Predicting the Air Stability of Phosphines.pdf

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Predicting the Air Stability of Phosphines Beverly Stewart, Anthony Harriman,* and Lee J. Higham* School of Chemistry, Newcastle University, Newcastle upon Tyne, NE1 7RU, U.K.

bS Supporting Information ABSTRACT: DFT calculations form the basis of a model capable of predicting the air stability of phosphines. The sensitivity of 18 primary phosphines is accounted for; the model also predicts the trend of increasing stability from phenylphosphine to triphenylphosphine. There is evidence that the radical cation SOMO energy for each corresponding phosphine may be key to its air stability/sensitivity.

’ INTRODUCTION Primary phosphines1 have a fearsome reputation as toxic, volatile compounds that are often spontaneously flammable in air. Air-stable primary phosphines are considered rare but for a few examples;1a
k we recently discovered a new member of this class2 based upon the binaphthyl backbone 1a,b (see Chart 1). With the exception of those possessing high steric hindrance such as 2, it is not possible to rationalize or predict which primary phosphines will exhibit air stability. Given that these highly versatile molecules allow access to derivatives with applications in fields as diverse as asymmetric catalysis,3a
e biomedicinals,1c polymer science,3f and carbohydrates,3g an ability to predict their sensitivity would bring significant safety benefits and enable researchers to better capitalize on their potential. To gain an insight into the stability of the phosphines in Chart 1 where steric hindrance cannot be the protecting factor, one must look at their electronic nature. The mechanism of phosphine oxidation4a,b by elemental oxygen has not been fully elucidated yet, but it is a topical subject.4c It was recently established that the formation of the corresponding radical cation of a phosphine by photolysis led to its oxidation via a radical mechanism.5a Scheme 1 shows the postulated oxidative process, although the exact fate of the peroxy radical cation formed is considered to be solvent dependent, earlier radiolysis experiments having proposed a different breakdown pathway to phosphine oxide formation.5b Our earlier report2 demonstrated that increasing the amount of conjugation in the molecular skeleton from phenyl 1f to binaphthyl 1a,b was sufficient to render the latter phosphines stable to air oxidation in both the neat and solution states (Figure 1). Note that 1b is the only example of a stable primary phosphine containing just a hydrocarbon backbone and therefore heteroatom presence cannot be the stabilizing factor; thus, r 2011 American Chemical Society

we reasoned that the source of this stabilization must be a conjugative effect, compounded by the fact that the saturated additional ring in 1e renders the molecule more prone to oxidation than its naphthyl counterpart 1c. We therefore wanted to consider in more detail the nature of the HOMO of each neutral phosphine and their radical cation counterparts, in order to try and establish the underlying factors which need to be considered to rationalize their air stability.

’ RESULTS AND DISCUSSION Photoelectron spectroscopy and molecular modeling suggest that conjugation between the phosphorus lone pair and the aromatic ring is minimal in phenylphosphine 1f and that the HOMO is a perturbed π orbital of the benzene ring.1m,6 In support of this view we note that both 1-naphthyl and 2-naphthyl substitutions lead to a similar stabilization in solution, whereas a conjugative effect to the lone pair might be expected to show a difference. Therefore, in order to establish the significance of how the orbital energies and distributions vary as the degree of conjugation in the phosphine changes, we carried out a series of DFT calculations using the B3LYP functional with a 6-31G* basis set on 1a
f (see the Supporting Information). First, the DFT models indicate a qualitative trend in orbital distribution which is in accord with experimental findings; primary phosphines which have an extended π-electron structural motif possess a HOMO with no significant phosphorus character and are those which demonstrate stability to air oxidation. In the case of 1a,b, calculations exclude the phosphorus from interaction with the HOMO (1a,
5.50 eV; 1b,
5.82 eV), HOMO-1 (1a,
5.74 eV; 1b,
5.90 eV) and HOMO-2 (1a, Received: January 25, 2011 Published: September 21, 2011 5338

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Chart 1. Air-Stable Primary Phosphinesa

a

Note the range of structural diversity present.

Figure 1. The 7 day neat (left bar) and d-chloroform solution (right bar) oxidation profiles of selected binaphthyl-, naphthyl- and phenylphosphines.

