Phosphine-Substituted (η5-Pentadienyl) Manganese Carbonyl

Dec 11, 2013 - Gabriel B. Hall , Rudresha Kottani , Greg A. N. Felton , Takuhei Yamamoto , Dennis H. Evans , Richard S. Glass , and Dennis L. Lichtenb...
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Phosphine-Substituted (η5‑Pentadienyl) Manganese Carbonyl Complexes: Geometric Structures, Electronic Structures, and Energetic Properties of the Associative Substitution Mechanism, Including Isolation of the Slipped η3‑Pentadienyl Associative Intermediate José Ignacio de la Cruz Cruz,† Patricia Juárez-Saavedra,† Brenda Paz-Michel,† Marco Antonio Leyva-Ramirez,† Asha Rajapakshe,‡ Aaron K. Vannucci,‡,∥ Dennis L. Lichtenberger,*,‡ and M. Angeles Paz-Sandoval*,† †

Departamento de Quı ́mica, Cinvestav, Av. IPN # 2508, Col. San Pedro Zacatenco, México D. F. 07360, México Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721, United States

‡

S Supporting Information *

ABSTRACT: The molecule (η5-Me2Pdl)Mn(CO)3 (η5-Me2Pdl = 2,4-dimethyl-η5-pentadienyl) has been prepared by a new method and used as a starting material to prepare the molecules (η5-Me2Pdl)Mn(CO)n(PMe3)3−n (n = 2, 1) by phosphine substitution for carbonyls. The first carbonyl substitution is achieved thermally in refluxing cyclohexane, and the second carbonyl substitution requires photolysis. At room temperature in benzene the associative intermediate (η3-Me2Pdl)Mn(CO)3(PMe3) that precedes the initial loss of carbonyl is observed. Single-crystal structures are reported for all complexes, including the associative intermediate of the first substitution in which the pentadienyl ligand has slipped to the η3 bonding mode. These molecules offer an opportunity to examine fundamental principles of the interactions between metals and pentadienyl ligands in comparison to the well-developed chemistry of metal cyclopentadienyl (Cp) complexes as a function of electron richness at the metal center. Photoelectron spectra of these molecules show that the Me2Pdl ligand has π ionizations at energy lower than that for the analogous Cp ligand and donates more strongly to the metal than the Cp ligand, making the metal more electron rich. Phosphine substitutions for carbonyls further increase the electron richness at the metal center. Density functional calculations provide further insight into the electronic structures and bonding of the molecules, revealing the energetics and role of the pentadienyl slip from η5 to η3 bonding in the early stages of the associative substitution mechanism. Computational comparison with dissociative ligand substitution mechanisms reveals the roles of dispersion interaction energies and the entropic free energies in the ligand substitution reactions. An alternative scheme for evaluating the computational translational and rotational entropy of a dissociative mechanism in solution is offered.

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INTRODUCTION

the bonding in comparison to that for the Cp ligand depending on the metal oxidation state, leading to differences in structure and reactivity of the low-valent and high-valent metal complexes.2,8a The Pdl ligand bonds more strongly to a metal center than does a Cp ligand in lower valent metal complexes, and the reverse is observed for high-valent metal complexes. This reversal is explained by a loss in δ bonding and a loss of metal−Pdl orbital overlap for higher oxidation state metal centers. The versatility of chemistry displayed by these transition metal pentadienyl systems has inspired further studies of d6 transition metal piano-stool complexes of the general forms CpMn(CO)39 and (Pdl)Mn(CO)3.10,11

Over the past decade various studies on the chemistry of the pentadienyl (Pdl = C5H7) ligand have given valuable insight into the reactivity of the Pdl ligand in comparison to the similar, and more well known, cyclopentadienyl ligand (Cp). A wide variety of Pdl ligands have been synthesized, and studies1−4 have revealed several unique properties of Pdl,2,5 leading to numerous possible applications of metal pentadienyl reaction chemistry.2,6 Metal complexes containing the Pdl ligand often exhibit interesting reactivity due to the accessibility of a range of η1, η3, and η5 bonding modes.7 While the Pdl ligand has an extreme tendency to bond to transition metals in low oxidation states as a result of the high δ acidities of these ligands, higher valent (≥+3) complexes have been reported.2,8 Previous studies on Pdl ligands also show a marked reversal in the favorability of © XXXX American Chemical Society

Received: October 17, 2013

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dx.doi.org/10.1021/om401017t | Organometallics XXXX, XXX, XXX−XXX

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Scheme 1. Substitution Reactions of (η5-Me2Pdl)Mn(CO)3 (1) with PMe3 Showing the Associative Mechanism between 1 and PMe3

The substitution of carbon monoxide in CpMn(CO)3 by a wide variety of two-electron donor ligands has been studied in great detail,9 while much less effort has given to the chemistry of the corresponding Cp*Mn(CO)3 (Cp* = η5-C5Me5).12,13 Both (cyclopentadienyl)manganese derivatives are quite inert to thermal substitution, requiring high temperature and longer reaction times, and consequently the majority of syntheses of the substitution products of the form CpMn(CO)2L have been carried out through a dissociative mechanism using photochemical procedures.14−17 In contrast, thermal reactions between (Pdl)Mn(CO)3 and phosphorus donor ligands afford the substituted complexes (Pdl)Mn(CO)2L (L = PMe3, PMe2Ph, PPh3, P(C6H11)3, P(OR)3)18 presumably through an associative mechanism favored by the more facile interconversion between η5 and η3 bonding modes in the open-chain pentadienyl ligands in comparison to the closed cyclopentadienyl ligand. The pentadienyl chemistry has also shown that the presence of methyl substituents at the 2- and 4positions helps to significantly stabilize Pdl in the “U” conformation.7b This study examines the complex (η5-Me2Pdl)Mn(CO)3 (1; 5 η -Me2Pdl = 2,4-dimethyl-η5-pentadienyl) and its reactivity toward phosphine substitutions. A reaction scheme is developed for the reaction of 1 with PMe3 through the intermediate species (η3-Me2Pdl)Mn(CO)3PMe3 (2; η3Me2Pdl = 2,4-dimethyl-η3-pentadienyl), in which the structure of the Me2Pdl ligand in an η3 orientation about the Mn center has been determined crystallographically. The structures of two phosphine-containing products, (η5-Me2Pdl)Mn(CO)2(PMe3) (3) and (η5-Me2Pdl)Mn(CO)(PMe3)2 (4), have also been determined by X-ray crystallography, and the electronic structures have been characterized by photoelectron spectroscopy and density functional theory (DFT) calculations. This study offers the opportunity to examine fundamental principles of the interactions between metals and pentadienyl ligands (Pdl) as a function of electron richness at the metal center and to examine the structural and energetic features of the associative substitution mechanism.

