Phosphaethynolate Dimerization and Carbonyl Migration in

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

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Phosphaethynolate Dimerization and Carbonyl Migration in Cyclopentadienyliron Carbonyl Systems: A Theoretical Study Chencheng Liu,† Yang Liu,† Ge Tian,† Liang Pu,† Zhong Zhang,*,† Song Yang,‡ and R. Bruce King*,§ †

College of Chemistry & Pharmacy, Northwest A&F University, Yangling, Shaanxi 712100, People’s Republic of China Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Changzhou University, Changzhou, Jiangsu 213164, People’s Republic of China § Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602, United States ‡

S Supporting Information *

ABSTRACT: Successive decarbonylation and cyclodimerization of the mono- and binuclear FeCp(PCO)(CO) 2 , Fe2Cp2(PCO)2(CO)4, and Fe2Cp2(CO)2(P2) complexes have been investigated using density functional theory calculations at the M06L/DZP level. For the mononuclear complexes, the lowest energy FeCp(PCO)(CO)m (m = 2, 1) structures always prefer the η-P(CO) bent configuration relative to other models. However, the lowest energy structure for the species of stoichiometry FeCp(PCO) is actually FeCp(P)(CO) with a formal FeP triple bond. The binuclear complexes are much more complicated. The Fe2Cp2(PCO)2(CO)4 structure, highly entropy controlled, has a P2C2 ring originating from cyclodimerization of two PCO groups from two FeCp(PCO)(CO)2 monomers. The lowest energy Fe2Cp2(PCO)2(CO)3 structure has a μ-P(CO) group donating two lone-pair electrons to each iron atom in a nonbonded Fe···Fe unit. The most favorable structure for the species of stoichiometry Fe2Cp2(PCO)2(CO)2 has an end-to-end diphosphene P2 bridge connecting two FeCp(CO) fragments in a trans arrangement. The lowest energy Fe2Cp2(PCO)2(CO) structure has μP(P) and μ-C(O) groups bridging an Fe−Fe single bond. The lowest energy structures for the species of stoichiometries Fe2Cp2(P2)(CO)2 and Fe2Cp2(P2)(CO) have a central P2Fe2 tetrahedron as well as two or one CO group(s) bridging Fe−Fe or FeFe bonds. The lowest energy carbonyl-free Fe2Cp2P2 structure has a rhombic Fe2P2 core with no direct P−P bond. charge on oxygen (1′ in Figure 1).6 Like the homologous pseudohalide NCO−, the PCO− anion is an ambidentate ligand which can bond to transition-metal centers through either the phosphorus atom (I in Figure 1) or the oxygen atom (II in Figure 1). The first experimental evidence for the ambident character of the PCO− anion was provided through reaction of Na(OCP) with silyl triflates.7 Subsequently, the first transitionmetal complex with a terminal phosphaethynolate ligand, namely Re(PCO)(CO)2(triphos), was synthesized by a simple metathesis reaction and shown to have a metallaphosphaketene (M−PCO) structure (I in Figure 1) with a highly covalent metal−phosphorus bond.8 However, complexation of PCO− with oxophilic uranium and thorium fragments leads to M−O− CP: species with a metal−oxygen bond (II in Figure 1).9 A third alternative bonding mode is the η2-PC(O) mode (III in Figure 1), which has been reported in a copper complex.10 In addition, the PC double bond in PCO− is reactive toward cyclodimerization, forming a P2C2 four-membered ring. This reflects the generally high reactivity of multiple bonds of thirdrow atoms such as phosphorus.

1. INTRODUCTION The 2-phosphaethynolate anion PCO − , which is the phosphorus analogue of the cyanate ion NCO−, was first synthesized by Becker and co-workers1 in 1992 by the reaction of LiP(SiMe3)2 with dimethyl carbonate. However, the chemistry of the PCO− anion remained dormant until 2011, when Grützmacher and co-workers2 reported a convenient synthesis of the PCO− anion as its sodium salt by heating NaPH2 with CO in dimethoxyethane solution under pressure. In 2013 Jupp and Goicoechoa3 found a new synthesis of the 2phosphaethynolate anion by direct carbonylation of bare polyphosphides at atmospheric pressure. In 2014 Grützmacher and co-workers4 reported an improved and even more convenient synthesis of NaPCO from red phosphorus, sodium, tert-butyl alcohol, and ethylene carbonate, thereby avoiding the need to handle the hazardous white phosphorus and carbon monoxide. Krummenacher and Cummins5 showed that the 2phosphaethynolate anion can be generated efficiently by reaction of carbon dioxide with the terminal niobium phosphide anion [(3,5-Me2C6H3NCH2CMe3)3NbP:]−. The phosphaethynolate anion has two important resonance structures, namely the phosphaketene structure P−=CO with the negative charge on phosphorus (1 in Figure 1) and the phosphaethynolate structure PC−O− with the negative © XXXX American Chemical Society

Received: June 20, 2017

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DOI: 10.1021/acs.organomet.7b00467 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

manganese carbonyl phosphides from Mn−PCO precursors through related decarbonylation reactions. It thus appears that the PCO ligand is an ideal phosphorus atom source through decarbonylation for synthesizing many interesting organophosphorus derivatives16 as well as novel metal phosphide complexes.17 However, many aspects of these processes are currently unclear, especially the mechanism for MP or M−P formation, dimerization mechanisms, and the generation of metal P2 complexes using two PCO ligands as the phosphorus source. We chose the cyclopentadienyliron carbonyl complexes FeCp(PCO)(CO)n (n = 2−0) and Fe2Cp2(PCO)2(CO)n (n = 4−0) discussed in this paper to explore theoretically the following three questions: (1) Can the PCO− anion be used as a phosphorus atom source to generate species with Fe−P multiple bonds? (2) Why do some PCO derivatives cyclodimerize to form P2C2 rings whereas other PCO derivatives are unreactive toward such cyclodimerization reactions? (3) Is it possible to generate a P2 ligand from decarbonylation of two PCO ligands in a metal complex? Figure 1. Phosphaethynolate bonding to metal fragments and related chemistry.

