Iron-57 NMR and Structural Study of - American Chemical Society

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Iron-57 NMR and Structural Study of [Fe(η5‑Cp)(SnPh3)(CO)(PR3)] (PR3 = Phosphine, Phosphite). Separation of Steric and Electronic σ and π Effects Richard M. Mampa, Manuel A. Fernandes, and Laurence Carlton* Molecular Science Institute, School of Chemistry, University of the Witwatersrand, Johannesburg 2050, Republic of South Africa S Supporting Information *

ABSTRACT: The complexes [Fe(Cp)(SnPh3)(CO)(PR3)] (PR3 = PMe3 (1), PnBu3 (2), PCy3 (3), PMe2Ph (4), PMePh2 (5), P(CH2Ph)3 (6), PPh3 (7), P(4-MeC6H4)3 (8), P(4-MeOC6H4)3 (9), P(4FC6H4)3 (10), P(4-CF3C6H4)3 (11), P(NMe2)3 (12), P(OMe)3 (13), P(OPh)3 (14)), which have been characterized by X-ray crystallography (except for 1 and 4), infrared spectroscopy (carbonyl stretching frequency, νCO), and NMR spectroscopy (13C, 31P, 57Fe, 119Sn) offer some insight into the response of the iron nucleus to changes in the electronic and steric properties of the PR3 ligand. A fairly good correlation is found between the 57Fe chemical shift and the Tolman cone angle θ for PR3 and a rather poorer correlation between δ(57Fe) and νCO. However, for the subseries of complexes 7−11 having PR3 = P(4-XC6H4)3 (X = H, Me, MeO, F, CF3), the correlation between δ(57Fe) and νCO is very good. Since the steric properties of these ligands, from the point of view of the metal, are identical (θ = 145°), this provides a means of separating the steric and electronic contributions of PR3 to δ(57Fe). The electronic contribution of PR3 to δ(57Fe) can be further separated into σ and π components by making use of the finding that the π component of the Fe−P bond has a negligible influence on δ(57Fe), unlike its influence on νCO. The ligands PMe3, PnBu3, PCy3, PMe2Ph, PMePh2, and P(NMe2)3 are found to be “pure” σ donors, P(OMe)3 and P(OPh)3 are found to be π acceptors of differing strength, and P(4-XC6H4)3 is found to show weak but clearly distinguishable π acceptor properties.

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commented on the methods of Drago28 and Suresh.29 Perspectives are given by Song and Alyea,30 Kühl,31 and York.32 Attempts to distinguish experimentally between the σ and π components of the M−P bond would, at first sight, appear to require a parameter that reflects only σ effects (neither pKa nor Kabachnik’s parameter is regarded as being entirely satisfactory27c,f,30b) or a parameter that, unlike the CO stretching frequency or the redox potential, has a distinguishable dependence on σ and π effects. It has been shown by Alyea and Song,33 using 95Mo NMR, that the transition-metal chemical shift can satisfy this requirement. In a number of studies phosphine electronic and steric effects have been distinguished and quantified but either without the separation of electronic σ and π effects34 or with measurement only of the electronic σ effect.35 The complete separation and quantification of steric, σ, π, and pendent group effects have been demonstrated by Prock, Giering, and co-workers using QALE, which avoids the need for a pure σ parameter.27,36 By means of a graphical method27c QALE can, in principle, distinguish between pure σ and σ donor/π acceptor ligands. While the differing properties of phosphines and phosphites emerge clearly from such a treatment, for ligands that are possible weak π acceptors (such as PPh3) the situation is less clear. The authors point to the fact that data for complexes [Fe(Cp)(COMe)(CO)(PR3)] (PR3 = P(4-XC6H4)3) lie on a welldefined gradient predicted for pure σ donors and consequently describe P(4-XC6H4)3 as having “...no appreciable π acidity”.27e

INTRODUCTION The relative influences of ligand electronic and steric effects on the reactivity of transition-metal complexes are of foremost importance in the design and optimization of homogeneous catalysts for a wide range of reactions.1 The ligands that have proved to be most amenable to electronic and steric modification and are most commonly used are tertiary phosphines and phosphites, but increasingly N-heterocyclic carbenes are finding a role as their electron-donating ability extends beyond the range accessible to phosphines.2 Measures of steric properties of a tertiary phosphine ligand in a metal complex include the widely used Tolman cone angle θ3 and more elaborate variants (ligand profile,4 cone angle radial profile,5 and solid angle6), Brown’s ligand repulsive energy,7 the accessible molecular surface (AMS) of Leitner and co-workers,8 Suresh’s molecular electrostatic potential,9 Nolan’s percentage buried volume,10 an NMR method,11 and averaged values of Xray data obtained from searches of the Cambridge Structural Database.12 Electronic effects have been quantified using infrared spectroscopy (CO stretching frequency and Tolman’s electronic parameter χ3b,13), pKa (of HPX3+),14 Kabachnik’s parameter,15 electrochemical,16 kinetic,17 and thermochemical18 measures, photoelectron spectroscopy,19 NMR spectroscopy,20 and computational methods.21 The nature of the metal− phosphorus bond has been analyzed in detail by computational22 (including Suresh’s molecular electrostatic potential21b,23), structural,24 and a combined structural−computational approach25 and by the covalent/ionic method of Drago26 and the more widely applicable quantitative analysis of ligand effects (QALE) of Prock and Giering,27 who have also © XXXX American Chemical Society

Received: November 28, 2013

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dx.doi.org/10.1021/om4011593 | Organometallics XXXX, XXX, XXX−XXX

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However, this result, by itself, need not preclude a role for π acceptance which, while weak, may remain fairly constant while the effects of varying X contribute largely to a change in σ donor ability. A number of studies find a significant π contribution to the bonding of P(4-XC6H4)3 to a metal.22g,h,24a In the present study of the steric and electronic influences of PR3 (phosphine, phosphite) ligands on structural and NMR parameters we have prepared a series of complexes [Fe(η5Cp)(SnPh3)(CO)(PR3)] that are sterically crowded, which have a metal and coordination sphere that are amenable to study by NMR spectroscopy (13C, 31P, 57Fe, 119Sn) and contain a CO ligand that can provide infrared data indicating the relative level of electron density on the transition metal. In addition to an examination of structural parameters, in particular the Fe−P bond length, we have attempted to separate the steric and electronic influences of PR3 on the 57Fe chemical shift and to distinguish between the electronic σ and π effects.

Cp)(SnPh3)(PR3)2] was also formed and separated from the desired product during crystallization. Of the series of complexes studied, X-ray structures (Figures 1−12) were found for all except the PMe3 and PMe2Ph derivatives, for which suitable crystals could not be grown. Crystal and structure refinement data are given in the Supporting Information and selected bond lengths and angles in Table 1. In all 12 structures the geometry at iron can best be described as distorted octahedral, with the Cp ring occupying three coordination sites, in the well-known “piano stool” geometry. The largest angles are those between the Cp centroid and the P, Sn, and C(O) substituents on iron. All are greater than 115° with Cp−Fe−C(O) > Cp−Fe−P > Cp−Fe−Sn (except for 2 and 3, where Cp−Fe−P is the largest angle). The relative sizes of the angles between the “legs” (CO, PR3, SnPh3) are P−Fe−Sn > P−Fe−C(O) > Sn−Fe−C(O) (except for 13 and 14), with the greatest degree of variation being found for P−Fe−Sn, a range of 11.4° (from 89.5 to 100.9°). Variations in P−Fe−C(O) (range 5.0°) and Sn−Fe−C(O) (range 5.6°) are significantly lower. Measurement of Steric Size of PR3 When Bound to a Metal Atom. The size of a PR3 ligand in a metal complex has been estimated in a variety of ways, the most well-known of which is Tolman’s cone angle (θ),3 in which a cone, with its origin at a metal atom positioned at a distance of 2.28 Å from the phosphorus, is drawn so as to enclose the substituents on the phosphorus. The concept has been further developed by Ferguson and co-workers4a,b in the form of the ligand profile, which takes account of the interleaving of substituents of adjacent ligands in sterically crowded environments in which bulky phosphine ligands “...do not behave as regular solid cones but are better described as irregular conic cogs.” A study by Müller and Mingos,12 based on a survey of the Cambridge Structural Database, showed that, for a sample of 1507 structures of metal complexes containing PPh3, values of the cone angle (θ) for PPh3 (calculated using a method based on that of Tolman) varied from 135 to 163°, with a mean value of 148.2° (standard deviation 4.9). In the same study complexes of other phosphines showed a similar variation in θ, with mean values (with the exception of that for PCy3, for which the

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RESULTS AND DISCUSSION The complexes [Fe(η5-Cp)(SnPh3)(CO)(PR3)] (Scheme 1) were obtained in moderate to good yield as crystals or Scheme 1a

a

PR3 = PMe3 (1), PnBu3 (2), PCy3 (3), PMe2Ph (4), PMePh2 (5), P(CH2Ph)3 (6), PPh3 (7), P(4-MeC6H4)3 (8), P(4-MeOC6H4)3 (9), P(4-FC6H4)3 (10), P(4-CF3C6H4)3 (11), P(NMe2)3 (12), P(OMe)3 (13), P(OPh)3 (14).

crystalline powders (except for 4 which gave an oil), varying in color from yellow to red, by photochemical reaction of [Fe(η5Cp)(SnPh3)(CO)2] with PR3 in toluene. With the phosphines PMe3 and PMePh2 and phosphites P(OMe)3 and P(OPh)3 (all of fairly low steric size) the disubstituted product [Fe(η5-

Figure 1. Molecular structure of [Fe(Cp)(SnPh3)(CO)(PnBu3)] (2). Thermal ellipsoids are shown at the 30% probability level. B


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Figure 2. Molecular structure of [Fe(Cp)(SnPh3)(CO)(PCy3)] (3). Thermal ellipsoids are shown at the 30% probability level.

