electron-poor than the bis-phosphonite. The calculated vav IR(CO) values were then plotted with respect to the number y of B extremities (with y = 2 − x, where x = 0, 1, or 2, is the number of NHC extremities), yielding the bundle of straight lines of Figure 3a. The latter is similar to the one obtained by Tolman (Figure 3b), upon replacement of the t Bu substituent in tri-tert-butylphosphine by three other X substituents in related [Ni(CO)3PXntBu3‑n] complexes.9a In the Tolman’s case, the linear correlations showed that the successive replacement of the P-tBu by another X substituent, results in a constant increment of vA1 IR (CO) values, allowing to assign to each P-substituent the contribution χi. The TEP value of a given phosphine ligand PX1X2X3, is then obtained by adding the contribution χi of each substituent Xi to the value of the stretching CO frequency of the Ni(CO)3PtBu3 reference complex.9a By analogy, the linear correlations of Figure 3a indicate that the successive replacement of a NHC by one or two other extremities B, results in a constant increment of vav IR(CO) values, suggesting that it is possible to assign to each extremity B, a contribution to the IR CO stretching frequency, ca. the slope of the linear correlation, namely, SB. This additive property was already anticipated above as the o-phenylene ring was shown to act as an insulating brigde linking two independent coordinating ends. A new ligand electronic parameter referred to as L2EP can be thus estimated from the slope value SB (Figure 3a). The L2EP acronym is selected to highlight that it refers primarily to bidentate ligands, although it may be also used in the case of two monodentate ligands. All straight lines of Figure 3a exhibit a linear correlation coefficient close to one and converge to the same intercept IB (ca. 2066.0 cm−1) which corresponds to the vav IR(CO) value of C


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Inorganic Chemistry

−1 Figure 2. Comparison of calculated infrared stretching frequencies (vav IR(CO) in cm ) of isostructural Rh(CO)2 complexes built from: electron-rich carbon donor extremities (a and b), and electron-poor phosphorus donor extremities (c and d). PBE/6-31G**/LANL2DZ*(Rh) level of calculation.

−1 Figure 3. (a) Linear variation of the calculated vav IR(CO) (in cm ) with the number y = 0, 1, or 2 of extremities B (y = 2 − x; x number of NHC extremities) in Rh(CO)2 complexes (see numbering in Figure 2); PBE/6-31G**/LANL2DZ*(Rh) level of calculation. (b) TEP equation involving the substitution contribution χi, in Ni(CO)3(PtBunX3−n) complexes (plotted from ref 9a).

D


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Inorganic Chemistry Table 1. Equations of the Straight Lines of Figure 3a (Intercept IB and Slope SB) for Various Extremities Ba extremity B

y=1

y=2

intercept IB

slope SB

Rb

NHC-P(OMe)2+

10 12 11 13 14 7 2 8 9′ 9 (R = CH2)

15 16 17 18 19 4 3 6 6′ 6 (R = H)

2066.1 2067.1 2066.5 2066.5 2066.3 2066.2 2067.5 2066.8 2065.3 2062.8

+30.9 +11.9 +8.5 −0.7 −2.1 −5.9 −17.6 −36.1 −49.5 −55.9

1.000 0.989 0.996 0.710 0.990 0.999 0.990 0.999 0.999 0.995

C6H4-P(OMe)2 NHC-PPh2+ C6H4-PPh2 C3N2-PPh2 a-NHC PPh2-CH2 C3N2− PPh2-CpTolSO− PPh2-CR− a

PBE/6-31G**/LANL2DZ*(Rh) level of calculation. bLinear correlation coefficient.

