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Comparison of binuclear phospholyl chromium carbonyl derivatives with their cyclopentadienyl analogues: Role of the phosphorus atom in ligand-metal bonding
Inorganica Chimica Acta 494 (2019) 194–203
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Research paper
Comparison of binuclear phospholyl chromium carbonyl derivatives with their cyclopentadienyl analogues: Role of the phosphorus atom in ligandmetal bonding
T
Xin Wana, Chunrong Fenga, Guiming Rena, Xiaohong Chena, Rong Jina, Quan Dua, Hao Fenga, Yaoming Xieb, R. Bruce Kingb a b
School of Science, Research Center for Advanced Computation, Xihua University, Chengdu 610039, China Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, GA 30602, USA
A B S T R A C T
The structures and energetics of the binuclear phospholyl chromium carbonyl derivatives (C4H4P)2Cr2(CO)n (n = 6, 5, 4, 3) have been investigated by density functional theory. The lowest energy (C4H4P)2Cr2(CO)n (n = 6, 4, 3) structures are completely analogous to their Cp2Cr2(CO)n analogues with two terminal pentahapto phospholyl rings. However, for the pentacarbonyl (C4H4P)2Cr2(CO)5 the lowest energy structure has a bridging seven-electron donor η5,η1-C4H4P ligand using the phosphorus lone pair in addition to the π-system of the phospholyl ligand as well as a Cr-Cr single bond. Thus, except for the pentacarbonyl the phosphorus lone pair of the phospholyl ligand is seen to play a minor role in the energetically preferred structures of the binuclear phospholyl chromium carbonyl derivatives.
1. Introduction The binuclear cyclopentadienylchromium carbonyls Cp2Cr2(CO)n (Cp = η5-C5H5; n = 6, 4) are known as stable compounds. The hexacarbonyl Cp2Cr2(CO)6 can be obtained by the mild oxidation of the anion CpCr(CO)3−, which can be obtained by heating Cr(CO)6 with the cyclopentadienide anion in a suitable ethereal solvent. The structure of Cp2Cr2(CO)6, as determined by X-ray crystallography, has a relatively long Cr-Cr bonding distance of 3.281 Å corresponding to the formal single bond suggested by the 18-electron rule (Fig. 1) [1]. Pyrolysis of Cp2Cr2(CO)6 leads to the stable tetracarbonyl Cp2Cr2(CO)4 shown by Xray crystallography to have a much shorter Cr≡Cr distance of 2.24 Å suggesting the formal triple bond required to give the chromium atom the favored 18-electron configuration [2]. The pentacarbonyl Cp2Cr2(CO)5 is shown spectroscopically to be generated from the reaction of Me3SiCHN2 with Cp2Cr2(CO)6 but is unstable with respect to disproportionation into Cp2Cr2(CO)6 + Cp2Cr2(CO)4 [3]. The infrared ν(CO) frequencies and theoretical studies suggest a singly bridged triplet spin state structure for Cp2Cr2(CO)5 with a Cr=Cr bond length of 2.68 Å intermediate between that of the Cr-Cr single bond in Cp2Cr2(CO)6 and the Cr≡Cr triple bond in Cp2Cr2(CO)4 (Fig. 1). Photolysis of Cp2Cr2(CO)4 in polyvinyl chloride [4] or in heptane solution [5] was shown to lead to further decarbonylation to give an unstable tricarbonyl Cp2Cr2(CO)3 suggested by its infrared ν(CO) frequencies and theoretical studies [6] to have three bridging carbonyl groups (Fig. 1). The lowest energy structure of the tricarbonyl is a triplet triply
bridged Cp2Cr2(µ-CO)3 with a Cr≡Cr distance of 2.31 Å slightly higher than the 2.24 Å Cr≡Cr triple bond distance in Cp2Cr2(CO)4 but still suggesting a formal triple bond. This gives each chromium atom a 17electron configuration consistent with a binuclear triplet spin state structure. The analogy between a bare phosphorus atom and a CH moiety [7] in the cyclopentadienyl group in terms of electron count and electronegativity makes the phospholyl (phosphacyclopentadienyl) ligand an analogue of the cyclopentadienyl ligand. This is indicated experimentally by synthesis of the very stable η5-phospholyl manganese carbonyl (η5-C4H4P)Mn(CO)3 (phosphacymantrene) as well as its more readily available 3,4-dimethyl derivative (η5-3,4-C4H2Me2P)Mn(CO)3 by thermal reaction of the corresponding P-phenylphosphole with Mn2(CO)10 [8,9]. Such reactions resulted in cleavage of the P-C6H5 bond in the original phosphole. Substitution of one CH group in the cyclopentadienyl ligand with a phosphorus to give a phospholyl ligand provides a phosphorus lone pair in addition to π-electrons from the ring atoms for donation to metal atoms. Thus a phospholyl ligand can formally function as a sevenelectron donor ligand (Fig. 2). However, the perpendicular orientation of a pentahapto bond from the C4P phosphole ring relative to the phosphorus lone pair prevents a neutral phospholyl ligand from functioning as a seven-electron donor to a single metal atom. Nevertheless, in a binuclear phospholyl metal carbonyl complex of the type (C4H4P)2M2(CO)n a neutral phospholyl ligand can function as a sevenelectron donor η1,η5-C4H4P ligand to a central bimetallic M2 unit. Such
E-mail address: [email protected] (R.B. King). https://doi.org/10.1016/j.ica.2019.05.018 Received 28 March 2019; Received in revised form 9 May 2019; Accepted 9 May 2019 Available online 10 May 2019 0020-1693/ © 2019 Elsevier B.V. All rights reserved.
Inorganica Chimica Acta 494 (2019) 194–203
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R R
R
R
O O C C
Cr
R
OC
C O
R
O C
Cr CR O
R
R
OO C C Cr
R
R
R
R
R
Cr
R
R
C O C R R O Cp2Cr2(CO)5
R
R
R
O C
R
R R
R
Cr
Cr
R
R
R
R
pair to displace a carbonyl group from a second (η5-C4H4P)Mn(CO)3 molecule. The lowest energy (C4H4P)2Mn2(CO)4 structure can be dissected into two (η5-C4H4P)Mn(CO)2 units using the phosphorus lone pairs of each such unit to bond to the manganese atom of the other (η5C4H4P)Mn(CO)2 thereby “biting the tail” of its partner. However, the lowest energy tricarbonyl (η5-C4H4P)2Mn2(CO)3 structure was analogous to its cyclopentadienyl analogue, an experimentally known species in its permethylated Cp*2Mn2(CO)3 form (Cp* = η5-Me5C5) [13]. In view of the occurrence of seven-electron donor η1,η5-C4H4P phospholyl ligands (Fig. 2) in the binuclear manganese carbonyl derivatives (C4H4P)2Mn2(CO)n (n = 5, 4; Fig. 3), we expected similar η1,η5C4H4P ligands in the lowest energy binuclear phospholyl iron and cobalt carbonyl structures, namely (C4H4P)2Fe2(CO)n (n = 4, 3, 2) and (C4H4P)2Co2(CO)n (n = 3, 2, 1), respectively. However, the lowest energy structures for all of these iron and cobalt systems were found to be completely analogous to their cyclopentadienyl analogues. The lack of involvement of the phosphorus lone pairs in the phospholyl-metal bonding in the low energy structures of the iron and cobalt derivatives (C4H4P)2M2(CO)n (M = Fe, Co) but their involvement in the bonding in the manganese derivatives (C4H4P)2Mn2(CO)n (n = 5, 4) suggested that such P → M bonding from bridging phosphole ligands might be a feature of binuclear phospholyl metal carbonyl derivatives of the early transition metals. In order to explore this possibility we have now extended such studies to the corresponding chromium derivatives (C4H4P)2Cr2(CO)n (n = 6, 5, 4, 3) for comparison with the corresponding cyclopentadienyl chromium carbonyl derivatives Cp2Cr2(CO)n discussed above. In this connection we have found the lowest energy (C4H4P)2Cr2(CO)n (n = 6, 4) structures to be analogous to those of the experimentally known stable Cp2Cr2(CO)n derivatives, discussed above. In addition, the lowest energy tricarbonyl structures (C4H4P)2Cr2(CO)3 and Cp2Cr2(CO)3 are analogous with the phospholyl ligands in terminal positions functioning only as a pentahapto ligand to a single metal atom. Only for the pentacarbonyl (C4H4P)2Cr2(CO)5 does the lowest energy structure have a bridging η1,η5-C4H4P ligand forming a dative P → Cr bond to one chromium atom as well as the usual pentahapto bond to the other chromium atom. The binuclear phospholyl chromium carbonyl derivatives discussed in this paper are potentially accessible from the phospholide anion and chromium hexacarbonyl using synthetic methods similar to those used to synthesize their binuclear cyclopentadienylchromium carbonyl analogues.
O C CO
R
R
Cr
C C O O
O C
R
Cp2Cr2(CO)6 R
R
R
R R
Cr
C C O O
R
R R
Cp2Cr2(CO)4
Cp2Cr2(µ-CO)3
Stable species
Detected spectroscopically
Fig. 1. Structures of the binuclear cyclopentadienylchromium carbonyls Cp2Cr2(CO)n (n = 6, 5, 4, 3).
Fig. 2. The phosphole ligand as a five-electron donor pentahapto ligand to a single metal atom and as a seven-electron donor to a pair of metal atoms. Note the perpendicular orientation of the pentahapto ring-metal bond and the phosphorus → metal dative bond.
a neutral phospholyl ligand can donate five electrons to one metal atom through a pentahapto bond and two electrons to another metal atom through the phosphorus lone pair. In view of the limited variety of experimentally known binuclear phospholyl metal carbonyl complexes, we have used well-established density functional methods to explore the involvement of the phosphorus lone pair in the ligand-metal bonding in binuclear phospholyl metal carbonyl derivatives of the type (C4H4P)2M2(CO)n (M = Mn [10], Fe [11], Co [12]). Our initial studies with the manganese derivatives (C4H4P)2Mn2(CO)n suggested that the phosphorus lone pairs might become involved in bonding to the manganese atoms. Thus the lowest energy (C4H4P)2Mn2(CO)n (n = 5, 4) structures by margins of more than 20 kcal/mol have one (n = 5) or two (n = 4) seven-electron donor η5,η1-C4H4P phospholyl ligands bridging the two manganese atoms with long non-bonding Mn…Mn distances of ∼4.3 Å (n = 5) or ∼3.7 Å (n = 4). The lowest energy (C4H4P)2Mn2(CO)5 structure (Fig. 3) can be generated by a (η5-C4H4P)Mn(CO)3 molecule using its phosphorus lone P O C P OC C O
Mn C O
Mn C O
CO
(C4H4P)2Mn2(CO)5
2. Theoretical methods Electron correlation effects were considered by employing density functional theory (DFT) methods, which have evolved as a practical and effective computational tool, especially for organometallic compounds [14–20]. Thus two DFT methods were used in this study. The first method uses the M06-L functional [21] of Zhao and Truhlar, which is suitable for applications in transition metal chemistry. The second DFT method used in the present paper is BP86, which combines Becke’s 1988 exchange functional with Perdew’s 1986 gradient corrected correlation functional method [22,23]. The M06-L and BP86 methods agree with each other fairly well in predicting the geometries of the (C4H4P)2Cr2(CO)n derivatives of interest. The M06-L and BP86 methods are also in reasonable agreement for most cases in predicting the relative energies of different structures of a given spin state, namely singlet or triplet. However, in some cases the two methods predict significantly different singlet-triplet splittings with the M06-L method energetically preferring higher spin states relative to the BP86 method. For the ν(CO) frequencies [24–26], the BP86 method is known to give values that are closer to the experimental values without using any scaling factors. This concurrence may be accidental, since the theoretical vibrational frequencies predicted by BP86 are harmonic frequencies, whereas the experimental fundamental frequencies are anharmonic. All computations were performed using the double-ζ plus
O C Mn
P P
Mn C O
C O
(C4H4P)2Mn2(CO)4
Fig. 3. Lowest energy structures of (C4H4P)2Mn2(CO)5 (=(OC)3Mn(η5,η1C4H4P)Mn(CO)2(η5-C4H4P) and (C4H4P)2Mn2(CO)4 (=[(η5,η1-C4H4P)Mn (CO)2]2 showing the seven-electron donor η5,η1-C4H4P ligands. 195
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direct bonding between the two chromium atoms. Placing a positive charge on the Cr(CO)4 group and a negative charge on the Cr(CO)2 group in 6S-3 gives each chromium atom the favored 18-electron configuration. In a sense 6S-3 is a zwitterion combining the [(C4H4P)Cr (CO)4]+ cation and a [(C4H4P)Cr(CO)2]− anion in the same molecule. The (C4H4P)2Cr2(CO)6 structure 6S-4, lying 10.5 kcal/mol (BP86) or 17.9 kcal/mol (M06-L) in energy above 6S-1, has two terminal CO groups bonded to one chromium atom and the other four terminal CO groups bonded to the other Cr atom. One of the phospholyl ligands in 6S-4 is a terminal pentahapto η5-C4H4P ligand similar to Cp whereas the other phospholyl ligand bridges the central Cr2 unit through only its phosphorus atom leaving two uncomplexed C=C double bonds. (Fig. 4 and Table 1). The bridging phospholyl ligand provides a single electron to the chromium atom of the (η5-C4H4P)Cr(CO)2 unit and two electrons to the Cr(CO)4 unit. The Cr=Cr distance of 2.730 Å (BP86) or 2.591 Å (M06-L) suggests a formal double bond, thereby giving each chromium atom the favored 18-electron configuration. The (C4H4P)2Cr2(CO)6 structure 6S-5 is a relatively high energy structure, lying 16.2 kcal/mol (BP86) or 26.2 kcal/mol (M06-L) above 6S-1 (Fig. 4 and Table 1). Structure 6S-5 has three terminal CO groups bonded to each chromium atom as well as a terminal pentahapto η5C4H4P ring and a bridging η1,η5-C4H4P ring. The long Cr…Cr distance > 5 Å implies the lack of a direct chromium-chromium bond in 6S-5 with the phosphorus atom of the bridging C4H4P ring providing the sole link between the two chromium atoms. Structure 6S-5 may be regarded as a zwitterion since the environment of the chromium atom bearing the terminal phospholyl group resembles that of a substituted [(C4H4P)Cr(CO)3]+ cation so this chromium atom bears a positive charge. Analogously the environment of the chromium atom bonded to all five atoms of the bridging phospholyl ligand resembles that of the [(C4H4P)Cr(CO)3]− anion and thus bears a negative charge. In this way each chromium atom in 6S-5 has the favored 18-electron configuration. The lowest energy triplet spin state (C4H4P)2Cr2(CO)6 structure 6T1, lying 11.2 kcal/mol (BP86) or 14.9 kcal/mol (M06-L) in energy above 6S-1, has four terminal CO groups bonded to one chromium atom, one terminal CO group bond to the other chromium atom, and one bridging CO group (Fig. 4 and Table 1). Structure 6T-1 has one terminal η5-C4H4P ring and one bridging η1,η1-C4H4P ring. The configuration of the ligands around the central Cr2 unit in the triplet structure 6T-1 is similar to that in the singlet structure 6S-4. The Cr=Cr distance of 2.715 Å (BP86) or 2.683 Å (M06-L) suggests a formal double bond, thereby giving each chromium atoms the favorable 18-electron
polarization (DZP) basis sets [27,28], which is consistent with our previous similar studies, and the DFT methods were reported to be less sensitive to the basis set size [29]. For the DZP basis sets used for hydrogen, a set of p polarization functions αp(H) = 0.75 is added to the Huzinaga–Dunning DZ set. For carbon, oxygen, and phosphorus one set of d functions was added to the Huzinaga-Dunning DZ sets with orbital exponents αd(C) = 0.75, αd(O) = 0.85, and αd(P) = 0.60. The loosely contracted DZP basis set for chromium is the Wachters primitive set [30] augmented by two sets of p functions and a set of d functions, contracted following Hood, Pitzer, and Schaefer [31], designated (14s11p6d/10s8p3d). The geometries of all structures were fully optimized using the DZP M06-L and DZP BP86 methods. The vibrational frequencies and the corresponding infrared intensities were determined at the same level of theory. All of the computations were carried out with the Gaussian 09 program [32], exercising the fine grid option (75 radial shells, 302 angular points) for evaluating integrals numerically [33]. 3. Results and discussion 3.1. Molecular structures 3.1.1 (C4H4P)2Cr2(CO)6 Five singlet structures and one triplet structure were found for (C4H4P)2Cr2(CO)6 (Fig. 4 and Table 1). All of them are predicted to be genuine minima. The global minimum of (C4H4P)2Cr2(CO)6, namely 6S-1, has exclusively terminal CO groups with the two C4H4P rings located in trans positions. Each η5-C4H4P ring is bonded to a single chromium atom as a pentahapto ligand similar to the Cp ligand. The CrCr distance of 3.293 Å (BP86) or 3.228 Å (M06-L) is similar to the experimental Cr-Cr distance [1] of 3.281 Å in Cp2Cr2(CO)6 thereby suggesting the formal single bond to give each chromium atom in 6S-1 the favored 18-electron configuration. The (C4H4P)2Cr2(CO)6 structure 6S2 (C2h), lying only 0.1 kcal/mol (BP86) or 0.2 kcal/mol (M06-L) in energy above 6S-1, differs only in the relative orientation of the phosphorus atoms in the η5-C4H4P rings. The (C4H4P)2Cr2(CO)6 structure 6S-3, lying 1.9 kcal/mol (BP86) or 13.7 kcal/mol (M06-L) in energy above 6S-1, has two terminal CO groups linked to one chromium atom and four terminal CO groups linked to the other chromium atom as well as one terminal η5-C4H4P ring and one bridging η1,η5-C4H4P ring (Fig. 4 and Table 1). The very long Cr…Cr distance of 4.867 Å (BP86) or 4.678 Å (M06-L) suggests no
6S-1 (C1)
6S-2 (C2h)
6S-3 (C1)
6S-4 (C1)
6S-5 (C1)
6T-1 (C1)
Fig. 4. The low-energy (C4H4P)2Cr2(CO)6 structures. In Figs. 4–7, the bond distances (in Å) are predicted with the BP86 (upper) and M06-L (lower) methods. 196
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Table 1 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), Cr-Cr distances (Å), and spin contamination values (〈S2〉) for the (C4H4P)2Cr2(CO)6 structures.