The infrared spectra exhibited, as expected, three (2021, 1954, 1934), two (1942, 1876), and one (1839 cm−1) strong carbonyl stretching frequencies for the molecules 1, 3, and 4, respectively. The shifts to lower stretching frequencies going from 1 to 4 are consistent with increased electron density at the Mn center from replacement of carbonyl ligands with phosphines. Comparison between the IR spectra of compound 1 and CpMn(CO)3 (CO frequencies 2028 and 1945 (twice) cm−1, cyclohexane) suggests that the metal center in 1 is slightly more electron rich than the metal center in CpMn(CO)3 (the average carbonyl stretching frequencies of 1 and CpMn(CO)3 are 1970 and 1973 cm−1, respectively). The photoelectron spectra discussed later illustrate more clearly the greater electron richness of 1 in comparison to CpMn(CO)3. The thermal reaction of 1 with PMe3 was also monitored by infrared spectroscopy, giving a complex spectrum with several bands, assigned to 1, 3, and the intermediate (η3-Me2Pdl)Mn(CO)3PMe3 (2) (2007, 1926, 1907 (CO) and 1621 (CC) cm−1; cyclohexane) as a result of an associative mechanism which occurs during the transformation of 1 into 3. Density functional theory calculations were able to account well for the carbonyl stretching frequencies and trends among all molecules, including the intermediate, without scaling the gas-phase values. The calculated frequencies were as follows (in cm−1): for 1, 2019, 1962, 1942; 2, 1990, 1927, 1905, 1614, 3, 1945, 1887; 4, 1848. This agreement shows that the calculations are able to properly account for the extent of back-bonding from the Mn center to the carbonyls. The intermediate 2 is analogous to the related nonmethylated pentadienyl complex (η 3 -Pdl)Mn(CO) 3 PMe 3 (2000, 1937, 1905 (CO) and 1612 (CC) cm−1; Nujol),18 which has been fully characterized from the substitution reaction of (η5-Pdl)Mn(CO)3 with PMe3. It was demonstrated that the pentadienyl ligand went through an η 5 → η 3 interconversion during PMe3 association, resulting in the pentadienyl ligand going from the U to the S geometric conformation.18 The reaction mechanism in this study was then analyzed with IR and NMR spectroscopy to determine if molecule 2 also converted between the U and S geometric conformations despite the possible steric constraints introduced by the methyl groups substituted at the 2- and 4-positions of the pentadienyl ligand. The presence of methyl substituents also increased the volatility and solubility of 2 in comparison to the analogue (η3-Pdl)Mn(CO)3PMe3,18 making it impossible to

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RESULTS AND DISCUSSION Synthesis of the Complexes 1−4. Reaction between the strong σ-donor phosphine PMe3 with (η5-Me2Pdl)Mn(CO)3 (1) in refluxing cyclohexane affords (η 5 -Me 2 Pdl)Mn(CO)2PMe3 (3) with a 72.7% yield. The reaction involves a displacement of a carbonyl ligand with PMe3. Attempts to substitute a second carbonyl in 1 with PMe3 by thermal reaction under longer reaction times and excess of PMe3 were unsuccessful. Instead, irradiation of a 1:7 mixture of 1 and PMe3 using a medium-pressure mercury lamp (125 W) for 7 h afforded a mixture of (η5-Me2Pdl)Mn(CO)(PMe3)2 (4) with traces of 3. The similar solubilities of 3 and 4 complicated the purification processes. A synthetic alternative, shown in Scheme 1, was to carry out the photochemical reaction from 3 and PMe3 in a 1:4 ratio. After 3 h compound 4 was the only new product formed, with traces of unreacted starting compound 3 and a substantial amount of brown decomposition product accounting for the remainder of the material (see Figure S7 in the Supporting Information). The purification process affords 4 in low yield (9%). According to the substitution reaction conditions, the increasing trend in energy required to remove a CO ligand in acyclic and cyclic pentadienyl manganese complexes can be arranged as follows: Pdl < Me2Pdl < Cp < Cp*. B

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constants of 23.6 and 18.7 Hz, respectively. On the basis of the above data, and considering the 31P NMR spectroscopy, it is proposed that the distribution of the phosphines in the halfsandwich complex 4 shows a piano-stool structure as described in Scheme 1, where both PMe3 phosphines reside in different chemical environments; P2 sits under the open “mouth”, while P1 is under an “edge” of the η5-Me2Pdl ligand, at 32.0 and 41.1 ppm, respectively. This is in agreement with trends observed for ruthenium,4,19 cobalt,20 and rhodium21 pentadienyl compounds. The 1H NMR spectra of 2 reveal seven different broad signals for the hydrogens of the η3-pentadienyl group. The H1anti and H1 syn hydrogens show very similar chemical shifts, and H3 resonates at 3.77 ppm, similar to the corresponding anti conformers in (η3-Pdl)Mn(CO)3PR3.18 Characteristic values of syn and anti conformations have been observed in many different kinds of ruthenium,22−25 iridium,26 rhodium,21,26 and manganese27 pentadienyl and heterodienyl complexes. The solid-state structure of 2 confirms that the anti (U shaped) geometry of 1 remains in a distorted U shape in the intermediate 2 (vide infra). The chemical shifts for H4, H5, and H5′ show a typical noncoordinated vinylic system. The 13 C{1H} NMR is in agreement with the assignment by 1H NMR, and the absence of phosphorus coupling to the η3pentadienyl ligand suggests that the phosphine is far away from this ligand, as was observed in the analogue (η3-C5H7)Mn(CO)3PMe3.18 31P NMR of 2 confirms the presence of a signal at 29.6 ppm for the coordinated PMe3. Structures. Molecular structures of 1−4 in the solid state were determined by X-ray crystallography and are presented in Figure 1 with crystal data, bond lengths, bond angles, and torsion angles provided in Table S1 (Supporting Information). The calculated structures of 1−4 were compared to the X-ray determined structures, and the comparisons are given in Tables S2−S5 (Supporting Information). The tables show that the calculated structures agree very well with the experimentally