Scheme 1. Postulated Steps in the Photolytic Oxidation of Phosphines: A Radical Cation Forms and Reacts with Dioxygen to Generate a Peroxy Radical That Ultimately Leads to 2 mol of Phosphine Oxide


6.33 eV; 1b,
6.64 eV). Conversely, phosphorus participation in the HOMO is generally significant when there is little conjugation present, which is consistent with the experimental observation that these phosphines are sensitive to oxidation (Chart 2, 1c,f). Work by others using the SARACEN1m and MP2/6-31G(d,p) methods6b found that the barrier to rotation for phenylphosphine 1f is low (1.5 kJ/mol in the latter case); phosphine 1a was found to have two low-energy conformers (the global minimum structure is shown in Chart 2). The most stable conformer has the phosphorus lone pair perpendicular to the naphthyl ring to which it is bound; the second form has the lone pair at an angle of 109° to this. The energy difference between the two was calculated to be only 3.33 kJ/mol and suggests that the possible localization of the lone pair electrons over the lower aromatic ring is not significant in this case; this has been argued to be an important factor in the relative air stability of dialkylbiaryl phosphines (see later).4 Further calculations on the series 1a
f indicate a general increase in the relative energy of the HOMO with increasing conjugation; for instance, the HOMO energy for 1a is
5.50 eV,
5.88 eV for 1c, and
6.41 eV for 1e. As expected, the HOMO
LUMO gap is smaller for 1a,b, calculated at 4.36 and 4.54 eV, respectively, than for the

remaining phosphines 1c
f, which have energy gaps ranging from 4.63 to 6.32 eV (Supporting Information); however, the calculations show the HOMO of the binaphthyl 1b to be comparable to that of the naphthyl derivative 1d (
5.82 eV versus
5.80 eV, respectively). We then modeled the radical cations for 1a
f and this time found that the phosphorus atom is incorporated into the SOMO surface in every case. The energies of these orbitals revealed that those above a threshold value of
10 eV correlate with the primary phosphines which are resistant to air-oxidation (Table 1, Figure 2, and the Supporting Information); this is consistent with increased conjugation raising the energy of the HOMO of the neutral phosphine and the SOMO of its radical cation counterpart (and in this case the trend holds as expected for the 1b versus 1d comparison, with SOMO energies of
9.29 eV versus
10.63 eV, respectively). One explanation of this phenomenon would be that the radical cation generated from a more stable SOMO would be more reactive to oxygen than one originating from a SOMO of lower stability. To test the limits of this principle, we examined other known examples of air-stable primary phosphines. Inspection of the diverse structures in Chart 1 suggests there may be many primary phosphines which are stable or only mildly sensitive to air handling. However, it is not obvious why the alkylated ferrocene 4 should be stable in air indefinitely, when FcPH2 (4b) and 1,10 Fc0 (PH2)2 are oxidized in 3
5 days.1g,h The alkyl spacer group in 4 is therefore important (as noted by the authors themselves), but its role has not yet been accounted for. The origins of the surprising stability of 5 were described as “unclear”; negative hyperconjugation from remote heteroatoms was considered as possibly responsible,1b,i
k with the same rationale applied to account for the remarkable stability of S(CH2S(CH2)3PH2)21a,b (6), a molecule lacking any apparent steric hindrance or aromaticity. Unpublished AM1 calculations on a related carboxylate 5339

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Chart 2. DFT Calculations Indicating the HOMO Distribution across the Molecule for the Primary Phosphines 1a,c,f and 2a

a

Increasing π conjugation eventually isolates the phosphino function from the HOMO; for 2, this phosphine is stabilized by the bulky tert-butyl groups.

Table 1. Calculateda Neutral (N) HOMO and Radical Cation (RC) SOMO Energies and the N
RC Optimized Energy Gap ΔEb of Selected Primary, Secondary, and Tertiary Phosphines in eVc

1a 1b 1c 1d 1e 1f 2 3 4 4b 5 6 7 8 9 10 PPh2H PEt2H PPh3 PMe3 PEt3

N HOMO (eV)

RC SOMO (eV)

N
RC Optimized Energy Gap (eV)b


5.50
5.82
5.88
5.80
6.41
6.87
5.79
5.91
5.17
5.34
5.90
5.99
6.88
6.52
6.30
5.47
6.14
6.77
5.75
5.98
5.72


9.02
9.29
10.64
10.63
10.96
11.73
10.28
10.00
9.58
10.67
9.94
9.51
11.83
11.71
11.94
9.47
10.36
12.67
9.50
12.43
11.72