BP86 −(E + 3762) ΔE Cr-Cr 〈S2〉 M06-L −(E + 3761) ΔE Cr-Cr 〈S2〉
6S-1 (C1, 1A)
6S-2 (C2h, 1Ag)
6S-3 (C1, 1Ag)
6S-4 (C1, 1A)
6S-5 (C1, 1A)
6T-1 (C1, 3A)
0.072471 0.0 3.293 0.0
0.072261 0.1 3.293 0.0
0.069435 1.9 4.867 0.0
0.055711 10.5 2.730 0.0
0.046596 16.2 5.019 0.0
0.054647 11.2 2.715 2.02
0.470205 0.0 3.228 0.0
0.469941 0.2 3.225 0.0
0.448433 13.7 4.678 0.0
0.441611 17.9 2.591 0.0
0.428505 26.2 5.009 0.0
0.446498 14.9 2.683 2.07
configuration. The triplet state arises from two unpaired electrons in the Cr=Cr double bond which is of the σ + 2/2π type with two unpaired electrons in orthogonal π components. Similar σ + 2/ 2 π double bonds are found in normal triplet dioxygen as well as the organometallic (η5-Me5C5)2Fe2(µ-CO)3, which has been characterized structurally by X-ray crystallography [34–37].
Table 2 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), and Cr-Cr distances (Å) for the four lowest energy singlet (C4H4P)2Cr2(CO)5 structures.
BP86 −(E + 3648) ΔE Cr-Cr M06-L −(E + 3648) ΔE Cr-Cr
3.1.2 (C4H4P)2Cr2(CO)5 Nine low-energy (C4H4P)2Cr2(CO)5 structures were found, namely four singlets and five triplets (Fig. 5 and Tables 2 and 3). The lowestenergy (C4H4P)2Cr2(CO)5 structure 5S-1 has three terminal CO groups on one chromium atom and two terminal CO groups on the other chromium atom. Structure 5S-1 has one five-electron donor terminal η5-C4H4P ring and a η1,η5-C4H4P ring bridging the central Cr2 unit. The bridging η1,η5-C4H4P ring considered as a neutral ligand is a sevenelectron donor with five π electrons going to one chromium atom through pentahapto coordination and two electrons going to the other
5S-1 (C1, 1A)
5S-2 (C1, 1A)
5S-3 (C1, 1A)
5S-4 (C1, 1A)
0.716659 0.0 3.533
0.689764 16.9 2.640
0.689223 17.2 2.703
0.677403 24.6 2.486
0.119587 0.0 3.192
0.098264 13.4 2.671
0.096466 14.5 2.681
0.071621 30.1 2.488
chromium atom through a P → Cr dative bond from the phosphorus lone pair (Fig. 2). The predicted Cr-Cr distance of 3.533 Å (BP86) or 3.192 Å (M06-L), is relatively long similar to the the experimentally
5S-1 (C1)
5S-2 (C1)
5S-3 (C1)
5S-4 (C1)
5T-1 (C1)
5T-2 (C1)
5T- 3 (C1)
5T-4 (C1)
5T-5 (C1)
Fig. 5. The low-energy (C4H4P)2Cr2(CO)5 structures. 197
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formal triple bond needed to give each chromium atom in 4S-1 the favored 18-electron configuration. Structures 4S-2 (C2h) and 4S-3 (Cs), lying within ∼2 kcal/mol in energy of 4S-1 are similar to 4S-1 (Ci), differing only in the relative orientation of the phosphorus atoms in the η5-C4H4P rings. All three structures 4S-1, 4S-2, and 4S-3 have a trans orientation of the η5-C4H4P rings relative to the Cr≡Cr triple bond. The significantly higher energy (C4H4P)2Cr2(CO)4 structure 4S-4, lying 11.2 kcal/mol (BP86, M06-L) above 4S-1, is similar to 4S-1, 4S-2, and 4S-3 except for a cis orientation of the η5-C4H4P rings and two terminal CO groups. The (C4H4P)2Cr2(CO)4 structure 4S-5, lying 17.4 kcal/mol (BP86) or 17.0 kcal/mol (M06-L) in energy above 4S-1, has three terminal CO groups, one four-electron donor bridging η2-µ-CO group with a short CrO distance of 2.256 Å (BP86) or 2.279 Å (M06-L)., one terminal η5C4H4P ring, and one bridging η1,η5-C4H4P ring (Fig. 6 and Table 4). The bridging η1,η5-C4H4P ring is a pentahapto five-electron donor to one chromium atom and bonds to the other chromium atom through the phosphorus lone pair with a P → Cr distance of 2.294 Å (BP86) or 2.301 Å (M06-L). Thus in 4S-5 the η1,η5-C4H4P ring is a seven-electron donor to the central Cr2 unit. The Cr-Cr distance in 4S-5 of 3.024 Å (BP86) or 2.961 Å (M06-L) suggests a single bond thereby giving each chromium atom the favored 18-electron configuration after considering the four-electron donor bridging η2-µ-CO group and the seven-electron donor bridging η1,η5-C4H4P ring. The triplet (C4H4P)2Cr2(CO)4 structure 4T-1, lying 14.0 kcal/mol (BP86) or 6.1 kcal/mol (M06-L) in energy above 4S-1, has one terminal η5-C4H4P ring and one bridging η1,η5-C4H4P ring (Fig. 6 and Table 5). The chromium atom in 4T-1 bonded to the terminal η5-C4H4P ring has one terminal CO groups whereas the other chromium atom has two terminal CO group. The fourth CO group in 4T-1 is a weakly semibridging carbonyl group with a short Cr-C distance to the chromium atom already bearing two terminal CO groups and a long almost nonbonding distance of ∼2.5 Å to the other chromium atom. The Cr=Cr distance of 4T-1 of 2.724 Å (BP86) or 2.696 Å (M06-L), suggests a formal double bond, thereby giving each chromium atom the favored 18-electron configuration. As in the triplet (C4H4P)2Cr2(CO)5 structures the two unpaired electrons of the triplet spin state in 4T-1 reside in two orthogonal single-electron “half-bond” π components so that these Cr=Cr formal double bonds are of the σ + 2 2 π type similar to that in Cp*2Fe2(µ-CO)3 [34–37]. The triplet (C4H4P)2Cr2(CO)4 structures 4T-2 and 4T-4 are similar to each other in geometry, differing only by the relative positions of the phosphorus atoms in the phospholyl rings (Fig. 6 and Table 5). Both structures have two terminal CO groups, two bridging CO groups, and two terminal η5-C4H4P rings. Structure 4T-2 lies 18.9 kcal/mol (BP86) or 12.9 kcal/mol (M06-L) in energy above 4S-1, whereas 4T-4 lies 22.5 kcal/mol (BP86) or 19.1 kcal/mol (M06-L) above 4S-1. Interpreting the Cr=Cr distances of ∼2.5 Å in 4T-2 and 4T-4 as formal double bonds gives each chromium atom in each structure a 17-electron configuration corresponding to a binuclear triplet. The triplet (C4H4P)2Cr2(CO)4 structure 4T-3, lying 20.8 kcal/mol (BP86) or 17.1 kcal/mol (M06-L) in energy above 4S-1, has three terminal CO groups, one bridging CO group, one terminal η5-C4H4P ring, and one bridging η1,η5-C4H4P ring (Fig. 6 and Table 5). The Cr=Cr distance of 2.641 Å (BP86) or 2.581 Å (M06-L) in 4T-3 suggests a formal double bond, thereby giving each chromium atom the favored 18-electron configurations. The two unpaired electrons of the triplet spin state of 4T-3 reside in the Cr=Cr double bond, which is of the σ + 2/2 π type similar to that in 4T-1.
Table 3 Total energies (E, in hartree), relative energies to 5S-1 (ΔE, in kcal/mol), the CrCr distances (Å), and spin contamination values (〈S2〉) for the five lowest-energy triplet (C4H4P)2Cr2(CO)5 structures.
BP86 −(E + 3648) ΔE Cr-Cr 〈S2〉 M06-L −(E + 3648) ΔE Cr-Cr 〈S2〉
5T-1 (C1, 3A)
5T-2 (C1, 3A)
5T-3 (C1, 3A)
5T-4 (C1, 3A)
5T-5 (C1, 3A)
0.703038 8.5 2.708 2.03
0.700922 9.9 2.717 2.03
0.700722 10.0 2.709 2.04
0.698134 11.6 2.711 2.04
0.693374 14.4 2.743 2.03
0.119020 0.4 2.719 2.10
0.117628 1.2 2.740 2.10
0.116258 2.1 2.726 2.10
0.113811 3.6 2.728 2.10
0.110067 6.0 2.756 2.10
determined Cr-Cr single bond distance of 3.281 Å in Cp2Cr2(CO)6. Interpreting this Cr-Cr distance in 5S-1 as a formal single bond gives each chromium atom the favorable 18-electron configuration. The singlet (C4H4P)2Cr2(CO)5 structures 5S-2 and 5S-3 and the five triplet structures 5T-1 through 5T-5 have similar geometries with three terminal CO groups, two semi-bridging CO groups, and two terminal η5C4H4P rings in a trans orientation (Fig. 5 and Tables 2 and 3). Other than the spin state these seven structures differ only in the relative orientations of the phosphorus atoms in the two η5-C4H4P rings. The singlet structures 5S-2 and 5S-3, lying 16.9 kcal/mol (BP86) or 13.4 kcal/mol (M06-L) and 17.2 kcal/mol (BP86) or 14.5 kcal/mol (M06-L), respectively, in energy above 5S-1, have Cr=Cr distances of ∼2.67 Å (M06-L), suggesting formal double bonds, thereby giving each chromium atom the favored 18-electron configuration. All five triplet (C4H4P)2Cr2(CO)5 structures (5T-1 through 5T-5 in Table 3) have Cr=Cr distances ranging from 2.71 to 2.76 Å (M06-L), also suggesting formal double bonds, thereby giving each chromium atom the favored 18-electron configuration. The two unpaired electrons in each of the triplet (C4H4P)2Cr2(CO)5 structures reside formally in the Cr=Cr double bond, which can be interpreted as a σ + 2/2 π bond with single unpaired electrons in two orthogonal π half-bonds similar to the experimentally known Cp*2Fe2(µ-(CO)3, characterized by X-ray crystallography [34–37]. The (C4H4P)2Cr2(CO)5 structure 5S-4, lying 24.6 kcal/mol (BP86) or 30.1 kcal/mol (M06-L) in energy above 5S-1, has two terminal CO groups bonded to one chromium atom and the remaining three terminal CO groups bonded to the other chromium atom (Fig. 5 and Table 2). Structure 5S-4 has a terminal η5-C4H4P ring and a bridging η1,η1-C4H4P ring bonded to the central Cr2 unit only through its phosphorus atom and thus having two uncomplexed C=C double bonds and donating a total of three electrons. The Cr≡Cr distance of 2.486 Å (BP86) or 2.488 Å (M06-L) in 5S-4 is ∼0.2 Å shorter than the formal Cr=Cr double bonds in the singlet (C4H4P)2Cr2(CO)5 structures 5S-2 and 5S-3 thereby suggesting a formal Cr≡Cr triple bond in 5S-4. This gives each chromium atom in 5S-4 the favored 18-electron configuration by placing a formal positive charge on the chromium atom bearing two terminal CO groups and the terminal η5-C4H4P ring and a formal negative charge on the other chromium atom.