2. THEORETICAL METHODS Electron correlation effects were considered using density functional theory (DFT) methods, which have evolved as a practical and effective computational tool, especially for organometallic compounds.18−24 The functional used in this work is a hybrid meta-GGA DFT method, M06-L, developed by Truhlar’s group.25,26 A recent detailed study of 35 different DFT methods suggests that the M06-L method coupled with suitable basis sets predicts structures very consistent with experiment for transition-metal organometallic and carbonyl derivatives.27 DFT methods are fortunately less basis set sensitive than higherlevel methods such as coupled cluster theory. The double-ζ plus polarization (DZP)28 basis set used here adds one set of pure spherical harmonic d functions with orbital exponents αd(C) = 0.75 and αd(O) = 0.85, respectively, to the Huzinaga−Dunning standard contracted DZ sets, designated as (9s5p1d/4s2p1d).29−31 For hydrogen, a set of p polarization functions (αp(H) = 0.75) was added to the Huzinaga− Dunning DZ sets. For iron, our loosely contracted DZP basis set (14s11p6d/10s8p3d) augments the Wachters primitive set32 by two sets of p functions and one set of d functions followed by contractions according to Hood, Pitzer, and Schaefer.33 The geometries of all structures were fully optimized with the M06L/DZP methods. We also report the vibrational frequencies and their corresponding infrared intensities. All of the computations were carried out with the Gaussian 09 program,34 in which the fine grid (75, 302) is the default for evaluating integrals numerically.35 The finer grid

Until recently, the chemistry of the PCO− anion remained relatively unexplored because of the low yields in the original methods for its synthesis. However, 4 years after the original phosphaethynolate synthesis,1 Weber and co-workers11 unexpectedly obtained a dimer of {FeCp*(PCO)(CO)2} (2 in Figure 1, Cp* = η5-Me5C5) containing a P2C2 ring connecting two FeCp*(CO)2 fragments through Fe−P bonds to each ring phosphorus atom. The more recent development of convenient syntheses of the PCO− anion has stimulated extensive study of its reactions with diverse metal derivatives. The P2C2 ring obtained from dimerization of PCO− can be stabilized by coordination of the oxygen atoms to B(C6F5)3 through B←O bond formation (3 in Figure 1) in the presence of LM(PCO) (M = Cu+, Au0; L = borylcarbene ligand).10 Another important reaction generating reactive phosphorus species is photochemical decarbonylation. Thus, exposure of LGe−PCO12−14 to UV light produced a species containing a digermadiphosphacyclobutadiene Ge2P2 ring (5 in Figure 1) as well as an LGePPGeL diphosphene (6 in Figure 1). The GeP or Ge−P phosphinidene (4 and 4′ in Figure 1) was proposed to be the key intermediate in this decarbonylation process. A recent theoretical study15 predicted the generation of

Figure 2. Optimized FeCp(PCO)(CO)2 structures. In Figures 2−9, the bond distances are given in Å. The symmetry point groups and the relative free energies (kcal/mol) are given in parentheses. B


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Organometallics (120, 974) was only used to check small imaginary vibrational frequencies. All structures are designated as ab-c, where a is the number of iron atoms (the same as the number of PCO groups), b is the number of CO groups, and c orders the structures according to their relative Gibbs free energies by the M06-L method. For both mononuclear and binuclear species, many triplet structures are reported only Figures S3 and S4 in the Supporting Information, since their relative energies are higher than the corresponding lowest energy singlet structures. The only triplet structure comparable in energy to its singlet isomers is the FeCp(PCO) structure designated as 10-4T and discussed below.

3. RESULTS 3.1. Mononuclear Derivatives FeCp(PCO)(CO)n (n = 2− 0). 3.1.1. FeCp(PCO)(CO)2. Four low-energy FeCp(PCO)(CO)2 structures were found (Figure 2). The lowest energy structure 12-1 is a genuine minimum with a phosphorusbonded PCO ligand in a trans arrangement bending away from the two CO groups. The PC distance of 1.658 Å is close to the value of 1.648 Å found in Re(PCO)(triphos)(CO)2 by Xray crystallography.8 The FeCp(PCO)(CO)2 isomer 12-2, lying only 0.8 kcal/mol in energy above 12-1, is similar to 12-1 except for the cis arrangement of the PCO group relative to the two CO groups. The similar energies of 12-1 and 12-2 suggest the possibility of FeCp(PCO)(CO)2 as a fluxional system between these two low-energy structures. In both 12-1 and 122, the PCO group is a η1-P(CO) ligand coordinated to the iron atom only through the phosphorus atom with a strongly bent Fe−P−C angle of 89.1° in 12-1 and 98.8° in 12-2. This is consistent with the highly covalent character of the M−P(CO) bond and the stereochemical activity of the phosphorus lone pair in a P-bonded PCO ligand functioning as a net oneelectron donor.7 The FeCp(PCO)(CO)2 structures 12-3 and 12-4 are relatively high energy structures, lying 21.8 and 23.5 kcal/mol above 12-1, respectively (Figure 2). Both 12-3 and 12-4 have the PCO ligand bonded to the iron atom through an Fe−O bond of length ∼2.01 Å. Thus, an O-bonded PCO ligand is clearly much less favored energetically than a P-bonded PCO ligand in this iron system. Attempts to optimize a FeCp(η2PCO)(CO)2 structure with a η2-PCO ligand (III in Figure 1) led to 12-1 or 12-2. 3.1.2. FeCp(PCO)(CO). Five FeCp(PCO)(CO) structures were found (Figure 3). The lowest energy structure 11-1 can be derived from the FeCp(PCO)(CO)2 structure 12-1 by removing an iron-bonded CO group and retaining the Pbonded PCO group with some distortion of the remaining CO group. The Fe−P−C angle of 83.1° indicates the neutral PCO ligand to be a one-electron donor with a stereochemically active phosphorus lone pair. This gives the iron atom in 11-1 a 16electron configuration. The FeCp(PCO)(CO) structure 11-2, lying only 0.3 kcal/mol in energy above 11-1, is a cis stereoisomer of 11-1. Note that the Fe−P distances are shortened from that of 2.41 ± 0.02 Å in 12-1 and 12-2 to 2.235 ± 0.01 Å in 11-1 and 11-2, suggesting stronger Fe−P bonding in 11-1 and 11-2 relating to the 16-electron iron configuration in these two FeCp(PCO)(CO) structures. The FeCp(PCO)(CO) structure 11-3, lying only 2.2 kcal/mol in energy above 11-1, has a η3-PCO ligand, indicated by the three bonding Fe− P, Fe−C, and Fe−O distances of 2.491, 2.040, and 2.118 Å, respectively. The η3-PCO ligand, formally considered to be neutral, becomes a net three-electron donor to give the iron atom the favored 18-electron configuration. Again, the small

Figure 3. Optimized FeCp(PCO)(CO) structures.

energy differences suggest the possibility of fluxionality for the FeCp(PCO)(CO) system. Isomeric FeCp(PCO)(CO) structures with O-bonded PCO ligands, namely 11-4 and 11-5 lying ∼22 kcal/mol in energy above 11-1, are significantly higher energy structures, similar to the situation with the FeCp(PCO)(CO)2 structures (Figure 2). The Fe−O bonds in 11-4 and 11-5 are shortened to 1.87 ± 0.01 Å relative to ∼2.01 Å for the FeCp(PCO)(CO)2 structures 12-3 and 12-4. This could be the effect of partial donation of the oxygen lone pairs into the empty orbital of the iron atom with only a 16-electron configuration. 3.1.3. FeCp(PCO). Four structures are found for FeCp(PCO), including one triplet and three singlet structures (Figure 4). The lowest energy singlet structure of composition

Figure 4. Optimized FeCp(PCO) structures.