6.65 7.00 7.44 7.41 7.57 8.00 7.10 7.28 6.05 6.19 7.48 7.31 8.19 7.95 7.93 6.43 7.28 7.95 6.71 7.64 7.25

a

See the Supporting Information. b This is the molecular energy difference between each neutral phosphine and its radical cation counterpart; the approach has been used as a measure of the first ionization potential of phenols.7 c The full data for these and some other common phosphines are given in the Supporting Information.

indicated sulfur participation in the HOMO, but no details were given.1b,k We therefore extended our calculations to phosphines 3
6 (Chart 3), aiming to rationalize this seemingly unpredictable behavior. For the neutral molecules, our calculations demonstrate that the HOMO is once again found to be quite remote from the phosphino function in phosphines 4
6 (Chart 3); the radical cation SOMO meanwhile is again relatively destabilized beyond
10 eV in each of these air-stable examples. Thus, it is also conceivable that rather than the previous rationale of solely steric protection,1a,f the triptycene 3 may also be stabilized, at least in part, by the significant conjugation provided by the three phenyl rings of the backbone, with a value of
10.00 eV calculated for the radical cation SOMO (Table 1, Figure 3). The article1g which describes 4 discusses the importance of the alkyl spacer group; when the phosphino group is directly bonded to

the ferrocene ring, the compound is air sensitive (4b). This can now be rationalized in terms of the ethyl group acting as “electronic insulation” for the phosphino functionality; its presence raises the energy of the radical cation SOMO from
10.67 eV in 4b to
9.58 eV in 4 (Table 1, Figure 3), beyond the apparent threshold. For the diprimary phosphine 51j,k we can now argue that the presence of the phenyl group provides sufficient conjugation to confer air stability; it destabilizes the SOMO of the radical cation from
10.55 eV for the modeled (and unknown)
CONH2 analogue 5b to
9.94 eV for the air-stable
CONHPh derivative 5 (Table 1, Figure 3). At this point we were curious to model the highly unusual diphosphine 6, which is also air stable but lacks both unsaturation and steric hindrance.1c Our model suggests we should expect air stability for this molecule, as found experimentally; the HOMO distribution of 6 and the SOMO energy of the corresponding radical cation (
9.51 eV) fit the emerging pattern (Chart 1, Figure 3) of what appears required to afford resistance to air oxidation. It is evident from Chart 4 and Table 1 that small primary phosphines lacking steric hindrance, appropriate conjugation, and/or sufficient heteroatom presence (structures 7
9) will oxidize rapidly; all have radical cation SOMO energies which are stabilized beyond the
10 eV threshold (Table 1, Figure 3). Compound 7 is used in an industrial safety demonstration,8 while 8 and 9 are extremely malodorous and oxygen sensitive, igniting on the tip of a syringe.9 These hazardous properties correlate well with most researchers’ expectations of this molecular family, but we demonstrate here that they are not necessarily an inherent feature of it. In fact, a wide variety of primary phosphines are likely to be air stable, if they incorporate an appropriate backbone. We next sought to establish if the utility of the model was confined to primary phosphines, or whether it could successfully predict the behavior of common secondary and tertiary phosphines. Table 1 and Figure 4 show that the model is also successful in accurately replicating the behavioral trend observed upon going from the air-sensitive phenylphosphine to the air-stable triphenylphosphine; 1f has a stabilized radical cation SOMO energy of
11.72 eV, while triphenylphosphine gave a corresponding energy of
9.51 eV. The related secondary phosphine diphenylphosphine gave an intermediate SOMO value (
10.36 eV), which reflects its moderate air sensitivity. Note that for triphenylphosphine the phosphorus atom is incorporated in the HOMO surface (Chart 4) of the neutral compound; thus, what is a useful qualitative tool for predicting the stability of primary phosphines appears inapplicable for the tertiary counterparts. The threshold energy of the radical cation SOMO remains valid for the 29 primary, secondary, and tertiary phosphines 5340

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Figure 2. Plot of the SOMO energy of the radical cations for 1a
f. The line indicates an apparent “air stability” threshold value of
10 eV; phosphines above or on this line are found to be air-stable.

Chart 3. Calculated HOMO Distribution for the Primary Phosphines 3
6a

a

All are surprisingly air-stable with the exception of 4b, and all except 4b have a SOMO energy of
10 eV or less.