3.1.3 (C4H4P)2Cr2(CO)4 Nine low-energy (C4H4P)2Cr2(CO)4 structures were found, namely five singlets and four triplets (Fig. 6 and Tables 4 and 5). The lowest energy such structure 4S-1 has all four CO groups as weak semi-bridges across the central Cr2 unit (long Cr-C distances of ∼2.5 Å) as well as one terminal η5-C4H4P ring bonded to each chromium atom with a short Cr≡Cr distance of 2.239 Å (BP86) or 2.248 Å (M06-L). This Cr≡Cr distance is very close to that of 2.24 Å determined by X-ray crystallography for the Cr≡Cr formal triple bond in the cyclopentadienyl analogue Cp2Cr2(CO)4 [2] and can likewise be interpreted as the
3.1.4 (C4H4P)2Cr2(CO)3 Eleven low-energy (C4H4P)2Cr2(CO)3 structures were found, namely six triplets and five singlets (Fig. 7 and Tables 6 and 7). The lowest energy (C4H4P)2Cr2(CO)3 structure 3T-1 has three bridging CO groups and two terminal five-electron donor η5-C4H4P rings. The Cr≡Cr distance of 2.292 Å (BP86) or 2.315 Å (M06-L) in 3T-1 is similar to that of 198
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4S-1 (Ci)
4S-2 (C2h)
4S-3 (Cs)
4S-4 (C1)
4S-5 (C1)
4T-1 (C1)
4T- 2 (C1)
4T-3 (C1)
4T-4 (C1)
Fig. 6. The nine low-energy (C4H4P)2Cr2(CO)4 structures. Table 4 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), and the Cr-Cr distances (Å) for the singlet (C4H4P)2Cr2(CO)4 structures.
BP86 −(E + 3535) ΔE Cr-Cr M06-L −(E + 3534) ΔE Cr-Cr
4S-1 (Ci, 1Ag)
4S-2 (C2h, 1Ag)
4S-3 (Cs, 1Á)
4S-4 (C1, 1A)
4S-5 (C1, 1A)
0.354244 0.0 2.239
0.353779 0.3 2.246
0.351288 1.9 2.246
0.336380 11.2 2.261
0.326548 17.4 3.024
0.771762 0.0 2.248
0.771066 0.4 2.257
0.768290 2.2 2.256
0.753837 11.2 2.276
0.744619 17.0 2.961
Table 5 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), Cr-Cr distances (Å), and spin contamination values (〈S2〉) for the triplet (C4H4P)2Cr2(CO)4 structures.
BP86 −(E + 3535) ΔE Cr-Cr 〈S2〉 M06-L −(E + 3534) ΔE Cr-Cr 〈S2〉
the 2.24 Å Cr≡Cr triple bond distance found experimentally in Cp2Cr2(CO)4 by X-ray crystallography [2] as well as to those predicted for the (η5-C4H4P)2Cr2(CO)4 structures 4S-1, 4S-2, and 4S-3 (Fig. 6 and Table 4). This suggests a formal Cr≡Cr triple bond in 3T-1 thereby giving each Cr atom in 3T-1 a 17-electron configuration for a binuclear triplet. The triplet structures 3T-2, 3T-3, 3T-4, and 3T-5, having energies within 4 kcal/mol (BP86) or 8 kcal/mol (M06-L) of 3T-1, are similar to 3T-1 differing only in the relative orientations of the phosphorus atoms in the η5-C4H4P rings. Spectroscopic studies, particularly the infrared ν(CO) frequencies, on the product from the photolysis of Cp2Cr2(CO)4 in polyvinyl chloride or heptane solution suggests a triply bridged Cp2Cr2(µ-CO)3 structure analogous to any of the triplet triply bridged (η5-C4H4P)2Cr(µ-CO)3 structures 3T-1 through 3T-5 [4,5].
4T-1 (C1, 3A)
4T-2 (C1, 3A)
4T-3 (C1, 3A)
4T-4 (C1, 3A)
0.331956 14.0 2.724 2.04
0.324160 18.9 2.544 2.04
0.321057 20.8 2.641 2.03
0.318336 22.5 2.482 2.05
0.762055 6.1 2.696 2.12
0.751183 12.9 2.556 2.10
0.744556 17.1 2.581 2.09
0.741390 19.1 2.456 2.12
The triplet (C4H4P)2Cr2(CO)3 structure 3T-6, lying 10.1 kcal/mol (BP86) or 7.8 kcal/mol (M06-L) in energy above 3T-1, is very different from the five lower energy triplet (C4H4P)2Cr2(CO)3 structures by having three terminal CO groups and two seven-electron donor bridging η1,η5-C4H4P rings rather than the opposite of bridging CO groups and terminal η5-C4H4P rings (Fig. 7 and Table 6). The Cr-Cr distance of 2.929 Å (BP86) or 2.830 Å (M06-L) can be interpreted as a formal single bond to give each chromium atom a 17-electron configuration for a binuclear triplet. The lowest energy singlet (C4H4P)2Cr2(CO)3 structure 3S-1, lying 199
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3T-1 (C1)
3T-2 (C1)
3T-3 (C1)
3T-4 (C1)
3T-5 (C1)
3T-6 (C1)
3S-1 (C1)
3S-2 (C1)
3S-3 (C1)
3S-4 (C1)
3S-5 (C1)
Fig. 7. The 11 low-energy (C4H4P)2Cr2(CO)3 structures.
7.4 kcal/mol (BP86) or 13.8 kcal/mol (M06-L) above 3T-1, has one terminal CO group, two semi-bridging CO groups, one terminal η5C4H4P ring, and one bridging η1,η5-C4H4P ring (Fig. 7 and Table 7). Interpreting the very short CrCr distance of 2.169 Å (BP86) or 2.198 Å (M06-L) as a formal quadruple bond gives each chromium atom the favored 18-electron configuration.
The other singlet (C4H4P)2Cr2(CO)3 structures 3S-2, 3S-3, 3S-4, and 3S-5 (Cs), lying 9 to 12 kcal/mol (BP86) or 15 to 18 kcal/mol (M06-L) in energy above 3T-1, have similar geometries to the five lowest energy triplet (C4H4P)2Cr2(CO)3 structures 3T-1 through 3T-5, i.e., with three bridging µ-CO groups and two terminal five-electron donor η5-C4H4P
Table 6 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), Cr-Cr distances (Å), and spin contamination values (〈S2〉) for the triplet (C4H4P)2Cr2(CO)3 structures.
BP86 −(E + 3421) ΔE Cr-Cr 〈S2〉 M06-L −(E + 3421) ΔE Cr-Cr 〈S2〉
3T-1 (C1, 3A)
3T-2 (C1, 3A)
3T-3 (C1, 3A)
3T-4 (C2, 3A)
3T-5 (C1, 3A)
3T-6 (C1, 3A)
0.957632 0.0 2.292 2.10
0.955412 1.4 2.303 2.09
0.954497 2.0 2.298 2.12
0.950205 4.7 2.267 2.02
0.951434 3.9 2.268 2.02
0.941544 10.1 2.929 2.08
0.397216 0.0 2.315 2.38
0.396940 0.2 2.324 2.26
0.396242 0.6 2.324 2.37
0.385923 7.1 2.281 2.05
0.384870 7.7 2.284 2.04
0.384706 7.8 2.830 2.27
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Table 7 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), Cr-Cr distances (Å) for the five singlet (C4H4P)2Cr2(CO)3 structures. 3S-1 (C1, 1A) BP86 −(E + 3421) ΔE Cr-Cr M06-L −(E + 3421) ΔE Cr-Cr
3S-2 (C1, 1A)
3S-3 (C1, 1A)
3S-4 (C1, 1A)
Table 9 Disproportionation energies (kcal/mol) for the reactions 2(C4H4P)2Cr2(CO)n → (C4H4P)2Cr2(CO)n+1 + (C4H4P)2Cr2(CO)n-1.
3S-5 (C1, 1A)
0.946792 7.4 2.169
0.943137 9.7 2.214
0.941402 10.8 2.214
0.941291 10.9 2.206
0.939160 12.2 2.213
0.375289 13.8 2.198
0.372823 15.3 2.232
0.372823 15.3 2.232
0.370823 16.6 2.233
0.368904 17.8 2.238
2(C4H4P)2Cr2(CO)5 (5S-1) → (C4H4P)2Cr2(CO)4 (4S-1) + (C4H4P)2Cr2(CO)6 (6S-1) 2(C4H4P)2Cr2(CO)4 (4S-1) → (C4H4P)2Cr2(CO)3 (3T-1) + (C4H4P)2Cr2(CO)5 (5S-1)
3.2. Thermochemistry. Table 8 lists the energies for the CO dissociation reactions (C4H4P)2Cr2(CO)n → (C4H4P)2Cr2(CO)n−1 + CO (n = 6, 5, 4). The largest energy for the dissociation of one CO group in a (C4H4P)2Cr2(CO)n derivative is for (C4H4P)2Cr2(CO)4 where CO dissociation is highly endothermic at 40.5 kcal/mol (BP86) or 33.2 kcal/mol (M06-L), suggesting a viable species. The CO dissociation energies for the other (C4H4P)2Cr2(CO)n species (n = 6, 5) are all positive in the range 15 to 20 kcal/mol, also suggesting viability towards CO dissociation. Table 9 lists the energies for the disproportionation reactions 2(C4H4P)2Cr2(CO)n → (C4H4P)2Cr2(CO)n+1 + (C4H4P)2Cr2(CO)n−1 (n = 5, 4). The energy required for disproportionation of (C4H4P)2Cr2(CO)4 is significant, i.e, 20.8 kcal/mol (BP86) or 15.7 kcal/ mol (M06-L), indicating that it is a viable species. Thus (C4H4P)2Cr2(CO)4 with a structure analogous to the known stable Cp2Cr2(CO)4 as a very favorable species and a reasonable synthetic target. However, the disproportionation energy for (C4H4P)2Cr2(CO)5 is essentially thermoneutral, namely slightly endothermic at 4.0 kcal/mol by BP86 but very slightly endothermic at −1.3 kcal/mol (M06-L). This suggests that (C4H4P)2Cr2(CO)5 is not a viable species. In this connection the cyclopentadienyl analogue Cp2Cr2(CO)5, synthesized under mild conditions by decarbonylation of Cp2Cr2(CO)6 with Me3SiN2 and identified by its ν(CO) frequencies, is found to undergo disproportionation into the two stable species Cp2Cr2(CO)6 + Cp2Cr2(CO)4 [3]. Also of interest is the dissociation of the (C4H4P)2Cr2(CO)n derivatives into mononuclear (C4H4P)Cr(CO)m fragments. In order to obtain such energetic data, the structures of the mononuclear (C4H4P)Cr(CO)m were optimized by the same DFT methods (Fig. 8). Using this information, the dissociation energies of the binuclear (C4H4P)2Cr2(CO)n into mononuclear fragments were determined (Table 10). The results were found to depend on the density functional used since the mononuclear (C4H4P)Cr(CO)n are necessarily doublet spin state structures and singlet-triplet splittings are dependent on the density functional used. This is particularly true for the dissociation of the saturated hexacarbonyl (C4H4P)2Cr2(CO)6 into two (C4H4P)Cr(CO)3 fragments for
(C4H4P)2Cr2(CO)6 (6S-1) → (C4H4P)2Cr2(CO)5 (5S-1) + CO (C4H4P)2Cr2(CO)5 (5S-1) → (C4H4P)2Cr2(CO)4 (4S-1) + CO (C4H4P)2Cr2(CO)4 (4S-1) → (C4H4P)2Cr2(CO)3 (3S-1) + CO
15.7 19.7 40.5
18.8 17.5 33.2
4.0
−1.3
20.8
15.7
3.3. NBO analysis of the chromium-chromium bonding Table 11 lists the natural atomic charges for the chromium atoms and the Wiberg Bond Indices (WBI) for the Cr-Cr bonds using NBO analysis [38] with the BP86 functional The Cr-Cr distances, the formal Cr-Cr bond orders, and the bridging groups are also listed for comparison. Only the singlet structures are considered since WBI analyses of higher spin state open shell structures appear to be less reliable. Most of the natural charges on the chromium atoms are negative since they accept electrons from the CO and phosphorus lone pairs without sufficient π back-bonding from the chromium atoms to the antibonding orbitals of the CO groups to remove completely this additional negative charge. The chromium natural charges depend on the number of CO groups and whether the lone pair of the phosphole ligand, considered formally as a neutral species, is coordinated to a chromium atom as a monohapto ligand, either as part of a seven-electron bridging η1,η5-C4H4P ligand or as a three-electron donor η1-C4H4P ligand with two uncomplexed double bonds as in 5S-4. Thus in the species 6S-3, 6S-4, 6S-5, 5S-1, and 5S-4, in which the phosphorus lone pair is coordinated to one or both chromium atoms, the atomic charges range from −0.70 to −0.88. In addition, the negative charge on the chromium atoms become more positive upon loss of CO groups. Chromium atoms bearing three CO groups, such as those in 6S-1 and 6S-2, have negative charges from −0.65 to −0.67. Chromium atoms bonded to 2 1 2 CO groups (i. e., two terminal CO groups and half of a bridging CO group), such as in 5S-2, have a negative charge of ∼−0.5. Chromium atoms bearing two CO groups, such as the four lowest energy singlet (C4H4P)2Cr2(CO)4 structures, have charges ranging from −0.41 to −0.46. The chromium atoms in the (C4H4P)2Cr2(CO)3 structures 3S-2, 3S-3, 3S-4, and 3S-5 bearing half of two bridging CO groups and no terminal CO groups have a charge close to zero. Previous studies on the Wiberg Bond Indices (WBIs) of metal carbonyls such as Fe2(CO)9 and Fe3(CO)12 show that the WBI values are relatively low [39] compared with the formal bond orders, particularly when the metal-metal bonds are bridged by CO groups. The WBI values for the Cr-Cr bonds in the (C4H4P)2Cr2(CO)n derivatives are related to the formal bond order and the number of bridging groups. Table 11 shows that for single bonds the Cr-Cr WBIs range from 0.23 to 0.31, for double bonds the Cr=Cr WBIs range from 0.37 to 0.52, for Cr≡Cr triple bonds the WBIs range from 0.91 to 0.97 and for CrCr quadruple bonds the WBI values range from 1.11 to 1.38. The WBIs for interactions between pairs of chromium atoms too far apart for direct bond formation, such as in 6S-3 and 6S-5, are close to zero ranging from 0.08
Table 8 Dissociation energies (kcal/mol) for the successive removal of carbonyl groups from the (C4H4P)2Cr2(CO)n derivatives. M06-L
M06-L
which the BP86 functional gives a relatively small dissociation energy of 8.7 kcal/mol whereas the M06-L functional gives a significantly higher dissociation energy of 17.5 kcal/mol. Either of these values, however, are low enough energies to predict that dissociation into two (C4H4P)Cr(CO)3 fragments will be significant in much of the reaction chemistry of (C4H4P)2Cr2(CO)6. Such is the case with the cyclopentadienyl analogue Cp2Cr2(CO)6. The dissociation energies of the unsaturated (C4H4P)2Cr2(CO)n into mononuclear fragments are very high at ∼44, ∼75, and ∼85 kcal/mol for n = 5, 4, 3, respectively, and increase with decreasing number of CO groups. This is consistent with the increase in the formal Cr-Cr bond order upon loss of CO groups.