separate 2 from 1 and 3 in solution. Nevertheless, full NMR and IR spectroscopic evidence of 2 was obtained from the reaction mixture. These results indicate, in solution and in the solid state, the exclusive presence of the U-shaped complex 2 and an associative mechanism in the reaction between 1 and PMe3. Following the course of the substitution reaction from 1 to 3 supports the expectation that the phosphine-associated molecule 2 is an intermediate before CO loss. In a pressure NMR tube molecule 1 was dissolved in C6D6 with PMe3 in a 1/ 1 ratio under a CO atmosphere. The CO atmosphere was used to increase the lifetime of 2. The results from the 1H and 13C NMR experiments indicated that the initial reaction involves the addition of the phosphine to the manganese (see Scheme 1), with an η5 → η3 interconversion of the pentadienyl ligand to afford 2. Molecule 2 then loses a CO and forms 3 as the final product. 31P NMR confirms the presence of a signal at 29.6 ppm, which is assigned to molecule 2. After ∼6 h at room temperature, the signal at 29.6 ppm decreases and a new signal at 38.7 ppm, corresponding to 3, arises. Also, during the reaction between 1 and PMe3, IR spectra show CO stretching frequency bands of 2 appearing first and then disappearing in the spectrum, with the appearance of CO stretching frequencies assigned to 3. Mass Spectra. The mass spectra show that the parent molecular ions of 1−4 were all observed, albeit in a low relative intensity. Compounds 1, 3, and 4 undergo their primary fragmentation by sequential loss of CO ligands by EI at low energy, while compound 2 was obtained by ESI(+)TOF. The most abundant fragments observed for molecules 1 and 3 were [C7H11Mn]+, and the most abundant fragment for molecule 4 was [C7H11MnPMe3]+. This fragmentation pattern is analogous to the reported fragmentation pattern of [PdlMn(CO)3].18 NMR. The 1H and 13C NMR spectra of the dicarbonyl molecule 3 are consistent with the presence of a symmetrical η5-pentadienyl ligand. The 1H NMR pattern is similar to that shown by the tricarbonyl complex 1, but the resonances of the syn and anti H1 atoms are at lower frequency. The anti hydrogens show coupling with the phosphorus on the order of 5.5 Hz, while the syn hydrogens were overlapped with the methyl groups. The 13C NMR is also similar to that of 1, but all resonances, except the methyl groups, lie to lower frequency. Coupling constants JPC are observed for the symmetric pentadienyl ligand at C1 (9.9 Hz), C2 (2.3 Hz), and Me groups (1.3 Hz), which confirms, along with JPH, that the PMe3 is on the open side of the acyclic ligand. The monocarbonyl complex 4 is fully asymmetric, and the 1H NMR spectrum reveals seven different signals for the hydrogens of the η5pentadienyl group. The spectrum was broad with the expected splittings obscured. The anti and syn hydrogens are at lower frequency in comparison to those of 3. The assignment was confirmed by a 2D HETCOR experiment. 13C{1H} NMR shows the same trend, where all nonequivalent resonances are shifted to high field in comparison to 3, and consequently to 1, which gives evidence of the higher electron density at the metal center by the systematic replacement of carbonyls for trimethylphosphines and demonstrates the π-acceptor character of the pentadienyl ligand. In molecule 4 there is a double coupling JPC with C5 of 13.5 and 7.5 Hz, where C1 shows one coupling of 12.5 Hz. The methyl group at C6 of the pentadienyl ligand, which is near C1, shows a coupling of 5 Hz and the PMe3 ligands show two doublets at 23.7 (P2) and 21.5 (P1) ppm with coupling

Figure 1. Structures of complexes 1−4 obtained by X-ray crystallography. Ellipsoids are at the 50% probability level. Hydrogen atoms are omitted for clarity. C

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torsion angles clearly indicated the different bonding modes between η3-Me2Pdl complex 2 and the η5-Me2Pdl complexes 1, 3, and 4: for C1−C2−C3−C4, 5.21(0.29)° (1), 44.88(0.46)° (2), −6.67(0.34)° (3), −7.60(0.92)° (4); C2−C3−C4−C5 or C2−C3−C2−C1, 5.21(0.29)° (1), 15.00(0.51)° (2), −6.67(0.34)° (3), 1.96(0.88)° (4); Mn−C3−C4−C5 or Mn−C3−C2−C1, 52.98(0.13)° (1), 94.12(0.34)° (2), −52.94(0.15)° (3), 48.77(0.49)° (4). The dihedral angle formed by the least-squares planes of C1−C2−C3 and C3− C4−C5 is 55°, which implies a structure distorted from planarity, as expected from an η3-pentadienyl ligand where there is a noncoordinated C4−C5 double bond (1.329(4) Å). Also, while a distorted U geometry is observed for the η3Me2Pdl ligand, an S geometry was established in the case of the Pdl ligand. The C2−C3−C4−C5 torsion angle is 155.40° for the S-shaped Pdl ligand,18 in comparison to only 15.00° for the U-shaped η3-Me2Pdl ligand. Photoelectron Spectroscopy. The valence gas-phase photoelectron spectra of complexes 1, 3, and 4 along with the previously reported spectrum30 of CpMn(CO)3 for comparison are shown in Figure 2. The arrows in Figure 2

determined molecular structures. The calculated gas-phase bond distances generally agree within 0.01 Å, the angles within 1°, and the dihedral angles within 3° with the crystal structures. The calculated structures are provided in the Supporting Information in a file format convenient for viewing in a molecular modeling program. The conformations in the crystal structures are the most stable conformations in the gas-phase computations. These conformations are also consistent with the solution spectroscopy, suggesting that crystal packing and solvation do not switch the preferred conformations. The metal atoms in molecules 1, 3, and 4 can be regarded as having a distorted-octahedral geometry if the η5-pentadienyl is considered to span three coordination positions. The pianostool structure of Mn with CO ligands shows that the angles between the ligands are in the range of 78.79(51)° for 4 and 95.01(6)° for 1, respectively. The bond angles reflect characteristic values for typical half-sandwich tripodal structures. The centroid of the η5 carbons of the pentadienyl ligand is directly opposite the centroid of the carbonyl carbons (the centroid(Pdl)−Mn−centroid(CO) angle is 178.5° from the crystal structure and 179.8° from the computations). The distance between these centroids is only 2.62 Å, and all of the nearest C(Pdl)−C(CO) distances are 2.8 Å or less, which is very short for nonbonded C−C contacts (the C−C van der Waals contact is ∼3.4 Å, which is also the interplanar spacing of graphite). The rotation barrier of the Mn(CO)3 portion of the molecule with respect to the η5-pentadienyl group is calculated to be 12 kcal/mol, with the transition state corresponding to the structure with the three carbonyls essentially eclipsed with the 1,3,5-carbons of the pentadienyl (see the Supporting Information). Apparently the bonding with the metal pulls the pentadienyl and the carbonyls into close contact and the carbonyls interdigitate as much as possible with the carbons of the pentadienyl, with one carbonyl under the open edge of the pentadienyl to minimize the nonbonded contacts. The phosphine substitutions in molecules 3 and 4 bring the pentadienyl ligand slightly closer to the manganese center according to the 3σ criterion, as observed from Mn−C1 (2.2133(15) Å, 1; 2.1975(16) Å, 3; 2.183(5) Å, 4) and Mn−C2 (2.1626(13) Å, 1; 2.1470(15) Å, 3; 2.117(4) Å, 4). The shortening of Mn−CO (1.8055(14) Å, 1; 1.7915(16) Å, 3; 1.750(3) Å, 4) and the consequent lengthening of the CO (1.1490(17) Å, 1; 1.160(2) Å, 3; 1.175(4) Å, 4) bond lengths are consistent with the increase in the π back-bonding, as evidenced in the previously mentioned infrared data and 13C NMR. The Mn−P bonds in 3 (2.2290(7) Å) and 4 (2.2453(14) and 2.2333(10) Å) are in the vicinity of analogous molecules, such as Cp*Mn(CO)2PMe3 (2.220(2) Å).12 Molecule 2 can also be considered to have distortedoctahedral coordination in which the η3-pentadienyl ligand is considered to span two coordination positions on the manganese atom, similar to the case for allyl-containing complexes,18,28 but considerably more distorted from ideal geometry. The manganese atom is bound to the phosphorus atom (2.2618(10) Å) with longer bond lengths in comparison to 3 (2.2290(7) Å) and 4 (2.2333(10) Å). Also, one of metal− carbon bonds of the carbonyl groups is significantly shorter Mn−C9 (1.780(3) Å) than the other two: Mn−C8 (1.827(4) Å) and Mn−C10 (1.817(3) Å). Similar trends in the coordination of PMe3 and CO ligands have been observed for the (η3-Pdl)Mn(CO)3PMe3 analogue; however, the η3Me2Pdl fragment is bonded to the metal in a less asymmetric fashion and with a different conformation.18,29 Indeed, the