FeCp(PCO) 10-1 is a Cs structure exhibiting the unprecedented feature of the PCO unit splitting into separate carbonyl and phosphide groups with each group bonded directly to the iron atom. The carbonyl group is a typical terminal metal carbonyl group with a ν(CO) frequency of 1965 cm−1 (Table S1 in the Supporting Information). The phosphide group is C


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Figure 5. Optimized Fe2Cp2(PCO)2(CO)4 structures and transition state for the dimerization of two FeCp(PCO)(CO)2 units. The activation free energy (kcal/mol) is given in parentheses.

that 10-1, in which the iron atom has the favored 18-electron configuration, is more stable than the triplet structure 10-4T, in which the iron atom has only a 16-electron configuration. 3.2. Binuclear Derivatives. 3.2.1. Fe2Cp2(PCO)2(CO)4. Only two structures are found for Fe2Cp2(PCO)2(CO)4 (241 and 24-2 in Figure 5). These two structures are stereoisomeric pairs with two cis Cp rings in 24-1 and two trans Cp rings in 24-2. Such a small structural difference leads to a small relative free energy of 2.2 kcal/mol, suggesting a potentially fluxional system. These structures result from dimerization of two FeCp(PCO)(CO)2 monomers (12-1 in Figure 2) by a [2 + 2] cycloaddition of the two PC bonds to form a fourmembered P2C2 ring with P−C bond lengths of ∼1.85 Å. Our predicted average Fe−C(O) (1.765 Å), Fe−P (2.302 Å), and P−C (1.850 Å) distances are very close to the average Fe− C(O) (1.770 Å), Fe−P (2.270 Å), and P−C (1.850 Å) distances determined by X-ray crystallography on the chromium carbonyl derivative Cp*2Fe2(CO)4(PCO)2Cr(CO)5.11 The Fe2Cp2(PCO)2(CO)4 structure 24-1 is the thermodynamic dimerization product, suggested by a slightly negative dimerization free energy (ΔG) of −2.1 kcal/mol at 298 K. Furthermore, the small value of the negative dimerization free energy also implies an 2FeCp(PCO)(CO)2 ↔ Fe2Cp2(PCO)2(CO)4 equilibrium. Indeed, the 2FeCp*(PCO)(CO)2 ↔ Fe2Cp*2(P2C2O2)(CO)4 equilibrium has been observed experimentally. Thus, the experimental ν(PCO) frequency of 1923 cm−1 for Fe2Cp*2(P2C2O2)(CO)4 purified by chromatography falls within the 1930−1919 cm−1 range for terminal ν(PCO) frequencies in the monomers 12-1 or 12-2 (see Table S2 in the Supporting Information). In addition, the experimental ν(CO) frequencies of 1635 and 1600 cm−1 correspond to the unsymmetric CO vibrations of the P2C2O2 rings in the two isomers 24-1 and 24-2 (see Table S2 in the Supporting Information). However, the enthalpy change (ΔH) of the dimerization process is predicted to be −16.7 kcal/mol. According to the Gibbs free energy change equation (ΔG = ΔH − TΔS), the dimerization process is highly entropy controlled because of a positive entropy change. Therefore, bigger monomers will lead to larger decreases in entropy and thus increased dimerization free energies. The free energy and enthalpy of formation for the experimentally observed dimer Fe2Cp*2(PCO)2(CO)4 from two discrete FeCp*(PCO)(CO)2 monomers are predicted to be −0.4 and −17.7 kcal/mol, respectively. The chromium carbonyl complex Fe2Cp*2(PCO)2(CO)4Cr(CO)5 is further stabilized by an additional P→Cr(CO)5 dative bond. However, in different [M](PCO) complexes with different electronic and steric effects of the surrounding ligands the dimerization free energies can become zero or even positive. This is consistent with the experimental observation that some mononuclear

clearly multiply bonded to the iron atom with a short FeP distance of 1.977 Å. Interpreting the FeP bond as a formal triple bond makes it a formal three-electron donor to the iron atom, thereby giving the iron atom in 10-1 the favored 18electron configuration. Interpretation of the FeP bond in 101 as a formal triple bond is also supported by its relatively high Wiberg bond index of 2.11 as well as its two occupied π molecular orbitals and an occupied σ molecular orbital (Figure S1 in the Supporting Information). Note that the HOMO− LUMO gap of 1.17 eV for 10-1 is large enough to reflect a stable electronic structure. The next FeCp(PCO) structure 10-2, lying only 2.0 kcal/ mol in energy above 10-1, has a η3-PCO ligand, as indicated by bonding Fe−P, Fe−C, and Fe−O distances of 2.273, 1.920, and 2.134 Å, respectively (Figure 4). Structure 10-2 thus can be derived from structure 11-3 by loss of a carbonyl group. The leads to a 16-electron configuration for the central iron atom in 10-2. The next FeCp(PCO) structure 10-3, lying 7.2 kcal/mol in energy above 10-1, has a η2-PC(O) ligand with Fe−P and Fe− C bond lengths of 2.131 and 1.977 Å, respectively (Figure 4). Such a formally neutral η2-PC(O) ligand functions as a threeelectron donor by donating one electron through the carbon atom and two electrons through the phosphorus atom. This gives the iron atom in 10-3 a 16-electron configuration. Structure 10-3 can be considered as an intermediate in the carbonyl migration from phosphorus to iron leading ultimately to 10-1. The transition state 10-ts for this process is found (Figure 4). This transition state 10-ts lies only 1.6 kcal/mol in energy above 10-3, implying lability of the P−C bond in 10-3 facilitating CO migration from phosphorus to iron, giving ultimately the lower energy structure 10-1. The lowest energy triplet FeCp(PCO) structure 10-4T (Figure 4) is the high-spin analogue of the singlet 10-3 with a η2-PC(O) ligand and a 16-electron configuration for the iron atom, which bears most of the spin density. However, this electron-deficient triplet structure 10-4T is predicted to lie 3.7 kcal/mol below the lowest energy singlet structure 10-1. This abnormal phenomenon leads us to suspect the reliability. The relative electronic energy of 10-4T is found to change to −2.5 kcal/mol by reoptimization at the more accurate M06L/cc-pvtz level. Furthermore, DFT methods can underestimate the orbital energy gaps, thereby leading to an overestimation of the stability of the triplet structure. In this connection, reoptimization at the MP2/cc-pvtz level indicates 10-4T to lie 9.0 kcal/ mol in energy above 10-1. Furthermore, single-point calculations at the CCSD/cc-pvtz level using the optimized MP2/cc-pvtz structure show 10-4T to lie 4.7 kcal/mol above 10-1. Therefore, the relative free energy of 10-4T at the M06L/ dzp level appears to be unreliable. From chemical intuition, we believe the more accurate computational methods indicating D


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Figure 6. Optimized Fe2Cp2(PCO)2(CO)3 structures.