rings (Fig. 7 and Table 7). Interpreting the short CrCr distances ranging from 2.206 to 2.214 Å (BP86) or 2.232 to 2.238 Å (M06-L) in 3S-1 to 3S-5, as formal quadruple bonds gives each chromium atom the favored 18-electron configuration.
BP86
BP86
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Fig. 8. The optimized geometries for the mononuclear structures (C4H4P)Cr(CO)m (m = 3, 2, 1).
derivatives of four adjacent first row transition metals (Cr, Mn, Fe, Co) to assess the role played by the phosphorus lone pair of the phospholyl ligand in the energetically preferred structures of such binuclear phospholyl derivatives. In other words, are the lowest-energy structures (C4H4P)2M2(CO)n (M = Cr, Mn, Fe, Co) similar to their cyclopentadienyl analogues Cp2M2(CO)n with a terminal pentahapto η5-C4H4P ligand analogous to the ubiquitous terminal pentahapto Cp ligand or do they exhibit other structural features in which the phosphorus lone pair of the phospholyl ligand plays a role? Our initial theoretical studies on binuclear phospholyl manganese carbonyl derivatives (C4H4P)2Mn2(CO)n [10] showed that for the pentacarbonyl and tetracarbonyl (n = 5, 4) the lowest energy structures had at least one bridging seven-electron donor η5,η1-C4H4P phospholyl ligand with energies more than 20 kcal/mol below the lowest energy (η5)2Mn2(CO)n isomers. We therefore expected similar situations with low-energy (C4H4P)2M2(CO)n structures of the transition metals surrounding manganese in the Periodic Table. Table 12 summarizes the relative energies of the binuclear phospholyl metal carbonyls (η5-C4H4P)2M2(CO)n with only terminal pentahapto phospholyl rings analogous to the lowest energy binuclear cyclopentadienylmetal carbonyls (η5-C5H5)2M2(CO)n {≡Cp2M2(CO)n}. A number of the lowest energy Cp2M2(CO)n derivatives are stable species that have been isolated and structurally characterized by X-ray crystallography (indicated in bold in Table 12). A “(0)” following a generic Q2M2(CO)n designation in Table 12 means that the lowest energy (C4H4P)2M2(CO)n and Cp2M2(CO)n structures are analogous having only terminal pentahapto η5-C4H4P rings in the phospholyl structure. The general observation from the information presented in Table 12 is that all of the phospholyl (η5-C4H4P)2M2(CO)n structures analogous to stable Cp2M2(CO)n structures (designed in bold in Table 12) are the lowest energy structures. Isomeric structures with seven-electron donor bridging η1,η5-phospholyl ligands as well as other types of phosphoryl ligands coordinating to the metal(s) through the phosphorus lone pair are found but at higher energies. The originally studied (C4H4P)2Mn2(CO)n (n = 5, 4) systems are seen to be exceptional since phospholyl structures with two terminal rings analogous to the lowest energy Cp2Mn2(CO)n structures lie more than 20 kcal/mol above their lowest energy isomers [10]. These lowest energy (C4H4P)2Mn2(CO)n (n = 5, 4) structures contain bridging seven-electron donor η5,η1-C4H4P rings (Fig. 3) and lack direct Mn-Mn bonds. The local environments of
Table 10 Dissociation energies of the binuclear (C4H4P)2Cr2(CO)n (n = 5, 4, 3, 2) into mononuclear fragments (kcal/mol).
(C4H4P)2Cr2(CO)6 (C4H4P)2Cr2(CO)5 (C4H4P)2Cr2(CO)4 (C4H4P)2Cr2(CO)3
(6S-1) → 2(C4H4P)Cr(CO)3 (5S-1) → (C4H4P)Cr(CO) 2+ (C4H4P)Cr(CO)3 (4S-1) → 2 (C4H4P)Cr(CO)2 (3T-1) → (C4H4P)Cr(CO)2 + (C4H4P)Cr(CO)
BP86
M06-L
8.7 43.9 75.0 83.6
17.5 45.3 74.4 86.4
Table 11 Atomic charges and Wiberg bond indices for the singlet (C4H4P)2Cr2(CO)n structures by the BP86 method.
6S-1 6S-2 6S-3 6S-4 6S-5 5S-1 5S-2 5S-3 5S-4 4S-1 4S-2 4S-3 4S-4 4S-5 3S-1 3S-2 3S-3 3S-4 3S-5
Natural Charge on Cr / Cr
Wiberg bond index
Cr-Cr distance, Å
Bridges
Formal Bond order
−0.67/−0.65 −0.65/−0.65 −0.84/ −0.86 −0.68/ −0.88 −0.70/−0.70 −0.71/−0.68 −0.50/−0.52 −0.65/−0.37 −0.83/−0.49 −0.44/−0.44 −0.44/ −0.44 −0.44/−0.46 −0.41/ −0.44 −0.71/ −0.34 −0.38/−0.22 −0.38/0.04 −0.38/0.04 −0.40/0.07 −0.39/0.09
0.24 0.23 0.08 0.37 0.15 0.28 0.58 0.43 0.52 0.94 0.91 0.91 0.97 0.31 1.38 1.11 1.11 1.15 1.12
3.293 3.293 4.867 2.730 5.019 3.533 2.640 2.703 2.486 2.239 2.246 2.246 2.261 3.024 2.169 2.214 2.214 2.206 2.213
4 semi-CO 4 semi-CO C4H4P C4H4P C4H4P C4H4P 2 semi-CO 2 semi-CO C4H4P none none none none CO/ C4H4P 2 CO 3 CO 3 CO 3 CO 3 CO
1 1 0 2 0 1 2 2 2 3 3 3 3 1 4 4 4 4 4
to 0.15. 4. Discussion The results reported here on the structures and energetics of the (C4H4P)2Cr2(CO)n (n = 6, 5, 4, 3) systems added to those from our previous studies provide information on such binuclear phospholyl
Table 12 Comparison of analogous binuclear phospholyl and cyclopentadienyl metal carbonyls Q2M2(CO)n (Q = C4H4P or C5H5). The figures in parentheses indicate the energy (in kcal/mol) of the phospholyl derivative (C4H4P)2M2(CO)n structurally analogous to the lowest energy cyclopentadienyl derivative (C5H5)2M2(CO)n relative to the lowest energy (C4H4P)2M2(CO)n isomer. The Cp2M2(CO)n structures in bold are stable isolable species structurally characterized by X-ray crystallography whereas those in italics have been detected spectroscopically. Bond Order
M = Cr
M = Mn
M = Fe
M = Co
1 2 3 4
Q2Cr2(CO)6 (0) Q2Cr2(CO)5 (17) Q2Cr2(CO)4 (0) Q2Cr2(CO)3 (0)
Q2Mn2(CO)5 (23) Q2Mn2(CO)4 (31) Q2Mn2(CO)3 (0) Q2Mn2(CO)2 (4)
Q2Fe2(CO)4 (0) Q2Fe2(CO)3 (1) Q2Fe2(CO) (0)
Q2Co2(CO)3 (0) Q2Co2(CO)2 (0) Q2Co2(CO) (12)
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the manganese atoms in these lowest energy (C4H4P)2Mn2(CO)n (n = 5, 4) are thus similar to that of the very stable CpMn(CO)3 with phospholyl phosphorus lone pairs substituting a carbonyl group. The high stability of the CpMn(CO)3 system and its carbonyl substitution species thus can account for the behavior of the (C4H4P)2Mn2(CO)n (n = 5, 4) systems, which is now seen to be anomalous by comparison with the other (C4H4P)2M2(CO)n (M = Cr, Mn, Fe, Co) systems.
[2] M.D. Curtis, W.M. Butler, J. Organometal. Chem. 155 (1978) 131. [3] G.C. Fortman, T. Kégl, Q.-S. Li, X. Zhang, H.F. Schaefer, Y. Xie, R.B. King, J. Telser, C.D. Hoff, J. Am. Chem. Soc. 129 (2007) 14388. [4] R.H. Hooker, A. Rest, J. Chem. Soc., Dalton Trans. (1990) 1221. [5] I.G. Virrels, T.F. Nolan, M.W. George, J.J. Turner, Organometallics 16 (1997) 5879. [6] X. Zhang, Q.-S. Li, Y. Xie, R.B. King, H.F. Schaefer, Dalton Trans. (2008) 4805. [7] K.B. Dillon, F. Mathey, J.F. Nixon, Phosphorus: the carbon copy: from organophosphorus to phospha-organic chemistry, Wiley VCH, New York, 1998. [8] F. Mathey, Tetrahedron Lett. (1976) 4155. [9] F. Mathey, A. Mitschler, R. Weiss, J. Am. Chem. Soc. 100 (1978) 5748. [10] X. Chen, Q. Du, R. Jin, H. Feng, Y. Xie, R.B. King, New J. Chem. 35 (2011) 1117. [11] X. Chen, L. Yuan, G. Ren, Q. Xi, R. King, Q. Du, H. Feng, Y. Xie, R.B. King, Inorg. Chim. Acta 445 (2016) 79. [12] X. Chen, R. Jin, Q. Du, H. Feng, Y. Xie, R.B. King, J. Organometal. Chem. 701 (2012) 1. [13] I. Bernal, J.D. Korp, W.A. Hermann, R. Serrano, Chem. Ber. 117 (1984) 434. [14] T. Ziegler, J. Autschbach, Chem. Rev. 105 (2005) 2695. [15] M. Bühl, H. Kabrede, J. Chem. Theory Comput. 2 (2006) 1282. [16] M. Brynda, L. Gagliardi, P.O. Widmark, P.P. Power, B.O. Roos, Angew. Chem. Int. Ed. 45 (2006) 3804. [17] N. Sieffert, M. Bühl, J. Am. Chem. Soc. 132 (2010) 8056. [18] P. Schyman, W. Lai, H. Chen, Y. Wang, S. Shaik, J. Am. Chem. Soc. 133 (2011) 7977. [19] R.D. Adams, W.C. Pearl, Y.O. Wong, Q. Zhang, M.B. Hall, J.R. Walensky, J. Am. Chem. Soc. 133 (2011) 12994. [20] R. Lonsdale, J. Olah, A.J. Mulholland, J.N. Harvey, J. Am. Chem. Soc. 133 (2011) 15464. [21] Y. Zhao, D.G. Truhlar, J. Chem. Phys. 125 (2006) 194101. [22] A.D. Becke, Phys. Rev. 38 (1988) 3098. [23] J.P. Perdew, Phys. Rev. B 33 (1986) 8822. [24] V. Jones, W. Thiel, J. Phys. Chem. 102 (1995) 8474. [25] I. Silaghi-Dumitrescu, T.E. Bitterwolf, R.B. King, J. Am. Chem. Soc. 128 (2006) 5432. [26] M.K. Assef, J.L. Dever, A.D. Brathwaite, J.D. Mosley, M.A. Duncan, Chem. Phys. Lett. 640 (2015) 175. [27] T.H. Dunning, J. Chem. Phys. 53 (1970) 2823. [28] S. Huzinaga, J. Chem. Phys. 42 (1965) 1293. [29] B.S. Narendrapurapu, N.A. Richardson, A.V. Copan, M.L. Estep, Z. Yang, H.F. Schaefer, J. Chem. Theory Comput. 9 (2013) 2930. [30] A.J.H. Wachters, J. Chem. Phys. 52 (1970) 1033. [31] D.M. Hood, R.M. Pitzer, H.F. Schaefer, J. Chem. Phys. 71 (1979) 705. [32] M.J. Frisch, et al., Gaussian 09, Revision A.02, Gaussian, Inc., Wallingford CT, 2009. [33] B.N. Papas, H.F. Schaefer, J. Mol. Struct. (THEOCHEM) 768 (2006) 175. [34] J.V. Caspar, T.J. Meyer, J. Am. Chem. Soc. 102 (1980) 7794. [35] R.H. Hooker, K.A. Mahmoud, A.J. Rest, Chem. Commun. (1983) 1022. [36] A.F. Hepp, J.P. Blaha, C. Lewis, M.S. Wrighton, M. S, Organometallics 3 (1984) 174. [37] J.P. Blaha, B.E. Bursten, J.C. Dewan, R.B. Frankel, C.L. Randolph, B.A. Wilson, M.S. Wrighton, J. Am. Chem. Soc. 107 (1985) 4561. [38] F. Weinhold, C.R. Landis, Valency and bonding: a natural bond order donor-acceptor perspective, Cambridge University Press, Cambridge, England, U. K., 2005, pp. 32–36. [39] H. Wang, Y. Xie, R.B. King, H.F. Schaefer, J. Am. Chem. Soc. 128 (2006) 11376.