Figure 2. He I spectra (black lines) of 1, 3, and 4 in comparison to the previously reported CpMn(CO)3. The metal-based ionization bands are given in red, and the ligand-based π bands are labeled Me2Pdl and Cp. The arrows point to the calculated first vertical ionization energies.

point to the DFT-calculated vertical first ionization energies for the molecules, where the structures of the cations in the calculations are frozen to the optimized geometries of the neutral molecules. The calculated values are in good agreement with the experimental results but slightly overestimate the shift to lower ionization energy with phosphine substitution. The valence ionization energies and relative areas for the ionizations of the molecules under study are given in Table 1. The photoelectron spectrum of CpMn(CO)3 has been discussed in detail previously.30 The group of three metal-based ionizations of the d6 metal center occurs in a broad asymmetric band between 7 and 9 eV, followed by the characteristic Cpbased ionizations between 9 and 10.5 eV. The valence ionization bands that are observed for molecules 1, 3, and 4 correspond similarly to the occupied valence metal d orbitals and the highest occupied π orbitals of the pentadienyl ligands, in addition to (in the case of 3 and 4) ionizations derived from the Mn−P σ orbitals.31 The relative shifts of the valence metalbased and pentadienyl-based ionizations give an indication of the overall electronic effects of pentadienyl and phosphine substitution on these molecules. (η5-Me2Pdl)Mn(CO)3 (1). The valence gas-phase photoelectron ionizations of 1 are all shifted to lower energy in D

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character and the third and fourth bands in the spectrum as predominantly ligand in character. The open pentadienyl ligand in 1 lowers the symmetry of the molecule in comparison to CpMn(CO)3, and hence the d6 metal electrons should occupy three orbitals without degeneracy. However, the first two ionizations are very close in energy and the ionizations are best fit with one band resulting in two metal-based ionization bands with a relative area of 2:1. This splitting is more pronounced in the previously reported (1-methylpentadienyl)manganese tricarbonyl derivative.33 DFT calculations support these assignments for molecule 1, as shown in Figure 4. The first ionization band

Table 1. Analytical Representations and Labels of Ionizationsa position

high, low half-width

rel areab

label

5

7.72 8.12 8.78 9.71 11.12 7.03 7.28 8.09 9.13 9.60 10.74 6.06 6.53 7.52 8.51 9.04 9.35 10.44

(η -Me2Pdl)Mn(CO)3 (1) 0.58, 0.38 1 M1 0.60, 0.30 1.04 M2 0.56, 0.29 0.56 ligand π (L1) 0.69, 0.38 0.56 ligand π (L2) 0.92, 0.33 0.39 ligand π (symmetric) (η5-Me2Pdl)Mn(CO)2(PMe3) (3) 0.55, 0.55 1 M1 0.57, 0.57 0.92 M2 0.54, 0.31 0.65 L1 0.48, 0.45 0.56 L2 0.55, 0.38 0.45 L3 0.54, 0.35 0.59 L4 (η5-Me2Pdl)Mn(CO)(PMe3)2 (4) 0.74, 0.49 1 M1 0.49, 0.49 1.04 M2 0.43, 0.28 0.68 L1 0.49, 0.49 0.90 L2 0.49, 0.32 0.35 L4 0.49, 0.34 0.92 L3 0.49, 0.50 0.41 L5

a

See the Experimental Section for a definition of the analytical representation. All energies are given in eV. bHe II/He I relative to peak assigned area of 1.

comparison to the ionizations of (Cp)Mn(CO)3.30 The shift of the metal-based ionizations indicates the greater donor ability of the Pdl ligand in comparison to the Cp ligand. Ionization assignments for this complex are confirmed by comparing the He I and He II spectra of 1 (Figure 3) and the spectra of

Figure 4. He I photoelectron spectrum of 1 with the corresponding calculated orbitals (isosurface ±0.05). The lines on the scale above the spectrum indicate the calculated Kohn−Sham orbital energies, with the energies shifted to align the HOMO energy with the first ionization.

contains ionizations from the calculated orbitals labeled M1 and M2. The calculated energies of these orbitals agree well with the experimental spectrum, as indicated by the dashes on the energy scale directly above the spectrum. After the first three ionizations with predominant metal character, the next three bands shown in Figure 4 correspond to the ionizations from the pentadienyl π orbitals. The calculations indicate that the pentadienyl π orbital with the lowest energy ionization (labeled L1) has a considerable amount of donation to the Mn center. This extensive mixing is not observed at this energy in the calculations of the Cp analogue and further illustrates the stronger bonding of pentadienyl ligands in comparison to Cp and related ligands.34 (η5-Me2Pdl)Mn(CO)2PMe3 (3) and (η5-Me2Pdl)Mn(CO)(PMe3)2 (4). The effect of phosphine group substitution on the electron richness of the molecule has been studied by systematically replacing carbonyl ligands with trimethylphosphine ligands in the (η5-Me2Pdl)Mn(CO)3−n(PMe3)n complexes. The assignments of the ionization bands in the photoelectron spectra of 3 and 4 are similar to those of the all-CO analogue 1. The first three ionizations, represented by two ionization bands in a 2/1 ratio, are the metal-based ionizations. The next two higher ionizations are pentadienyl π based, and then there is one additional ionization in the

Figure 3. Comparison of the He I and He II spectra of 1, showing the difference in the relative intensities of the metal-based ionization bands in comparison to the ligand-based ionization bands.

previously reported complexes3,8a and through DFT calculations. A comparison of the relative ionization peak intensities in the He I and He II spectra provides insight into the atomic character associated with each ionization band. The change in relative intensities as a function of the change in source photon energy from He Iα to He IIα is due primarily to the different inherent photoionization cross sections of the atomic orbitals.31,32 The amount of metal−ligand mixing modifies the relative intensity changes according to the character of the molecular orbital. Figure 3 shows that the relative intensities of the ionization envelope around 8 eV increase in comparison to the higher energy ionizations (9−11 eV) on going from the He I to the He II spectrum. This observation aids in identifying the first two ionization bands in Table 1 as primarily metal in E

