Synthesis and Structure of Metal Complexes of P-Stereogenic Chiral

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Synthesis and Structure of Metal Complexes of P‑Stereogenic Chiral Phosphiranes: An EDA-NOCV Analysis of the Donor−Acceptor Properties of Phosphirane Ligands Meaghan M. Deegan,† Jake A. Muldoon,† Russell P. Hughes,*,† David S. Glueck,*,† and Arnold L. Rheingold‡ †

Department of Chemistry, Dartmouth College, 6128 Burke Laboratory, Hanover, New Hampshire 03755, United States Department of Chemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States

‡

S Supporting Information *

ABSTRACT: Reaction of the enantiomerically enriched P-stereogenic phosphiranes syn(RP,SC)-Mes*PCH2CH(Ph) (syn-1) and anti-(SP,SC)-Mes*PCH2CH(Ph) (anti-2, Mes* = 2,4,6-(t-Bu)3C6H2) with metal complex precursors gave Au(L)(Cl) (L = 1 (3); L = 2 (4)), trans-ML2Cl2 (L = 1, M = Pd (5), Pt (6)), Pd(η3-C3H5)(L)(Cl) (L = 1 (7)), and transRhL2(CO)(Cl) (L = 1 (8); L = 2 (9)); 3, 4, 7, and 9 were crystallographically characterized. Phosphirane coordination resulted in shortening of the P−C bonds and increased bond angles at P, consistent with rehybridization at phosphorus. A comparison of complexes of phenylphosphirane and phenyldimethylphosphine using IR spectra, coupled with DFT studies using electronic decomposition analysis (EDA) and natural orbitals for chemical valence (NOCV), indicated that phosphiranes are slightly poorer σ-donors than the analogous phosphines and that the π-acceptor properties of these ligands are similar. Pauli repulsion, dispersion, and electrostatic attraction are also important factors in determining the strength of these metal−ligand interactions.

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INTRODUCTION Steric and electronic tuning of phosphine ligands is valuable in optimizing the properties of metal complexes for catalytic applications.1 Extreme structural perturbations are potentially useful in developing ligands with unique properties.2 For example, the three-membered ring in the highly pyramidalized phosphiranes results in smaller cone angles and higher barriers to pyramidal inversion than in other phosphines.3 However, their coordination chemistry and use in catalysis have been little explored.4 Chiral phosphiranes are especially unusual5 and have rarely been used in asymmetric catalysis.6 We recently reported a diastereoselective and enantioselective synthesis of the isomeric P-stereogenic phosphiranes syn-Mes*PCH2CH(Ph) (1; Mes* = 2,4,6-(t-Bu)3C6H2) and anti-Mes*PCH2CH(Ph) (2) (Scheme 1) from enantiomerically enriched styrene oxide.7 The sterically demanding Mes* group made the synthesis and handling of these air-stable phosphiranes convenient,8 but its large size might hinder metal binding, despite the reduced cone angle expected from the tiedback nature of the phosphirane ring. To investigate the ligand properties of these newly accessible chiral phosphiranes for their potential use in asymmetric catalysis, we report here the synthesis and structure of several derivatives. To complement this experimental survey, we include a more general computational assessment of metal−phosphirane and metal−phosphine interactions, including the relative donor− acceptor properties of these ligands. Walsh diagram analysis suggests that phosphiranes are poor σ-donors (low-energy HOMO, with high s-character in the P lone pair) and good π-acceptors (low-energy LUMO).9 To test this hypothesis, bonding in © XXXX American Chemical Society

metal−phosphirane complexes has been studied by multiple techniques. In X-ray crystal structures, the high s character of the P lone pair should give rise to a short M−P distance, facilitated by the small cone angle, which limits steric repulsions. However, weak σ-donation from the phosphirane might counteract this effect. It has been suggested that if π-back-bonding from the metal to phosphine P−C σ* MOs is important, the P−C bonds should lengthen on coordination.10 However, rehybridization at P on binding should result in shorter P−C bonds, and this effect might be larger in magnitude than that of π-bonding.11 Measurement of 1JP−M for spin 1/2 metals such as 195Pt and 103 Rh in 31P NMR spectra offers another experimental probe of the s-character of the M−P bond.12,13 IR spectra of metal carbonyl complexes provide information on back-bonding to CO, which is affected by the donor−acceptor properties of the phosphirane, but this analysis does not permit deconvolution of σ- and π-effects.14 The thermodynamics of ligand substitution can show preferential binding of phosphines vs phosphiranes, but does not explain the origin of the energy differences.15 Finally, as described in more detail below, we have applied DFT computational energy decomposition analysis (EDA)16 and deformation densities arising from the natural orbitals for chemical valence method17 to assess the σ-donor and π-acceptor properties of phosphiranes separately and quantitatively. As in other systems, these orbital contributions to bonding, although familiar to chemists, are only part of the story, as Pauli repulsion and Received: February 28, 2018

A

DOI: 10.1021/acs.organomet.8b00123 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

cis-Pt(phosphirane)2Cl2 complexes displayed significantly larger couplings (∼4100 Hz),21,6d while JP−Pt in trans-Pt(PH2Mes*)2Cl2, a model complex containing a Mes*-phosphine with two small P substituents, was 2707 Hz.22 The larger coupling in 6, in comparison to that in the primary phosphine complex, is consistent with the expected high s-character of the P lone pair donor.21,6d The crystal structures of complexes 3, 4, 7, and 9 are shown in Figures 1−4, respectively, with additional details in the figure

Scheme 1. Synthesis of Phosphirane Complexes

attractive dispersive and electrostatic interactions are also important.18

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RESULTS AND DISCUSSION Synthesis and Structure of Metal Complexes of P-Stereogenic Chiral Phosphiranes syn-1 and anti-2. Reaction of phosphiranes syn-1 and anti-2 with standard starting materials gave complexes 3−9; palladium allyl complex 7 was formed as a 1:1 mixture of exo and endo isomers19 (Scheme 1). All of the reactions occurred at room temperature to give air-stable products, which were characterized by spectroscopy and by elemental analyses. In contrast, syn-1 did not react with Pt(COD)Cl2 or with [Rh(COD)2][BF4], even on heating to 50 °C. This unusual behavior is consistent with a kinetic barrier to formation of complexes containing cis-Mes*phosphiranes, presumably due to steric repulsions. Metal binding resulted in large 31P NMR coordination chemical shifts (Table 1). The trans-phosphirane coordination