Figure 3. Molecular structure of [Fe(Cp)(SnPh3)(CO)(PMePh2)] (5). Thermal ellipsoids are shown at the 30% probability level.

phosphine and phosphite electronic properties in the form of the Tolman electronic factor.3b,13 The CO stretching frequency decreases as the σ-donor ability of the coligand (PR3) increases and increases as the π-acceptor ability of the coligand increases and therefore, in the absence of other evidence, the value of νCO does not permit a distinction between the effects of diminished σ donation and increased π acceptance (or vice versa). Values of νCO for 1−14 are given in Table 2. Iron−Phosphorus Bond Lengths. The iron−phosphorus bond lengths for complexes 2, 3, and 5−14 are shown plotted against the carbonyl stretching frequencies (νCO) in Figure 13 and against the Tolman cone angles θ for PR3 in Figure 14. The general trend is for an increase in d(Fe−P) to be associated with a decrease in νCO and an increase in θ. The shortest Fe−P bonds are found for the complexes of the phosphites 13 and 14, where a significant π contribution to the bond can be expected; the longest Fe−P bond is found for the complex of the sterically demanding PCy3 (3). On closer inspection of Figures 13 and 14, however, features are found which depart from these general trends. In Figure 13 the PnBu3 complex 2 is found to

sample size was small) close to the values given by Tolman. The values of cone angles calculated from crystallographic data using the ligand profile method show a variation (a range of 20−30°) similar to those of Mingos, as do values obtained using the solid angle method, which, however, gives values of θ significantly lower than those of Tolman. The iron-57 chemical shifts are measured from samples in solution. It is not unreasonable to suppose that the cone angle θ of the PR3 ligand in a dissolved complex takes an average value, with conformations rapidly changing during molecular tumbling in solution, that is approximated by the average of the values obtained from solid-state data. The averaged solid-state data are in generally good agreement with the Tolman values, and it is on this basis that the Tolman values (Table 2) are used throughout the following discussion. Measurement of Relative Electron-Donating Ability of PR3. The infrared stretching frequency of a carbonyl ligand (νCO) has been widely used as a relative measure of the electron-donating properties of a coligand in a transition-metal complex and has found specific application in the assessment of C


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Figure 4. Molecular structure of [Fe(Cp)(SnPh3)(CO)(P(CH2Ph)3)] (6). Thermal ellipsoids are shown at the 30% probability level. Two cocrystallized dichloromethanes are omitted.

Figure 5. Molecular structure of [Fe(Cp)(SnPh3)(CO)(PPh3)] (7). Thermal ellipsoids are shown at the 30% probability level.

have a significantly shorter d(Fe−P) than is found for complexes 5−10 and 12, although sterically it is only slightly smaller than 5 and electronically (νCO) it does not differ from 5−10 and 12 (all eight complexes have νCO values in the range 1900−1908 cm−1). Complexes 5−10 and 12 form a cluster lying within a narrow range of bond lengths (2.196−2.206 Å) and νCO in Figure 13, while the same complexes are found in Figure 14 to occupy a fairly wide range of θ (from 136° (PMePh2) to 165° (P(CH2Ph)3)) for the same narrow range of d(Fe−P), indicating that here the Fe−P bond length is largely independent of the steric property (θ) of PR3. In contrast to this, a number of studies relating metal−phosphorus bond length to the cone angle of the PR3 ligand have found a moderate to good correlation between bond length and θ for complexes of cobalt,37 molybdenum,38 and ruthenium.39 The decrease in d(Fe−P) of 0.015 Å on going from 7 (PPh3) to 11 (P(4-CF3C6H4)3) almost exactly matches that found by

Prock, Giering, and co-workers for the same ligands in the complexes [Fe(Cp)(COMe)(CO)(PR3)], which is attributed to a slight contraction of the phosphorus covalent radius as the substituents on phosphorus become more electronegative.40 For the complexes [Fe(Cp)(COMe)(CO)(PR3)] it was found that, although Fe−P bond lengths for the phosphite complexes are significantly shorter than for the phosphine complexes, indicating the contribution of π acceptance in the former, the correlation between Fe−P bond length and either Tolman χ or θ was poor.40 The authors draw the conclusion that in the absence of π bonding and steric encumbrance the length of the M−P bond is essentially constant and independent of the electron donor capacity of the ligand. From the foregoing evidence it would seem clear that the use of metal−phosphorus bond lengths in the analysis of ligand stereoelectronic properties should be approached with caution. D


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Figure 6. Molecular structure of [Fe(Cp)(SnPh3)(CO)(P(4-MeC6H4)3)] (8). Thermal ellipsoids are shown at the 30% probability level.

Figure 7. Molecular structure of [Fe(Cp)(SnPh3)(CO)(P(4-MeOC6H4)3)] (9). Thermal ellipsoids are shown at the 30% probability level. Two cocrystallized dichloromethanes are omitted.

(P(NMe2)3, θ = 157°). While the differences are not large, it is clear that there is no increase in the P−Fe−Sn bond angle accompanying the increase in cone angle of the phosphines. The increased volume of the phosphine is evidently taken up elsewhere (see below). A prominent feature of Figure 15 is the variation in P−Fe−Sn bond angle that is found as X is changed in the complexes of P(4-XC6H4) (X = H (7), Me (8), OMe (9), F (10), CF3 (11), extending from P−Fe−Sn = 96.501(17)° (R = OMe) to 100.95(2)° (R = H). Although the cone angles are all regarded as being equal (145°), packing forces in the solid state will vary according to the substituent X, exerting an influence on the P−Fe−Sn angle amounting to 4.45° as X is varied. This is almost 40% of the full range (11.40°) of variation in P−Fe−Sn. If this is the magnitude of the contribution of packing forces to the observed geometry, then a discussion of the influences of ligand electronic and steric effects on the geometry can, at best, be only approximate.

Bond Angles. In Figure 13 the cluster of data points in the region 2.20 (±0.01) Å and 1900−1910 cm−1 (complexes 5−10 and 12, having PR3 = PMePh2, P(CH2Ph)3, P(4-RC6H4)3 (R = H, Me, MeO, F), P(NMe2)3), with the PnBu3 complex (2) having a similar νCO value (1901 cm−1) but shorter Fe−P distance, can be viewed as consisting of complexes having phosphines with similar electronic properties. In a plot of P− Fe−Sn bond angle against cone angle θ (Figure 15) four of these complexes (PR3 = PnBu3 (2), P(4-MeC6H4)3 (8), P(4FC6H4)3 (10), P(NMe2)3 (12)), their cone angles extending from 132 to 157°, have values of P−Fe−Sn that fall within a range of 0.87°. The cone angle of PR3 increases with very little change in the Fe−P bond length (Figure 14) or the electronic properties of the phosphine (νCO, Figure 13), and yet the P− Fe−Sn bond angle (Figure 15) actually decreases slightly: 99.34(5)° (PnBu3, θ = 132°) > 98.67(2)° (P(4-MeC6H4)3, θ = 145°), 98.56(3)° (P(4-FC6H4)3, θ = 145°) > 98.47(2)° E


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Figure 8. Molecular structure of [Fe(Cp)(SnPh3)(CO)(P(4-FC6H4)3)] (10). Thermal ellipsoids are shown at the 30% probability level.

Figure 9. Molecular structure of [Fe(Cp)(SnPh3)(CO)(P(4-CF3C6H4)3)] (11). Thermal ellipsoids are shown at the 30% probability level.

Nevertheless, one or two structural trends are worthy of comment. The major geometrical changes which occur in the complex as the phosphine is exchanged in the sequence PnBu3 (2), P(4MeC6H4)3 (8), P(4-FC6H4)3 (9), P(NMe2)3 (12), for which the P−Fe−Sn angle remains almost constant (Figure 15), are an increase in the P−Fe−C(O) bond angle from 91.33(18)° (2) to 96.10(6)° (12) (the full observed range of change of P− Fe−C(O)) and a decrease in the Sn−Fe−C(O) bond angle (from 89.31(16)° to 85.70(6)°) on going from the PnBu3 complex (2) to the P(NMe2)2 complex (12), as the cone angle θ of PR3 is increased. Accompanying these changes is a progressive change in the Cp−Fe−P bond angle, which for the P(OMe)3 (13) complex (θ = 107°) has a value of 127.44(3)°, decreasing as θ increases to a minimum (at a cone angle θ = 145°) and then increasing, with increasing θ, to a maximum of 128.73(5)° for the PCy3 complex (3) (Figure 16). In Figures 15 and 16 the range of variation arising from the differing substituents on the phosphines in complexes 7−11 can be

taken as clear evidence of the relative contribution of packing forces to the observed differences in structural parameters for all of the complexes studied. From these findings it would appear that the response of the complex to changes in ligand size is not necessarily smooth and continuous but may occur in steps, with one mode of change (increasing bond angle) being superseded by another as the ligand reaches a certain size. Influences on the 57Fe Chemical Shift. The 57Fe chemical shifts (Table 3) of the 14 complexes under study extend across a range of 542 ppm, demonstrating the high sensitivity of the iron nucleus to its chemical environment. Iron-57 NMR studies of a variety of organoiron complexes have been reported by von Philipsborn,41 Benn,42 and Wrackmeyer,43 all of whom comment on the sensitivity of the 57Fe chemical shift to steric and electronic effects. More generally, for a transition-metal nucleus, influences on the chemical shift that arise from a ligand such as PR3 can be rationalized as follows. The influences are of two types: (i) σ donor/π acceptor F


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Figure 10. Molecular structure of [Fe(Cp)(SnPh3)(CO)(P(NMe2)3)] (12). Thermal ellipsoids are shown at the 30% probability level.

Figure 11. Molecular structure of [Fe(Cp)(SnPh3)(CO)(P(OMe)3)] (13). Thermal ellipsoids are shown at the 30% probability level.