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the solvent, and the account for entropy change in the solvation model. In order to judge whether the 0.5 scale factor is reasonable for this system, we considered modeling the dissociative reaction in two different ways, as shown in Scheme 2. The first

spectrum of 3 and two additional ionizations in the spectrum of 4 near 9 eV. The additional ionizations correspond to Mn−P σbond ionizations.31 In the monophosphine complex 3, the metal-based ionizations are shifted to roughly 0.7 eV lower ionization energies in comparison to the all-CO complex 1, as can be seen in Figure 2. The pentadienyl ligand ionizations going from 1 to 3 shifted to lower ionization energies by a slightly smaller magnitude: 0.6 eV. The ionizations shifted even further in molecule 4. The metal-based ionizations shifted lower by nearly 1 eV between 3 and 4, and the pentadienyl ligands ionizations once again shifted lower by roughly 0.6 eV. The lowered ionization energies are a direct measure of the increased electron density on the Mn center that occurs from replacing the π-accepting CO ligands with strong σ-donating PMe3 ligands. The calculations were also able to properly account for the increase of electron density at the metal center, as is evident by the calculated vertical ionization energies shown in Figure 2. Free Energies of Ligand Substitution. Experimental determination of the activation energies and free energies of intermediates of these phosphine substitution reactions is complicated due to the evolution of CO, and such studies are beyond the scope of this study. On the other hand, these systems provide a near-ideal opportunity to explore the energetics of ligand substitution reactions computationally. As has already been shown, the computations are able to account well for the geometric structures, the vibrational frequencies as a function of electron richness at the metal centers, and the electronic energies as a function of electron distribution and bonding as determined by photoelectron spectroscopy. Additional validations of the computational method are provided in the Experimental Section and the Supporting Information. Thus, for purposes of probing the trends in free energies along the reaction coordinate, the computations can provide information on the relative electronic energies, zero-point vibrational energies, and the thermal contributions to the vibrational enthalpy and vibrational entropy changes. Furthermore, relative solvation effects are minimal in comparison to most other systems because all of the species are neutral molecules with uniform charge distributions, and the chemistry is carried out in cyclohexane, which has a low polarizability and only weak explicit solvent interactions with the molecules, both of which can be reasonably modeled with solvent continuum models in the first case and current dispersion energy models in the second case. The remaining challenge for the computations is estimation of the solution translational and rotational entropies. Several papers have noted that the calculated gas-phase translational and rotational entropies significantly overestimate the solution translational and rotational entropies, and different approaches have been used to adjust the gas-phase entropy values for the solution free energies.35−52 This is particularly important when the number of reactant molecules is different from the number of product molecules, and the error is in the opposite direction for a dissociative process from that for an associative process. In the model used in this study, the gas-phase translational and rotational entropies give a free energy for dissociation of CO from molecule 1 that is too low by ∼7 kcal/mol, and the free energy for association of PMe3 is too high by ∼9 kcal/mol, leading to a combined error in the relative energies for the processes of ∼16 kcal/mol. A common approach is to scale the calculated gas-phase entropy by a factor of 0.5 for the solution entropy. However, the correct scale depends on the molecule,

Scheme 2. Two Approaches to Modeling the M−L Ligand Dissociation Energy of Solvated Species

method employs only a continuum solvation model for all species, whereas the second method includes the explicit interaction of one solvent molecule with the vacant site. The primary free energy difference between the two approaches is that in the first case one molecule of reactant results in two molecules of product, while in the second approach the number of reactant and product molecules is the same. In the limit of a nonbonding interaction (but including electrostatic and dispersive interactions) of the metal complex with a solvent molecule, designated by species M(S) which is also solvated, the free energies calculated by each approach should be approximately the same if the solvation and entropy terms are consistent with each other. In the calculation of the free energy by the second approach it was found that dispersion energies were necessary to obtain a reasonable description of the weakly associated M(S) species. For the calculated dissociation of CO from molecule 1 in cyclohexane ΔG(1) = ΔG(2) if the gasphase translational and rotational entropies are scaled by 0.57. Other functionals that include dispersion (see the Supporting Information) yielded factors ranging from 0.48 to 0.65 and the corresponding contribution to the solution entropy varied over a range of only ±1 kcal/mol. Thus, for the purposes of this study a scale factor of 0.5 is reasonable for the translational and rotational entropies and was used throughout. Overall uncertainties in the calculated relative free energies of reactions (further details in the Experimental Section and in the Supporting Information) are estimated to be on the order of 2−3 kcal/mol, with a smaller uncertainty in the relative energies between similar reactions. Figure 5 shows the calculated free energies at key points along the reaction coordinate for PMe3 substitution for carbonyl in molecule 1, and the subsequent photochemical reaction to synthesize molecule 4. A similar diagram of the enthalpies is shown in Figure S10. Also shown schematically in Figure 5 are the structures of the molecules at key minima and transition states on the potential energy surface. The optimized coordinates of these structures are provided in a file in the Supporting Information in a form that can be imported easily into many molecular modeling programs for 3-dimensional visualization. PMe3 substitution for a CO in 1 to form 3. Both the associative substitution mechanism that proceeds through intermediate 2 and the dissociative mechanism that proceeds through initial loss of carbonyl were compared computationally. The transition state for the association of PMe3 with 1 to form 2 is designated [1→2]‡. In this transition state two carbon atoms of the η5-pentadienyl group that are proximal to the incoming PMe3 molecule pull away from the metal center as the PMe3 molecule approaches, thus distorting the pentadienyl to an η3 bonding geometry. After the Mn−PMe3 bond is fully F

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Figure 5. Calculated free energies for the substitution of CO by PMe3. The curved lines clarify which structures are stable energy minima and which structures are transition state energies (one imaginary frequency along the reaction coordinate). The observed room temperature (RT) reaction is shown in green, the reaction in refluxing cyclohexane is shown in red, and the photochemical reaction is shown in blue.