Figure 1. ORTEP diagram of [Au(syn-(RP,SC)-Mes*PCH2CH(Ph))(Cl)]2·2(toluene) (3·2(toluene)), with solvent molecules omitted. Selected bond lengths (Å) and angles (deg): Au(1)−P(2) 2.2379(17), Au(2)−P(1) 2.2356(16), Au(1)−Cl(2) 2.2909(15), Au(2)−Cl(1) 2.2831(16), Au(1)−Au(2) 3.1851(4); P(2)−Au(1)−Cl(2) 176.57(6), P(1)−Au(2)−Cl(1) 174.83(6).

captions, Table 2, and the Supporting Information. The gold complexes 3 and 4 showed the expected linear coordination. However, 3 formed an aurophilic dimer (Figure 1),23 while no close Au···Au interactions were observed in 4 (Figure 2), perhaps because of increased steric hindrance with the phosphirane substituents on opposite sides of the ring. The Au−P and Au−Cl distances in 3 and 4 were similar to those in the primary and secondary phosphine complexes Au(PH2Mes*)(Cl)24 and Au(PHPhMes*)(Cl), which also crystallized as dimers with Au−Au interactions.25 Although two diastereomers of Pd−allyl complex 7 were observed in solution, the crystal investigated contained only one, with the expected approximate square-planar geometry at Pd(II) (Figure 3). The Pd−P and Pd−Cl distances were similar to those in related phosphine complexes; disorder in the allyl group precludes further analysis of Pd−C distances.26 Despite the large size of the Mes* substituent, the Rh−P distance in the structure of 9 (2.3164(12) Å) was similar to those in analogues with PMe2Ph (2.314(2) Å)27 or PPh3 ligands (2.322(1) Å),28 and shorter than in complexes with the bulkier PCy3 (2.355(3) Å)29 or P(t-Bu)3 (2.427(1) Å).30 As often observed in these Rh−Vaska complexes, the Cl and CO ligands were disordered (Figure 4).28

Table 1. 31P{1H} NMR Data for Phosphiranes 1 and 2 and Their Metal Complexes 3−9a ligand/complex (no.)

δ(31P) (ppm)

syn-(RP,SC)-Mes*PCH2CH(Ph) (syn-1) anti-(SP,SC)-Mes*PCH2CHPh (anti-2) Au(syn-1)(Cl) (3) Au(anti-2)(Cl) (4) trans-Pd(syn-1)2Cl2 (5) trans-Pt(syn-1)2Cl2 (6) Pd(syn-1)(η3-C3H5)(Cl) (7) trans-Rh(syn-1)2(CO)(Cl) (8) trans-Rh(anti-2)2(CO)(Cl) (9)

−155.2 −172.5 −104.2 −101.3 −113.7 −116.6 (JP−Pt = 3292) −120.4, −120.6 (1:1) −111.2 (d, JP−Rh = 161) −121.7 (d, JP−Rh = 168)

a31 P NMR chemical shifts are reported with respect to 85% H3PO4; coupling constants are given in Hz. Solvents: CD2Cl2 for 1 and 2, CDCl3 for 3, 4, and 9, THF for 5 and 6, CH2Cl2 for 8.

in Pt complex 6 (and, by analogy, in Pd complex 5), was assigned from the JP−Pt coupling constant of 3292 Hz.20 Related B


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Figure 2. ORTEP diagram of Au(anti-(SP,SC)-Mes*PCH2CHPh)(Cl) (4). Disorder in a tert-butyl group is not shown. Selected bond lengths (Å) and angles (deg): Au(1)−Cl(1) 2.2884(11), Au(1)−P(1) 2.2307(11); P(1)−Au(1)−Cl(1) 176.10(4).

Figure 4. ORTEP diagram of Rh((SP,SC)-anti-Mes*PCH2CHPh)2(CO)(Cl), showing the Cl/CO disorder and the symmetry-equivalent phosphirane ligands. Selected bond lengths (Å) and angles (deg): Rh(1)−P(1) 2.3164(12), Rh(1)−Cl(1) 2.405(5), Rh(1)−C(1) 1.823(18); P(1)−Rh(1)−P(1)#1 175.09(7), P(1)−Rh(1)−Cl(1) 89.40(13), P(1)−Rh(1)−Cl(1)#1 91.20(13), Cl(1)#1-Rh(1)−Cl(1) 166.0(5), C(1)−Rh(1)−P(1) 90.9(4), C(1)#1-Rh(1)−P(1) 89.0(4), C(1)−Rh(1)−C(1)#1 178.2(7).

observed previously; they can be rationalized by considering a change in hybridization at phosphorus on complexation.9,15 In the free phosphiranes, the highly pyramidalized phosphorus may be described as unhybridized, with a lone pair of high s-character and P−C bonds of high p-character. Quaternization at phosphorus results in hybridization closer to sp3, with increased s-character in the now-shorter P−C bonds and exocyclic bond angles closer to the ideal tetrahedral values, with smaller changes in the constrained ring.31 In principle, these structural changes could be used as a probe of metal to phosphirane π-back-bonding, in which occupation of the P−C σ* LUMO should result in longer P−C bonds in the metal complexes.10 The experimental observation of shorter P−C bonds suggests that rehybridization effects are more important than this π-acceptor behavior. As an alternative way to assess the donor−acceptor properties of phosphiranes, we considered the IR spectra of metal carbonyl complexes containing arylphosphirane and aryldimethylphosphine (Table 3). In both Rh−Vaska complexes and fac-Mo(CO)3(PR3)3, ArPMe2 ligands gave rise to lower carbonyl stretching frequencies in comparison to the analogous phosphiranes.6d,32,33 Although the structures of our chiral phosphirane complexes 8 and 9 are significantly different, with the bulky Mes* group and CHPh instead of CH2, their CO stretching frequencies of 1972 and 1973 cm−1 are similar in magnitude to those of the simpler models. Higham and co-workers suggested that the observations for rhodium complexes could be explained if the PCH2CH2 group in phosphiranes, because of the pyramidalized structure and increased s-character of the donor orbitals, was a poorer σ-donor than PMe2.6d,34 A similar rationalization is plausible for the Mo complexes. In contrast, no difference in the CO stretching frequencies for W(CO)5 complexes was observed.35,36 However, this system is particularly insensitive to phosphine