5. Summary The lowest energy structures for the binuclear phospholyl chromium carbonyls (C4H4P)2Cr2(CO)n (n = 6, 4) are singlet spin state structures with two terminal pentahapto phospholyl rings and only terminal CO groups analogous to their experimentally known cyclopentadienyl analogues Cp2Cr2(CO)n. However, the lowest-energy structure for the pentacarbonyl (C4H4P)2Cr2(CO)5 is a singlet spin state structure with a seven-electron donor bridging η1,η5-C4H4P phospholyl ring and a formal Cr-Cr single bond. Nevertheless isomeric (C4H4P)2Cr2(CO)5 structures with two terminal phospholyl rings and either a Cr=Cr double bond in a singlet spin state or a Cr-Cr single bond in a triplet spin state lie at only slightly higher energies. The lowest energy structure for the tricarbonyl (C4H4P)2Cr2(CO)3 is a triplet structure with two terminal pentahapto phospholyl rings, three bridging CO groups, and a formal Cr≡Cr triple bond analogous to the lowest energy cyclopentadienyl analogue Cp2Cr2(µ-CO)3 detected spectroscopically in the photolysis product of Cp2Cr2(CO)4. Thus electron donation from the phosphorus lone pair of the phospholyl ring in addition to or in place of pentahapto coordination of the π system is seen to play a relatively minor role in the energetically preferred structures of the binuclear (C4H4P)2Cr2(CO)n (n = 6, 5, 4, 3). Acknowledgment We are indebted to the Scientific Research Fund of the Key Laboratory of the Education Department of Sichuan Province (Grant No. 10ZX012) for the support of this research. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ica.2019.05.018. References [1] R.D. Adams, D.E. Collins, F.A. Cotton, J. Am. Chem. Soc. 96 (1974) 749.
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Research paper
Comparison of binuclear phospholyl chromium carbonyl derivatives with their cyclopentadienyl analogues: Role of the phosphorus atom in ligandmetal bonding
T
Xin Wana, Chunrong Fenga, Guiming Rena, Xiaohong Chena, Rong Jina, Quan Dua, Hao Fenga, Yaoming Xieb, R. Bruce Kingb a b
School of Science, Research Center for Advanced Computation, Xihua University, Chengdu 610039, China Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, GA 30602, USA
A B S T R A C T
The structures and energetics of the binuclear phospholyl chromium carbonyl derivatives (C4H4P)2Cr2(CO)n (n = 6, 5, 4, 3) have been investigated by density functional theory. The lowest energy (C4H4P)2Cr2(CO)n (n = 6, 4, 3) structures are completely analogous to their Cp2Cr2(CO)n analogues with two terminal pentahapto phospholyl rings. However, for the pentacarbonyl (C4H4P)2Cr2(CO)5 the lowest energy structure has a bridging seven-electron donor η5,η1-C4H4P ligand using the phosphorus lone pair in addition to the π-system of the phospholyl ligand as well as a Cr-Cr single bond. Thus, except for the pentacarbonyl the phosphorus lone pair of the phospholyl ligand is seen to play a minor role in the energetically preferred structures of the binuclear phospholyl chromium carbonyl derivatives.
1. Introduction The binuclear cyclopentadienylchromium carbonyls Cp2Cr2(CO)n (Cp = η5-C5H5; n = 6, 4) are known as stable compounds. The hexacarbonyl Cp2Cr2(CO)6 can be obtained by the mild oxidation of the anion CpCr(CO)3−, which can be obtained by heating Cr(CO)6 with the cyclopentadienide anion in a suitable ethereal solvent. The structure of Cp2Cr2(CO)6, as determined by X-ray crystallography, has a relatively long Cr-Cr bonding distance of 3.281 Å corresponding to the formal single bond suggested by the 18-electron rule (Fig. 1) [1]. Pyrolysis of Cp2Cr2(CO)6 leads to the stable tetracarbonyl Cp2Cr2(CO)4 shown by Xray crystallography to have a much shorter Cr≡Cr distance of 2.24 Å suggesting the formal triple bond required to give the chromium atom the favored 18-electron configuration [2]. The pentacarbonyl Cp2Cr2(CO)5 is shown spectroscopically to be generated from the reaction of Me3SiCHN2 with Cp2Cr2(CO)6 but is unstable with respect to disproportionation into Cp2Cr2(CO)6 + Cp2Cr2(CO)4 [3]. The infrared ν(CO) frequencies and theoretical studies suggest a singly bridged triplet spin state structure for Cp2Cr2(CO)5 with a Cr=Cr bond length of 2.68 Å intermediate between that of the Cr-Cr single bond in Cp2Cr2(CO)6 and the Cr≡Cr triple bond in Cp2Cr2(CO)4 (Fig. 1). Photolysis of Cp2Cr2(CO)4 in polyvinyl chloride [4] or in heptane solution [5] was shown to lead to further decarbonylation to give an unstable tricarbonyl Cp2Cr2(CO)3 suggested by its infrared ν(CO) frequencies and theoretical studies [6] to have three bridging carbonyl groups (Fig. 1). The lowest energy structure of the tricarbonyl is a triplet triply
bridged Cp2Cr2(µ-CO)3 with a Cr≡Cr distance of 2.31 Å slightly higher than the 2.24 Å Cr≡Cr triple bond distance in Cp2Cr2(CO)4 but still suggesting a formal triple bond. This gives each chromium atom a 17electron configuration consistent with a binuclear triplet spin state structure. The analogy between a bare phosphorus atom and a CH moiety [7] in the cyclopentadienyl group in terms of electron count and electronegativity makes the phospholyl (phosphacyclopentadienyl) ligand an analogue of the cyclopentadienyl ligand. This is indicated experimentally by synthesis of the very stable η5-phospholyl manganese carbonyl (η5-C4H4P)Mn(CO)3 (phosphacymantrene) as well as its more readily available 3,4-dimethyl derivative (η5-3,4-C4H2Me2P)Mn(CO)3 by thermal reaction of the corresponding P-phenylphosphole with Mn2(CO)10 [8,9]. Such reactions resulted in cleavage of the P-C6H5 bond in the original phosphole. Substitution of one CH group in the cyclopentadienyl ligand with a phosphorus to give a phospholyl ligand provides a phosphorus lone pair in addition to π-electrons from the ring atoms for donation to metal atoms. Thus a phospholyl ligand can formally function as a sevenelectron donor ligand (Fig. 2). However, the perpendicular orientation of a pentahapto bond from the C4P phosphole ring relative to the phosphorus lone pair prevents a neutral phospholyl ligand from functioning as a seven-electron donor to a single metal atom. Nevertheless, in a binuclear phospholyl metal carbonyl complex of the type (C4H4P)2M2(CO)n a neutral phospholyl ligand can function as a sevenelectron donor η1,η5-C4H4P ligand to a central bimetallic M2 unit. Such
E-mail address: [email protected] (R.B. King). https://doi.org/10.1016/j.ica.2019.05.018 Received 28 March 2019; Received in revised form 9 May 2019; Accepted 9 May 2019 Available online 10 May 2019 0020-1693/ © 2019 Elsevier B.V. All rights reserved.
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R R
R
R
O O C C
Cr
R
OC
C O
R
O C
Cr CR O
R
R
OO C C Cr
R
R
R
R
R
Cr
R
R
C O C R R O Cp2Cr2(CO)5
R
R
R
O C
R
R R
R
Cr
Cr
R
R
R
R
pair to displace a carbonyl group from a second (η5-C4H4P)Mn(CO)3 molecule. The lowest energy (C4H4P)2Mn2(CO)4 structure can be dissected into two (η5-C4H4P)Mn(CO)2 units using the phosphorus lone pairs of each such unit to bond to the manganese atom of the other (η5C4H4P)Mn(CO)2 thereby “biting the tail” of its partner. However, the lowest energy tricarbonyl (η5-C4H4P)2Mn2(CO)3 structure was analogous to its cyclopentadienyl analogue, an experimentally known species in its permethylated Cp*2Mn2(CO)3 form (Cp* = η5-Me5C5) [13]. In view of the occurrence of seven-electron donor η1,η5-C4H4P phospholyl ligands (Fig. 2) in the binuclear manganese carbonyl derivatives (C4H4P)2Mn2(CO)n (n = 5, 4; Fig. 3), we expected similar η1,η5C4H4P ligands in the lowest energy binuclear phospholyl iron and cobalt carbonyl structures, namely (C4H4P)2Fe2(CO)n (n = 4, 3, 2) and (C4H4P)2Co2(CO)n (n = 3, 2, 1), respectively. However, the lowest energy structures for all of these iron and cobalt systems were found to be completely analogous to their cyclopentadienyl analogues. The lack of involvement of the phosphorus lone pairs in the phospholyl-metal bonding in the low energy structures of the iron and cobalt derivatives (C4H4P)2M2(CO)n (M = Fe, Co) but their involvement in the bonding in the manganese derivatives (C4H4P)2Mn2(CO)n (n = 5, 4) suggested that such P → M bonding from bridging phosphole ligands might be a feature of binuclear phospholyl metal carbonyl derivatives of the early transition metals. In order to explore this possibility we have now extended such studies to the corresponding chromium derivatives (C4H4P)2Cr2(CO)n (n = 6, 5, 4, 3) for comparison with the corresponding cyclopentadienyl chromium carbonyl derivatives Cp2Cr2(CO)n discussed above. In this connection we have found the lowest energy (C4H4P)2Cr2(CO)n (n = 6, 4) structures to be analogous to those of the experimentally known stable Cp2Cr2(CO)n derivatives, discussed above. In addition, the lowest energy tricarbonyl structures (C4H4P)2Cr2(CO)3 and Cp2Cr2(CO)3 are analogous with the phospholyl ligands in terminal positions functioning only as a pentahapto ligand to a single metal atom. Only for the pentacarbonyl (C4H4P)2Cr2(CO)5 does the lowest energy structure have a bridging η1,η5-C4H4P ligand forming a dative P → Cr bond to one chromium atom as well as the usual pentahapto bond to the other chromium atom. The binuclear phospholyl chromium carbonyl derivatives discussed in this paper are potentially accessible from the phospholide anion and chromium hexacarbonyl using synthetic methods similar to those used to synthesize their binuclear cyclopentadienylchromium carbonyl analogues.
O C CO
R
R
Cr
C C O O
O C
R
Cp2Cr2(CO)6 R
R
R
R R
Cr
C C O O
R
R R
Cp2Cr2(CO)4
Cp2Cr2(µ-CO)3
Stable species
Detected spectroscopically
Fig. 1. Structures of the binuclear cyclopentadienylchromium carbonyls Cp2Cr2(CO)n (n = 6, 5, 4, 3).
Fig. 2. The phosphole ligand as a five-electron donor pentahapto ligand to a single metal atom and as a seven-electron donor to a pair of metal atoms. Note the perpendicular orientation of the pentahapto ring-metal bond and the phosphorus → metal dative bond.