the [(η2-o-C6H4(NHC)2Rh(CO)2]+ complex. The latter can be considered as the reference of the present L2EP scale, similarly to the tri-tert-butylphosphine in the Tolman’s TEP scale (Table 1). The slopes of the straight lines of Figure 3a, namely, SB, are therefore equivalent to the substitution contribution χi of the TEP equation as illustrated in Figure 4, and can be used to

predict the electron-donation of any bidentate ligand. The more negative the slope SB, the stronger the electron-donation of the ligand and vice versa (Table 1). From the calculated slope SB values (Table 1), the coordinating ends may be classified according to their donating character. While diphenylphosphonium yldiides are the most electron-donating ligands (SB = −55.9), amidiniophosphonites are the less electron-donating ligands of the series (SB = +30.9). The latter are indeed more electron-poor than amidiniophosphines (SB = +8.5) and phosphonites (SB = +11.9), well-known as a reference of electron-poor ligands. As expected, the electron-donation of triarylphosphine (SB = −0.7) is very similar to that of NHC (SB = 0), although slightly lower. Strengths and Limitations. The classification of the donating ability of chelating ligands, based on the SB and related L2EP values, obtained for a large diversity of ligands (Table 1), are in good agreement with previously reported theoretical studies, relying, for example, on the near-frontier molecular orbitals in weakly donating amidinio-phosphine and phosphonite ligands of Rh(CO)2 complexes 10−11, 15, and 17.31b The main strength of the L2EP scale is to allow the estimation of the donating character of any chelating ligand featuring two coordinating extremities which can be different in nature. In the case of two distinct donor ends A and B (with A and B different from NHC) acting in an independent way, the

Figure 4. Analogy between the Tolman’s electronic parameter (TEP) and the electronic parameter for chelating ligands (L2EP), estimated from the slope SB of the linear variation of the calculated vav IR(CO) with the number of extremities B (y) different from NHC in Rh(I)dicarbonyl complexes.

Table 2. Illustration of the Performance of L2EP for Hybrid Bidentate Ligands in RhCO2 Complexes

SA

A −

PPh2-CH C3N2− C3N2− PPh2-CH− PPh2-CH2 PPh2-CH2 PPh2-CH2 C3N2− C3N2−PPh2 C3N2−PPh2 NHC-P(OMe)2+ a

−55.9 −36.1 −36.1 −55.9 −17.6 −17.6 −17.6 −36.1 −2.1 −2.1 +30.9

SB

B −

−49.5 −17.6 −5.9 +30.9 −2.1 −0.7 +8.5 +30.9 −0.7 +8.5 +8.5

PPh2-CHpTol PPh2-CH2 a-NHC NHC-P(OMe)2+ C3N2−PPh2 C6H4−PPh2 NHC-PPh2+ NHC-P(OMe)2+ C6H4−PPh2 NHC-PPh2+ NHC-PPh2+

SAB

L2EP

a vav IR(CO)

−105.4 −53.7 −42.0 −25.0 −1.4 −18.2 −9.0 −5.2 −2.8 +6.4 +39.4

1961 2012 2024 2041 2046 2048 2057 2061 2063 2072 2105

1958 2019 2026 2038 2047 2056 2064 2069 2062 2076 2112

Means vav IR(CO) calculated at the PBE/6-31G**/LANL2DZ*(Rh) level. E


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Inorganic Chemistry

indicative of the flexibility of the metallacycle linking both coordinating atoms39 and of limited steric effects (Table 3).