Figure 3. ORTEP diagram of Pd(syn-(RP,SC)-Mes*PCH2CHPh)(C3H5)(Cl), showing one of the three independent molecules in the unit cell. Disorder in the allyl group is not shown. Selected bond lengths (Å) and angles (deg), with average values for the three independent molecules: Pd(1)−Cl(1) 2.3583(15), Pd(1)−P(1) 2.3049(15), Pd(1)−C(27) 2.181(7), Pd(1)−C(28) 2.125(8), Pd(1)−C(29) 2.122(7); P(1)−Pd(1)−Cl(1) 92.84(6), C(27)− Pd(1)−Cl(1) 98.7(2), C(27)−Pd(1)−P(1) 168.4(2), C(29)− Pd(1)−Cl(1) 165.9(2), C(29)−Pd(1)−P(1) 100.3(2), C(29)− Pd(1)−C(27) 68.2(3).

Donor−Acceptor Properties of Phosphiranes in Metal Complexes. Table 2 shows selected structural data for phosphiranes syn-1 and anti-2 and their complexes.7 Complexation of either phosphirane to a metal resulted in shorter P−C bonds and larger angles at phosphorus, with only small changes in the angles in the ring. Similar structural changes have been C


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Organometallics Table 2. Selected Bond Lengths (Å) and Angles (deg) in Phosphiranes syn-1 and anti-2 and Their Complexesa P−CH2 P−CHPh P−Mes* CH2−CHPh CH2−P−CHPh CH2−P−Mes* Mes*−P−CHPh P−CH2−CHPh P−CHPh−CH2 Mes*−P−M CH2−P−M CHPh−P−M

syn-1b

anti-2

Au(syn-1)(Cl) (3·2C7H8)c

Au(anti-2)(Cl) (4)

Pd(syn-1)(C3H5)(Cl) (7)b

Rh(anti-2)2(CO)(Cl) (9)

1.839(3) 1.902(3) 1.849(2) 1.498(4) 47.17(12) 101.48(11) 104.09(11) 68.64(16) 64.20(15)

1.8477(11) 1.8674(11) 1.8574(9) 1.5065(14) 47.84(4) 102.29(4) 99.61(4) 66.76(6) 65.40(6)

1.804(6) 1.856(7) 1.822(6) 1.516(9) 49.0(3) 106.0(3) 108.9(3) 63.7(3) 67.3(3) 127.9(2) 119.7(2) 119.2(2)

1.807(4) 1.840(4) 1.830(4) 1.523(6) 49.34(19) 106.20(17) 106.48(16) 66.5(2) 64.2(2) 126.00(12) 122.77(13) 121.73(12)

1.817(6) 1.859(6) 1.830(5) 1.513(8) 48.6(2) 103.8(3) 106.9(2) 67.2(3) 64.2(3) 130.23(19) 120.04(19) 118.5(2)

1.817(5) 1.848(5) 1.848(5) 1.509(7) 48.6(2) 102.4(2) 103.5(2) 66.8(3) 64.6(3) 129.55(17) 121.95(16) 123.09(16)

a

Data for syn-1 and anti-2 from ref 7. bAverage values for the three independent molecules. cAverage values for the two independent molecules; the crystals contained two toluene molecules.

for Ni(CO)3(L) complexes, we used it to calculate νCO for L = PhPMe2 (2072 cm−1) and for L = PhPCH2CH2 (2082 cm−1), showing the same trend as with the other functionals.38 Energy decomposition analysis (EDA) allows partitioning of the overall attractive interaction (Etot) between two fragments into attractive and repulsive components, with the fragments in their geometries present in the complex as the reference state; clearly the outcome of the analysis depends on the choice of fragmentation.39 Fragment wave functions are evaluated independently in their molecular geometries, which are almost invariably different from those in their ground states. The reorganizational energy required to distort the fragment from its ground state to that in the full molecule, defined as Eprep, is positive. At this point the energy of the two fragments consists of an overall (for neutral fragments) repulsive interaction (Esteric), the sum of Pauli repulsion (EPauli) between valence electrons occupying overlapping regions of space and subject to the exclusion principle, offset by attractive electrostatic interactions (Eestat) between the charge distributions in the two fragments. While the EDA process separates these components computationally, they may be combined as Esteric to represent an overall repulsive “steric wall” limiting distance between fragments in the molecule. Relaxation of the wave function and allowing orbital mixing between fragments gives a net stabilization (Eorb) resulting from electron sharing by orbital overlap and polarization of electrons in the resultant molecular orbitals. An additional attractive interaction due to dispersive forces (Edisp)40 completes the partitioning, so that

Table 3. Carbonyl Stretching Frequencies in Experimental IR Spectra of Rhodium, Molybdenum, and Tungsten Arylphosphirane and Aryldimethylphosphine Complexes

complex trans-Rh(H-binaphthylPCH2CH2)2(CO)(Cl) trans-Rh(H-binaphthylPMe2)2(CO)(Cl) trans-Rh(MeO-binaphthylPCH2CH2)2(CO)(Cl) trans-Rh(MeO-binaphthylPMe2)2(CO)(Cl) trans-Rh(syn-Mes*PCH2CHPh)2(CO)(Cl) trans-Rh(anti-Mes*PCH2CHPh)2(CO)(Cl) fac-Mo(CO)3(PhPCH2CH2)3 fac-Mo(CO)3(PhPMe2)3 W(CO)5(PhPCH2CH2) W(CO)5(PhPMe2)