CO and supportive of the computational modeling of the bonding interactions and entropy. PMe3 substitution for a CO in 3 to form 4. The associative mechanism for phosphine substitution of CO in 3 to form 4 is shown in gray in Figure 5 because it is thermally nonproductive. The transition state energy for PMe3 association with 3 is not much different from that with 1, which follows because the energy for the pentadienyl to slip from η5 to η3 coordination and open a coordination site is not much different. The difference is that the associated intermediate (η3-Me2Pdl)Mn(CO)2(PMe3)2 is less stable than 3 and is not favored in the equilibrium, presumably due to the electron richness at the metal center and perhaps steric repulsions. Furthermore, the electron richness at the metal center significantly increases the transition state energy for carbonyl dissociation from the intermediate. Molecule 4 is significantly less thermodynamically stable than the molecules and intermediates that precede it in the reaction diagram. Consequently, 4 is prepared by photochemical dissociation of CO followed by phosphine substitution, as shown by the dotted line path in Figure 5. A similar associative substitution mechanism for the analogous cyclopentadienyl complex CpMn(CO)3 is much less favorable. Figure 6 compares the free energies of the first substitution of CO by PMe3 for the pentadienyl and cyclopentadienyl complexes. After the free energies of the reactants are aligned to zero in both cases, the energies of the final substituted products relative to the reactants are not much different. However, the path for substitution is much higher in energy for the cyclopentadienyl complex. The slip of the cyclopentadienyl to η3 coordination is 20−25 kcal/mol less effective at accommodating the associate intermediate than the slip of the pentadienyl, and the cyclopentadienyl also is less effective at stabilizing the intermediate that results from

formed in molecule 2, the computations show that the most stable conformation has the two carbon atoms distal (trans) to the Mn−PMe3 bond more distant from the metal center, as observed in the crystal structure of 2. Thus, the proximal distorted conformation of 2 is the kinetic product and the distal distorted conformation of 2 is the thermodynamic product. The difference in free energy is on the order of 7 kcal/mol, with the distal conformation higher for the [1→2]‡ transition state and the proximal conformation higher for the molecule 2. An additional structure of 1 was optimized in which the metal−pentadienyl portion was constrained to the η3 bonding geometry of 2: that is, without the incoming PMe3 molecule. This structure is designated 1(η3) in Figure 5. As can be seen, this structure is only slightly less stable than the optimized transition state structure [1→2]‡. Therefore, the transition state is largely formed by distortion of the pentadienyl from the η5 to the η3 bonding configuration. Because this distortion essentially opens up a coordination site for the incoming phosphine, it might be considered a low-energy intramolecular dissociative process. The transition state for carbonyl dissociation from 2 to form 3, designated by the notation [2→3]‡, is calculated to be about 5 kcal/mol higher in energy than the transition state [1→2]‡. The alternative mechanism for formation of 3 from 1 involving carbonyl dissociation from 1 is about 15 kcal/mol higher in energy. The dissociation of carbonyl from 2 is compensated by the pentadienyl relaxing back from η3 to the η5 bonding and filling the coordination saturation. The difference in energy of these two carbonyl dissociation transition states is closely similar to the difference in energy of 1 and 1(η3). Figure 5 shows that molecule 2 is slightly more stable than both molecule 1 and molecule 3, consistent with the ability to observe this intermediate while being susceptible to the loss of G

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coordination site at the metal and inviting the phosphine ligand in. In a sense this is an intramolecular dissociative mechanism as the pentadienyl ligand distorts from formally occupying three coordination sites (η5) to formally occupying two coordination sites (η3). The relative strengths of the carbonyl and phosphine bonds are dependent on the electron richness at the metal center. In somewhat electron deficient molecules such as molecule 2, where there are three carbonyl ligands, the σ-donor phosphine ligand bonds well and molecule 2 is more stable than molecule 1 and molecule 3 according to these computations. However, phosphine association to the more electron-rich molecule 3 is not as favored, carbonyl dissociation from 3 is high in energy, and the diphosphine molecule 4 is significantly less stable than the other molecules. Thus, the relative bonding capabilities of both Pdl vs Cp and PMe3 vs CO depend on the electron richness at the metal center.

Figure 6. Calculated free energies for the first substitution of CO by PMe3 (same as Figure 5 for molecules 1−3) with comparison of the corresponding calculated free energies for the analogous cyclopentadienyl complex CpMn(CO)3 ([Cp]) in blue.

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EXPERIMENTAL SECTION

General Procedures. All preparative work was carried out under a nitrogen atmosphere using conventional Schlenk techniques. Solvents were carefully dried; tetrahydrofuran and Et2O were freshly distilled from Na/benzophenone, while hexane and cyclohexane were distilled from CaH2. The 1H and 13C NMR spectra were recorded on JEOL GSX-270, JEOL Eclipse-400, and Bruker Avance DPX 300 MHz spectrometers using deuterated solvents and TMS as an internal reference. Mass spectra were run on Hewlett-Packard HP-5990A (compounds 1, 3, and 4) and ESI spectra recorded in positive ion mode using an Agilent G1969A spectrometer (compound 2). Compounds MnBr(CO)553 and 2,4-dimethyl-1-trimethylstannyl-2,4pentadiene25 were prepared according to literature procedures. Trimethylphosphine and the deuterated solvents were used as obtained from Strem Chemicals, Fluka, or Sigma-Aldrich. Elemental analyses were performed at Cinvestav, using a Thermo-Finnigan Flash 1112 elemental analyzer. IR spectra were recorded on a PerkinElmer 6FPC-FT spectrophotometer in cyclohexane solution. Photochemical reactions were performed in a reaction vessel fitted with a quartz water-cooled immersion well, a reflux condenser, and a cannula for admission of nitrogen. The irradiation source was a Hanovia 125 W medium-pressure mercury arc lamp. Synthesis of (η5-CH2C(Me)CHC(Me)CH2)Mn(CO)3 (1). Molecule 1 has already been reported, but by different synthetic procedures.29 MnBr(CO)5 (485 mg, 1.76 mmol) in THF (45 mL) was heated under reflux with (2,4-dimethylpentadienyl)trimethyltin (457 mg, 1.76 mmol). After 2.5 h the brown-orange solution turned yellow and the solution was cooled, filtered, and evaporated until dryness. The solid product was dissolved in hexane and purified using an alumina column. The lemon yellow band obtained from the column afforded crystals in 75.6% yield (312 mg, 1.13 mmol). Single crystals, as pale yellow prisms, were obtained from a saturated solution in pentane at −20 °C. Mp: 71.1−72.9 °C. IR (CO, ν, cm−1): 2021, 1954, 1934. 1H NMR (C6D6): δ 0.14 (d, J = 2.7 Hz, 1H, H1anti), 2.12 (d, J = 2.0 Hz, 1H, H1syn), 4.77 (s, 1H, H3), 1.61 (s, 6H, Me). 13C NMR (C6D6): δ 58.4 (C1), 113.7 (C2), 91.3 (C3), 27.7 (Me), 221.0−223.0 (CO). MS: m/z (20 eV) 234 (11) [M+], 206 (13), 178 (18), 150 (100), 95 (20), 55 (4). Obtainment of the Intermediate (η3-CH2C(Me)CHC(Me)CH2)Mn(CO)3PMe3 (2). A solution of PMe3 (0.53 mL, 5.12 mmol) in 5.0 mL of benzene was added to another solution of molecule 1 (150 mg, 0.64 mmol) in 5.0 mL of benzene with stirring, in a glass reactor, and CO was introduced at 1 atm. After 3 h at room temperature, the solution changed from yellow-green to orange-yellow. It was filtered and evaporated until dryness. The yellow-orange solid was crystallized in pentane and after 2 days at −20 °C afforded a mixture of compounds 1−3 in a 0.45/0.45/1.0 ratio. Several attempts to purify 2 by sublimation or recrystallizations were unsuccessful; therefore, the sample was not submitted to elemental analysis. However, single crystals of 2, as small yellow plates, were selected from this mixture.

carbonyl dissociation. These energy trends account for the fact that substitution reactions of CpMn(CO)3 are typically carried out photochemically rather than thermally to dissociate a carbonyl and create an open site.