[M](PCO) complexes7,8 resist dimerization partially because of the bulky triphos ligand. More importantly, the equilibrium is highly affected by temperature, since a lower temperature favors dimer formation because of a lower decrease in entropy. Thus, RPCO (R = t-Bu,36 2,4,6-t-Bu3C6H237) molecules undergo reversible dimerization below 258 K. More organic or organometallic (P2C2O2) dimers are expected by using the interaction of phosphorus lone pair electrons with Lewis acid or electron-deficient metal fragments. A transition state (24-ts in Figure 5) has been found for the dimerization of the FeCp(PCO)(CO)2 structure 12-1 to give 24-1 with a predicted activation free energy of 25.4 kcal/mol, suggesting a feasible process. However, the corresponding activation enthalpy of 24-ts is found to be only 9.0 kcal/mol. Therefore, formation of a P2C2 ring structure (24-1 in Figure 5) from two isolated monomers (12-1 or 12-2 in Figure 2) is again a highly entropy controlled process, since the dimerization process also leads to a negative entropy change. Formation of 24-ts generates two new P···C bonds of lengths 2.143 and 2.837 Å. The shorter P···C bond (2.143 Å) can contract further to form a P−C single bond through donation of electrons from the occupied π orbital of one PC fragment to the π* antibonding orbital of the other PC fragment (see Figure S1 in the Supporting Information). A similar process involving the other new P···C interaction can then result in closure of the P2C2 ring to give ultimately structure 24-1. 3.2.2. Fe2Cp2(PCO)2(CO)3. Six structures are found within 22 kcal/mol for Fe2Cp2(PCO)2(CO)3 (Figure 6). The lowest energy structure 23-1 has a bridging μ-P(CO) ligand, which donates two electrons to one FeCp(PCO)(CO) unit and one electron to the other FeCp(CO)2 unit. Alternatively 23-1 can be considered as a derivative of FeCp(PCO)(CO)2 (12-1) in which one of its CO groups has been replaced by a second FeCp(PCO)(CO)2 unit functioning as a ligand through its phosphorus lone pair to form a P→Fe dative bond. The Fe···Fe distance of 4.180 Å in 23-1 indicates the absence of a direct iron−iron bond in 23-1. Despite the absence of an iron−iron bond in 23-1 each iron atom has the favored 18-electron configuration.

The next lowest energy Fe2Cp2(PCO)2(CO)3 structure 23-2, lying 8.7 kcal/mol in energy above 23-1, is derived from 24-1 by loss of a terminal CO group (Figure 6). This gives the iron bearing only a single terminal CO group only a 16-electron configuration, whereas the other iron atom has the favored 18electron configuration. The next lowest energy Fe2Cp2(PCO)2(CO)3 structure 23-3, lying 15.1 kcal/mol in energy above 23-1, has a μ-CO group bridging two FeCp(PCO)(CO) units with two nearly equivalent Fe−C distances of 1.868 ± 0.015 Å and a trans arrangement of the two Cp rings (Figure 6). The Fe−Fe distance of 2.709 Å in 23-3 suggests a formal single bond, thereby giving each iron atom the favored 18-electron configuration. The high energy of 23-3 relative to 23-1 indicates that a μ-P(CO) bridge with two Fe−P bonds is highly favored over a μ-C(O) bridge with two Fe−C bonds. The Fe2Cp2(PCO)2(CO)3 structure 23-6, lying 22.0 kcal/mol in energy above 23-1, is similar to 23-3, but with a cis arrangement of the Cp rings. The higher energy of the cis isomer 23-6 relative to the trans isomer 23-3 could be related to increased steric repulsion between the Cp rings in the cis configuration relative to the trans configuration. The remaining two Fe2Cp2(PCO)2(CO)3 structures, namely 23-4 and 23-5 lying 15.5 and 17.8 kcal/mol, respectively, in energy above 23-1 (Figure 6), are found to have tiny imaginary vibrational frequencies. However, they are still true local minima, since the imaginary frequency decreases using an ultrafine integration grid. The long Fe···Fe distances in both 234 and 23-5 indicate the absence of direct iron−iron bonds. Structure 23-4 has a PCO bridge donating one electron to an FeCp(CO)2 unit through an P−Fe bond and two electrons to an FeCp(PCO)(CO) unit through an O→Fe dative bond. Structure 23-5 has a η2-μ-CO group bridging two FeCp(CO)(PCO) units by forming an O→Fe dative bond to one iron atom and a C→Fe dative bond to the other iron atom. Both 234 and 23-5 have the favored 18-electron configuration for both iron atoms. 3.2.3. Fe2Cp2(PCO)2(CO)2. Seven Fe2Cp2(PCO)2(CO)2 structures were found within 20 kcal/mol of energy (Figure E


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Figure 7. Optimized Fe2Cp2(PCO)2(CO)2 structures.

on P→Fe dative bond of length 2.334 Å to one iron atom and a η3-PPC linkage to the other iron atom with Fe−P, Fe−P, and Fe−C distances of 2.376, 2.369, and 1.935 Å, respectively (Figure 7). The P−C distance in 22-2 is predicted to be 1.832 Å, consistent with a P−C single bond. Structure 22-2 can be derived from 22-1 by bonding the carbon atom of a terminal CO group to a phosphorus atom so that an Fe−P bond is bridged by this CO group. This P−C single bond originates from donation of a phosphorus lone pair in a diphosphene to the carbon atom, thereby converting a CO triple bond to a CO double bond as indicated by a longer CO bond distance of 1.195 Å. The low energy of 22-2 at only 0.8 kcal/ mol above 22-1 suggests high reactivity of the diphosphene complex owing to the highly nucleophilic lone pairs on the diphosphene phosphorus atoms. The next two Fe2Cp2(PCO)2(CO)2 structures, namely 22-4 and 22-5 lying 11.5 ± 0.6 kcal/mol in energy above 22-1, are a trans−cis stereoisomer pair with two bridging μ-P(CO) groups and two terminal CO groups (Figure 7). The long Fe···Fe distances in 22-4 and 22-5 of ∼3.5 Å indicate the absence of a direct iron−iron bond. Each bridging μ-P(CO) group, considered formally as a neutral ligand, donates two electrons to one iron atom and one electron to the other iron atom. This gives each iron atom in 22-4 and 22-5 the favored 18-electron configuration. The remaining two Fe2Cp2(PCO)2(CO)2 structures, namely 22-6 and 22-7 lying ∼20 kcal/mol in energy above 22-1, are another trans−cis stereoisomer pair, derived from 22-4 and 22-