a neutral phospholyl ligand can donate five electrons to one metal atom through a pentahapto bond and two electrons to another metal atom through the phosphorus lone pair. In view of the limited variety of experimentally known binuclear phospholyl metal carbonyl complexes, we have used well-established density functional methods to explore the involvement of the phosphorus lone pair in the ligand-metal bonding in binuclear phospholyl metal carbonyl derivatives of the type (C4H4P)2M2(CO)n (M = Mn [10], Fe [11], Co [12]). Our initial studies with the manganese derivatives (C4H4P)2Mn2(CO)n suggested that the phosphorus lone pairs might become involved in bonding to the manganese atoms. Thus the lowest energy (C4H4P)2Mn2(CO)n (n = 5, 4) structures by margins of more than 20 kcal/mol have one (n = 5) or two (n = 4) seven-electron donor η5,η1-C4H4P phospholyl ligands bridging the two manganese atoms with long non-bonding Mn…Mn distances of ∼4.3 Å (n = 5) or ∼3.7 Å (n = 4). The lowest energy (C4H4P)2Mn2(CO)5 structure (Fig. 3) can be generated by a (η5-C4H4P)Mn(CO)3 molecule using its phosphorus lone P O C P OC C O
Mn C O
Mn C O
CO
(C4H4P)2Mn2(CO)5
2. Theoretical methods Electron correlation effects were considered by employing density functional theory (DFT) methods, which have evolved as a practical and effective computational tool, especially for organometallic compounds [14–20]. Thus two DFT methods were used in this study. The first method uses the M06-L functional [21] of Zhao and Truhlar, which is suitable for applications in transition metal chemistry. The second DFT method used in the present paper is BP86, which combines Becke’s 1988 exchange functional with Perdew’s 1986 gradient corrected correlation functional method [22,23]. The M06-L and BP86 methods agree with each other fairly well in predicting the geometries of the (C4H4P)2Cr2(CO)n derivatives of interest. The M06-L and BP86 methods are also in reasonable agreement for most cases in predicting the relative energies of different structures of a given spin state, namely singlet or triplet. However, in some cases the two methods predict significantly different singlet-triplet splittings with the M06-L method energetically preferring higher spin states relative to the BP86 method. For the ν(CO) frequencies [24–26], the BP86 method is known to give values that are closer to the experimental values without using any scaling factors. This concurrence may be accidental, since the theoretical vibrational frequencies predicted by BP86 are harmonic frequencies, whereas the experimental fundamental frequencies are anharmonic. All computations were performed using the double-ζ plus
O C Mn
P P
Mn C O
C O
(C4H4P)2Mn2(CO)4
Fig. 3. Lowest energy structures of (C4H4P)2Mn2(CO)5 (=(OC)3Mn(η5,η1C4H4P)Mn(CO)2(η5-C4H4P) and (C4H4P)2Mn2(CO)4 (=[(η5,η1-C4H4P)Mn (CO)2]2 showing the seven-electron donor η5,η1-C4H4P ligands. 195
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direct bonding between the two chromium atoms. Placing a positive charge on the Cr(CO)4 group and a negative charge on the Cr(CO)2 group in 6S-3 gives each chromium atom the favored 18-electron configuration. In a sense 6S-3 is a zwitterion combining the [(C4H4P)Cr (CO)4]+ cation and a [(C4H4P)Cr(CO)2]− anion in the same molecule. The (C4H4P)2Cr2(CO)6 structure 6S-4, lying 10.5 kcal/mol (BP86) or 17.9 kcal/mol (M06-L) in energy above 6S-1, has two terminal CO groups bonded to one chromium atom and the other four terminal CO groups bonded to the other Cr atom. One of the phospholyl ligands in 6S-4 is a terminal pentahapto η5-C4H4P ligand similar to Cp whereas the other phospholyl ligand bridges the central Cr2 unit through only its phosphorus atom leaving two uncomplexed C=C double bonds. (Fig. 4 and Table 1). The bridging phospholyl ligand provides a single electron to the chromium atom of the (η5-C4H4P)Cr(CO)2 unit and two electrons to the Cr(CO)4 unit. The Cr=Cr distance of 2.730 Å (BP86) or 2.591 Å (M06-L) suggests a formal double bond, thereby giving each chromium atom the favored 18-electron configuration. The (C4H4P)2Cr2(CO)6 structure 6S-5 is a relatively high energy structure, lying 16.2 kcal/mol (BP86) or 26.2 kcal/mol (M06-L) above 6S-1 (Fig. 4 and Table 1). Structure 6S-5 has three terminal CO groups bonded to each chromium atom as well as a terminal pentahapto η5C4H4P ring and a bridging η1,η5-C4H4P ring. The long Cr…Cr distance > 5 Å implies the lack of a direct chromium-chromium bond in 6S-5 with the phosphorus atom of the bridging C4H4P ring providing the sole link between the two chromium atoms. Structure 6S-5 may be regarded as a zwitterion since the environment of the chromium atom bearing the terminal phospholyl group resembles that of a substituted [(C4H4P)Cr(CO)3]+ cation so this chromium atom bears a positive charge. Analogously the environment of the chromium atom bonded to all five atoms of the bridging phospholyl ligand resembles that of the [(C4H4P)Cr(CO)3]− anion and thus bears a negative charge. In this way each chromium atom in 6S-5 has the favored 18-electron configuration. The lowest energy triplet spin state (C4H4P)2Cr2(CO)6 structure 6T1, lying 11.2 kcal/mol (BP86) or 14.9 kcal/mol (M06-L) in energy above 6S-1, has four terminal CO groups bonded to one chromium atom, one terminal CO group bond to the other chromium atom, and one bridging CO group (Fig. 4 and Table 1). Structure 6T-1 has one terminal η5-C4H4P ring and one bridging η1,η1-C4H4P ring. The configuration of the ligands around the central Cr2 unit in the triplet structure 6T-1 is similar to that in the singlet structure 6S-4. The Cr=Cr distance of 2.715 Å (BP86) or 2.683 Å (M06-L) suggests a formal double bond, thereby giving each chromium atoms the favorable 18-electron
polarization (DZP) basis sets [27,28], which is consistent with our previous similar studies, and the DFT methods were reported to be less sensitive to the basis set size [29]. For the DZP basis sets used for hydrogen, a set of p polarization functions αp(H) = 0.75 is added to the Huzinaga–Dunning DZ set. For carbon, oxygen, and phosphorus one set of d functions was added to the Huzinaga-Dunning DZ sets with orbital exponents αd(C) = 0.75, αd(O) = 0.85, and αd(P) = 0.60. The loosely contracted DZP basis set for chromium is the Wachters primitive set [30] augmented by two sets of p functions and a set of d functions, contracted following Hood, Pitzer, and Schaefer [31], designated (14s11p6d/10s8p3d). The geometries of all structures were fully optimized using the DZP M06-L and DZP BP86 methods. The vibrational frequencies and the corresponding infrared intensities were determined at the same level of theory. All of the computations were carried out with the Gaussian 09 program [32], exercising the fine grid option (75 radial shells, 302 angular points) for evaluating integrals numerically [33]. 3. Results and discussion 3.1. Molecular structures 3.1.1 (C4H4P)2Cr2(CO)6 Five singlet structures and one triplet structure were found for (C4H4P)2Cr2(CO)6 (Fig. 4 and Table 1). All of them are predicted to be genuine minima. The global minimum of (C4H4P)2Cr2(CO)6, namely 6S-1, has exclusively terminal CO groups with the two C4H4P rings located in trans positions. Each η5-C4H4P ring is bonded to a single chromium atom as a pentahapto ligand similar to the Cp ligand. The CrCr distance of 3.293 Å (BP86) or 3.228 Å (M06-L) is similar to the experimental Cr-Cr distance [1] of 3.281 Å in Cp2Cr2(CO)6 thereby suggesting the formal single bond to give each chromium atom in 6S-1 the favored 18-electron configuration. The (C4H4P)2Cr2(CO)6 structure 6S2 (C2h), lying only 0.1 kcal/mol (BP86) or 0.2 kcal/mol (M06-L) in energy above 6S-1, differs only in the relative orientation of the phosphorus atoms in the η5-C4H4P rings. The (C4H4P)2Cr2(CO)6 structure 6S-3, lying 1.9 kcal/mol (BP86) or 13.7 kcal/mol (M06-L) in energy above 6S-1, has two terminal CO groups linked to one chromium atom and four terminal CO groups linked to the other chromium atom as well as one terminal η5-C4H4P ring and one bridging η1,η5-C4H4P ring (Fig. 4 and Table 1). The very long Cr…Cr distance of 4.867 Å (BP86) or 4.678 Å (M06-L) suggests no
6S-1 (C1)
6S-2 (C2h)
6S-3 (C1)
6S-4 (C1)
6S-5 (C1)
6T-1 (C1)
Fig. 4. The low-energy (C4H4P)2Cr2(CO)6 structures. In Figs. 4–7, the bond distances (in Å) are predicted with the BP86 (upper) and M06-L (lower) methods. 196
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Table 1 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), Cr-Cr distances (Å), and spin contamination values (〈S2〉) for the (C4H4P)2Cr2(CO)6 structures.
BP86 −(E + 3762) ΔE Cr-Cr 〈S2〉 M06-L −(E + 3761) ΔE Cr-Cr 〈S2〉
6S-1 (C1, 1A)
6S-2 (C2h, 1Ag)
6S-3 (C1, 1Ag)
6S-4 (C1, 1A)
6S-5 (C1, 1A)
6T-1 (C1, 3A)
0.072471 0.0 3.293 0.0
0.072261 0.1 3.293 0.0
0.069435 1.9 4.867 0.0
0.055711 10.5 2.730 0.0
0.046596 16.2 5.019 0.0
0.054647 11.2 2.715 2.02
0.470205 0.0 3.228 0.0
0.469941 0.2 3.225 0.0
0.448433 13.7 4.678 0.0
0.441611 17.9 2.591 0.0
0.428505 26.2 5.009 0.0
0.446498 14.9 2.683 2.07
configuration. The triplet state arises from two unpaired electrons in the Cr=Cr double bond which is of the σ + 2/2π type with two unpaired electrons in orthogonal π components. Similar σ + 2/ 2 π double bonds are found in normal triplet dioxygen as well as the organometallic (η5-Me5C5)2Fe2(µ-CO)3, which has been characterized structurally by X-ray crystallography [34–37].
Table 2 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), and Cr-Cr distances (Å) for the four lowest energy singlet (C4H4P)2Cr2(CO)5 structures.
BP86 −(E + 3648) ΔE Cr-Cr M06-L −(E + 3648) ΔE Cr-Cr
3.1.2 (C4H4P)2Cr2(CO)5 Nine low-energy (C4H4P)2Cr2(CO)5 structures were found, namely four singlets and five triplets (Fig. 5 and Tables 2 and 3). The lowestenergy (C4H4P)2Cr2(CO)5 structure 5S-1 has three terminal CO groups on one chromium atom and two terminal CO groups on the other chromium atom. Structure 5S-1 has one five-electron donor terminal η5-C4H4P ring and a η1,η5-C4H4P ring bridging the central Cr2 unit. The bridging η1,η5-C4H4P ring considered as a neutral ligand is a sevenelectron donor with five π electrons going to one chromium atom through pentahapto coordination and two electrons going to the other
5S-1 (C1, 1A)
5S-2 (C1, 1A)
5S-3 (C1, 1A)
5S-4 (C1, 1A)
0.716659 0.0 3.533
0.689764 16.9 2.640
0.689223 17.2 2.703
0.677403 24.6 2.486
0.119587 0.0 3.192
0.098264 13.4 2.671
0.096466 14.5 2.681
0.071621 30.1 2.488
chromium atom through a P → Cr dative bond from the phosphorus lone pair (Fig. 2). The predicted Cr-Cr distance of 3.533 Å (BP86) or 3.192 Å (M06-L), is relatively long similar to the the experimentally
5S-1 (C1)
5S-2 (C1)
5S-3 (C1)
5S-4 (C1)
5T-1 (C1)
5T-2 (C1)
5T- 3 (C1)
5T-4 (C1)
5T-5 (C1)
Fig. 5. The low-energy (C4H4P)2Cr2(CO)5 structures. 197
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formal triple bond needed to give each chromium atom in 4S-1 the favored 18-electron configuration. Structures 4S-2 (C2h) and 4S-3 (Cs), lying within ∼2 kcal/mol in energy of 4S-1 are similar to 4S-1 (Ci), differing only in the relative orientation of the phosphorus atoms in the η5-C4H4P rings. All three structures 4S-1, 4S-2, and 4S-3 have a trans orientation of the η5-C4H4P rings relative to the Cr≡Cr triple bond. The significantly higher energy (C4H4P)2Cr2(CO)4 structure 4S-4, lying 11.2 kcal/mol (BP86, M06-L) above 4S-1, is similar to 4S-1, 4S-2, and 4S-3 except for a cis orientation of the η5-C4H4P rings and two terminal CO groups. The (C4H4P)2Cr2(CO)4 structure 4S-5, lying 17.4 kcal/mol (BP86) or 17.0 kcal/mol (M06-L) in energy above 4S-1, has three terminal CO groups, one four-electron donor bridging η2-µ-CO group with a short CrO distance of 2.256 Å (BP86) or 2.279 Å (M06-L)., one terminal η5C4H4P ring, and one bridging η1,η5-C4H4P ring (Fig. 6 and Table 4). The bridging η1,η5-C4H4P ring is a pentahapto five-electron donor to one chromium atom and bonds to the other chromium atom through the phosphorus lone pair with a P → Cr distance of 2.294 Å (BP86) or 2.301 Å (M06-L). Thus in 4S-5 the η1,η5-C4H4P ring is a seven-electron donor to the central Cr2 unit. The Cr-Cr distance in 4S-5 of 3.024 Å (BP86) or 2.961 Å (M06-L) suggests a single bond thereby giving each chromium atom the favored 18-electron configuration after considering the four-electron donor bridging η2-µ-CO group and the seven-electron donor bridging η1,η5-C4H4P ring. The triplet (C4H4P)2Cr2(CO)4 structure 4T-1, lying 14.0 kcal/mol (BP86) or 6.1 kcal/mol (M06-L) in energy above 4S-1, has one terminal η5-C4H4P ring and one bridging η1,η5-C4H4P ring (Fig. 6 and Table 5). The chromium atom in 4T-1 bonded to the terminal η5-C4H4P ring has one terminal CO groups whereas the other chromium atom has two terminal CO group. The fourth CO group in 4T-1 is a weakly semibridging carbonyl group with a short Cr-C distance to the chromium atom already bearing two terminal CO groups and a long almost nonbonding distance of ∼2.5 Å to the other chromium atom. The Cr=Cr distance of 4T-1 of 2.724 Å (BP86) or 2.696 Å (M06-L), suggests a formal double bond, thereby giving each chromium atom the favored 18-electron configuration. As in the triplet (C4H4P)2Cr2(CO)5 structures the two unpaired electrons of the triplet spin state in 4T-1 reside in two orthogonal single-electron “half-bond” π components so that these Cr=Cr formal double bonds are of the σ + 2 2 π type similar to that in Cp*2Fe2(µ-CO)3 [34–37]. The triplet (C4H4P)2Cr2(CO)4 structures 4T-2 and 4T-4 are similar to each other in geometry, differing only by the relative positions of the phosphorus atoms in the phospholyl rings (Fig. 6 and Table 5). Both structures have two terminal CO groups, two bridging CO groups, and two terminal η5-C4H4P rings. Structure 4T-2 lies 18.9 kcal/mol (BP86) or 12.9 kcal/mol (M06-L) in energy above 4S-1, whereas 4T-4 lies 22.5 kcal/mol (BP86) or 19.1 kcal/mol (M06-L) above 4S-1. Interpreting the Cr=Cr distances of ∼2.5 Å in 4T-2 and 4T-4 as formal double bonds gives each chromium atom in each structure a 17-electron configuration corresponding to a binuclear triplet. The triplet (C4H4P)2Cr2(CO)4 structure 4T-3, lying 20.8 kcal/mol (BP86) or 17.1 kcal/mol (M06-L) in energy above 4S-1, has three terminal CO groups, one bridging CO group, one terminal η5-C4H4P ring, and one bridging η1,η5-C4H4P ring (Fig. 6 and Table 5). The Cr=Cr distance of 2.641 Å (BP86) or 2.581 Å (M06-L) in 4T-3 suggests a formal double bond, thereby giving each chromium atom the favored 18-electron configurations. The two unpaired electrons of the triplet spin state of 4T-3 reside in the Cr=Cr double bond, which is of the σ + 2/2 π type similar to that in 4T-1.