Figure 3. Plot of the radical cation SOMO energies for the air-stable primary phosphines whose stability has not yet been fully rationalized (4, 5, and 6). All of these phosphines have SOMO energies which lie above the threshold value of
10 eV; note that 4b is air-sensitive. Phosphine 2 is stabilized by steric encumbrance; the same argument was used to rationalize the air stability of 3. The highly air-sensitive 7
9 are also depicted, showing relatively stabilized radical cation SOMO energies.

examined (Supporting Information), in terms of its correlation with a given phosphine’s air stability.

The behavior of the highly air-sensitive secondary phosphine diethylphosphine and the related, readily oxidized tertiary phosphines 5341

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Chart 4. Calculated HOMO Distribution for the Highly Air-Sensitive Primary Phosphines 7
9, the Air-Sensitive Secondary Phosphine PPh2H, and the Air-Stable Tertiary Phosphine PPh3a

a For 7
9 and Ph2PH, the associated radical cations have a SOMO which is relatively stabilized (Table 1, Figures 3 and 4). Unlike the air-stable primary phosphines, the phosphorus atom of PPh3 is prominent in the HOMO, but the corresponding radical cation is destabilized beyond the apparent threshold limit (Table 1, Figure 4).

Figure 4. SOMO energies of the radical cations for PhPH2, Ph2PH, and Ph3P replicating the trend toward air stability. The radical cation energies of airsensitive secondary and tertiary phosphines are also plotted, as is the SOMO energy for oxidation resistant 10.

triethyl- and trimethylphosphine is accounted for (Table 1, Figure 4), as is the case for other primary, secondary, and tertiary phosphines of this type (see the Supporting Information). We can now return at this point to the work on the surprising air stability of many dialkylbiaryl phosphines by Buchwald;4 their stability was attributed to the inhibition of phosphorus lone pair rotation, as the result of a lone pair
aryl ring interaction. Whether the manifestation of this interaction is responsible for destabilizing the SOMO or not, one can see from Figure 4 that the radical cation SOMO for 10 again meets the
10 eV threshold seemingly required. The final column in Table 1 corresponds to the gap between the optimized energy of the neutral molecule and that of its radical cation and can be used to estimate the ionization energies of these phosphines; Shimizu7 has used this approach for phenols, and our values for 1f, trimethylphosphine, and PH3 correlate well with those reported in the literature (see the Supporting Information).

’ CONCLUSIONS The data allows us to draw key conclusions. (i) Steric hindrance is important in stabilizing an otherwise sensitive phosphine. (ii) Air stability can similarly be achieved by the incorporation of sufficient conjugation/heteroatom presence in the molecular

skeleton which, for the primary phosphines, is accompanied by a shift in the localization of the neutral molecular HOMO away from the phosphorus. (iii) The radical cations of the air-stable phosphines have a higher energy SOMO than their air-sensitive counterparts, with a calculated “threshold” value of
10 eV seemingly being significant. This may imply that a radical cation generated from a stabilized SOMO has sufficient reactivity to react with molecular oxygen and generate a peroxy radical which continues along the oxidative pathway to the phosphine oxide. (iv) The model universally predicted the air stability/sensitivity of primary, secondary, and tertiary phosphines.10 (v) The study will now be expanded to incorporate an even wider range of phosphine structures in order to better understand the oxidative phenomena and ascertain any limits of the model.

’ ASSOCIATED CONTENT

bS

Supporting Information. Tables, figures, and text giving the detailed calculation methodology, consistency and accuracy of the output values, relationship between ionization energy and the optimized energy gap, plots of HOMO
LUMO gaps for the neutral molecules, and HOMO
SOMO gaps of the neutral molecules

5342

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Organometallics and their associated radical cations. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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’ ACKNOWLEDGMENT We thank Newcastle University (B.S.) and an EPSRC Career Acceleration Fellowship (L.J.H.: EP/G005206/1) for funding and Dr. Ben Horrocks (NU), Professor Declan Gilheany (University College Dublin), Dr. Gordon Docherty (Rhodia), and Professor Warren Hehre (Wavefunction Inc.) for helpful discussions. ’ REFERENCES (1) (a) Brynda, M. Coord. Chem. Rev. 2005, 249, 2013–2034. (b) Katti, K. V.; Pillarsetty, N.; Raghuraman, K. New Vistas in Chemistry and Applications of Primary Phosphines. In Topics in Current Chemistry; Majoral, J.-P., Ed.; Springer-Verlag: Berlin, Heidelberg, Germany, 2003; Vol. 229, pp 121
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