14). A more advanced treatment of transition-metal chemical shifts can be found in ref 45. In the discussion below, the influences on the iron chemical shift that will be examined are steric (leading to a realignment of bonding orbitals and consequent change in ΔE), quantified in terms of the Tolman cone angle θ for PR3, and electronic, represented by the carbonyl ligand CO stretching frequency, νCO. The possible influence on the iron chemical shift of local magnetic fields generated by ring currents in phenyl groups (where PR3 has aryl substituents) was calculated, using dimensions from structures, by means of the method of Pople.46 Ring current effects were found to influence the magnetic field at iron to an extent of less than 1 ppm. Iron Chemical Shift and Steric and Electronic Parameters. The relationship between the electronic properties of PR3, as quantified by the value of νCO, for complexes 1− 14 and the iron chemical shift are shown in Figure 17. A general trend is found from high νCO and low δ(57Fe) to low

effects and (ii) hard/soft (polarizability) effects, the former influencing the separation (ΔE) between occupied and unoccupied orbitals on the metal and the latter influencing the average d orbital radius (⟨rd⟩). In a simplified form,44 applicable when the oxidation state and coordination geometry are effectively constant, the chemical shift of the metal can be represented as δM ∝ ΔE−1 (⟨rd−3⟩). It follows that ligands that are good σ donors and/or poor π acceptors will increase the chemical shift of the metal by decreasing ΔE (the filled orbitals repel each other and their energy will rise) and soft/polarizable ligands will decrease the chemical shift of the metal by increasing ⟨rd⟩. For a complex of a ligand such as PCy3, which exhibits both properties, there will be two opposing influences on the chemical shift, the dominant effect being the lowering of ΔE and the consequent increase in δM (the PCy3 complex (9) has the highest δ(57Fe) value of the series). A complex of P(OPh)3 (good π acceptor, low polarizability, leading to high ΔE and low ⟨rd⟩) will have a low δM value (as seen for complex G


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Figure 12. Molecular structure of [Fe(Cp)(SnPh3)(CO)(P(OPh)3)] (14). Thermal ellipsoids are shown at the 30% probability level. A cocrystallized toluene is omitted.

Table 1. Selected Bond Lengths (Å) and Angles (°) for Complexes [Fe(Cp)(SnPh3)(CO)(PR3)] (2, 3, 5−14) 2

3

5

PR3 Fe−P Fe−Sn Fe−C(O) C−O P−Fe−Sn P−Fe−C(O) Sn−Fe−C(O) Cp−Fe−P Cp−Fe−Sn Cp−Fe−C(O)

PnBu3 2.1678(17) 2.5330(7) 1.745(5) 1.146(6) 99.34(5) 91.33(18) 89.31(16) 125.50(6) 117.64(3) 125.2(2) 9

PCy3 2.2681(14) 2.5569(9) 1.729(6) 1.158(6) 99.80(4) 93.15(16) 86.87(16) 128.73(5) 115.21(4) 123.5(2) 10

PMePh2 2.1957(8) 2.5458(5) 1.738(3) 1.151(3) 94.13(2) 91.73(9) 89.63(9) 126.77(3) 116.20(2) 128.34(9) 11

PR3 Fe−P Fe−Sn Fe−C(O) C−O P−Fe−Sn P−Fe−C(O) Sn−Fe−C(O) Cp−Fe−P Cp−Fe−Sn Cp−Fe−C(O)

P(4-MeOC6H4)3 2.1962(6) 2.5403(3) 1.732(2) 1.160(3) 96.501(17) 94.09(8) 88.72(7) 123.58(2) 118.78(1) 126.48(8)

P(4-FC6H4)3 2.1988(8) 2.5518(5) 1.727(3) 1.166(3) 98.56(3) 95.51(10) 87.48(11) 123.49(3) 117.02(2) 127.41(11)

6

P(4-CF3C6H4)3 2.1831(13) 2.5417(7) 1.732(5) 1.157(6) 100.37(4) 92.74(15) 84.57(15) 124.80(5) 117.35(3) 127.56(17)

7

P(CH2Ph)3 2.2060(7) 2.5347(4) 1.728(3) 1.165(3) 97.76(2) 91.30(8) 83.98(8) 125.60(2) 119.66(2) 127.91(8) 12 P(NMe2)3 2.2034(9) 2.5299(7) 1.7215(16) 1.158(2) 98.47(2) 96.10(6) 85.70(6) 124.50(2) 117.53(2) 125.50(5)

PPh3 2.1981(5) 2.5557(3) 1.735(2) 1.157(2) 100.95(2) 93.67(6) 87.64(7) 123.05(2) 116.62(1) 127.02(7) 13 P(OMe)3 2.1204(8) 2.5236(4) 1.745(3) 1.142(4) 89.55(2) 91.07(10) 88.12(11) 127.44(3) 118.82(2) 129.67(11)

8 P(4-MeC6H4)3 2.2032(8) 2.5532(5) 1.730(3) 1.153(3) 98.67(2) 94.79(9) 86.90(9) 124.34(3) 116.70(2) 126.66(10) 14 P(OPh)3 2.1079(8) 2.5487(5) 1.739(4) 1.147(4) 91.70(2) 96.11(11) 86.08(11) 125.82(3) 119.79(2) 126.37(11)

increasing σ donor ability, to the PCy3 complex (strong σ donor). Figure 18 shows a plot of δ(57Fe) against Tolman θ for 1− 14. The plot shows a good (with the exception of the data point for 6, discussed below) but in some ways deceptive correlation in which δ(57Fe) increases with increasing θ of PR3. The role played by electronic effects can be clearly seen for complexes 7−11 (P(4-XC6H4)3), in which θ has a constant value (in solution, at least) of 145°. Here δ(57Fe) increases from 541 ppm (X = CF3) to 673 ppm (X = OMe). The role of electronic effects is less clearly seen for the phosphite

νCO and high δ(57Fe), with a cluster of data points in the middle region with νCO in the range 1900−1910 cm−1, where there appears to be little correlation. On closer inspection the data points for complexes of P(4-XC6H4)3 (7−11) can be seen to display, to a good approximation, a linear relationship between νCO and δ(57Fe), which is examined in detail below. With the exception of data points for complexes 1,2, and 4−6 (PR3 = PMe3, PnBu3, PMe2Ph, PMePh2, P(CH2Ph)3), for which there is no clear relationship between νCO and δ(57Fe), Figure 17 shows a clear trend extending from the phosphite complexes (good π acceptors) via the P(4-XC6H4) complexes in order of H


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Table 2. Infrared (CO Stretching Frequency) and Steric Data (Cone Angle of PR3) for Complexes [Fe(η5Cp)(SnPh3)(CO)(PR3)] (1−14) 1 PMe3 1903 118

PR3 νCO (cm−1)a θ (deg)b 8 PR3 νCO (cm−1) θ (deg) a

P(4-MeC6H4)3 1904 145

2 n

P Bu3 1901 132 9 P(4-MeOC6H4)3 1903 145

3

4

5

6

7

PCy3 1894 170

PMe2Ph 1903 122

PMePh2 1907 136

P(CH2Ph)3 1908 165 13

PPh3 1904 145 14

10

11

12

P(4-FC6H4)3 1908 145

P(4-CF3C6H4)3 1914 145

P(NMe2)3 1900 157

P(OMe)3 1922 107

P(OPh)3 1936 128

Recorded from chloroform solution. bTolman cone angle.3b

Figure 13. Plot of νCO against Fe−P bond length for [Fe(Cp)(SnPh3)(CO)(PR3)] (PR3 = PnBu3 (2), PCy3 (3), PMePh2 (5), P(CH2Ph)3 (6), PPh3 (7), P(4-MeC6H4)3 (8), P(4-MeOC6H4)3 (9), P(4-FC6H4)3 (10), P(4-CF3C6H4)3 (11), P(NMe2)3 (12), P(OMe)3 (13), P(OPh)3 (14)).

Figure 15. Plot of PR3 cone angle θ against P−Fe−Sn bond angle for [Fe(Cp)(SnPh3)(CO)(PR3)] (PR3 = PnBu3 (2), PCy3 (3), PMePh2 (5), P(CH2Ph)3 (6), PPh3 (7), P(4-MeC6H4)3 (8), P(4-MeOC6H4)3 (9), P(4-FC6H4)3 (10), P(4-CF3C6H4)3 (11), P(NMe2)3 (12), P(OMe)3 (13), P(OPh)3 (14)).

Figure 14. Plot of PR3 cone angle θ against Fe−P bond length for [Fe(Cp)(SnPh3)(CO)(PR3)] (PR3 = PnBu3 (2), PCy3 (3), PMePh2 (5), P(CH2Ph)3 (6), PPh3 (7), P(4-MeC6H4)3 (8), P(4-MeOC6H4)3 (9), P(4-FC6H4)3 (10), P(4-CF3C6H4)3 (11), P(NMe2)3 (12), P(OMe)3 (13), P(OPh)3 (14)).