νCO (cm−1) 1983 1965 1985 1963 1972 1973 1950, 1939, 2070, 2071,

1856 1832 1940 1947,

ref

6d 6d 6d 6d this work this work 32 33 35 1938 36

Etot = E Pauli + Eestat + Eorb + Edisp = Esteric + Eorb + Edisp

Thus, Etot is the energy required to “snap” the bond, without allowing the resultant fragments to relax to their ground state structures. Inclusion of Eprep affords a calculated bond dissociation energy (BDE):

structure, so that electronic differences among PPh3, PMe3, and the mixed arylmethylphosphines PPh3−nMen (n = 0−3) could not be determined by vibrational spectroscopy.37 For comparison, we computed the structures and IR spectra of phenylphosphirane and phenyldimethylphosphine complexes in these Rh, Mo, and W systems, as well as for Ni(CO)3L, using two different density functionals (Table 4). Consistent with the experimental IR data from Table 3, phenylphosphirane complexes had larger νCO values in comparison to their phenyldimethylphosphine analogues. As expected, the difference in these values was smallest for W(CO)5 complexes. Although these computations reproduced the experimental trends, they were less successful in matching the νCO values. Therefore, because the MPW1PW91 functional was reported to yield an excellent linear correlation of experimental and computed stretching frequencies

BDE = Etot + Eprep

Details of this energy decomposition and its strengths and weaknesses have been addressed at length in the literature.16 Particularly useful to chemists is the ability to deconvolute the covalent part of the total bonding picture (Eorb) into components having different symmetries, in this case σ and π. Application of the NOCV methodology allows these components to be quantified in terms of the number of electrons “transferred” from one fragment to the other as covalent bonding is “turned on”.17,41 These are expressed as eigenvalues for each NOCV pair and illustrated pictorially as electron deformation densities; D


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Table 4. Calculated Comparison of Phenylphosphirane and Phenyldimethylphosphine Bonding Energetics and CO Stretching Frequencies using B3LYP-D3 and BP86-D3 (in Italics)a EDA-NOCV B3LYP-D3 νCO (scaled *0.961)

BP86-D3 νCO (scaled *0.991)

total bonding energy Etot

EPauli

Eestat

Esteric

EDisp

EOrb

σdonor

1933.8 1873.0 1872.4 1918.2 1852.1 1851.7 2034.9 1981.6 1979.4 2025.5 1971.3 1969.8 2036.4 1955.5 1936.2 1926.8 1925.1 2031.3 1948.8 1933.8 1919.6 1913.9 1951.2

−38.9 −45.2

83.0 90.5

−66.9 −70.6

16.1 19.9

−15.2 −18.5

−39.8 −46.7

−21.7 −24.8

−13.6 −16.0

0 −1.2

−42.5 −48.7

89.6 95.6

−76.4 −79.1

13.3 16.5

−16.5 −19.5

−39.3 −45.7

−21.9 −25.0

−13.5 −14.7

0 −0.8

−30.8 −38.3

89.6 94.1

−77.1 −80.9

12.5 13.2

−6.9 −7.8

−36.4 −43.6

−19.7 −23.9

−10.1 −13.1

−5.3 −5.7

−38.5 −46.7

106.6 111.9

−96.2 −101.0

10.5 10.9

−8.4 −9.5

−40.6 −48.1

−24.7 −29.6

−10.6 −12.5

−4.9 −5.2

−45.1 −50.0

90.5 93.1

−79.2 −80.4

11.3 12.7

−9.9 −11.7

−46.5 −51.0

−30.3 −33.0

−13.1 −14.2

0 −0.8

−53.4 −58.7

104.7 107.2

−96.9 −97.6

7.8 9.7

−12.3 −14.2

−49.0 −54.1

−33.5 −37.0

−13.7 −12.8

0 −0.9

Rh(CO)(Cl)P2

1947.1 1880.9 1881.2 1931.2 1858.1 1859.1 2059.6 2000.3 1998.1 2049.7 1989.1 1987.8 2052.6 1970.4 1944.5 1938.4 1935.9 2048.2 1963.9 1941.9 1931.9 1925.1 1970.1

Rh(CO)(Cl)(PPhMe2)2

1948.7

1930.5

−40.6 −45.0 −48.2 −52.9 −57.4 −63.9 −67.6 −74.2

124.8 131.5 138.6 143.8 150.8 155.3 173.3 177.3

−104.4 −108.0 −122.2 −124.6 −144.9 −147.2 −174.9 −177.1

20.5 23.6 16.4 19.2 5.9 8.1 −1.6 0.2

−6.9 −7.8 −9.0 −10.1 −4.6 −5.0 −5.4 −5.9

−54.1 −60.8 −55.6 −62.0 −58.6 −66.9 −60.6 −68.6

−34.7 −38.0 −36.6 −39.8 −34.1 −38.8 −37.0 −41.9

−12.3 −15.1 −11.2 −13.6 −15.7 −18.7 −14.6 −17.3

−4.4 −4.8 −3.9 −4.4 −6.7 −6.7 −6.4 −6.4

complexb Mo(CO)3P3

Mo(CO)3(PPhMe2)3

Ni(CO)3P

Ni(CO)3(PPhMe2)

W(CO)5P

W(CO)5(PPhMe2)

AuClP AuCl(PMe2Ph)

πσacceptor acceptor

a

All data for EDA-NOCV are in kcal/mol and represent attractive and repulsive energies involved in dissociation of a single phosphine or phosphirane (MLnP → MLn + P) at the fragment geometry of the complex (Figure 5). bP = phenylphosphirane.

stabilization energies associated with each NOCV pair can be calculated. Consequently the EDA-NOCV method provides a powerful means to address the various components of M−P bonding and has been applied to other transition-metal phosphine complexes.42,17b,18 A related approach (charge decomposition analysis) was used earlier to compare bonding in Pt complexes of the model aminophosphirane H2NPCH2CH2 and PH3.15 For each EDA-NOCV study the reference states shown in Chart 1 were chosen, corresponding to dissociative fragmentation into a single phosphorus ligand and an MLn fragment, each with a singlet ground state. Numerical EDA-NOCV values for all compounds are presented in Table 4. Figure 5 illustrates the four principal NOCV orbitals for the AuCl(phenylphosphirane) complex. The antibonding combination for each NOCV pair, which is easiest to visualize, is shown in column 1. The largest single bonding component is NOCV1, the combination of the P lone pair with the empty Au−Cl σ*-orbital, involving net σ-donation to the metal and providing −34.1 kcal/mol stabilization. The P−R σ*-orbitals of the ligands mix to give two combinations of π-symmetry, usually invoked as the acceptor component in M−P backbonding; NOCV2 and NOCV3 illustrate the interactions of these acceptor orbitals with the Au d-orbitals to give two Au → P

Chart 1. Fragmentation Reference States Used in the EDANOCV Analysisa

a

The dashed line represents the fragment separation. P = phenylphosphirane or phenyldimethylphosphine.