Table 3 Total energies (E, in hartree), relative energies to 5S-1 (ΔE, in kcal/mol), the CrCr distances (Å), and spin contamination values (〈S2〉) for the five lowest-energy triplet (C4H4P)2Cr2(CO)5 structures.
BP86 −(E + 3648) ΔE Cr-Cr 〈S2〉 M06-L −(E + 3648) ΔE Cr-Cr 〈S2〉
5T-1 (C1, 3A)
5T-2 (C1, 3A)
5T-3 (C1, 3A)
5T-4 (C1, 3A)
5T-5 (C1, 3A)
0.703038 8.5 2.708 2.03
0.700922 9.9 2.717 2.03
0.700722 10.0 2.709 2.04
0.698134 11.6 2.711 2.04
0.693374 14.4 2.743 2.03
0.119020 0.4 2.719 2.10
0.117628 1.2 2.740 2.10
0.116258 2.1 2.726 2.10
0.113811 3.6 2.728 2.10
0.110067 6.0 2.756 2.10
determined Cr-Cr single bond distance of 3.281 Å in Cp2Cr2(CO)6. Interpreting this Cr-Cr distance in 5S-1 as a formal single bond gives each chromium atom the favorable 18-electron configuration. The singlet (C4H4P)2Cr2(CO)5 structures 5S-2 and 5S-3 and the five triplet structures 5T-1 through 5T-5 have similar geometries with three terminal CO groups, two semi-bridging CO groups, and two terminal η5C4H4P rings in a trans orientation (Fig. 5 and Tables 2 and 3). Other than the spin state these seven structures differ only in the relative orientations of the phosphorus atoms in the two η5-C4H4P rings. The singlet structures 5S-2 and 5S-3, lying 16.9 kcal/mol (BP86) or 13.4 kcal/mol (M06-L) and 17.2 kcal/mol (BP86) or 14.5 kcal/mol (M06-L), respectively, in energy above 5S-1, have Cr=Cr distances of ∼2.67 Å (M06-L), suggesting formal double bonds, thereby giving each chromium atom the favored 18-electron configuration. All five triplet (C4H4P)2Cr2(CO)5 structures (5T-1 through 5T-5 in Table 3) have Cr=Cr distances ranging from 2.71 to 2.76 Å (M06-L), also suggesting formal double bonds, thereby giving each chromium atom the favored 18-electron configuration. The two unpaired electrons in each of the triplet (C4H4P)2Cr2(CO)5 structures reside formally in the Cr=Cr double bond, which can be interpreted as a σ + 2/2 π bond with single unpaired electrons in two orthogonal π half-bonds similar to the experimentally known Cp*2Fe2(µ-(CO)3, characterized by X-ray crystallography [34–37]. The (C4H4P)2Cr2(CO)5 structure 5S-4, lying 24.6 kcal/mol (BP86) or 30.1 kcal/mol (M06-L) in energy above 5S-1, has two terminal CO groups bonded to one chromium atom and the remaining three terminal CO groups bonded to the other chromium atom (Fig. 5 and Table 2). Structure 5S-4 has a terminal η5-C4H4P ring and a bridging η1,η1-C4H4P ring bonded to the central Cr2 unit only through its phosphorus atom and thus having two uncomplexed C=C double bonds and donating a total of three electrons. The Cr≡Cr distance of 2.486 Å (BP86) or 2.488 Å (M06-L) in 5S-4 is ∼0.2 Å shorter than the formal Cr=Cr double bonds in the singlet (C4H4P)2Cr2(CO)5 structures 5S-2 and 5S-3 thereby suggesting a formal Cr≡Cr triple bond in 5S-4. This gives each chromium atom in 5S-4 the favored 18-electron configuration by placing a formal positive charge on the chromium atom bearing two terminal CO groups and the terminal η5-C4H4P ring and a formal negative charge on the other chromium atom.
3.1.3 (C4H4P)2Cr2(CO)4 Nine low-energy (C4H4P)2Cr2(CO)4 structures were found, namely five singlets and four triplets (Fig. 6 and Tables 4 and 5). The lowest energy such structure 4S-1 has all four CO groups as weak semi-bridges across the central Cr2 unit (long Cr-C distances of ∼2.5 Å) as well as one terminal η5-C4H4P ring bonded to each chromium atom with a short Cr≡Cr distance of 2.239 Å (BP86) or 2.248 Å (M06-L). This Cr≡Cr distance is very close to that of 2.24 Å determined by X-ray crystallography for the Cr≡Cr formal triple bond in the cyclopentadienyl analogue Cp2Cr2(CO)4 [2] and can likewise be interpreted as the
3.1.4 (C4H4P)2Cr2(CO)3 Eleven low-energy (C4H4P)2Cr2(CO)3 structures were found, namely six triplets and five singlets (Fig. 7 and Tables 6 and 7). The lowest energy (C4H4P)2Cr2(CO)3 structure 3T-1 has three bridging CO groups and two terminal five-electron donor η5-C4H4P rings. The Cr≡Cr distance of 2.292 Å (BP86) or 2.315 Å (M06-L) in 3T-1 is similar to that of 198
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4S-1 (Ci)
4S-2 (C2h)
4S-3 (Cs)
4S-4 (C1)
4S-5 (C1)
4T-1 (C1)
4T- 2 (C1)
4T-3 (C1)
4T-4 (C1)
Fig. 6. The nine low-energy (C4H4P)2Cr2(CO)4 structures. Table 4 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), and the Cr-Cr distances (Å) for the singlet (C4H4P)2Cr2(CO)4 structures.
BP86 −(E + 3535) ΔE Cr-Cr M06-L −(E + 3534) ΔE Cr-Cr
4S-1 (Ci, 1Ag)
4S-2 (C2h, 1Ag)
4S-3 (Cs, 1Á)
4S-4 (C1, 1A)
4S-5 (C1, 1A)
0.354244 0.0 2.239
0.353779 0.3 2.246
0.351288 1.9 2.246
0.336380 11.2 2.261
0.326548 17.4 3.024
0.771762 0.0 2.248
0.771066 0.4 2.257
0.768290 2.2 2.256
0.753837 11.2 2.276
0.744619 17.0 2.961
Table 5 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), Cr-Cr distances (Å), and spin contamination values (〈S2〉) for the triplet (C4H4P)2Cr2(CO)4 structures.
BP86 −(E + 3535) ΔE Cr-Cr 〈S2〉 M06-L −(E + 3534) ΔE Cr-Cr 〈S2〉
the 2.24 Å Cr≡Cr triple bond distance found experimentally in Cp2Cr2(CO)4 by X-ray crystallography [2] as well as to those predicted for the (η5-C4H4P)2Cr2(CO)4 structures 4S-1, 4S-2, and 4S-3 (Fig. 6 and Table 4). This suggests a formal Cr≡Cr triple bond in 3T-1 thereby giving each Cr atom in 3T-1 a 17-electron configuration for a binuclear triplet. The triplet structures 3T-2, 3T-3, 3T-4, and 3T-5, having energies within 4 kcal/mol (BP86) or 8 kcal/mol (M06-L) of 3T-1, are similar to 3T-1 differing only in the relative orientations of the phosphorus atoms in the η5-C4H4P rings. Spectroscopic studies, particularly the infrared ν(CO) frequencies, on the product from the photolysis of Cp2Cr2(CO)4 in polyvinyl chloride or heptane solution suggests a triply bridged Cp2Cr2(µ-CO)3 structure analogous to any of the triplet triply bridged (η5-C4H4P)2Cr(µ-CO)3 structures 3T-1 through 3T-5 [4,5].
4T-1 (C1, 3A)
4T-2 (C1, 3A)
4T-3 (C1, 3A)
4T-4 (C1, 3A)
0.331956 14.0 2.724 2.04
0.324160 18.9 2.544 2.04
0.321057 20.8 2.641 2.03
0.318336 22.5 2.482 2.05
0.762055 6.1 2.696 2.12
0.751183 12.9 2.556 2.10
0.744556 17.1 2.581 2.09
0.741390 19.1 2.456 2.12
The triplet (C4H4P)2Cr2(CO)3 structure 3T-6, lying 10.1 kcal/mol (BP86) or 7.8 kcal/mol (M06-L) in energy above 3T-1, is very different from the five lower energy triplet (C4H4P)2Cr2(CO)3 structures by having three terminal CO groups and two seven-electron donor bridging η1,η5-C4H4P rings rather than the opposite of bridging CO groups and terminal η5-C4H4P rings (Fig. 7 and Table 6). The Cr-Cr distance of 2.929 Å (BP86) or 2.830 Å (M06-L) can be interpreted as a formal single bond to give each chromium atom a 17-electron configuration for a binuclear triplet. The lowest energy singlet (C4H4P)2Cr2(CO)3 structure 3S-1, lying 199
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3T-1 (C1)
3T-2 (C1)
3T-3 (C1)
3T-4 (C1)
3T-5 (C1)
3T-6 (C1)
3S-1 (C1)
3S-2 (C1)
3S-3 (C1)
3S-4 (C1)
3S-5 (C1)
Fig. 7. The 11 low-energy (C4H4P)2Cr2(CO)3 structures.
7.4 kcal/mol (BP86) or 13.8 kcal/mol (M06-L) above 3T-1, has one terminal CO group, two semi-bridging CO groups, one terminal η5C4H4P ring, and one bridging η1,η5-C4H4P ring (Fig. 7 and Table 7). Interpreting the very short CrCr distance of 2.169 Å (BP86) or 2.198 Å (M06-L) as a formal quadruple bond gives each chromium atom the favored 18-electron configuration.
The other singlet (C4H4P)2Cr2(CO)3 structures 3S-2, 3S-3, 3S-4, and 3S-5 (Cs), lying 9 to 12 kcal/mol (BP86) or 15 to 18 kcal/mol (M06-L) in energy above 3T-1, have similar geometries to the five lowest energy triplet (C4H4P)2Cr2(CO)3 structures 3T-1 through 3T-5, i.e., with three bridging µ-CO groups and two terminal five-electron donor η5-C4H4P
Table 6 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), Cr-Cr distances (Å), and spin contamination values (〈S2〉) for the triplet (C4H4P)2Cr2(CO)3 structures.
BP86 −(E + 3421) ΔE Cr-Cr 〈S2〉 M06-L −(E + 3421) ΔE Cr-Cr 〈S2〉
3T-1 (C1, 3A)
3T-2 (C1, 3A)
3T-3 (C1, 3A)
3T-4 (C2, 3A)
3T-5 (C1, 3A)
3T-6 (C1, 3A)
0.957632 0.0 2.292 2.10
0.955412 1.4 2.303 2.09
0.954497 2.0 2.298 2.12
0.950205 4.7 2.267 2.02
0.951434 3.9 2.268 2.02
0.941544 10.1 2.929 2.08
0.397216 0.0 2.315 2.38
0.396940 0.2 2.324 2.26
0.396242 0.6 2.324 2.37
0.385923 7.1 2.281 2.05
0.384870 7.7 2.284 2.04
0.384706 7.8 2.830 2.27
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Table 7 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), Cr-Cr distances (Å) for the five singlet (C4H4P)2Cr2(CO)3 structures. 3S-1 (C1, 1A) BP86 −(E + 3421) ΔE Cr-Cr M06-L −(E + 3421) ΔE Cr-Cr
3S-2 (C1, 1A)
3S-3 (C1, 1A)
3S-4 (C1, 1A)
Table 9 Disproportionation energies (kcal/mol) for the reactions 2(C4H4P)2Cr2(CO)n → (C4H4P)2Cr2(CO)n+1 + (C4H4P)2Cr2(CO)n-1.