Figure 16. Plot of PR3 cone angle θ against Cp−Fe-P bond angle for [Fe(Cp)(SnPh3)(CO)(PR3)] (PR3 = PnBu3 (2), PCy3 (3), PMePh2 (5), P(CH2Ph)3 (6), PPh3 (7), P(4-MeC6H4)3 (8), P(4-MeOC6H4)3 (9), P(4-FC6H4)3 (10), P(4-CF3C6H4)3 (11), P(NMe2)3 (12), P(OMe)3 (13), P(OPh)3 (14)).

complexes. P(OPh)3 has a cone angle only 10° larger than that of PMe3, and the 57Fe chemical shifts of their complexes are almost identical, but in terms of electronic properties the ligands differ considerably, with complex 1 (PMe3) having a νCO value of 1903 cm−1 and complex 14 (P(OPh)3) a νCO value

of 1936 cm−1. A possible reason for the lack of impact of this electronic difference on the iron chemical shift is discussed below. Also in Figure 18 one outlying point, arising from the P(CH2Ph)3 complex (6), appears a distance of ∼−250 pm in I


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Table 3. NMR Data for Complexes [Fe(Cp)(SnPh3)(CO)(PR3)] complexa

R

δ(13C)b

δ(31P)c

δ(57Fe)d

δ(119Sn)e

J(31P,13C)f

J(57Fe,31P)f

J(119Sn,31P)f

1 2 3 4 5 6 7 8 9 10 11 12 13 14

PMe3 PnBu3 PCy3 PMe2Ph PMePh2 P(CH2Ph)3 PPh3 P(4-MeC6H4)3 P(4-MeOC6H4)3 P(4-FC6H4)3 P(4-CF3C6H4)3 P(NMe2)3 P(OMe)3 P(OPh)3

218.92 219.82 221.96 219.24 219.29 219.73 220.50 220.70 220.74 220.23 219.74 222.09 217.47 216.86

29.16 50.55 74.18 40.24 56.81 58.49 75.34 72.63 70.15 74.63 79.74 171.56 185.41 171.11

447 536 858 492 513 555 651 661 673 613 541 706 316 454

46.71 32.75 6.05 44.12 34.77 24.98 9.29 10.36 9.81 10.30 14.08 17.79 47.76 46.65

30.0 28.8 27.0 29.1 29.1 28.0 28.6 28.8 29.3 29.2 27.8 38.7 41.5 40.6

53 53 54 53 54 56 56 56 55 57 59 73 100 102

428 391 330 409 387 365 369 373 378 366 353 461 534 496

a Solution (∼0.02 M) in CDCl3 at 300 K. bChemical shifts in ppm from zero defined using CDCl3 at 77.00 ppm (TMS 0 ppm). cChemical shifts in ppm from a frequency defined by 85% H3PO4 (external standard). dChemical shifts in ppm from a frequency defined by Fe(CO)5 (neat liquid), equivalent to an absolute frequency Ξ(57Fe) = 3.237797 MHz (in a field in which the protons of TMS resonate at exactly 100 MHz). eChemical shift in ppm from a frequency defined by SnMe4 (external standard). fCoupling constants (absolute magnitude) in Hz. Signs are probably as follows: 2 31 13 J( P, C)cis positive (as for [Fe(CO)5‑x(PF3)x]47), 1J(57Fe,31P) positive (as for [Fe(Cp)(H)(PR3)2]42c), and 2J(119Sn,31P)cis positive (as for cis[Pt(SnPh3)2(diphos)] and related compounds48 and for [Rh(NCBPh3)(H)(SnPh3)(PPh3)2(py)]49).

Figure 17. Plot of νCO against 57Fe chemical shift for [Fe(Cp)(SnPh3)(CO)(PR3)] (PR3 = PMe3 (1), PnBu3 (2), PCy3 (3), PMe2Ph (4), PMePh2 (5), P(CH2Ph)3 (6), PPh3 (7), P(4-MeC6H4)3 (8), P(4MeOC6H4)3 (9), P(4-FC6H4)3 (10), P(4-CF3C6H4)3 (11), P(NMe2)3 (12), P(OMe)3 (13), P(OPh)3 (14)).

Figure 18. Plot of PR3 cone angle θ against 57Fe chemical shift for [Fe(Cp)(SnPh3)(CO)(PR3)] (PR3 = PMe3 (1), PnBu3 (2), PCy3 (3), PMe2Ph (4), PMePh2 (5), P(CH2Ph)3 (6), PPh3 (7), P(4-MeC6H4)3 (8), P(4-MeOC6H4)3 (9), P(4-FC6H4)3 (10), P(4-CF3C6H4)3 (11), P(NMe2)3 (12), P(OMe)3 (13), P(OPh)3 (14)).

the 57Fe chemical shift dimension and ∼+25° in the cone angle dimension from the position which it might be expected to occupy, in terms of the observed trend of the plot. A cone angle of 137° is given for the P(CH2Ph)3 ligand by a solid angle calculation50 on the data for 6. Although calculated values for phosphines in other complexes were similarly low, we find evidence from 13C and 57Fe data (see below) that this value (θ = 137°) may more accurately reflect the steric properties of P(CH2Ph)3 in 6 than Tolman’s value of 165°. A precedent for the observed anomalous data point for the P(CH2Ph)3 complex (6) can be found in the results of the 103Rh NMR study of complexes [Rh(Cl)(cod)(PR3)] reported by Elsevier and coworkers.34d If 103Rh chemical shifts from this work are plotted against cone angle θ of PR3, then the data point for the tribenzylphosphine complex is found to be located ∼120 ppm to lower chemical shift of the position at which it might be expected according to the trend of the plot. In a different context another complex of P(CH2Ph)3 has been found to

display anomalous behavior.34c For these reasons complex 6 is omitted from a more detailed examination of the results of the present study. Separation of Steric and Electronic Effects. The separation of electronic and steric influences on the iron chemical shift requires that, for a number of complexes, one of the influences remains constant while the other is varied. It can readily be seen that the subseries of complexes with ligands P(4-XC6H4)3 (7−11) satisfies this requirement. The cone angle θ of the phosphine remains constant at 145°, while the electronic properties of the complexes vary according to the substituent X. A plot of iron chemical shift against the observed stretching frequency νCO for these complexes (Figure 19a) is linear, with νCO decreasing and δ(57Fe) increasing as the substituent X becomes more strongly electron donating. On going from X = CF3 to X = OMe the stretching frequency decreases from 1914 to 1903 cm−1 and the chemical shift increases by 132 ppm. The relationship between δ(57Fe) and J


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(here 1900 cm−1). A plot of δ(57Fe)st against cone angle θ of PR3 for each complex (Figure 19b) shows the dependence of the iron chemical shift on steric effects alone. In Figure 19b data points for the complexes 1−5 and 12 lie, to a good approximation, on a straight line having a gradient from which the relationship 5.9(±0.3) ppm per degree of θ can be obtained. The iron chemical shift increases as the cone angle of PR3 increases. Data points which lie a significant distance from this line are those corresponding to P(4-XC6H4)3, P(OMe)3, and P(OPh)3, for which the displacements from the gradient (in ppm of δ(57Fe)) are 67, 161, and 342, respectively. The data points that define the line arise from complexes of ligands PR3 that are considered to be either “pure” σ donors or very weak π acceptors (PR3 = PMe3 (1), PnBu3 (2), PCy3 (3), PMe2Ph (4), PMePh2 (5), P(NMe2)3 (12)). The discrepancies between the data points for complexes of P(4-XC6H4)3 (7−11), P(OMe)3 (13), and P(OPh)3 (14) and the gradient would appear to arise from π effects. The phosphites are generally regarded as being π acceptor ligands, while the ligands P(4-XC6H4)3 have been described as either pure σ donors27c (a term taken by some authors to imply a contribution of up to 20% π acceptance22d) or weak π acceptors.22h In order to ensure that the gradient obtained using eq 1 and Figure 19b is attributable to σ donor effects alone (the gradient in Figure 19a was obtained using data from complexes 7−11 with PR3 ligands (P(4-XC6H4)3) now regarded as likely weak π acceptors), data from complexes having the “pure” σ donor ligands PMe3 (1), PnBu3 (2), PMe2Ph (4), and P(NMe2)3 (12) were used in a plot (see Supporting Information) of δ(57Fe)obs against θ. The four complexes have νCO values lying in the narrow range of 1900−1903 cm−1, and so electronic differences between their Fe−P bonds will be small. From the plot a value of 6.4 ppm/deg is obtained, which considering that electronic differences, although small, have not been taken into account is a good match for the value of 5.9 ppm/deg obtained above. In order to place the infrared data on a broader footing (data from the sterically crowded complexes 1−14 might contain possible steric influences), plots (see Supporting Information) of δ(57Fe)obs for 7−10 against Tolman νCO for P(4-XC6H4)33b and of δ(57Fe)obs against νCO of the less sterically crowded [Fe(Cp)(COMe)(CO){P(4-RC6H4)3}]27c (R = H, Me, OMe, F, CF3) give, to within 5%, the same gradient as that found in Figure 19a. Separation of σ and π Effects. In a broad sense information about the presence or absence of π effects in a metal−phosphorus bond can be obtained from CO stretching frequencies and from metal−phosphorus bond lengths (the former higher and the latter shorter where a significant π component is involved), but neither method can distinguish between pure σ donors and weakly π accepting σ donors and, with bond length information, anomalous data (as found for 2) can further obscure the issue. The quantitative analysis of ligand effects (QALE) makes use of a combination of parameters, including CO stretching frequencies, pKa values (for HPR3+), reduction potentials, and enthalpies of reduction in the examination of the M−P bond in complexes such as [Fe(Cp)(COMe)(CO)(PZ3)]27c,e and identifies the four components θ (steric), σ, π, and Ear (aryl or pendent group effect), which have been quantitatively separated. Transitionmetal NMR (95Mo) has been used by Alyea and Song33a in conjunction with Kabachnik’s parameter to distinguish between PCl3 and PF3, among a series of related ligands, in complexes fac-[Mo(CO)3L3]. The use of a two (or three36)-dimensional

Figure 19. Plots of (a) νCO against 57Fe chemical shift for [Fe(Cp)(SnPh3)(CO)(P(4-XC6H4)3)] (X = H (7), Me (8), MeO (9), F (10), CF3 (11)) and (b) PR3 cone angle θ against δ(57Fe)st (see text) for [Fe(Cp)(SnPh3)(CO)(PR3)] (PR3 = PMe3 (1), PnBu3 (2), PCy3 (3), PMe2Ph (4), PMePh2 (5), 7−11 as for (a), P(NMe2)3 (12), P(OMe)3 (13), P(OPh)3 (14)).