π-interactions worth a combined −15.7 kcal/mol. In addition, an often overlooked combination of P−R σ*-orbitals has σ-symmetry and interacts with the filled Au−Cl σ-orbital to give NOCV4, in which electrons are σ-back-donated from Au→P; in this case this σ-acceptor interaction is stabilizing by −6.7 kcal/mol. This ligand σ-acceptor component of M−P bonding has been recognized previously18 and has been shown to be essential in quantifying charge flow in M−P interactions.42a We note also that the sum of all these covalent contributions to the M−P bonding is relatively small in comparison to the electrostatic component. These observations are similar to previous E


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Figure 5. Natural orbitals for chemical valence (NOCV) contributions (B3LYP-D3/TZVP) (column 1) to Eorb for [AuCl(phenylphosphirane)] (for clarity the antibonding member of the NOCV pair is shown), the resulting deformation densities (column 2), and eigenvalues (in italics) and energies (column 3) associated with each. In deformation density plots the sense of electron “flow” is from red to blue regions.

Similarly, the total metal−ligand bonding energy was consistently larger for phosphines than for phosphiranes. This analysis considers dissociation of a single P ligand at the fragment geometry of the complex. Therefore, these are not bond dissociation energies, because relaxation energies of the fragments are not included (see below). The difference is due in part to the donor part of Eorb but also reflects significant changes in EPauli and Eestat, whose sum is given in Table 4 as Esteric. As we go from phosphirane to phosphine, the M−P−C angles become more acute, as the Me groups move closer to the metal (and its ancillary ligands). Consequently, EPauli increases (more repulsion) but so does Eestat (closer distances between local dipoles → more attraction); the latter is larger, with the counterintuitive result that opening up of the phosphorus angles, increasing the ligand steric profile, gives less net repulsion, as evaluated by Esteric. Values for Eprep for the ligand and metal fragments are provided in Table 5. Surprisingly, given the more significant reorganization observed on coordination of the phosphine, in comparison to the more angularly constrained phosphirane (Figure 6) the values of Eprep for the two ligands are remarkably similar and rather small. The reorganization of the metal fragment is more variable, as expected. Trends in BDE mirror those of Etot, with significantly larger values predicted using BP86-D3; this

calculations for M−P interactions in [M(CO)5(PR3)] (M = Cr, Mo, W; R = H, Me) for which Eestat is significantly greater than Eorb, constituting ∼70% of the total attractive interaction, and the partition between σ and π contributions to Eorb is ∼70/30.43 The data in Table 4 illustrate some dependence of the bonding properties of the phosphirane and phosphine ligands on the metal fragments to which they are bound, and some dependence on the functional used; absolute numbers vary, but the trends are the same. As expected from the qualitative considerations described above, the σ-donor interaction was consistently stronger for the phosphine than the phosphirane. However, the differences were small (maximum 5−6 kcal/mol) or even negligible for Mo(CO)3L3. Energy differences for π-acceptor contributions were even smaller and showed greater dependence on the functional; these data provide little support for the description of phosphiranes as excellent π-acceptor ligands. Even in Au(L)(Cl), without CO ligands competing for π-back-bonding, the phosphirane π-acceptor contribution was preferred over the phosphine only by about 1 kcal/mol. These trends, and the greater magnitude of σ- vs π-interactions, make the total orbital contribution to M−L bonding larger for the phosphine vs the phosphirane. In general, this method of analysis suggests that phosphines are slightly better σ-donors and that the two ligands have similar π-acceptor properties. F


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Table 5. Calculated Values for the Fragment Preparation Energies (Eprep) for Phenylphosphirane and Phenyldimethylphosphine Complexes using B3LYP-D3 and BP86-D3 (in Italics) Eprep (MLnfragment)

a

Eprep (P ligand)

total Eprep

BDE = Etot + Eprep

complexa

B3LYP-D3

BP86-D3

B3LYP-D3

BP86-D3

B3LYP-D3

BP86-D3

B3LYP-D3

BP86-D3

Mo(CO)3P3 Mo(CO)3(PPhMe2)3 Ni(CO)3P Ni(CO)3(PPhMe2) W(CO)5P W(CO)5(PPhMe2) Rh(CO)(Cl)P2 Rh(CO)(Cl)(PPhMe2)2 AuClP AuCl(PMe2Ph)