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CONCLUSIONS In comparison to the more extensively studied Cp ligands, Pdl ligands are more readily able to bond to metal centers through η1, η3, and η5 bonding modes. Also, Pdl has been shown to form stronger bonds to low-valent metal centers in comparison to the analogous Cp ligand. This study on the molecules of the general form (η5-Me2Pdl)Mn(CO)3−n(PMe3)n (where n = 1, 2) has utilized NMR, IR, and photoelectron spectroscopy and crystallography to explore ligand substitution reactions in which the Pdl ligand can interconvert between η5 and η3 bonding modes. These systems also provided an ideal opportunity to explore the computational aspects of reactivity at metal centers. The structural and spectroscopic information obtained on these systems allows validation of the computational methods. The ligands (CO, PMe3, pentadienyl) are relatively small with strong internal bonding, so that the focus is on their interactions. All species are neutral, and the chemistry is carried out in low-dielectric solvents, so that solvent contributions are minimal. Current challenges for computational modeling of reactivity and catalysis concern the handling of entropy in solution and the handling of dispersion energies. Even with the near-ideal situation offered by these systems, uncertainties in the relative free energies calculated in this study are estimated to be about 2−3 kcal/mol. This uncertainty is inconsequential for the conclusions of this study. It has been shown that a PMe3 ligand displaces a carbonyl on 1 through an associative mechanism, forming molecule 2 as an intermediate. The η3-Me2Pdl ligand in molecule 2 bonds to the metal in an η3 bonding mode and takes on a unique distorted U shaped geometry. The energy of the transition state to this associated molecule is approximately the energy for the pentadienyl to slip and distort to the η3 bonding mode. Therefore, it is not necessary to think of the associative mechanism as a concerted process of metal−phosphine bond formation, forcing the slippage and distortion of the pentadienyl ligand to the η3 bonding mode, but rather a process of the pentadienyl ligand distorting and opening a H

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Mp: 68.8−69.3 °C. IR (CO, ν, cm−1): 2007 (m), 1926 (vs), 1907 (vs) and 1621 (CC, vw). 1H NMR (C6D6): δ 2.24 (1H, H1anti), 2.31 (H1syn), 3.77 (1H, H3), 4.30 (1H, H5), 4.55 (1H, H5′), 1.82 (3H, Me6), 2.03 (3H, Me7), 1.02 (d, JP−H = 7.2 Hz, 3H, PMe3). 13C NMR (C6D6): δ 47.3 (C1), 100.8 (C2), 65.99 (C3), 148.1 (C4), 103.1 (C5), 24.9 (Me6), 29.5 (Me7), 20.9 (d, JP−C = 28.5 Hz, PMe3), 224.0 (CO). 31 P NMR (C6D6): δ 29.6 (s, br). ESI -TOF: m/z 311.0603; error ppm −0.0992; DBE 4.0. Fragmentation pattern: 311 (10), 297 (100), 269 (5), 256 (31), 213 (2), 204 (11). Synthesis of (η5-CH2C(Me)CHC(Me)CH2)Mn(CO)2PMe3 (3). A mixture of 1 (150 mg, 0.64 mmol) and PMe3 (1.0 M in THF, 1.9 mL, 1.9 mmol) was stirred in cyclohexane (40 mL) at reflux for 14 h. After it was cooled, the reaction mixture was filtered and evaporated under vacuum until dryness, resulting in a canary yellow solid. The product was dissolved in pentane, filtered, and evaporated under vacuum, affording a 72.7% yield (131.4 g, 0.47 mmol) of yellow crystals which sublimed at 48 °C/0.05 mmHg. Single crystals, as yellow prisms, were obtained from a saturated solution in pentane at −20 °C. Mp: 85.3− 85.9 °C. IR (CO, ν, cm−1): 1942, 1876. 1H NMR (C6D6): δ −0.21 (d, JP−H = 5.5 Hz, 1H, H1anti), 2.01 (overlap, H1syn), 4.88 (s, 1H, H3), 2.01 (s, 6H, Me), 1.23 (d, JP−H = 8.5 Hz, 3H, PMe3). 13C NMR (C6D6): δ 56.8 (d, JP−C = 9.9 Hz, C1), 112.2 (d, JP−C = 2.3 Hz, C2), 89.6 (C3), 28.7 (d, JP−C = 1.3 Hz, Me), 22.7 (d, JP−C = 27.7 Hz, PMe3), 224.0 (CO). 31P NMR (C6D6): δ = 38.7. MS: m/z (20 eV) 282 (11) [M+], 254 (0.3), 226 (78), 150 (100), 95 (10), 55 (6). Anal. Calcd: C, 51.07; H, 7.14. Found: C, 51.10; H, 7.29. Synthesis of (η5-CH2C(Me)CHC(Me)CH2)Mn(CO)(PMe3)2 (4). A mixture of 3 (251 mg, 0.89 mmol) and PMe3 (0.36 mL, 3.56 mmol) dissolved in cyclohexane (190 mL) was photolyzed at room temperature for 3 h. The yellow-brown solution was filtered through Celite and evaporated until dryness. The orange-yellow oily solid was extracted with pentane, and the extract was filtered and reduced in volume (∼5 mL), giving a 9% yield (0.03 g, 0.08 mmol) of orangeyellow crystals at −20 °C, which decomposed at 92 °C without melting. IR (CO, ν, cm−1): 1839. 1H NMR (C6D6): δ −0.97 (s,br, 1H, H1anti), −0.39 (s,br, 1H, H5anti), 1.57 (s, br, H5syn), 1.92 (overlap, H1syn), 4.78 (s, br, 1H, H3), 1.92 (s, 3H, Me7), 2.28 (s, 3H, Me6), 0.98 (d, JP−H = 6.0 Hz, 3H, PMe3, P1), 1.33 (d, JP−H = 6.7 Hz, 3H, PMe3, P2). 13C NMR (C6D6): δ 52.2 (dd, JP−C = 13.5, 7.5 Hz, C1), 100.8 (s, C2), 88.0 (s, C3), 109.9 (s, C4), 49.0 (d, JP−C = 12.5 Hz, C5), 28.0 (d, JP−C = 5.0 Hz, Me6), 28.6 (s, Me7), 21.5 (d, JP−C = 18.7 Hz, PMe3, B), 23.7 (d, JP−C = 23.6 Hz, PMe3, A), 229.0 (br, CO). 31P NMR (C6D6): δ 32.0 (A), 41.1 (B). MS: m/z (20 eV) 330 (4) [M+], 302 (26), 226 (100), 150 (77), 96 (13), 55 (9). Anal. Calcd: C, 50.92; H, 8.85. Found: C, 50.58; H, 8.93. X-ray Structure Determination of Molecules 1−4. Single crystals of molecules 1−4 were mounted on glass fibers. X-ray data were collected on an Enraf-Nonius Kappa CCD difractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at low temperature. Structure solutions and refinements were carried out using SHELXS-9754 included in the package Wingx.55 The manganese positions were determined by direct methods. The remaining nonhydrogen atoms were found by successive full-matrix least-squares refinement and difference Fourier map calculations. In general, nonhydrogen atoms were refined anisotropically, while hydrogen atoms were placed at idealized positions. Absorption correction was applied for all molecules. Data collection parameters and structure refinement details are summarized in Table 1. Photoelectron Data Collection. Photoelectron spectra were recorded using an instrument that features a 36 cm hemispherical analyzer56 and custom-designed photon source, sample cells, and detection and control electronics as described previously.57 The ionization energy scale was calibrated using the 2P3/2 ionization of argon (15.759 eV) and the 2E1/2 ionization of methyl iodide (9.538 eV). The argon 2P3/2 ionization also was used as an internal calibration lock of the absolute ionization energy to control spectrometer drift to less than ±0.005 eV throughout data collection. During data collection the instrument resolution, measured using the full width at halfmaximum of the argon 2P3/2 ionization, was 0.024−0.030 eV. All of the spectra were corrected for the presence of ionizations caused by other