νCO, obtained from the gradient, is −11.9(±0.6) ppm per wavenumber. With the assumption that this conversion factor is valid across the full range of stretching frequencies and chemical shifts, it is then possible to compensate for differences in the electronic influence of PR3 on the chemical shifts of the complexes under study and to examine the steric influence in isolation from the electronic influence. This can be done by first choosing a value of νCO that will serve as a reference point, e.g. 1900 cm−1, and subtracting this value from the observed νCO to give the difference ΔνCO, which may be positive or negative. When this value is multiplied by the conversion factor of −11.9 ppm/cm−1, it gives the quantity Δδ(57Fe)el (subscript indicating “electronic”), which can be used to adjust the observed iron chemical shift δ(57Fe)obs so as to exclude the electronic differences between the complexes. This procedure is summarized in eq 1, where Δδ(57Fe)el = −11.9[ΔνCO]. δ(57Fe)st = δ(57Fe)obs − Δδ(57Fe)el

(1)

The value of δ(57Fe)st (subscript indicating “steric”) reflects relative steric differences in isolation from electronic influences and should not be regarded as having absolute meaning, as its value depends on the value of νCO chosen as a reference point K


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for the complexes of the pure σ donors, this would imply that for 7−11 the π contribution has a constant value and that the measured differences between the νCO values for these complexes (7−11) arise from σ effects alone. A feature of the 57Fe NMR data which requires some comment is the fact that complexes having ligands PR3 with similar cone angles but with very different νCO values can have δ(57Fe) values that differ only slightly. This is clearly shown by complexes 4 (PR3 = PMe2Ph, θ = 122°, νCO 1903 cm−1, δ(57Fe) 492 ppm) and 14 (PR3 = P(OPh)3, θ = 128°, νCO 1936 cm−1, δ(57Fe) 454 ppm). In Figure 20, where steric influences have been normalized, it can be seen that the iron chemical shift appears to be entirely insensitive to the difference between 11 and 13 (ΔνCO = 4 cm−1) or the difference between 2 and 10 (ΔνCO = 7 cm−1), differences which in terms of νCO are quite significant and are attributed to π-bonding effects. This phenomenon, seen for complexes which differ only in the πbonding ability of a ligand (PR3, P(OR)3), has been observed for [Os(p-cymene)Cl2(PR3)] (PR3 = PnBu3 (θ = 132°, Tolman ν 2060.3 cm−1, δ(187Os) −2081 ppm), P(OiPr)3 (θ = 130°; Tolman ν 2075.9 cm−1, δ(187Os) −2076 ppm))34e and for trans-[W(CO)4(PPh3)(PR3)] (PR3 = PnBu3 (θ = 132°, Tolman ν 2060.3 cm−1, δ(183W) 789 ppm), P(OPh)3 (θ = 128°, ν 2085.3 cm−1, δ(183W) 776 ppm)),51 where large differences in νCO (up to 25 cm−1) translate into negligible differences in the transition-metal chemical shift. The origin of the effect has been identified by von Philipsborn as being “...probably due to an increase in the spectrochemical ΔE parameter caused by a larger metal-ligand orbital interaction of more strongly π accepting P(OR)3 ligands. This offsets the deshielding effect expected by way of the associated decrease in the nephelauxetic rd parameter.”34e,52 Thus, an increase in π acceptance increases ΔE (δmetal decreases) and the withdrawal of electron density from the π-accepting phosphorus by its substituents (OR), making it less polarizable, decreases ⟨rd⟩ (δmetal increases). The two effects are related and can (see above) exactly match and cancel each other. If the π contribution to the Fe−P bond does not influence the observed iron chemical shift, then the difference in δ(57Fe) between any two complexes can be fully accounted for by steric (θ) and σ (νCO) effects. Since for some of the complexes π effects are clearly involved, a measure of the π contribution to the Fe−P bond must come indirectly from the CO stretching frequency via the empirical relationship Δδ(57Fe)el = −11.9[ΔνCO] (see above). The displacement of data points for 7−11, 13, and 14 from the gradient obtained from the complexes of the pure σ donors in Figures 19b and 20 gives a measure of the π contribution to the Fe−P bond expressed in ppm of iron chemical shift (Figure 19b) or in wavenumbers of CO stretching frequency (Figure 20). The relative contributions of θ, σ, and π effects, which are valid only for dif ferences between any two complexes, can then be calculated. Thus, on going from complex 1 (PMe3: θ = 118°, νCO 1903 cm−1, δ(57Fe) 447 ppm) to complex 13 (P(OMe)3: θ = 107°, νCO 1922 cm−1, δ(57Fe) 316 ppm) the changes in the contributions to the Fe−P bond, in terms of 57Fe chemical shift, are θ = −(118° − 107°) × 5.9 ppm/deg = −65 ppm, π = −(ΔνCO(π) (=13.3 cm−1 from Figure 20) × 11.9 ppm/cm−1 = −158 ppm, and σ = −(ΔνCO(obs) − ΔνCO(π)) × 11.9 ppm/ cm−1 = −68 ppm. The negative signs arise because the effects of decreased cone angle, of increased π acceptance, and of decreased σ donation are all to reduce the magnitude of δ(57Fe). It can be seen that the θ and σ contributions (−65 and

approach can provide a basis for separating the σ and π contributions to the metal−phosphorus bond. The transitionmetal chemical shift alone can be quite unhelpful with regard to identifying the π contribution (see below). The relative contribution of steric effects to the iron chemical shift can be excluded by making use of the relationship derived from Figure 19b: namely that 1° of cone angle is equivalent to 5.9 ppm in the iron chemical shift. A value of θ can be chosen as a reference point, e.g. 145°, and subtracted from the cone angle of the PR3 ligand to give a difference ±Δθ. When multiplied by the conversion factor of 5.9 ppm/deg, this gives the quantity Δδ(57Fe)st, which can be used to adjust the iron chemical shifts so as to exclude the influence of the steric differences between the complexes. This is shown in eq 2, where Δδ(57Fe)st = 5.9[Δθ]. δ(57Fe)el = δ(57Fe)obs − Δδ(57Fe)st

(2)

The value of δ(57Fe)el reflects relative electronic differences in the absence of steric influences and has a numerical value that varies with the choice of reference θ (here 145°). A plot of δ(57Fe)el against observed νCO is shown in Figure 20. In this

Figure 20. Plot of νCO against δ(57Fe)el (see text) for [Fe(Cp)(SnPh3)(CO)(PR3)] (PR3 = PMe3 (1), PnBu3 (2), PCy3 (3), PMe2Ph (4), PMePh2 (5), P(CH2Ph)3 (6), PPh3 (7), P(4-MeC6H4)3 (8), P(4MeOC6H4)3 (9), P(4-FC6H4)3 (10), P(4-CF3C6H4)3 (11), P(NMe2)3 (12), P(OMe)3 (13), P(OPh)3 (14)).

figure two rows of data points define (to a good approximation) equal gradients separated, in the νCO dimension, by 5.5 cm−1. At higher wavenumbers are data points for complexes of the phosphites 13 and 14, separated from the lower gradient by 13.3 and 28.5 cm−1, respectively. The lower of the two parallel gradients is defined by data points for complexes of the PR3 ligands regarded as pure σ donors (PMe3 (1), PnBu3 (2), PCy3 (3), PMe2Ph (4), PMePh2 (5), and P(NMe2)3 (12)), and the upper gradient by data points for complexes of P(4-XC6H4)3 (X = H (7), Me (8), OMe (9), F (10), CF3 (11)). If the differences in νCO and δ(57Fe) that are associated with complexes 1−5 and 12 in the lower gradient arise from purely σ effects, then it would appear reasonable to attribute any further differences in νCO to π effects, as already noted for the phosphites. In view of the fact that the gradient for the complexes of P(4-XC6H4)3 (7−11) lies parallel to the gradient L


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−68 pm, respectively) to the iron chemical shift difference between 1 and 13 combine to give −133 ppm, which is, within experimental error, equal to the observed difference of −131 ppm. As noted above, the π contribution to the iron chemical shift is, to a good approximation, exactly matched by a counterbalancing influence, believed to originate in the nephelauxetic effect, which nullifies the π contribution to the metal (here iron) chemical shift but not, of course, to the Fe−P bond. The relative contributions of θ, σ, and π effects to the Fe−P bond on going from 1 to 13, expressed as percentages, are θ (22), σ (23), and π (55). Similarly, on going from 1 to 14 (P(OPh)3) the relative contributions are θ (13), σ (12), and π (75) and on going from 1 to 10 (P(4-MeOC6H4)3) are θ (55), σ (22.5), π (22.5). Carbon-13 Chemical Shift and Tin−phosphorus Spin Coupling. The carbon-13 chemical shift of the carbonyl in 1− 14 shows a moderate correlation with cone angle θ (plot shown in the Supporting Information). The correlation improves considerably if data points for 6 (P(CH2Ph)3) and 14 (P(OPh)3) are omitted (both appear ∼2 ppm to lower chemical shift of positions that would match the trend of the data). In Figure 21, which shows a plot of δ(57Fe) against

the RO substituent on phosphorus (Fermi contact model of spin coupling). In a plot (see the Supporting Information) of J(119Sn,31P) against νCO the general trend in a poor correlation is to higher J(119Sn,31P) with increasing νCO, but with a reverse trend to lower J (as νCO increases) for complexes of P(4XC6H4)3 (7−11). On the other hand, the correlation between J(119Sn,31P) and θ (Figure 22) is quite good if data points for

Figure 22. Plot of PR3 cone angle θ against J(Sn,P) for [Fe(Cp)(SnPh3)(CO)(PR3)] (PR3 = PMe3 (1), PnBu3 (2), PCy3 (3), PMe2Ph (4), PMePh2 (5), P(CH2Ph)3 (6), PPh3 (7), P(4-MeC6H4)3 (8), P(4MeOC6H4)3 (9), P(4-FC6H4)3 (10), P(4-CF3C6H4)3 (11), P(NMe2)3 (12), P(OMe)3 (13), P(OPh)3 (14)).

12−14 are omitted, with J increasing as θ decreases. In Figure 22 the increase in J(119Sn,31P) that accompanies the change in X from CF3 to OMe in the complexes of P(4-XC4)3 (7−11) can be seen to occur as the complexes become increasingly electron rich.