global L2EP can be obtained from the addition of the slope of each individual extremity, as the following: SAB = SA + SB. The relevance of the latter equation is illustrated in Table 2 for various coordinating extremities and a wide range of donating character. L2EP values calculated from the SAB values are shifted by a few cm−1, but in very good agreement with the vav IR(CO) values calculated at the PBE/6-31G**/LANL2DZ*(Rh) level (Table 2). The relative electron-donation of the corresponding bidentate ligands is therefore accurately described, thus validating the L2EP classification method. Noteworthy, although most of these ligands are not yet experimentally available, the determination of their electronic properties through the L2EP scale opens new prospects, as the design of charge transfer (CT) metal complexes of the LD[M]LACT type.35 Influence of Steric Factors. Since electronic and steric effects are generally interconnected, the estimation of the electron-donating character of a ligand requires the consideration of the possible steric constraints affecting the metal− ligand interactions. This indeed led Tolman to introduce the cone angle θ as a measure of the bulkiness of a trialkylphosphine.9 In the present rhodium-dicarbonyl complexes based on carbon ligands (Figure 2, series a and b) involving a similar boat-shaped seven-membered metallacycle, steric effects are expected to be negligible.28 This is however not the case, in the series based on phosphorus ligands (Figure 2, series c and d), exhibiting metallacycles ranging from five- to nine-membered rings. In the case of the larger metallacycles (15, 17, and 19), the flexibility of the rhodacycle is expected to minimize the steric effects. In contrast, for the smaller metallacycles (16 and 18), ring strain might be anticipated from the smaller PRhP bite angle values [15 (98.1°); 17 (94.9°); 19 (92.4°); 16 (82.5°); 18 (82.5°)]. The decrease of the bite angle might indeed affect the CO stretching frequency in complexes 16 and 18, similarly to the increase of the infrared CO stretching frequencies reported in diphosphine nickel complexes with decreasing PNiP bite angles.27 Following this line, the lower linear correlation coefficients 0.989 and 0.710 obtained for complexes 12, 16 and 13, 18 respectively, and the upshift of the intercept from the reference value of 2066.0 cm−1 (1) to 2067.1 cm−1 (12 and 16) (Table 1) may be assigned to steric contraints due to the presence of five- or six-membered rhodacycles with low PRhP bite angle values. Moreover, in some cases, several quasi-degenerate conformations of RhCO2 complexes may be encountered, and a conformational analysis might be thus required to compute the most relevant infrared CO stretching frequency for the best accuracy of the L2EP parameter values. The yaw distortion was defined by Crabtree et al. as the half of the difference in Rh−C−N angle values to measure the inplane distortion of the NHC resulting from the steric constraint imposed by the metallacycle in bis-NHC Rh(I) complexes.36 The chelating bis-NHC complexes with shorter linkers were generally shown to present the highest yaw distortions.37 Although Crabtree et al. found no evidence for any effect of the yaw distortion on the donating character of the NHC ligand,36a on the basis of energy decomposition analysis, such steric distortion was recently associated with the decrease in the M−> NHC π-back-donation (M = Rh, Ir).38 In the series b−c of RhCO2 complexes where at least one NHC extremity is present, the yaw distortion θ appears to be very low, thus rather

Table 3. Yaw Distortion Angle θ of the NHC Moiety (Half of the Difference in Rh−C−N angles α and β) in Selected NHC Rh(CO)2 Complexes (Series b−c)a NHC Rh(CO)2 complex

αb

βb

θb

1 1 (expl.) 2 7 8 9 10 11 12 13 14

124.8 125.1 127.4 126.2 126.8 127.8 132.8 132.8 127.4 128.1 130.5

130.6 130.2 128.0 129.5 128.9 127.4 122.1 122.1 128.0 127.3 124.6

−2.9 −2.6 −0.3 −1.6 −1.0 0.2 5.3 5.3 −0.3 0.4 2.9

a PBE/6-31G**/LANL2DZ*(Rh) level of calculation. bAngle values are given in degrees.