4.2 4.7 7.8 8.3 1.3 1.6 5.2 6.3 0.5 0.6

4.9 5.4 6.7 6.8 1.8 2.0 6.1 6.6 0.7 0.9

1.2 0.3 0.1 0.3 1.3 1.0 1.8 1.7 1.6 2.0

0.6 0.1 0.1 0.9 1.2 1.0 1.5 1.2 1.3 2.3

5.4 5.0 7.7 8.7 2.6 2.7 7.0 8.0 2.1 2.6

5.5 5.5 6.6 7.7 3.0 3.0 7.6 7.8 1.9 3.1

−33.5 −37.5 −23.1 −29.9 −42.5 −50.8 −33.6 −40.3 −55.3 −65.0

−39.7 −43.1 −31.6 −39.0 −47.0 −55.7 −37.4 −45.1 −62.0 −71.1

P = phenylphosphirane. 300 or 500 and Bruker 500 or 600 MHz spectrometers. 1H or 13C NMR chemical shifts are reported vs Me4Si and were determined by reference to the residual 1H or 13C solvent peaks. 31P NMR chemical shifts are reported vs H3PO4 (85%) used as an external reference. Coupling constants are reported in Hz, as absolute values unless noted otherwise. Unless indicated, peaks in NMR spectra are singlets. Elemental analyses were provided by Quantitative Technologies Inc. or Atlantic Microlab, Norcross, GA. Mass spectra were recorded at the University of Illinois, Urbana−Champaign. Reagents were from commercial suppliers. These compounds were prepared by the literature methods: Pd(NCPh)2Cl2 and Pt(NCPh)2Cl2,45 Au(THT)(Cl),46 and the phosphiranes syn-1 and anti-2.7 Sample purity was established by a combination of elemental analyses and NMR spectroscopy (see the Supporting Information for NMR spectra). Au(syn-(RP,SC)-Mes*PCH2CHPh)Cl (3). A solution of syn-1 (20 mg, 0.053 mmol, 1.0 equiv) in THF (0.5 mL) was added to a suspension of Au(THT)(Cl) (17 mg, 0.053 mmol, 1.0 equiv) in THF (0.5 mL). The mixture was stirred until all solid was dissolved. The product was observed by 31P{1H} NMR spectroscopy, and the solvents were removed, yielding the crude product as a white solid (27 mg, 84%), which was recrystallized from toluene/methanol to give a white crystalline solid (5 mg, 16%). In a similar experiment, recrystallization from CH2Cl2/ pentane gave a sample for elemental analysis. Anal. Calcd for C26H37AuClP: C, 50.95; H, 6.08. Found: C, 50.55; H, 6.08. 31P{1H} NMR (CDCl3): δ −104.2. 1H NMR (CDCl3): δ 7.56 (dd, J = 4, 2, 1H, Mes* CH), 7.10 (dd, J = 8, 6, 1H, para Ph), 7.03 (dd, J = 2, 5, 1H, Mes* CH), 6.97 (t, J = 8, 2H, Ph), 6.02 (d, J = 8, 2H, Ph), 3.11 (t, J = 10, 1H, CH), 2.38 (ddd, J = 10, 3, 1H, CH2), 1.96 (q, J = 10, 1H, CH2), 1.75 (9H, ortho t-Bu), 1.34 (9H, para t-Bu), 1.23 (9H, ortho t-Bu). 13C{1H} NMR (CDCl3): δ 159.8 (quat Ar), 159.4 (d, J = 11, quat Ar), 152.9 (quat Ar), 134.7 (d, J = 7, quat Ar), 128.0 (d, J = 2, Ph CH), 127.4 (d, J = 2.5, Ph CH), 127.2 (d, J = 2.5, Ph CH), 125.5 (d, J = 10, Mes* CH), 123.6 (d, J = 11, Mes* CH), 40.6 (quat t-Bu), 39.7 (quat t-Bu), 34.9 (quat t-Bu), 34.7 (d, J = 2, CH3 ortho t-Bu), 34.4 (d, J = 12, CH), 34.3 (d, J = 3, CH3 ortho t-Bu), 31.2 (CH3 para t-Bu), 22.2 (d, J = 33, CH2). One quat Ar peak was not observed. Au(anti-(SP,SC)-Mes*PCH2CHPh)(Cl) (4). A solution of anti-2 (20 mg, 0.053 mmol, 1.0 equiv) in THF (1 mL) was added to a suspension of Au(THT)(Cl) (17 mg, 0.053 mmol, 1.0 equiv) in THF (1 mL). The solvent was removed under vacuum, and the product, a white solid (25 mg, 80%), was recrystallized from CH2Cl2/pentane. Anal. Calcd for C26H37AuClP: C, 50.95; H, 6.08. Found: C, 51.41; H, 6.32. Although these results are outside the range viewed as establishing analytical purity, they are provided to illustrate the best values obtained to date; please see the Supporting Information for NMR spectra. 31P{1H} NMR (CDCl3): δ −101.3. 1H NMR (CDCl3): δ 7.39 (m, 2H, Ph), 7.34 (1H, Mes*), 7.33 (3H, Ph), 7.29 (m, 1H, Mes*), 2.70 (apparent q, J = 8, 1H, CH2), 2.15 (t, J = 9, 1H, CH), 1.94 (apparent q, J = 8, 1H, CH2), 1.72 (9H, t-Bu), 1.63 (9H, t-Bu), 1.31 (9H, t-Bu). 13C{1H} NMR (CDCl3): δ 157.7 (d, J = 6, quat Mes*),

Figure 6. Calculated (B3LYP-D3) geometries for phosphine and phosphirane ligands in their uncoordinated ground states (top) and these ligands bound to the AuCl fragment (bottom).

difference arises principally due to the larger values of Eorb (Table 4) calculated using this functional.44

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CONCLUSIONS Despite the bulky Mes* group, the P-stereogenic phosphiranes syn-1 and anti-2 readily formed gold, palladium, platinum, and rhodium complexes, which can now be investigated for potential applications in asymmetric catalysis, for comparison to previous work with P-stereogenic phosphiranes6a and the more recently reported binaphthyl-based air-stable derivatives.6c−e Metal coordination resulted in shorter P−C bonds and larger angles at phosphorus, consistent with rehybridization on quaternization. More generally, we have used electronic decomposition analysis (EDA-NOCV) to quantitatively evaluate the hypothesis that phosphiranes are poor σ-donors and good π-acceptors. Comparison of phenylphosphirane and phenyldimethylphosphine complexes of Rh, Mo, W, Ni, and Au suggested that the phosphirane was a slightly inferior σ-donor, as proposed, but the π-acceptor properties of the ligands were similar. Beyond these orbital contributions to covalent bonding, electrostatic and dispersive attraction and Pauli repulsion are significant, with the surprising conclusion that the tied-back phosphirane results in greater net repulsions. These computational analyses may be useful in further studies of metal−phosphirane bonding.