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CONCLUSION The structural study of 12 of the complexes [Fe(Cp)(SnPh3)(CO)(PR3)] shows, other than for PR3 = PCy3 (longest Fe−P bond) and PR3 = phosphite (shortest Fe−P bonds), a poor correlation between Fe−P bond length and either Tolman θ or infrared stretching frequency νCO. As the size of PR3 increases, changes occur in bond angles, most notably P−Fe−Sn and Cp−Fe−P, in a manner which is not in all cases smooth, unidirectional, and continuous. The relative contributions of θ, σ, and π effects to changes in the iron chemical shift for complexes 1−14 can be derived from plots of 57Fe chemical shift against Tolman θ (for PR3) and against νCO. The 57Fe chemical shift is found to reflect changes in both the θ and σ properties of PR3 but to be unresponsive to changes in the πacceptor ability of PR3, unlike the CO stretching frequency, thus permitting the separation of σ and π contributions to νCO and, indirectly, to δ(57Fe). It is found that the ligands PMe3, PnBu3, PCy3, PMe2Ph, PMePh2, and P(NMe2)3 behave as “pure” σ donors, P(OMe)3 and P(OPh)3 behave as π acceptors of differing strength, and P(4-XC6H4)3 behaves as having weak but clearly distinguishable π acceptor properties which, within the limits of experimental error, do not vary with X. The relative contributions of θ, σ, and π effects can be calculated as dif ferences between any two complexes. With PMe3 (complex 1) as a reference point, the contribution of π effects to changes in the Fe−P bond that occur on going from 1 to 7−11 (complexes of P(4-XC6H4)3) is ∼20% (which varies as the σ contribution independently varies with X).

Figure 21. Plot of CO δ(13C) against 57Fe chemical shift for [Fe(Cp)(SnPh3)(CO)(PR3)] (PR3 = PMe3 (1), PnBu3 (2), PCy3 (3), PMe2Ph (4), PMePh2 (5), P(CH2Ph)3 (6), PPh3 (7), P(4-MeC6H4)3 (8), P(4-MeOC6H4)3 (9), P(4-FC6H4)3 (10), P(4-CF3C6H4)3 (11), P(NMe2)3 (12), P(OMe)3 (13), P(OPh)3 (14)).

δ(13C) of the carbonyl, if data points are considered only for complexes of the P−C-bonded phosphines 1−11 and data for complexes 12−14 are disregarded, a surprisingly good correlation is found between the iron and carbon chemical shifts. The data point for the P(CH2Ph)3 complex 6 in Figure 21 now agrees well with the trend, indicating that the cause of the discrepancies observed for the complex in plots of δ(57Fe) against θ and of δ(13C) against θ is likely to be steric in origin. The tin-119 chemical shift correlates poorly with νCO and only moderately with θ (plots shown in Supporting Information), δ(119Sn) decreasing with increasing θ. The tin−phosphorus coupling constant is sensitive to the electronic properties of PR3, with much higher values for complexes of the phosphites 13 and 14 than for complexes of the phosphines. The greater magnitude of J(119Sn,31P) for the phosphite complexes can be interpreted in terms of a greater degree of s character in the phosphorus−iron bond induced by M


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cm−1; 1H NMR (CDCl3 300 K) δ 7.59 (m, Sn sat. 3JSnH ∼35, 6H, PhSn), 7.28−7.18 (m, 9H, PhSn), 4.99 (s, 5H, Cp), 1.52 (m, 3H, CH2), 0.80 (t, 3JHH 7.1, 9H, Me); 13C NMR (CDCl3 300 K) δ 219.82 (d, 2JPC 28.8, CO), 148.21 (s, PhSn), 137.26 (s, Sn sat. JSnC 32.1, PhSn), 127.62 (s, Sn sat. JSnC 34.6, PhSn), 127.02 (s, Sn sat. JSnC 9.1 PhSn), 79.15 (s, Cp), 30.44 (d, 1JPC 25.4, CH2 α), 26.02 (d, 3JPC 2.4, CH2 γ), 24.22 (d, 2JPC 12.5. CH2 β), 13.71 (s, Me). Anal. Calcd for C36H47FeOPSn: C, 61.66 H, 6.76. Found: C, 61.94; H, 6.88. Preparation of [Fe(η5-Cp)(SnPh3)(CO)(PCy3)] (3). A stirred solution of [Fe(η5-Cp)(SnPh3)(CO)2] (0.059 g, 0.095 mmol) and tricyclohexylphosphine (0.040 g, 0.14 mmol) in toluene (10 mL) at room temperature was irradiated for 18 h, during which time a color change from yellow to orange was observed. The solvent was removed under vacuum, and the residue was extracted with dichloromethane. The extract was concentrated and treated with hexane to give the product as orange-red crystals: yield 0.047 g (60%); IR (chloroform) ν(CO) 1894 cm−1; H NMR (CDCl3, 300 K) δ 7.64 (m, Sn. sat.3JSnH ∼34, 6H, PhSn), 7.26−7.17 (m, 9H, PhSn), 4.58 (d, 3JPH 1.0, 5H, Cp), 1.89 (m, 3H, CH), 1.69 (m, 9H, CH2), 1.58 (m, 6H, CH2), 1.48 (m, 3H, CH2), 1.27 (m, 3H, CH2), 1.12 (m, 3H, CH2), 1.90 (m, 6H CH2); 13 C NMR (CDCl3 300 K) δ 221.96 (d, 2JPC 27.0, CO), 148.87 (d, Sn sat. 3JPC 0.8, 1JSnC 224, PhSn), 137.39 (s, Sn sat. JSnC 30.0, PhSn), 127.53 (s, Sn sat. JSnC 33.4, PhSn), 126.80 (s, Sn sat. JSnC 8.7, PhSn), 78.73 (s, Cp), 39.04 (d, 1JPC 17.7, CH), 30.78 (s, CH2), 29.47 (d, JPC 2.2, CH2), 27.34 (d, JPC 9.1, CH2), 27.18 (d, JPC 10.2, CH2), 26.42 (s, CH2). Anal. Calcd for C43H55FeOPSn: C, 64.72; H, 6.85. Found: C, 64.89, H, 6.76. Preparation of [Fe(η5-Cp)(SnPh3)(CO)(PMe2Ph)]·CH2Cl2 (4). A stirred solution of [Fe(η5-Cp)(SnPh3)(CO)2] (0.075 g, 0.142 mmol) and dimethylphenylphosphine (0.015 g, 0.109 mmol) in toluene (10 mL) at room temperature was irradiated for 8 h, during which time a color change from yellow to orange was observed. The solvent was removed under vacuum to give a sticky red-orange residue, which was redissolved in dichloromethane. This mixture was centrifuged to remove suspended solid and the solvent again removed to give a viscous red oil: yield 0.056 g (55%). Attempts to crystallize the product from CH2Cl2, benzene, toluene, Et2O, and THF using hexane were unsuccessful: IR (chloroform) ν(CO) 1903 cm−1; 1H NMR (CDCl3 300 K) δ 7.75−7.15 (m, 20H, Ph), 4.32 (s, 5H, Cp), 1.62 (d, 2 JPH 8.7, CH3), 1.32 (d, 2JPH 8.9, CH3′); 13C NMR (CDCl3 300 K) δ 219.24 (d, 2JPC 29.1, CO), 147.54 (s, Sn sat. 1JSnC 253, PhSn), 143.16 (d, 1JPC 39.5, PhP), 137.25 (s, Sn sat. JSnC 32.9, PhSn), 128.94 (d, JPC 2.0, PhP), 128.80 (d, JPC 9.0, PhP), 128.43 (d, JPC 8.9, PhP), 127.74 (s, Sn sat. JSnC 35.8, PhSn), 127.17 (s, Sn sat. JSnC 8.7, PhSn), 80.25 (s, Cp), 21.29 (d, 1JPC 30.4, Me), 21.05 (d, 1JPC 33.0, Me′). Anal. Calcd for C33H33Cl2FeOPSn: C, 54.89; H, 4.61. Found: C, 55.22; H, 4.77. Preparation of [Fe(η5-Cp)(SnPh3)(CO)(PMePh2)] (5). A stirred solution of [Fe(η5-Cp)(SnPh3)(CO)2] (0.050 g, 0.095 mmol) and methyldiphenylphosphine (0.040 g, 0.20 mmol) in toluene (10 mL) at room temperature was irradiated for 20 h, during which time a color change from yellow to orange was observed. The solvent was removed under vacuum, and the residue was extracted with dichloromethane. The extract was concentrated and treated with hexane to give the product as a mixture of orange and red crystals, which were separated by hand. The red product (0.021 g, 25%) was identified as the disubstituted complex [Fe(η5-Cp)(SnPh3)(PMePh2)2]: yield of orange product (5) 0.033 g (50%); IR (chloroform) ν(CO) 1907 cm−1; 1H NMR (CDCl3, 300 K) δ 7.9−6.9 (m, 35H, Ph), 4.36 (d, 3JPH 1.4, 5H, Cp), 1.40 (d, 2JPH 8.4, Me); 13C NMR (CDCl3, 300 K) δ 219.29 (d, 2 JPC 29.1, CO), 147.32 (s, Sn sat. 1JSnC 258, PhSn), 141.96 (d, 1JPC 40.9, PhP), 138.58 (d, 1JPC 39.3, Ph′P), 137.24 (s, Sn sat. JSnC 33.0, PhSn), 131.71 (d, JPC 9.8, PhP), 131.05 (d, JPC 9.9, Ph′P), 129.77 (d, JPC 2.2, PhP), 129.15 (d, JPC 2.0, Ph′P), 128.45 (d, JPC 9.3, PhP), 128.14 (d, JPC 9.4, Ph′P), 127.56 (s, Sn sat. JSnC 36,1, PhSn), 127.06 (s, Sn sat. JSnC 8.8, PhSn), 80.91 (s, Cp), 19.66 (d, 1JPC 34.1, Me). Anal. Calcd for C22H37FeOPSn: C, 63.57; H, 4.76. Found: C, 63.56; H, 4.84. Preparation of [Fe(η5-Cp)(SnPh3)(CO)(P(CH2Ph)3)] (6). A stirred solution of [Fe(η5-Cp)(SnPh3)(CO)2] (0.058 g, 0110 mmol) and tribenzylphosphine (0.035 g 0.115 mmol) in toluene (10 mL) at room temperature was irradiated for 16 h, during which time a color