Influence of the Total Charge. The donating ability scale relying on L2EP values is based on IR stretching frequencies of CO, which are expected to be mostly related to the πacceptation of CO and to the electron-richness of the metal center. However, it has been reported that charge polarization of the molecular orbitals of CO can also affect the C−O bond distance and thus the IR stretching frequency.40 On the basis of NBO analyses, Zobi suggested that “the effect probed by high CO stretching frequencies in the IR spectrum relates primarily to a decreased polarization of the CO bonds”. He designed thus a new ligand parameter [IRp(L)] for the calculation of the symmetric CO stretching frequency in fac-[M(CO) 3 ] + complexes.41 Such electrostatic effects affecting the IR stretching frequency of CO have also been evidenced more recently by Chaquin et al., using the derivatives of the energies of canonical MOs.42 However, the very good linear correlations of Table 1, exhibiting linear correlation coefficients very close to 1, suggest that the effect of the variation of the charge polarization of CO along the rhodium-dicarbonyl series is negligible. The L2EP scale is thus reliable whatever the total charge of the Rh(CO)2 complexes 1−19, varying from −1 to +3 (Table 1). A weak effect is however noticeable for complexes 6 and 9 featuring anionic phosphonium yldiide ligands, with an intercept value slightly downshifted (2062.8 cm−1) from the reference value of 2066.0 cm−1. The introduction of a p-toluene-sulfonyl substituent (pTolSO) at the ylidic position results in a smaller intercept downshift (2065.3 cm−1) in the corresponding yldiide Rh(CO)2 complexes 6′ and 9′ (Table 1). Considering the difference of bulkiness of the ylidic substituent (H vs CH2 vs pTol-SO), the observed shift of the intercept values might be a priori rather assigned to steric factors than to the variation of the total charge of the complex. Moreover, this analysis must be tempered, since it involves a non-homogeneous series of bisphosphonium yldiide RhCO2 complexes: the parent complex [η2-o-C6H4ABRh (CO)2][TfO] (A, B = (P+Ph2CH2−)) could not be isolated as a minimum on the potential energy surface and was indeed replaced by the bridged isomer 6 (A, B = (P+Ph2C2−)2CH2) for extracting the corresponding linear correlation intercept and slope (Table 1). F


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Inorganic Chemistry

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CONCLUSIONS AND PERSPECTIVES In the present work, DFT computations of infrared stretching frequencies of CO have been performed for the quantification of global metal−ligand electronic exchanges in a series of isostructural Rh(I)-dicarbonyl complexes. A ligand electronic parameter, referred to as L2EP, calculated at the PBE/6-31G** level, has been disclosed and illustrated for the classification of a wide range of bidentate chelating ligands. This L2EP parameter allows for the first time the evaluation of the donating character of ligands featuring two dif ferent coordinating ends. This new computational tool enables thus an accurate ranking, from the electron-rich anionic phosphonium yldiides to the electronpoor cationic amidiniophosphonites located at the upper and the lower limits of the “donating ability scale”, respectively. The data obtained here for weakly donating phosphorus ligands is particularly attractive, since to the best of our knowledge, very few data related to the quantification of the donating ability of electron-poor ligands was indeed available up to now. The extension of the L2EP parameter to chelating ligands featuring more than two donor extremities may be envisioned. “Pincers”, which refer to tridentate ligands of the LXL type, are natural candidates.43

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COMPUTATIONAL DETAILS

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AUTHOR INFORMATION

Geometries were fully optimized under symmetry constraint whenever possible at the B3PW91/6-31G**/LANL2DZ*(Rh) level or at the PBE/6-31G**/LANL2DZ*(Rh) level using Gaussian03 or Gaussian09.44 LANL2DZ*(Rh) means that f-polarization functions derived by Ehlers et al.45 for Rh have been added to the pseudopotential LANL2DZ(Rh) basis set. Vibrational analysis was performed at the same level as the geometry optimization. Linear fits, intercepts, slopes, and linear correlation coefficients were obtained using Kaleidagraph 4.1.3.

Corresponding Authors

*(Y.C.) E-mail: [email protected]. *(C.L.) E-mail: [email protected]. ORCID

Christine Lepetit: 0000-0002-0008-9506 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Computational studies were performed using HPC resources from CALMIP (Grant 2009-2011 and 2016 [0851]) and from GENCI-[CINES/IDRIS] (Grant 2009-2011 and 2016 [085008]).

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DEDICATION This report is dedicated to our friend and colleague Professor Henry Chermette for his contributions to density functional theory.

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REFERENCES

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Classification of the Electronic Properties of Chelating Ligands in cis

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