3S-5 (C1, 1A)
0.946792 7.4 2.169
0.943137 9.7 2.214
0.941402 10.8 2.214
0.941291 10.9 2.206
0.939160 12.2 2.213
0.375289 13.8 2.198
0.372823 15.3 2.232
0.372823 15.3 2.232
0.370823 16.6 2.233
0.368904 17.8 2.238
2(C4H4P)2Cr2(CO)5 (5S-1) → (C4H4P)2Cr2(CO)4 (4S-1) + (C4H4P)2Cr2(CO)6 (6S-1) 2(C4H4P)2Cr2(CO)4 (4S-1) → (C4H4P)2Cr2(CO)3 (3T-1) + (C4H4P)2Cr2(CO)5 (5S-1)
3.2. Thermochemistry. Table 8 lists the energies for the CO dissociation reactions (C4H4P)2Cr2(CO)n → (C4H4P)2Cr2(CO)n−1 + CO (n = 6, 5, 4). The largest energy for the dissociation of one CO group in a (C4H4P)2Cr2(CO)n derivative is for (C4H4P)2Cr2(CO)4 where CO dissociation is highly endothermic at 40.5 kcal/mol (BP86) or 33.2 kcal/mol (M06-L), suggesting a viable species. The CO dissociation energies for the other (C4H4P)2Cr2(CO)n species (n = 6, 5) are all positive in the range 15 to 20 kcal/mol, also suggesting viability towards CO dissociation. Table 9 lists the energies for the disproportionation reactions 2(C4H4P)2Cr2(CO)n → (C4H4P)2Cr2(CO)n+1 + (C4H4P)2Cr2(CO)n−1 (n = 5, 4). The energy required for disproportionation of (C4H4P)2Cr2(CO)4 is significant, i.e, 20.8 kcal/mol (BP86) or 15.7 kcal/ mol (M06-L), indicating that it is a viable species. Thus (C4H4P)2Cr2(CO)4 with a structure analogous to the known stable Cp2Cr2(CO)4 as a very favorable species and a reasonable synthetic target. However, the disproportionation energy for (C4H4P)2Cr2(CO)5 is essentially thermoneutral, namely slightly endothermic at 4.0 kcal/mol by BP86 but very slightly endothermic at −1.3 kcal/mol (M06-L). This suggests that (C4H4P)2Cr2(CO)5 is not a viable species. In this connection the cyclopentadienyl analogue Cp2Cr2(CO)5, synthesized under mild conditions by decarbonylation of Cp2Cr2(CO)6 with Me3SiN2 and identified by its ν(CO) frequencies, is found to undergo disproportionation into the two stable species Cp2Cr2(CO)6 + Cp2Cr2(CO)4 [3]. Also of interest is the dissociation of the (C4H4P)2Cr2(CO)n derivatives into mononuclear (C4H4P)Cr(CO)m fragments. In order to obtain such energetic data, the structures of the mononuclear (C4H4P)Cr(CO)m were optimized by the same DFT methods (Fig. 8). Using this information, the dissociation energies of the binuclear (C4H4P)2Cr2(CO)n into mononuclear fragments were determined (Table 10). The results were found to depend on the density functional used since the mononuclear (C4H4P)Cr(CO)n are necessarily doublet spin state structures and singlet-triplet splittings are dependent on the density functional used. This is particularly true for the dissociation of the saturated hexacarbonyl (C4H4P)2Cr2(CO)6 into two (C4H4P)Cr(CO)3 fragments for
(C4H4P)2Cr2(CO)6 (6S-1) → (C4H4P)2Cr2(CO)5 (5S-1) + CO (C4H4P)2Cr2(CO)5 (5S-1) → (C4H4P)2Cr2(CO)4 (4S-1) + CO (C4H4P)2Cr2(CO)4 (4S-1) → (C4H4P)2Cr2(CO)3 (3S-1) + CO
15.7 19.7 40.5
18.8 17.5 33.2
4.0
−1.3
20.8
15.7
3.3. NBO analysis of the chromium-chromium bonding Table 11 lists the natural atomic charges for the chromium atoms and the Wiberg Bond Indices (WBI) for the Cr-Cr bonds using NBO analysis [38] with the BP86 functional The Cr-Cr distances, the formal Cr-Cr bond orders, and the bridging groups are also listed for comparison. Only the singlet structures are considered since WBI analyses of higher spin state open shell structures appear to be less reliable. Most of the natural charges on the chromium atoms are negative since they accept electrons from the CO and phosphorus lone pairs without sufficient π back-bonding from the chromium atoms to the antibonding orbitals of the CO groups to remove completely this additional negative charge. The chromium natural charges depend on the number of CO groups and whether the lone pair of the phosphole ligand, considered formally as a neutral species, is coordinated to a chromium atom as a monohapto ligand, either as part of a seven-electron bridging η1,η5-C4H4P ligand or as a three-electron donor η1-C4H4P ligand with two uncomplexed double bonds as in 5S-4. Thus in the species 6S-3, 6S-4, 6S-5, 5S-1, and 5S-4, in which the phosphorus lone pair is coordinated to one or both chromium atoms, the atomic charges range from −0.70 to −0.88. In addition, the negative charge on the chromium atoms become more positive upon loss of CO groups. Chromium atoms bearing three CO groups, such as those in 6S-1 and 6S-2, have negative charges from −0.65 to −0.67. Chromium atoms bonded to 2 1 2 CO groups (i. e., two terminal CO groups and half of a bridging CO group), such as in 5S-2, have a negative charge of ∼−0.5. Chromium atoms bearing two CO groups, such as the four lowest energy singlet (C4H4P)2Cr2(CO)4 structures, have charges ranging from −0.41 to −0.46. The chromium atoms in the (C4H4P)2Cr2(CO)3 structures 3S-2, 3S-3, 3S-4, and 3S-5 bearing half of two bridging CO groups and no terminal CO groups have a charge close to zero. Previous studies on the Wiberg Bond Indices (WBIs) of metal carbonyls such as Fe2(CO)9 and Fe3(CO)12 show that the WBI values are relatively low [39] compared with the formal bond orders, particularly when the metal-metal bonds are bridged by CO groups. The WBI values for the Cr-Cr bonds in the (C4H4P)2Cr2(CO)n derivatives are related to the formal bond order and the number of bridging groups. Table 11 shows that for single bonds the Cr-Cr WBIs range from 0.23 to 0.31, for double bonds the Cr=Cr WBIs range from 0.37 to 0.52, for Cr≡Cr triple bonds the WBIs range from 0.91 to 0.97 and for CrCr quadruple bonds the WBI values range from 1.11 to 1.38. The WBIs for interactions between pairs of chromium atoms too far apart for direct bond formation, such as in 6S-3 and 6S-5, are close to zero ranging from 0.08
Table 8 Dissociation energies (kcal/mol) for the successive removal of carbonyl groups from the (C4H4P)2Cr2(CO)n derivatives. M06-L
M06-L
which the BP86 functional gives a relatively small dissociation energy of 8.7 kcal/mol whereas the M06-L functional gives a significantly higher dissociation energy of 17.5 kcal/mol. Either of these values, however, are low enough energies to predict that dissociation into two (C4H4P)Cr(CO)3 fragments will be significant in much of the reaction chemistry of (C4H4P)2Cr2(CO)6. Such is the case with the cyclopentadienyl analogue Cp2Cr2(CO)6. The dissociation energies of the unsaturated (C4H4P)2Cr2(CO)n into mononuclear fragments are very high at ∼44, ∼75, and ∼85 kcal/mol for n = 5, 4, 3, respectively, and increase with decreasing number of CO groups. This is consistent with the increase in the formal Cr-Cr bond order upon loss of CO groups.
rings (Fig. 7 and Table 7). Interpreting the short CrCr distances ranging from 2.206 to 2.214 Å (BP86) or 2.232 to 2.238 Å (M06-L) in 3S-1 to 3S-5, as formal quadruple bonds gives each chromium atom the favored 18-electron configuration.
BP86
BP86
201
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Fig. 8. The optimized geometries for the mononuclear structures (C4H4P)Cr(CO)m (m = 3, 2, 1).
derivatives of four adjacent first row transition metals (Cr, Mn, Fe, Co) to assess the role played by the phosphorus lone pair of the phospholyl ligand in the energetically preferred structures of such binuclear phospholyl derivatives. In other words, are the lowest-energy structures (C4H4P)2M2(CO)n (M = Cr, Mn, Fe, Co) similar to their cyclopentadienyl analogues Cp2M2(CO)n with a terminal pentahapto η5-C4H4P ligand analogous to the ubiquitous terminal pentahapto Cp ligand or do they exhibit other structural features in which the phosphorus lone pair of the phospholyl ligand plays a role? Our initial theoretical studies on binuclear phospholyl manganese carbonyl derivatives (C4H4P)2Mn2(CO)n [10] showed that for the pentacarbonyl and tetracarbonyl (n = 5, 4) the lowest energy structures had at least one bridging seven-electron donor η5,η1-C4H4P phospholyl ligand with energies more than 20 kcal/mol below the lowest energy (η5)2Mn2(CO)n isomers. We therefore expected similar situations with low-energy (C4H4P)2M2(CO)n structures of the transition metals surrounding manganese in the Periodic Table. Table 12 summarizes the relative energies of the binuclear phospholyl metal carbonyls (η5-C4H4P)2M2(CO)n with only terminal pentahapto phospholyl rings analogous to the lowest energy binuclear cyclopentadienylmetal carbonyls (η5-C5H5)2M2(CO)n {≡Cp2M2(CO)n}. A number of the lowest energy Cp2M2(CO)n derivatives are stable species that have been isolated and structurally characterized by X-ray crystallography (indicated in bold in Table 12). A “(0)” following a generic Q2M2(CO)n designation in Table 12 means that the lowest energy (C4H4P)2M2(CO)n and Cp2M2(CO)n structures are analogous having only terminal pentahapto η5-C4H4P rings in the phospholyl structure. The general observation from the information presented in Table 12 is that all of the phospholyl (η5-C4H4P)2M2(CO)n structures analogous to stable Cp2M2(CO)n structures (designed in bold in Table 12) are the lowest energy structures. Isomeric structures with seven-electron donor bridging η1,η5-phospholyl ligands as well as other types of phosphoryl ligands coordinating to the metal(s) through the phosphorus lone pair are found but at higher energies. The originally studied (C4H4P)2Mn2(CO)n (n = 5, 4) systems are seen to be exceptional since phospholyl structures with two terminal rings analogous to the lowest energy Cp2Mn2(CO)n structures lie more than 20 kcal/mol above their lowest energy isomers [10]. These lowest energy (C4H4P)2Mn2(CO)n (n = 5, 4) structures contain bridging seven-electron donor η5,η1-C4H4P rings (Fig. 3) and lack direct Mn-Mn bonds. The local environments of
Table 10 Dissociation energies of the binuclear (C4H4P)2Cr2(CO)n (n = 5, 4, 3, 2) into mononuclear fragments (kcal/mol).
(C4H4P)2Cr2(CO)6 (C4H4P)2Cr2(CO)5 (C4H4P)2Cr2(CO)4 (C4H4P)2Cr2(CO)3
(6S-1) → 2(C4H4P)Cr(CO)3 (5S-1) → (C4H4P)Cr(CO) 2+ (C4H4P)Cr(CO)3 (4S-1) → 2 (C4H4P)Cr(CO)2 (3T-1) → (C4H4P)Cr(CO)2 + (C4H4P)Cr(CO)
BP86
M06-L
8.7 43.9 75.0 83.6
17.5 45.3 74.4 86.4
Table 11 Atomic charges and Wiberg bond indices for the singlet (C4H4P)2Cr2(CO)n structures by the BP86 method.
6S-1 6S-2 6S-3 6S-4 6S-5 5S-1 5S-2 5S-3 5S-4 4S-1 4S-2 4S-3 4S-4 4S-5 3S-1 3S-2 3S-3 3S-4 3S-5
Natural Charge on Cr / Cr
Wiberg bond index
Cr-Cr distance, Å
Bridges
Formal Bond order
−0.67/−0.65 −0.65/−0.65 −0.84/ −0.86 −0.68/ −0.88 −0.70/−0.70 −0.71/−0.68 −0.50/−0.52 −0.65/−0.37 −0.83/−0.49 −0.44/−0.44 −0.44/ −0.44 −0.44/−0.46 −0.41/ −0.44 −0.71/ −0.34 −0.38/−0.22 −0.38/0.04 −0.38/0.04 −0.40/0.07 −0.39/0.09
0.24 0.23 0.08 0.37 0.15 0.28 0.58 0.43 0.52 0.94 0.91 0.91 0.97 0.31 1.38 1.11 1.11 1.15 1.12
3.293 3.293 4.867 2.730 5.019 3.533 2.640 2.703 2.486 2.239 2.246 2.246 2.261 3.024 2.169 2.214 2.214 2.206 2.213
4 semi-CO 4 semi-CO C4H4P C4H4P C4H4P C4H4P 2 semi-CO 2 semi-CO C4H4P none none none none CO/ C4H4P 2 CO 3 CO 3 CO 3 CO 3 CO
1 1 0 2 0 1 2 2 2 3 3 3 3 1 4 4 4 4 4
to 0.15. 4. Discussion The results reported here on the structures and energetics of the (C4H4P)2Cr2(CO)n (n = 6, 5, 4, 3) systems added to those from our previous studies provide information on such binuclear phospholyl
Table 12 Comparison of analogous binuclear phospholyl and cyclopentadienyl metal carbonyls Q2M2(CO)n (Q = C4H4P or C5H5). The figures in parentheses indicate the energy (in kcal/mol) of the phospholyl derivative (C4H4P)2M2(CO)n structurally analogous to the lowest energy cyclopentadienyl derivative (C5H5)2M2(CO)n relative to the lowest energy (C4H4P)2M2(CO)n isomer. The Cp2M2(CO)n structures in bold are stable isolable species structurally characterized by X-ray crystallography whereas those in italics have been detected spectroscopically. Bond Order
M = Cr
M = Mn
M = Fe
M = Co
1 2 3 4
Q2Cr2(CO)6 (0) Q2Cr2(CO)5 (17) Q2Cr2(CO)4 (0) Q2Cr2(CO)3 (0)
Q2Mn2(CO)5 (23) Q2Mn2(CO)4 (31) Q2Mn2(CO)3 (0) Q2Mn2(CO)2 (4)
Q2Fe2(CO)4 (0) Q2Fe2(CO)3 (1) Q2Fe2(CO) (0)
Q2Co2(CO)3 (0) Q2Co2(CO)2 (0) Q2Co2(CO) (12)
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the manganese atoms in these lowest energy (C4H4P)2Mn2(CO)n (n = 5, 4) are thus similar to that of the very stable CpMn(CO)3 with phospholyl phosphorus lone pairs substituting a carbonyl group. The high stability of the CpMn(CO)3 system and its carbonyl substitution species thus can account for the behavior of the (C4H4P)2Mn2(CO)n (n = 5, 4) systems, which is now seen to be anomalous by comparison with the other (C4H4P)2M2(CO)n (M = Cr, Mn, Fe, Co) systems.
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5. Summary The lowest energy structures for the binuclear phospholyl chromium carbonyls (C4H4P)2Cr2(CO)n (n = 6, 4) are singlet spin state structures with two terminal pentahapto phospholyl rings and only terminal CO groups analogous to their experimentally known cyclopentadienyl analogues Cp2Cr2(CO)n. However, the lowest-energy structure for the pentacarbonyl (C4H4P)2Cr2(CO)5 is a singlet spin state structure with a seven-electron donor bridging η1,η5-C4H4P phospholyl ring and a formal Cr-Cr single bond. Nevertheless isomeric (C4H4P)2Cr2(CO)5 structures with two terminal phospholyl rings and either a Cr=Cr double bond in a singlet spin state or a Cr-Cr single bond in a triplet spin state lie at only slightly higher energies. The lowest energy structure for the tricarbonyl (C4H4P)2Cr2(CO)3 is a triplet structure with two terminal pentahapto phospholyl rings, three bridging CO groups, and a formal Cr≡Cr triple bond analogous to the lowest energy cyclopentadienyl analogue Cp2Cr2(µ-CO)3 detected spectroscopically in the photolysis product of Cp2Cr2(CO)4. Thus electron donation from the phosphorus lone pair of the phospholyl ring in addition to or in place of pentahapto coordination of the π system is seen to play a relatively minor role in the energetically preferred structures of the binuclear (C4H4P)2Cr2(CO)n (n = 6, 5, 4, 3). Acknowledgment We are indebted to the Scientific Research Fund of the Key Laboratory of the Education Department of Sichuan Province (Grant No. 10ZX012) for the support of this research. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ica.2019.05.018. References [1] R.D. Adams, D.E. Collins, F.A. Cotton, J. Am. Chem. Soc. 96 (1974) 749.
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