7). The lowest energy structure 22-1 of stoichiometry Fe2Cp2(PCO)2(CO)2 is actually trans-CpFe(CO)2PPFe(CO)2Cp in which the carbonyl unit in each PCO group has migrated from the phosphorus atom to an iron atom leaving an end-to-end PP unit bridging the two iron atoms. The Fe···Fe distance of 5.555 Å is much too long for a direct iron−iron bond. Structure 22-1 is thus a trans-diphosphene with a CpFe(CO)2 substituent on each phosphorus atom. The PP distances of 2.034 Å in the original diphosphene38 Mes*P PMes* (Mes* = 2,4,6-t-Bu3C6H2), and 2.029 Å in 22-1 are comparable to the experimental PP distance of 2.045 Å in the diphosphene LGe−PP−GeL,13 as determined by X-ray crystallography. The Fe−P distances of 2.351 Å to the FeCp(CO)2 units correspond to formal covalent single bonds. Therefore, each phosphorus atom in the diphosphene has a stereochemically active lone pair causing bending of the Fe−P−P angle to ∼104°. Each iron atom in 22-1 has the favored 18-electron configuration. Since the first discovery of an RPPR diphosphene by Yoshifuji in 1981,38 several purely organic diphosphenes have been synthesized. However, no diphosphenes of the general type [M]PP[M] with transition metal rather than organic substituents have been synthesized. The cis-diphosphene stereoisomer related to 22-1, namely 22-3, lies only ∼1.2 kcal/mol in energy above 22-1. This suggests the possibility of a fluxional system. Another relatively low energy Fe2Cp2(PCO)2(CO)2 structure, 22-2, lying only 0.8 kcal/mol in energy above 22-1, has a novel η3,η1-P2CO group bridging the two iron atoms by an endF


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Organometallics

Figure 8. Optimized Fe2Cp2(PCO)2(CO) structures.

5 by replacing one μ-P(CO) bridge with a two-electron-donor μ−C(O) bridge. The Fe−Fe bonding distance of 2.595 ± 0.01 Å gives each iron atom the favored 18-electron configuration. 3.2.4. Fe2Cp2(PCO)2(CO). Seven Fe2Cp2(PCO)2(CO) structures were found within 21 kcal/mol of the lowest energy structure (Figure 8). In the lowest energy Fe2Cp2(PCO)2(CO) structure 21-1 the carbonyls in both PCO groups have migrated completely from the phosphorus atom to an iron atom, leaving a two-electron donor μ-P(P) bridge as well as a μ-C(O) bridge. The short PP distance of 1.926 Å combined with a Wiberg bond index of 2.42 and three localized natural P−P bond orbitals suggests a formal triple bond. The Fe−Fe distance of 2.607 Å corresponds to a formal doubly bridged Fe−Fe single bond, giving each iron atom the favored 18electron configuration. A bridging PP unit of the type found in 21-1 has not yet been found experimentally, despite many experimentally known P2 complexes.39 The Fe2Cp2(PCO)2(CO) structure 21-2, lying 8.1 kcal/mol in energy above 21-1, has two bridging μ-CO ligands and a terminal P2 ligand with a short PP distance of 1.906 Å, suggesting a formal triple bond (Figure 8). The significantly higher energy of 21-2 relative to 21-1 suggests that a μ-PP bridge is more favorable than a μ-CO bridge. The Fe−Fe bond distance of 2.534 Å can be interpreted as a formal single bond, thereby giving each iron atom the favored 18-electron configuration. The Fe2Cp2(PCO)2(CO) structure 21-3, lying 8.9 kcal/mol in energy above 21-1, has a four-electron-donor η2-μ-P2 group

bridging an FeCp(CO)2 unit to an FeCp(CO) unit (Figure 8). The η2-μ-P2 group is σ-bonded to the FeCp(CO)2 iron atom through a single P−Fe bond of length 2.274 Å, resulting in the net donation of one electron. Furthermore, the η2-μ-P2 group π-bonded to the FeCp(CO) iron through two PP π electrons and an Fe−P σ bond leading to Fe−P distances of 2.192 and 2.426 Å corresponding to formal double and single Fe−P bonds, respectively, results in the net donation of three electrons. The long Fe···Fe distance of 4.274 Å in 21-3 implies the absence of any direct Fe···Fe bonding. This bonding interpretation for 21-3 gives each iron atom the favored 18electron configuration. The PP distance of 1.984 Å in 21-3 is somewhat longer than that in 21-1 or 21-2 but shorter than the PP distance in the diphosphene 22-1 (Figure 7). The elongated PP distance in 21-3 may be a consequence of donation of the P2 π electrons to one iron atom (the right iron atom in Figure 8) as well as back-donation of electrons from filled iron d orbitals to a P2 π* orbital. The Fe2Cp2(PCO)2(CO) structure 21-4, lying 13.0 kcal/mol in energy above 21-1, is predicted to have a bridging μ-CO group and a bridging η2-μ-PP diphosphorus unit (Figure 8). The bridging PP unit functions as a four-electron donor by donation of an electron pair from each of two perpendicular occupied π orbitals of the PP unit to an iron atom. This forward bonding is accompanied by back-bonding of iron d electrons into vacant π* orbitals of the bridging η2-μ-PP unit. In this way, both π → Fe forward donation and Fe → π* backdonation reduce the effective PP bond order. In this G