emission lines from the discharge source.58 The He I spectra were corrected for the He Iβ line (1.866 eV higher in energy and 3% of the intensity of the He Iα line), and the He II spectra were corrected for the He IIβ line (7.568 eV higher in energy and 12% of the intensity of the He IIα line). All data also were intensity corrected with an experimentally determined instrument analyzer sensitivity function. In the figures of the data, the vertical length of each data mark represents the experimental variance of that point.59 The samples sublimed cleanly at room temperature with no visible changes in the spectra during data collection. Photoelectron Data Analysis. The valence ionization bands are represented analytically with the best fit of asymmetric Gaussian peaks.59,60 The Gaussians are defined with the position, amplitude, and half-width for the high binding energy side of the peak and the halfwidth for the low binding energy side of the peak. A minimum number of Gaussian peaks were used to model the ionization band contours. The peak positions and half-widths are reproducible to about ±0.02 eV (∼3σ level). The parameters describing an individual ionization are less certain when two or more peaks are close in energy and are overlapping. Confidence limits for the relative integrated peak areas are about 5%, with the primary source of uncertainty being the determination of the baseline under the peaks. The baseline is caused by electron scattering and taken to be linear over the small energy range of these spectra. The total area under a series of overlapping peaks is known with the same confidence, but the individual peak areas are less certain. He II spectra are modeled with the same Gaussian peaks (energies and shapes) as obtained from the analysis of the He I spectra. Only the relative intensities of the Gaussian peaks are allowed to vary to account for the changes in ionization cross sections with excitation photon energy. Density Functional Theory Calculations. All calculations were performed using ADF2013.01.61−63 Different levels of density functional models were tested, including the local density approximation (LDA), the generalized gradient approximation (GGA) with and without dispersion, a hybrid functional with and without dispersion, a metaGGA functional, and a meta-hybrid functional. A summary is provided in the Supporting Information. All of the functionals that were examined lead to the same trends and conclusions for these molecules, except for weak interactions (e.g., explicit solvent molecule association with the metal complex and some transition states) where dispersion is necessary. Initial geometry optimizations and frequency calculations were carried out efficiently using the VWN functional with the Stoll correction.64 The optimized geometries of all reactants, intermediates, and products were true minima devoid of imaginary frequencies, and all transition state structures had a single imaginary frequency. In addition, possible conformers and rotational isomers other than those indicated by the crystal structures were evaluated, and it was found in each case that the conformations in the crystal structures led to the global minima in the computations. Also, the intrinsic reaction coordinates from the transition states to reactants and products were traced to ensure connectivity on the potential energy surface. The calculated stretching frequencies agreed well with the experimental frequencies in solution and were used without scaling to determine the zero-point vibrational energies and the thermal vibrational enthalpy and vibrational entropy contributions to the free energy. Solvation was taken into account with single-point computations using the conductor-like screening model (COSMO65) with default parameters as implemented in the ADF package. For the electronic energies the initial geometries above were further optimized at the M06-L level66 to account for weak interactions using the Becke grid67 with good precision to aid convergence, and the final reported energies were determined with the M06 functional.66 A triple-ζ Slater-type orbital (STO) basis set with one polarization function (TZP) was used in all calculations. For dissociative steps the basis set superposition error (BSSE) was estimated by the standard counterpoise method. The largest BSSE found for this basis set on these molecules was 1.5 kcal/mol, but this approach is known to give an overestimate68 in general and especially in this study, because rearrangement of the metal complex toward the vacant site was not included in the BSSE estimate. In any event the I

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BSSE is small in comparison to the other uncertainties and for consistency is not included in the energies. Relativistic effects, although minor, were taken into account in all calculations by using the scalar ZORA formalism69 implemented as part of the ADF program. All electronic structures with unpaired spin (the positive ions that result from photoelectron ionization) were calculated using an unrestricted framework. Only low-spin molecules have been analyzed, consistent with the strong ligand fields. As a check of the ability of this model to account for a metal−phosphine bond energy, the bond enthalpy for dissociation of P(n-Bu)3 from trans-Cr(CO)4(P(n-Bu)3)2 in decane at 388 K was calculated. The experimental value is 42.5 kcal/mol,70 and the calculated value is 44 kcal/mol. Figures of the valence orbitals were created using the program Molekel.71

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ASSOCIATED CONTENT

S Supporting Information *

Figures, tables, CIF files and a .txt file giving 1H, 13C, and 31P NMR and IR spectra of the intermediate molecule (η3Me2Pdl)Mn(CO)3PMe3 (2), a comparison of He I and He II spectra of 3 and 4, crystallographic data, including atomic coordinates, bond lengths and angles, anisotropic displacement parameters, hydrogen coordinates, isotropic displacement parameters, and torsion angles for molecules 1−4 (CCDC 934831−934834), a comparison of calculated structures, the energy profile for rotation of Mn(CO)3 in molecule 1, an example input file, and calculated molecular structures in a file format convenient for viewing in molecular modeling programs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for D.L.S.: [email protected]. *E-mail for M.A.P.-S.: [email protected]. Present Addresses §

Merck & Co., Inc. One Merck Drive, Whitehouse Station, NJ, USA 08889. ∥ Department of Chemistry, University of North Carolina, Chapel Hill, NC, USA 27599. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dedicated to Professor Dietmar Seyferth for his role in the development of organometallic chemistry and for his support and encouragement to organometallic chemists. M.A.P.-S. thanks Conacyt (Mexico) for financial support through projects 46556-E and 152280. J.I.d.l.C.C. and B.P.-M. give thanks for graduate scholarships. D.L.L. thanks the National Science Foundation through the project CHE-1111570.

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