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EXPERIMENTAL SECTION

General Experimental Details. Unless otherwise noted, all manipulations were carried out in air. NMR spectra were recorded using Varian G


Article

Organometallics

the two diastereomers appeared to overlap in the 1H NMR and the C{1H} NMR spectra, and one ortho t-Bu CH3 peak was not observed; it is likely overlapping with another signal. trans-Rh(syn-(RP,SC)-Mes*PCH2CHPh)2(CO)(Cl) (8). A solution of syn-1 (20 mg, 0.051 mmol, 2.0 equiv per Rh) in CH2Cl2 (0.5 mL) was added to a solution of [Rh(CO)2Cl]2 (5 mg, 0.013 mmol, 1.0 equiv) in THF (0.5 mL). The reaction proceeded overnight (24 h), and product formation was observed by 31P{1H} NMR. The solvents were removed, yielding a yellow solid which was recrystallized from THF/ether (16 mg, 67%). A sample recrystallized from chloroform/pentane for elemental analysis gave results consistent with the presence of CHCl3. Anal. Calcd for C53H74ClOP2Rh: C, 68.64; H, 8.04. Calcd for C53H74ClOP2Rh·0.25CHCl3: C, 66.81; H, 7.82. Found: C, 66.81; H, 7.84. 31P{1H} NMR (CH2Cl2): δ −111.2 (d, JRh−P = 161). 1H NMR (CDCl3): δ 7.56 (1H, Mes* CH), 7.04 (overlapping, 1H, Mes* CH, m, 1H, Ph), 6.95 (t, J = 8, 2H, Ph), 6.17 (br, 2H, Ph), 3.59 (t, J = 10, 1H, CH), 2.70 (dd, J = 8, 8, 1H, CH2), 1.87 (9H, t-Bu), 1.82 (m, br, 1H, CH2), 1.33 (9H, t-Bu), 1.25 (9H, t-Bu). 13C{1H} NMR (CDCl3): δ 157.9 (quat Ar), 157.7 (quat Ar), 150.9 (quat Ar), 137.6 (quat Ar), 128.0 (Ar CH), 127.6 (Ar CH), 126.0 (Ar CH), 125.4 (Ar CH), 124.5 (Ar CH), 121.7 (quat Ar), 40.7 (quat t-Bu), 40.5 (quat t-Bu), 35.7 (t-Bu CH3), 35.2 (t-Bu CH3), 34.7 (quat t-Bu), 31.9 (br, CH), 31.2 (t-Bu CH3), 22.9 (br, CH2). The CO and one quat Ar 13 C NMR signals were not observed. Evaporation of a CH2Cl2 solution on NaCl plates gave a sample for IR: νCO 1972 cm−1. trans-Rh(anti-(SP,SC)-Mes*PCH2CHPh)2(CO)(Cl) (9). A solution of anti-2 (20 mg, 0.051 mmol, 2.0 equiv per Rh) in CH2Cl2 (0.5 mL) was added to a solution of [Rh(CO)2Cl]2 (5 mg, 0.013 mmol, 1.0 equiv) in THF (0.5 mL). The reaction proceeded overnight (24 h), and product formation was observed by 31P{1H} NMR spectroscopy. The solvents were removed, yielding the product as a yellow solid (19 mg, 79%). Recrystallization from THF/ether gave a sample for elemental analysis. Anal. Calcd for C53H74ClOP2Rh: C, 68.64; H, 8.04. Found: C, 68.12; H, 8.20. Although these results are outside the range viewed as establishing analytical purity, they are provided to illustrate the best values obtained to date; please see the Supporting Information for NMR spectra. 31P{1H} NMR (CDCl3): δ −121.7 (d, JRh−P = 168). 1 H NMR (CDCl3): δ 7.68 (d, J = 7, 2H, Ph), 7.39 (d, J = 9, 2H, Mes* CH), 7.23 (overlapping m, 3H, Ph), 3.02 (apparent t, J = 10, 1H, CH), 2.62 (apparent t, J = 9, 1H, CH2), 1.83 (9H, t-Bu), 1.69 (overlapping, m, 1H, CH2, 9H, t-Bu), 1.32 (9H, t-Bu). 13C{1H} NMR (CDCl3): δ 155.5 (quat Ar), 150.2 (quat Ar), 138.0 (quat Ar), 129.7 (Ph CH), 128.1 (Ph CH), 126.4 (Ph CH), 125.2 (Mes* CH), 124.4 (Mes* CH), 40.4 (quat t-Bu), 39.9 (quat t-Bu), 35.9 (CH), 35.4 (t-Bu CH3), 34.9 (t-Bu CH3), 34.7 (quat t-Bu), 31.1 (t-Bu CH3), 21.9 (CH2). Signals due to CO and two quat Ar were not observed. Evaporation of a CH2Cl2 solution on NaCl plates gave a sample for IR: νCO 1973 cm−1.