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Organometallics connection, the Wiberg bond index for the PP interaction decreases to 1.48 and the PP bond elongates to 2.030 Å. Interpreting the relatively long Fe···Fe distance of 3.062 Å as the absence of a direct Fe···Fe bond gives each iron atom the favored 18-electron configuration. A similar central M2(η2-μPP) unit is found in the experimentally known Co2(CO)6(P2) complex.40 The next two Fe2Cp2(PCO)2(CO) structures, namely 21-5 and 21-6 lying 14.2 and 17.6 kcal/mol in energy, respectively, above 21-1, are a cis−trans stereoisomer pair on the basis of the relative orientation of the bridging μ-PCO groups (Figure 8). Both 21-5 and 21-6 also have a μ-CO bridge. The formally neutral bridging μ-P(CO) ligands are net three-electron donors to each of the iron atoms that they bridge. The Fe−Fe distances of ∼2.44 Å are reasonable for triply bridged formal Fe−Fe single bonds, thereby giving each iron atom in 21-5 and 22-6 the favored 18-electron configuration. These triply bridged Fe− Fe single bonds in 21-5 and 22-6 are shorter by ∼0.1−0.15 Å than the doubly bridged Fe−Fe single bonds in the isomeric Fe2Cp2(PCO)2(CO) structures 21-1 and 21-2. The species Fe2Cp2(μ-PPh2)2(μ-CO) closely related to 21-5 and 21-6 but with bridging μ-PPh2 groups rather than μ-PCO groups has been synthesized and characterized by its infrared and NMR spectra.41 However, its structure apparently has not been determined by X-ray crystallography. The last of the seven lowest energy Fe2Cp2(PCO)2(CO) structures, namely 21-7 lying 20.7 kcal/mol in energy above 211, can be derived from the cis-diphosphene Fe2Cp2(CO)4(μ-P2) structure 22-3 by removal of the axial CO group with some distortion of the remaining structure (Figure 8). The Fe−P bond length of 2.379 Å to the FeCp(CO)2 unit (the left iron atom in Figure 8) is similar to the Fe−P bond distances in 22-3 and can likewise be considered as a formal single bond consistent with an Fe−P WBI of 0.73. This gives this iron atom the favored 18-electron configuration. The P−P−Fe angle to the FeCp(CO)2 unit in 21-7 is bent to 105.1°, consistent with stereochemical activity of the phosphorus lone pair. However, the FeP bond length of 2.047 Å to the FeCp(CO) unit in 21-7 is shortened by more than ∼0.3 Å from the Fe−P bond to the FeCp(CO)2 unit. This suggests the formal triple bond required to give this iron atom also the favored 18-electron configuration. Interpretation of the FeP bond to the FeCp(CO) unit as a formal triple bond is supported by the nearly linear P−PFe angle of 171.4° to this iron atom. 3.2.5. Fe2Cp2(CO)x(P2) (x = 2−0). Only the lowest energy structure 20-1 is reported for Fe2Cp2(PCO)2 in Figure 9, since the other structures (Figure S2 in the Supporting Information) are predicted to lie more than 24 kcal/mol in energy above 201. Structure 20-1, formulated as Fe2Cp2(CO)2(μ-P2), can be derived from 21-4 (Figure 8) by removing a terminal CO group and converting the other terminal CO group into a bridging μCO group. The Fe−Fe distance in 20-1 is shortened from the nonbonding Fe···Fe distance of 3.062 Å in 21-4 to 2.411 Å, corresponding to a formal Fe−Fe single bond. This gives each iron atom in 20-1 the favored 18-electron configuration. The Fe2Cp2(CO)2(P2) structure 20-1 still has two carbonyl groups. Loss of a bridging CO group from 20-1 gives the Fe2Cp2(CO)(P2) structure 2a-1 (Figure 9). The FeFe distance in 2a-1 is shortened from 2.411 Å for the triply bridged Fe−Fe single bond in 20-1 to 2.347 Å in the doubly bridged structure 2a-1. However, the P−P distance of 2.071 Å in 2a-1 is relatively unchanged from that in its precursor 20-1. This suggests the formal FeFe double bond required to give

Figure 9. Optimized Fe 2 Cp 2 (PCO) 2 , Fe 2 Cp 2 (CO)(P 2 ), and Fe2Cp2(P2) structures.

each iron atom the favored 18-electron configuration. A related Fe2Cp′′′2(CO)(P2) (Cp′′′ = η5-1,2,4-tBu3C5H2) has been isolated from the pyrolysis of [FeCp′′′(CO)2]2P4 at 190 °C.42 The experimental FeFe double bond length of 2.394 Å, determined by X-ray crystallography, is somewhat longer than our predicted FeFe distance of 2.347 Å for 2a-1, possibly because of steric hindrance from the three tert-butyl groups on each Cp ring in the experimental structure. However, the experimental P−P distance of 2.064 Å and average Fe−P distance of 2.291 Å agree nearly perfectly with our predicted results of 2.071 and 2.298 Å, respectively, for 2a-1. In addition, our predicted ν(CO) frequency of 1790 cm−1 for 2a-1 (BP86/ DZP level) agrees well with the experimental ν(CO) frequency of 1769 cm−1 for Fe2Cp′′′2(CO)(P2)42 (Table S1 in the Supporting Information). Further decarbonylation of 2a-1 leads to two carbonyl-free Fe2Cp2(P2) structures, namely 2b-1 and 2b-2 (Figure 9). The lower energy Cp2Fe2(P2) structure 2b-1 has two isolated P ligands in a P2Fe2 rhombus with an FeFe bond of length 2.479 Å across the diagonal and four equivalent Fe−P distances of 2.019 Å. These may arise from partial PFe double-bond character originating from averaging two Kekulé-type structures with alternating PFe double bonds in the P2Fe2 rhombus (Figure 10). As a result of this bonding scheme, each

Figure 10. Two Kekulé-like resonance structures for the Fe2Cp2(P2) structure 2b-1.

phosphorus atom can donate a total of three electrons to the central Fe2 unit, leaving an external lone pair on each phosphorus. Interpreting the FeFe bond as a formal double bond gives each iron atom in 2b-1 the favored 18-electron configuration. The other Fe2Cp2(P2) structure, namely 2b-2 lying 16.6 kcal/mol in energy above 2b-1, can be derived directly from 2a1 by removal of its remaining CO group (Figure 9). This has a negligible effect on the FeFe distance but increases the P−P distance from 2.071 to 2.223 Å. Further, lengthening of the P− H


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Organometallics

vored. The dissociation energy of Fe2Cp2(PCO)2(CO)3 is also endothermic, but only by ∼9 kcal/mol. This may be at least partially a consequence of the favored 18-electron configuration of one of its dissociation products, namely FeCp(PCO)(CO)2.

P distance in 2b-2 can lead to rupture of the phosphorus− phosphorus bond and ultimately to the lower energy structure 2b-1. Thus, 2b-2 can be regarded as an unstable intermediate in the decarbonylation of 2a-1 to give 2b-1. 3.3. Dissociation Energies. The predicted dissociation energies (ΔEs) for releasing one CO group from the lowest energy mononuclear FeCp(PCO)(CO) m and binuclear Fe2Cp2(PCO)2(CO)n structures according to the following equations are given in Table 1.