EXPERIMENTAL SECTION

Materials. [Fe(η5-Cp)(SnPh3)(CO)2] was prepared from Na[Fe(Cp)(CO)2] and SnClPh3 according to the method of Gorsich.53 [Fe(Cp)(CO) 2]2 , SnClPh3, phosphines, and phosphites were purchased from Aldrich and from Fluka and were used without further purification. THF was distilled from sodium/benzophenone, dichloromethane was distilled from P2O5, and toluene, n-hexane, acetonitrile, and diethyl ether were distilled from calcium hydride. All operations were performed under a nitrogen atmosphere. Synthetic Method. Complexes 1−14 were prepared by the widely used photolytic method for displacement of a coordinated CO. Reagents in toluene solution in sealed Schlenk tubes were irradiated by means of a 400 W medium-pressure mercury UV lamp positioned at a distance of ∼2 cm from the reaction vessel. Complex 7 has previously been reported.54 NMR Spectroscopy. Spectra were recorded on Bruker DRX 400 and Avance III 400 spectrometers equipped with 5 mm tripleresonance inverse probes with dedicated 31P channel and extended decoupler range operating at 400.13 MHz (1H), 100.61 MHz (13C), 161.98 MHz (31P), 12.95 MHz (57Fe), and 149.19 MHz (119Sn). Twodimensional 57Fe−31P spectra were obtained using the pulse sequence π/2(31P)−1/[2J(57Fe,31P)]−π/2(57Fe)−τ−π(31P)−τ−π/2(57Fe)−1/ [2J(57Fe,31P)]−Acq(31P).55 Chemical shifts were referenced to the generally accepted standards of TMS (1H), CDCl3 at 77.00 ppm (13C), 85% H3PO4 (external standard; 31P), Fe(CO)5 (external standard; 57Fe, equivalent to Ξ(57Fe) = 3.237797 MHz), and SnMe4 (external standard; 119Sn). Chemical shifts are reported in ppm and coupling constants (J) in Hz. Infrared Spectroscopy. Spectra were recorded on Bruker Vector 22 FT-IR and Varian FTS 800 FT-IR spectrometers from samples in chloroform solution. X-ray Structure Determination. Intensity data were collected on Bruker SMART and APEX II CCD area detector diffractometers with graphite-monochromated Mo Kα radiation (50 kV, 30 mA) using respectively SMART-NT56 and APEX257 data collection software. The collection method involved ω scans of width 0.5° and 512 × 512 bit data frames. Data reduction was carried out using the program SAINT+,58 and face-indexed absorption corrections were made using XPREP.58 The crystal structures were solved by direct methods using SHELXTL.59 Non-hydrogen atoms were first refined isotropically, followed by anisotropic refinement by full-matrix least-squares calculations based on F2 using SHELXTL. Hydrogen atoms were first located in the difference map and then positioned geometrically and allowed to ride on their respective parent atoms. Diagrams were generated using ORTEP-3 for Windows.60 Preparation of [Fe(η5-Cp)(SnPh3)(CO)(PMe3)] (1). A stirred solution of [Fe(η5-Cp)(SnPh3)(CO)2] (0.067 g, 0.127 mmol) and trimethylphosphine (0.0076 g, 0.100 mmol) in toluene (10 mL) at room temperature was irradiated for 15 h, during which time a color change from yellow to orange was observed. The solution was concentrated and treated with hexane to give the product as a yellow microcrystalline powder: yield 0.034 g (59%); IR (chloroform) νCO 1903 cm−1; 1H NMR (CDCl3 300 K) δ 7.59 (m, Sn sat. 3JSnH 35, 6H, PhSn), 7.30−7.21 (m, 9H, PhSn), 4.47 (s, 5H, Cp), 1.24 (d, 2JPH 9.0, 9H, Me); 13C NMR (CDCl3 300 K) δ 218.92 (d, 2JPC 30.0, CO), 147.68 (d, 3JPC 0.8, PhSn), 137.22 (s, Sn sat. JSnC 33.0, PhSn), 127.74 (s, Sn sat. JSnC 35.4, PhSn), 127.13 (s, Sn sat. JSnC 9.2, PhSn), 79.49 (s, Cp), 23.22 (d 1JPC 29.7, Me). Anal. Calcd for C27H29FeOPSn: C, 56.39; H, 5.08. Found: C, 56.50; H, 5.09. A low quantity of phosphine (less than 1 equiv) was used in order to minimize the extent to which a disubstituted product was formed. Preparation of [Fe(η5-Cp)(SnPh3)(CO)(PnBu3)] (2). A stirred solution of [Fe(η5-Cp)(SnPh3)(CO)2] (0.053 g, 0.101 mmol) and trin-butylphosphine (0.03 g, 0.15 mmol) in toluene (10 mL) at room temperature was irradiated for 16 h, during which time a color change from yellow to orange was observed. The solvent was removed under vacuum, and the residue was extracted with dichloromethane. The extract was concentrated and treated with hexane to give the product as orange crystals: yield 0.061 g (87%); IR (chloroform) ν(CO) 1901 N


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change from yellow to orange was observed. The solvent was removed under vacuum, and the residue was extracted with dichloromethane. The extract was concentrated and treated with hexane to give the product as orange crystals: yield 0.075 g (85%); IR (chloroform) νCO 1908 cm−1; 1H NMR (CDCl3 300 K) δ7.9−6.9 (m, 25H, Ph), 3.99 (d, 3 JPH 1.3, 5H, Cp), 3.12 (dd, 2JHH 14.6, 2JPH 9.7, 3H, CH2), 2.86 (dd, 2 JHH 14.6, 2JPH 6.4, 3H, CH2); 13C NMR (CDCl3 300 K) δ 219.73 (d, 2 JPC 28.0, CO), 148.01 (d, 3JPC 1.0, PhSn), 137.50 (s, Sn sat. JSnC 31.9, PhSn), 135.36 (d, JPC 5.3, benzyl), 130.23 (d, JPC 5.3, benzyl), 128.49 (d, JPC 1.9, benzyl), 127.93 (s, Sn sat. JSnC 35.6, PhSn), 127.35 (s, Sn sat. JSnC 9.0, PhSn), 126.67 (d, JPC 2.3, benzyl), 79.23 (s, Cp), 38.00 (d, 1 JPC 19.1, CH2). Anal. Calcd for C45H41 FeOPSn: C, 67.28; H, 5.14. Found: C, 67.03; H, 5.15. Preparation of [Fe(η5-Cp)(SnPh3)(CO)(PPh3)] (7). A stirred solution of [Fe(η5-Cp)(SnPh3)(CO)2] (0.050 g, 0.095 mmol) and triphenylphosphine (0.037 g, 0.141 mmol) in toluene (10 mL) at room temperature was irradiated for 24 h, during which time a color change from yellow to red-orange was observed. The solvent was removed under vacuum, and the residue was extracted with dichloromethane. The extract was concentrated and treated with hexane to give the product as dark orange crystals: yield 0.038 g (50%); IR (chloroform) ν(CO) 1904 cm−1; 1H NMR (CDCl3 300 K) δ 7.49−7.05 (m, 30H, Ph), 4.33 (d, 3J PH 1.4, 5H, Cp); 13C NMR (CDCl3 300 K) δ 220.50 (d, 2JPC 28.6, CO), 147.90 (d, Sn sat. 3JPC 1.0, 1 JSnC 256, PhSn), 137.35 (d, 1JPC 40.4, PhP), 137.15 (s, Sn sat. JSnC 31.8, PhSn), 133.12 (d, JPC 10.2, PhP), 129.44 (d, JPC 1.9, PhP), 127.89 (d, JPC 9.6, PhP), 127.54 (s, Sn sat. JSnC 35.8, PhSn), 126.81 (s, Sn sat. JSnC 8.8, PhSn), 81.72 (s, Cp). Anal. Calcd for C42H35FeOPSn: C, 66.27; H, 4.63. Found: C, 66.07; H, 4.74. Preparation of [Fe(η5-Cp)(SnPh3)(CO)(P(4-MeC6H4)3)] (8). A stirred solution of [Fe(η5-Cp)(SnPh3)(CO)2] (0.050 g, 0.095 mmol) and tris(p-tolyl)phosphine (0.043 g, 0.14 mmol) in toluene (10 mL) at room temperature was irradiated for 20 h, during which time a color change from yellow to red-orange was observed. The solvent was removed under vacuum, and the residue was extracted with dichloromethane. The extract was concentrated and treated with hexane to give the product as red-orange crystals: yield 0.051 g (63%), IR (chloroform) ν(CO) 1904 cm−1; 1H NMR (CDCl3 300 K) δ 7.37 (m, Sn sat. 3JSnH ∼35, 6H, PhSn), 7.19−7.09 (m, 15H, Ar), 6.89, (m, 6H, C6H4P), 4.32 (d, 3JPH 1.4, 5H, Cp), 2.28 (s, 9H, Me); 13C NMR (CDCl3 300 K) δ 220.70 (d, 2JPC 28.8, CO), 147.98 (d, Sn sat. 3JPC 1.1, 1 JSnC 250, PhSn), 139.32 (d, JPC 1.8, C6H4P), 137.18 (s, Sn sat. JSnC, 31.8, PhSn), 134.49 (d, 1JPC 44.4, C6H4P), 132.99 (d, JPC 10.3, C6H4P), 128.56 (d, JPC 10.0, C6H4P), 127.41 (s, Sn sat. JSnC 35.4, PhSn), 126.61 (s, Sn sat. JSnC 8.8, PhSn), 81.62 (s, Cp), 21.24 (s, Me). Anal. Calcd for C45H41FeOPSn: C, 67.28 H, 5.14. Found: C, 67.30; H, 5.09. Preparation of [Fe(η5-Cp)(SnPh3)(CO)(P(4-MeOC6H4)3)] (9). A stirred solution of [Fe(η5-Cp)(SnPh3)(CO)2] (0.050 g, 0.095 mmol) and tris(4-methoxyphenyl)phosphine (0.050 g, 0.14 mmol) in toluene (10 mL) at room temperature was irradiated for 20 h, during which time a color change from yellow to red-orange was observed. The solvent was removed under vacuum, and the residue was extracted with dichloromethane. The extract was concentrated and treated with hexane to give the product as orange crystals: yield 0.074 g (87%), IR (chloroform) ν(CO) 1903 cm−1; 1H NMR (CDCl3 300 K) δ 7.38 (m, Sn sat. 3JSnH ∼35, 6H, PhSn), 7.19−7.08 (m, 15H, Ar), 6.60 (m, 6H, C6H4P), 4.32 (d, 3JPH 1.4, 5H, Cp), 3.74 (s, 9H, Me); 13C NMR (CDCl3 300 K) δ 220.74 (d, JPC 29.3, CO), 160.14 (d, JPC 1.6, C6H4P), 148.08 (d, 3JPC 1.1, 1JSnC 247, PhSn), 137.07 (s, Sn sat. JSnC 31.5, PhSn), 134.37 (d, JPC 11.6, C6H4P), 129.16 (d, 1JPC 47.1, C6H4P), 127.36 (s, Sn sat. JSnC 35.2, PhSn), 126.59 (s, Sn sat. JSnC 8.9, PhSn), 113.16 (d, JPC 10.6, C6H4P), 81.48 (s, Cp), 58.32 (s, Me). Anal. Calcd for C45H41FeOPSn: C, 63.49; H, 4.86. Found: C, 63.35, H, 4.85. Preparation of [Fe(η5-Cp)(SnPh3)(CO)(P(4-FC6H4)3)] (10). A stirred solution of [Fe(η5-Cp)(SnPh3)(CO)2] (0.050 g, 0.095 mmol) and tris(4-fluorophenyl)phosphine (0.045 g, 0.14 mmol) in toluene (10 mL) at room temperature was irradiated for 24 h, during which time a color change from yellow to red-orange was observed. The