157.3 (d, J = 4, quat Mes*), 152.7 (quat Mes*), 136.2 (quat Ph), 129.1 (Ph), 127.7 (Ph), 127.0 (d, J = 6, Ph), 125.1 (d, J = 9, Mes* CH), 124.5 (d, J = 10, Mes* CH), 40.2 (quat t-Bu), 40.0 (quat t-Bu), 34.95 (quat t-Bu), 34.95 (d, J = 2, ortho t-Bu), 34.90 (d, J = 2, ortho t-Bu), 32.6 (d, J = 8, CH), 31.1 (para t-Bu), 23.0 (d, J = 7, P-CH2). One quat Ar peak was not observed. Pd(syn-(RP,SC)-Mes*PCH2CHPh)2Cl2 (5). A solution of syn-1 (20 mg, 0.053 mmol, 2.0 equiv) in THF (0.5 mL) was added to a solution of Pd(NCPh)2Cl2 (10 mg, 0.027 mmol, 1.0 equiv) in THF (0.5 mL). The product was observed by 31P{1H} NMR spectroscopy, and the solvents were removed. The yellow residue was washed with CH3OH (3 mL) to give an analytically pure yellow solid (12 mg, 48%). Anal. Calcd for C52H74Cl2P2Pd: C, 66.55; H, 7.95. Found: C, 66.59; H, 7.99. 31P{1H} NMR (THF): δ −113.7. 1H NMR (CDCl3): δ 7.54 (1H, Mes* CH), 7.05 (t, J = 7.5, 1H, para Ph), 7.03 (1H, Mes* CH), 6.95 (t, J = 7.5, 2H, Ph), 6.18 (br, 2H, Ph), 3.63 (t, J = 10, 1H, CH), 2.72 (dd, J = 11, 8, 1H, CH2), 1.86 (overlapping, 9H, t-Bu, m, 1H, CH2), 1.33 (9H, t-Bu), 1.21 (9H, t-Bu). 13C{1H} NMR (CDCl3): δ 158.2 (d, J = 10, quat Ar), 151.4 (quat Ar), 136.9 (quat Ar), 128.1 (Ph CH), 127.7 (Ph CH), 126.4 (para Ph CH), 125.5 (Mes* CH), 124.4 (Mes* CH), 40.6 (quat t-Bu), 40.4 (quat t-Bu), 35.0 (t-Bu CH3), 34.7 (quat t-Bu), 34.4 (t-Bu CH3), 33.0 (br, CH), 31.2 (t-Bu CH3), 22.6 (CH2). Two quat Ar peaks were not observed. Pt(syn-(RP,SC)-Mes*PCH2CHPh)2Cl2 (6). A solution of syn-1 (32 mg, 0.084 mmol, 2.0 equiv) in THF (0.5 mL) was added to a slurry of Pt(NCPh)2Cl2 (20 mg, 0.042 mmol, 1.0 equiv) in THF (1 mL). The reaction proceeded to completion in 24 h, as observed by 31P{1H} NMR spectroscopy. The solvents were removed, and the residue was washed with methanol to give a yellow-white solid (31 mg, 72%). Recrystallization from THF/ether gave a sample for elemental analysis. Anal. Calcd for C52H74Cl2P2Pt: C, 60.81; H, 7.26. Found: C, 61.09; H, 7.55. 31P{1H} NMR (THF): δ −116.6 (JPt−P = 3292). 1H NMR (CDCl3): δ 7.56 (br, 1H, Mes* CH), 7.05 (overlapping, 1H, Mes* CH, m, 1H para Ph), 6.96 (t, J = 7, 2H, Ph), 6.18 (br, 2H, Ph), 3.60 (t, J = 10, 1H, CH), 2.73 (dd, J = 11, 8, 1H, CH2), 1.86 (overlapping, 9H, t-Bu, m, 1H, CH2), 1.33 (9H, t-Bu), 1.22 (9H, t-Bu). 13C{1H} NMR (CDCl3): δ 158.2 (quat Ar), 151.4 (quat Ar), 136.8 (quat Ar), 133.7 (br, quat Ar), 129.1 (d, J = 12, quat Ar), 128.0 (br, Ph CH), 127.7 (Ph CH), 126.3 (Ph CH), 125.6 (Mes* CH), 124.6 (Mes* CH), 40.7 (quat t-Bu), 40.5 (quat t-Bu), 35.0 (t-Bu CH3), 34.7 (quat t-Bu), 34.3 (t-Bu CH3), 31.2 (t-Bu CH3), 30.7 (br, CH), 20.6 (br, CH2). Pd(syn-(RP,SC)-Mes*PCH2CHPh)(C3H5)(Cl) (7). A solution of syn-1 (40 mg, 0.11 mmol, 1 equiv per Pd) in CH2Cl2 (2 mL) was added to a yellow solution of [Pd(C3H5)Cl]2 (20 mg, 0.053 mmol, 0.5 equiv of complex, 1 equiv of Pd) in CH2Cl2 (2 mL); the mixture immediately turned lighter yellow. The solvent was removed, and the product was recrystallized from pentane, yielding a yellow solid (35 mg, 58%) as a 1:1 mixture of diastereomers A and B. Recrystallization from THF/ ether gave a sample for elemental analysis. Anal. Calcd for C29H42ClPPd: C, 61.81; H, 7.51. Found: C, 61.75; H, 7.61. 31P{1H} NMR (CH2Cl2): δ −120.4, −120.6 (1:1). 1H NMR (CDCl3): δ 7.51 (1H, Mes* CH A), 7.50 (1H, Mes* CH B), 7.00 (overlapping, 2H, Mes* CH, 2H, Ph), 6.89 (4H, Ph), 6.05 (br, 4H, Ph), 5.57 (overlapping, br, 2H, allyl CH A and B), 4.63 (br, 2H, allyl CH2 A1), 3.81 (br, 2H, allyl CH2 B1), 3.50 (overlapping, m, 2H, allyl CH2 A2), 2.96 (m, 2H, allyl CH2 B2), 2.67 (t, J = 8, 1H, CH2 A1), 2.56 (t, J = 8, 1H, CH2 A2), 1.74 (overlapping, 9H, ortho t-Bu, 1H, CH2 B1), 1.65 (overlapping, 9H, ortho t-Bu, 1H CH2 B2), 1.30 (overlapping, 18H, para t-Bu A and B), 1.10 (9H, ortho t-Bu), 1.03 (9H, ortho t-Bu). 13C{1H} NMR (CDCl3): δ 158.7 (quat Ar), 158.3 (quat Ar), 158.2 (quat Ar), 158.0 (quat Ar), 151.2 (quat Ar), 137.2 (quat Ar), 137.1 (quat Ar), 127.9 (br, Ph CH), 127.8 (Ph CH), 126.4 (Ar CH), 126.3 (Ar CH), 125.35 (Mes* CH), 125.32 (Mes* CH), 124.31 (Ar CH), 124.26 (Ar CH), 121.7 (d, J = 33, quat Ar), 116.3 (br m, allyl CH), 75.2 (overlapping d, J = 37, allyl CH2), 62.9 (allyl CH2), 62.6 (allyl CH2), 40.4 (quat t-Bu), 40.3 (quat t-Bu), 40.2 (quat t-Bu), 40.0 (quat t-Bu), 35.2 (br, 2 ortho t-Bu CH3), 35.1 (ortho t-Bu CH3), 35.0 (ortho t-Bu CH3), 34.8 (quat t-Bu), 34.5 (quat t-Bu), 33.1 (d, J = 20, CH), 32.8 (d, J = 29, CH), 31.2 (overlapping, 2 para t-Bu CH3), 22.9 (d, J = 18, CH2), 22.8 (d, J = 15, CH2). The para t-Bu signals for

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00123. Additional crystallographic and computational details and NMR spectra (PDF) Cartesian coordinates for calculated structures (XYZ) Accession Codes

CCDC 1579387−1579390 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for R.P.H.: [email protected]. *E-mail for D.S.G.: [email protected]. H


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Russell P. Hughes: 0000-0002-1891-6530 David S. Glueck: 0000-0002-8438-8166 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation for funding (CHE126578 and -1562037) and the Department of Education (GAANN, J.A.M.) and Dartmouth College (M.M.D.) for fellowship support.

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REFERENCES

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Article

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