4. DISCUSSION 4.1. Mononuclear Complexes. Comparison of the relative energies of P-bonded FeCp(PCO)(CO)2 structures 12-1 and 12-2 and their O-bonded FeCp(OCP)(CO)2 isomers 12-3 and 12-4 indicates that the P-bonded isomers are more favorable by more than 20 kcal/mol than the O-bonded isomers. This is consistent with previous studies on phosphaethynolate complexes of the d-block metals, including experimental work on the rhenium derivative8 Re(PCO)(CO)2(triphos) as well as theoretical work on the manganese derivative15 Mn(PCO)(CO)5. In this respect the phosphaethynolate complexes of the d-block transition metals differ from those of the more oxophilic f-block metals such as thorium and uranium, where O-bonding rather than P-bonding phosphaethynolate ligands are found in the experimental structures.9 This could be attributed to the stronger covalent P−M bonding character for d-block transition metals but stronger ionic P−M bonding character for f-block transition metals. A comparison of the lowest energy FeCp(PCO)(CO)n (n = 2−0) structures shows how the bonding mode of the PCO ligand changes to maintain the favored 18-electron configuration as CO groups are lost (Figure 11). Thus, loss of CO

FeCp(PCO)(CO)m → FeCp(PCO)(CO)m − 1 + CO m = 2, 1

Fe2Cp2 (PCO)2 (CO)n → Fe2Cp2(PCO)2 (CO)n − 1 + CO n = 4−1

Table 1. Predicted CO Dissociation Energies (kcal/mol) ΔE FeCp(PCO)(CO)2 → FeCp(PCO)(CO) + CO FeCp(PCO)(CO) → FeCp(PCO) + CO Fe2Cp2(PCO)2(CO)4 → Fe2Cp2(PCO)2(CO)3 + CO Fe2Cp2(PCO)2(CO)3→ Fe2Cp2(PCO)2(CO)2 + CO Fe2Cp2(PCO)2(CO)2 → Fe2Cp2(PCO)2(CO) + CO Fe2Cp2(PCO)2(CO) → Fe2Cp2(PCO)2 + CO Fe2Cp2(PCO)2 → Fe2Cp2(CO)P2 + CO Fe2Cp2(CO)P2 → Fe2Cp2P2 + CO

27.0 26.4 19.7 −7.7 10.4 −5.1 20.2 38.1

The CO dissociation energies of FeCp(PCO)(CO)2 and FeCp(PCO)(CO) are predicted to be 27.0 and 26.4 kcal/mol, respectively (Table 1). These are similar to the experimental 27.0 kcal/mol CO dissociation energy for the stable Ni(CO)4.43 For the binuclear derivatives the CO dissociation energy of Fe2Cp2(PCO)2(CO)3 (23-1) is found to be exothermic at −7.7 kcal/mol, indicating that it is not a viable species. This exothermic CO dissociation is a reflection of the stability of the diphosphene isomer of Fe2Cp2(PCO)2(CO)2, namely CpFe(CO)2PPFe(CO)2Cp (22-1). In addition, the CO dissociation of the lowest energy Fe2Cp2(PCO)2(CO) isomer (21-1), namely [CpFe(CO)]2(μ-CO)(μ-PP), is found to be exothermic at −5.1 kcal/mol. This reflects the reactivity of the lone pair on the dangling phosphorus atom in the bridging μ-PP: ligand in 21-1. The CO dissociation energies of the lowest energy Fe 2 Cp 2 (PCO) 2 (CO) 4 , Fe2Cp2(PCO)2(CO)2, Fe2Cp2(PCO)2, and Fe2Cp2(CO)P2 structures are all endothermic, suggesting viability toward CO dissociation. The energies for dissocation of the binuclear Fe2Cp2(PCO)2(CO)n complexes into mononuclear fragments are given in Table 2. All such dissociation processes are strongly endothermic by at least 43 kcal/mol except for that of Fe2Cp2(PCO)2(CO)3, indicating that they are highly disfa-

Figure 11. Conversion of FeCp(PCO)(CO)2 to FeCp(PCO)(CO) and FeCp(CO)(P) by successive loss of CO groups.

from FeCp(PCO)(CO)2 (12-1 or 12-2) converts the P-bonded η1-PCO ligand into a η3-PCO ligand in which all three ligand atoms are within bonding distance of the iron atom. Such a neutral ligand is a net three-electron donor, thereby preserving the favored 18-electron iron configuration after loss of the CO group. The two slightly lower energy FeCp(PCO)(CO) structures by only ∼2 kcal/mol, namely 11-1 and 11-2, preserve the η1 P-bonded PCO ligand and thus have a coordinately unsaturated iron atom with only a 16-electron configuration. Further loss of the final CO ligand from the FeCp(η3PCO)(CO) structure 11-3 results in rupture of the P−C bond in the PCO ligand with concurrent migration of the CO ligand in the PCO group from the phosphorus atom to the iron atom (Figure 11) thereby giving the FeCp(CO)(P) complex 10-1 (Figure 4). This leaves a lone phosphorus atom bonded to the iron atom. The FeP bond in 10-1 is relatively short at only 1.977 Å and thus is interpreted as a triple bond using three of the phosphorus valence electrons, leaving a lone pair on the phosphorus atom. This makes the lone phosphorus atom in 101 a three-electron donor to the central iron atom thereby giving it the favored 18-electron configuration. A higher energy FeCp(PCO) structure 10-3 is found with a η2-dihapto PC

Table 2. Predicted Energies (kcal/mol) for the Dissociation of Binuclear Complexes into Mononuclear Fragments ΔE Fe2Cp2(PCO)2(CO)3 → FeCp(PCO)(CO)2 + FeCp(PCO)(CO) Fe2Cp2(PCO)2(CO)3 → FeCp(PCO)(CO)2 + FeCp(PCO) Fe2Cp2(PCO)2(CO)2 →2FeCp(PCO)(CO) Fe2Cp2(PCO)2(CO) →FeCp(PCO)(CO) + FeCp(PCO)