solvent was removed under vacuum, and the residue was extracted with dichloromethane. The extract was concentrated and treated with hexane to give the product as red-brown crystals: yield 0.044 g (57%), IR (chloroform) ν(CO) 1908 cm−1; 1H NMR (CDCl3 300 K) δ 7.36 (m, Sn sat. 3JSnH ∼36, 6H, PhSn), 7.23−7.12 (m, 15H, Ar), 6.80 (m, 6H, C6H4P), 4.34 (s, 5H, Cp); 13C NMR (CDCl3 300 K) δ 220.23, (d, 2 JPC 29.2, CO), 163.41 (dd, 1JFC 252, JPC 2.4, C6H4P), 147.23 (s, PhSn), 136.99 (s, Sn sat. JSnC, 31.6, PhSn), 134.93 (dd, JFC 8.4, JPC 11.6, C6H4P), 133.04 (dd, 1JPC 47.4, JFC 3.6, C6H4P), 127.74 (s, Sn sat. JSnC 36.3 PhSn), 127.09 (s, Sn sat. JSnC 8.8, PhSn), 115.27 (dd, JFC 21.1, J PC , 10.6, C 6 H 4 P), 81.68 (s, Cp). Anal. Calcd for C42H32F3FeOPSn: C, 61.88; H, 3.96. Found: C, 61.64; H, 3.94. Preparation of [Fe(η5-Cp)(SnPh3)(CO)(P(4-CF3C6H4)3)] (11). A stirred solution of [Fe(η5-Cp)(SnPh3)(CO)2] (0.063 g, 0.120 mmol) and tris(4-trifluoromethylphenyl)phosphine (0.058 g, 0.124 mmol) in toluene (10 mL) at room temperature was irradiated for 16 h, during which time a color change from yellow to orange was observed. The solvent was removed under vacuum, and the residue was extracted with dichloromethane. The extract was concentrated and treated with hexane to give the product as orange crystals: yield 0.088 g (76%), IR (chloroform) 1914 cm−1; 1H NMR (CDCl3 300 K) δ 7.4−7.15 (m, 27H, Ar), 4.38 (d, 3JPH 1.4, 5H, Cp); 13C NMR (CDCl3 300 K) δ 219.74 (d, 2JPC 27.8, CO), 146.34 (d, 3JPC 0.9, PhSn), 140.61 (d, 1JPC 40.0, C6H4P), 136.81 (s, Sn sat. JSnC 32.0, PhSn), 133.09 (d, JPC 10.8, C6H4P), 131.90 (dq, JPC 2.3, JFC 32.8, C6H4P, 127.90 (s, Sn sat. JSnC 37.7, PhSn), 127.40 (s, Sn sat. JSnC 9.6, PhSn), 125.15 (dq, JPC 9.8, JFC 3.8, C6H4P), 123.47 (dq, JPC 1.0, 1JFC 272.7, CF3), 81.83 (d, 2JPC 0.7, Cp). Anal. Calcd for C45H32F9FeOPSn: C, 55.99; H, 3.34. Found: C, 55.86; H, 3.41. Preparation of [Fe(η5-Cp)(SnPh3)(CO)(P(NMe2)3)] (12). A stirred solution of [Fe(η5-Cp)(SnPh3)(CO)2] (0.050 g, 0.095 mmol) and tris(dimethylamine)phosphine (0.080 g, 0.50 mmol) in toluene (10 mL) at room temperature was irradiated for 16 h, during which time a color change from yellow to orange was observed. The solvent was removed under vacuum, and the residue was extracted with dichloromethane. The extract was concentrated and treated with hexane to give the product as an orange microcrystalline powder: yield 0.065 g (95%). X-ray quality crystals were grown from a CH2Cl2/ hexane solution at −20 °C: IR (chloroform) ν(CO) 1900 cm−1; 1H NMR (CDCl3 300 K) δ 7.57 (m, Sn sat. 3JSnH ∼34, 6H, PhSn), 7.28− 7.17 (m, 9H, PhSn), 4.66 (s, 5H, Cp), 2.42 (d, 3JPH 9.2, 18H, Me); 13C NMR (CDCl3 300 K) δ 222.09 (d, 2JPC 38.7, CO), 148.89 (d, Sn sat. 3 JPC 2.1, 1JSnC 226, PhSn), 137.40 (s, Sn sat. JSnC 31.4, PhSn), 127.30 (s, Sn sat. JSnC 34.0, PhSn), 126.74 (s, Sn sat. JSnC 8.8, PhSn), 79.23 (s, Cp), 39.01 (d, 2JPC 4.2, Me). Anal. Calcd for C30H38FeN3OPSn: C, 54.42; H, 5.78; N, 6.35. Found: C, 54.32; H, 5.89; N, 6.55. Preparation of [Fe(η5-Cp)(SnPh3)(CO)(P(OMe)3)] (13). A stirred solution of [Fe(η5-Cp)(SnPh3)(CO)2] (0.053 g, 0.106 mmol) and trimethyl phosphite (0.013 g, 0.105 mmol) in toluene (10 mL) at room temperature was irradiated for 20 h, during which time a color change from yellow to light orange was observed. The solvent was removed under vacuum, and the residue was extracted with dichloromethane. The extract was concentrated and treated with hexane to give the product as a bright yellow microcrystalline powder: yield 0.025 g (40%). X-ray-quality crystals were grown from a CH2Cl2/ hexane solution at −20 °C: IR (chloroform) ν(CO) 1922 cm−1; 1H NMR (CDCl3 300 K) δ 7.58 (m, Sn sat. 3JSnH ∼39, 6H, PhSn), 7.30− 7.20 (m, 9H, PhSn), 4.56 (d, 3JPH 0.7, 5H, Cp), 3.39 (d, 2JPH 11.4, 9H, Me); 13C NMR (CDCl3 300 K) δ 217.47 (d, 2JPC 41.5, CO), 147.53 (d, Sn sat. 3JPC 1.8, 1JSnC 280, PhSn), 137.15 (s, Sn sat. JSnC 33.6, PhSn), 127.48 (s, Sn sat. JSnC 38.1, PhSn), 127.02 (s, Sn sat. JSnC 9.5, PhSn), 79.80 (s, Cp), 52.09 (d, 2JPC 6.1, Me). Anal. Calcd for C27H29FeOPSn: C, 52.05; H, 4.69. Found: C, 51.94; H, 5.03. Preparation of [Fe(η5-Cp)(SnPh3)(CO)(P(OPh)3)] (14). A stirred solution of [Fe(η5-Cp)(SnPh3)(CO)2] (0.053 g, 0.106 mmol) and P(OPh)3 (0.040 g, 0.129 mmol) in toluene at room temperature was irradiated for 16 h, during which time a color change from yellow to light orange was observed. The solvent was removed under vacuum, and the residue was extracted with dichloromethane. The extract was concentrated and treated with hexane to give the product as yellow O


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crystals: yield 0.056 g (62%); IR (chloroform) ν(CO) 1936 cm−1; 1H NMR (CDCl3 300 K) δ 7.63 (m, Sn sat. 3JSnH ∼40, 6H, PhSn), 7.25 (m, 9H, PhSn), 7.18 (m, 6H, PhO), 7.08 (m, 3H, PhO), 6.78 (m, 6H, PhO), 4.26 (s, 5H, Cp); 13C NMR (CDCl3 300 K) δ 216.86 (d, 2JPC 40.6, CO), 151.75 (d, JPC 10.6, PhO), 146.65 (s, PhSn), 137.30 (s, Sn sat. JSnC 33.9, PhSn), 129.35 (s, PhO), 127.76 (s, Sn sat. JSnC 39.4, PhSn), 127.28 (s, Sn sat. JSnC 9.8, PhSn), 124.03 (s, PhO), 121.38 (d, JPC 4.2, PhO), 80.17 (s, Cp). Anal. Calcd for C42H35FeO4PSn: C, 62.36; H, 4.36. Found: C, 62.38; H, 4.47.

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ASSOCIATED CONTENT

S Supporting Information *

Tables giving crystal data and structure refinement, figures giving additional plots, and CIF files giving crystal coordinates for compounds 2, 3, and 5−14. This material is available free of charge via the Internet at http://pubs.acs.org. The CIF files can also be obtained from the Cambridge Crystallographic Data Centre (CCDC) as file numbers 973675−973686.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank the University of the Witwatersrand and the NRF for financial support. REFERENCES

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