9.4 43.4 44.0 60.0 I


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Organometallics O ligand bonded to the iron atom through the PC double bond (Figure 4). This may correspond to an intermediate in the migration of CO from phosphorus to iron in forming the FeCp(CO)(P) structure 10-1. Further lengthening of the P−C bond in the PCO ligand prior to its rupture into P + CO ligands is found in the transition state structure 10-ts. 4.2. Binuclear Complexes. A P-bonded PCO ligand such as that in FeCp(−PCO)(CO)2 has a reactive PC double bond subject to cycloaddition reactions. The cycloaddition is a thermodynamically controlled process because of a slightly negative dimerization free energy as well as a negative entropy change. The monomer/dimer equilibrium is controlled by themodynamics. In fact, much of the organic chemistry36,37 of the PCO group reflects the susceptibility of the PC bond toward cycloaddition reactions below 258 K. The lowest energy Fe2Cp2(PCO)2(CO)4 structure 24-1 (Figure 5) reflects this reactivity of the PC bond, since it contains a 1,3-diphospha2,4-cyclobutanone four-membered ring obtained from 2 + 2 cyclodimerization of the PC bond in FeCp(PCO)(CO)2. Migration of CO groups from PCO ligands to iron atoms in binuclear derivatives occurs readily as CO groups are lost. A key species is the diphosphene derivative CpFe(CO)2PPFe(CO)2Cp, which is observed as both trans and cis stereoisomers in low-energy structures of similar relative energies within 1 kcal/mol (22-1 and 22-3 in Figure 7). These diphosphene structures are closely related to the stable diaryldiphosphene (tBu3C6H2)PP(C6H2tBu3) originally discovered by Yoshifuji and co-workers38 but with the bulky aryl groups in Yoshifuji’s diphosphene replaced by CpFe(CO)2 groups forming Fe−P bonds to the central diphosphene unit. Formation of CpFe(CO)2PPFe(CO)2Cp from the carbonyl richer Fe2Cp2(PCO)2(CO)n necessarily requires migration of the CO units from both PCO ligands to iron atoms. The Fe2Cp2(PCO)2(CO)2 structure 22-2 (Figure 7), lying in the same energy range, can be viewed as having a CO group bridging a phosphorus atom to an iron atom and thus may represent an intermediate in the migration of the CO group from the second PCO ligand to the iron atom. Note also that the CO dissociation energy from Fe2Cp2(PCO)2(CO)3 is exothermic by ∼8 kcal/mol (Table 1), indicating the energetic favorability of CO migrations from phosphorus to iron to form diphosphene derivatives. The further loss of CO groups from the diphosphene stereoisomers 22-1 and 22-3 can lead to the lowest energy structures of the carbonyl poorer systems (Figure 12). The first step can be a migration of an FeCp(CO)2 unit from one phosphorus atom to the other so that one phosphorus atoms bears two FeCp(CO)2 units, leaving a bare phosphorus atom. This places the iron atoms close enough together to form an iron−iron bond. Loss of a CO group to form an Fe−Fe bond gives the Fe2Cp2(CO)2(μ-CO)(μ-PP:) structure 21-1, which is the lowest energy structure for the stoichiometry Fe2Cp2(PCO)2(CO) by a significant ∼8 kcal/mol. The PP distance of 1.926 Å in 21-1 is somewhat shorter than the PP distances in the diphosphene 22-1 and 22-3, suggesting a formal PP triple bond in 21-1. The bare triply bonded phosphorus atom in the μ-PP: ligand of 21-1 is potentially very reactive, having a lone pair and being multiply bonded to the other phosphorus atom. Therefore, it can attack the Fe2 unit of 21-1 to displace a CO group to give the first Fe2P2 tetrahedrane structure 20-1 (Figure 12), which is the lowest energy structure of stoichiometry Fe2Cp2(PCO)2. The two bridging CO groups

Figure 12. Conversion of the CpFe(CO)2PPFe(CO)2Cp diphosphene structure to the lowest energy carbonyl-poorer structures.

in the tetrahedrane 20-1 can be lost in succession, retaining the central Fe2P2 unit as far as 2b-2 (Figure 12). Loss of the first CO group from 20-1 to give 2a-1 shortens the iron−iron distance from 2.411 Å, interpreted as a formal Fe−Fe single bond shortened by the two μ-CO bridges, to 2.347 Å in 2a-1, interpreted as a formal FeFe double bond. Loss of the second CO group converts 2a-1 into the completely carbonyl free 2b-2 with no change in the FeFe distance but with the P−P distance lengthened from 2.071 Å in 2a-1 to 2.223 Å in 2b-2. Structure 2b-2 can then rearrange to the lower-energy isomer 2b-1 by complete rupture of the P−P bond, releasing ∼17 kcal/mol. The central Fe2P2 unit in 2b-1 can be viewed as two triangles sharing an FeFe edge or as a rhombus of alternating iron and phosphorus atoms having equal 2.109 Å edges with the FeFe diagonal bridged by a formal double bond. In order to give each iron atom in 2b-1 the favored 18electron configuration, one of its iron−phosphorus bonds must be a formal single Fe−P bond and the other a formal FeP double bond. In order to account for the predicted equality of all four Fe−P edges, structure 2b-1 can be considered as a resonance hybrid of two Kekulé structures (Figure 10) analogous to the classical example of benzene. A Fe2Cp2(μ-CO)(μ-P2) structure 2a′′′ corresponding to 2a1 but with the 1,2,4-tritert-butylcyclopentadienyl ligand (Cp′′′) has been obtained by pyrolysis of a [FeCp′′′(CO)2]2P4 derivative in decalin at 190 °C.42 Photolysis of 2a′′′ results in loss of a CO group to give the completely carbonyl-free Fe2Cp′′′2P2 structure 2b-1′′′.

5. SUMMARY Isomers of FeCp(PCO)(CO)2 with P-bonded phosphaketene lignds are energetically favored by ∼20 kcal/mol over isomers with O-bonded phosphaethynolate ligands. The PCO ligand can convert to a η3 ligand upon decarbonylation of FeCp(PCO)(CO)2 to give FeCp(η3-PCO)(CO). Further decarbonylation results in carbonyl migration from phosphorus to iron through rupture of the P−C bond to give FeCp(CO)(P:) with separate carbonyl and phosphide ligands. The favored 18J


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Organometallics

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electron configuration for the central iron atom is maintained throughout this decarbonylation sequence. The PCO ligand in FeCp(PCO)(CO)2 is susceptible toward dimerization through 2 + 2 cycloaddition of the PC double bonds in the PCO ligand, leading to the dimer Fe2Cp2(PCO)2(CO)4 containing a 1,3-diphosphacyclobutanedione ring. Another energetically favored binuclear species is the diphosphene CpFe(CO)2PPFe(CO)2Cp, in which the CO groups of both PCO ligands have migrated from phosphorus to iron, leaving behind a P2 unit. This diphosphene is closely related to the original RPPR (R = 2,4,6-tBu3C6H2) diphosphene first synthesized by Yoshifuji and co-workers.38 Further decarbonylation of the binuclear system first results in 1,2-migration of one of the FeCp(CO)2 units so that both iron substituents are on the same phosphorus atom. Iron−iron bond formation can then ensue upon further decarbonylation leading to Cp2{Fe2P2}(CO)n (n = 2−0) species containing central Fe2P2 tetrahedrane units. Species of this type are experimentally known with bulky 1,2,4-tri-tert-butylcyclopentadienyl ligands.42 However, the carbonyl-free diferradiphosphatetrahedrane Cp2Fe2P2 with a P−P bond of length 2.223 Å is energetically disfavored by ∼16 kcal/mol over an isomer in which the P−P bond is broken to give an isomer with a central Fe2P2 unit described as a rhombus with an additional FeFe double bond across a diagonal.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00467. Some important orbitals, optimized Fe2Cp2(PCO)2 structures, optimized triplet FeCp(PCO)(CO)2 and FeCp(PCO) structures, optimized triplet Fe2Cp2(PCO)2(CO)n and Fe2Cp2(P)2(CO)m (n = 4, 2, 1; m = 2−0) structures, and predicted ν(CO) vibrational frequencies and infrared intensities for all reported structures at the BP86/DZP level (PDF) Coordinates of the low-energy structures (XYZ)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for Z.Z.: [email protected]. *E-mail for R.B.K.: [email protected]. ORCID

R. Bruce King: 0000-0001-9177-5220 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are indebted to the National Natural Science Foundation of China (Grants 21303138 and 21406180), Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (Grant BM2012110), and the Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) for support of this work.

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REFERENCES

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