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1,3-Diphosphacyclobutadiene sandwich compounds as bidentate ligands in metal carbonyl chemistry: Binuclear chromium derivatives
Inorganica Chimica Acta 498 (2019) 119123
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1,3-Diphosphacyclobutadiene sandwich compounds as bidentate ligands in metal carbonyl chemistry: Binuclear chromium derivatives
T
Xin Wana, Xuefeng Wanga, Xiaohong Chena, Rong Jina, Quan Dua, Yaoming Xieb, R. Bruce Kingb a b
School of Sciences, Research Center for Advanced Computation, Xihua University, Chengdu 610039, China Department of Chemistry and Center for Computational Quantum Chemistry, University of Georgia, Athens, GA 30602, USA
ABSTRACT
The structures and thermochemistry of the binuclear 1,3-diphosphacyclobutadiene chromium carbonyl complexes (Me2C2P2)2Cr2(CO)n (n = 6, 5, 4, 3) have been examined by density functional theory. The only low-energy structure for the hexacarbonyl (Me2C2P2)2Cr2(CO)6 consists of a bidentate chelating (η4-Me2C2P2)2Cr (CO)2 sandwich diphosphine ligand bonded to a Cr(CO)4 unit with a long ∼4.0 Å Cr⋯Cr distance indicating lack of a direct metal-metal bond. The lowest energy structure for the pentacarbonyl (Me2C2P2)2Cr2(CO)5 is a triplet structure derived from this singlet hexacarbonyl structure by loss of a CO group from the sandwiched chromium atom. The bridging η1,4-Me2C2P2 ligands found in these two structures, using the four π-electrons of the P2C2 ring to bond to one chromium atom and a phosphorus lone pair to bond to the other chromium atom but not sandwiching a chromium atom, are features of the next lowest energy triplet and singlet (Me2C2P2)2Cr2(CO)5 structures. In contrast to the hexa- and pentacarbonyls, the lowest energy structures of the more highly unsaturated tetra- and tricarbonyls (Me2C2P2)2Cr2(CO)n (n = 4, 3) have terminal tetrahapto η4-Me2C2P2 rings and chromium-chromium multiple bonds.
1. Introduction Dimerization of phosphaalkynes, RC^P:, on transition metal sites provides routes to metal complexes of substituted 1,3-diphosphacyclobutadienes [1]. Using this approach the sandwich compounds (η4-tBu2C2P2)2M (M = Fe [2], Co [3], Ni [4]) as well as the very stable iron carbonyl derivative (η4-tBu2C2P2)Fe(CO)3 [5] have been synthesized by reactions of tBuC^P: with [(η4-anthracene)2M]– (M = Co, Fe), (η2,2-C8H12)2Ni, and Fe(CO)5, respectively (Fig. 1). The analogy between a lone phosphorus atom and a CH moiety based on their valence isoelectronic relationship and similar electronegativities suggests a close similarity between mononuclear cyclobutadiene and diphosphacyclobutadiene metal complexes. However, in certain systems the diphosphacyclobutadiene metal complexers are more readily synthesized than the corresponding butadiene metal complexes. Thus the cyclobutadiene sandwich compounds (η4-R4C4)2M (M = Fe, Co) analogous to the 1,3-diphosphacyclobutadiene sandwich compounds (η4tBu2C2P2)2M remain unknown. The heterocyclic 1,3-diphosphacyclobutadiene and the homocyclic cyclobutadiene systems become distinctly different when binuclear metal complexes are considered. Complexation of a 1,3-diphosphacyclobutadiene ring to one metal atom as a tetrahapto ligand does not disturb the external phosphorus lone pairs which can coordinate to additional metal atoms. Therefore the chemistry of binuclear 1,3-diphosphacyclobutadiene metal complexes is likely to be significantly different from that of binuclear cyclobutadiene metal complexes
because of the accessibility of phosphorus lone pairs in the former systems. Thus a metal complex with a single 1,3-diphosphacyclobutadiene ligand can act as an electron pair donor to a second metal atom. This mode of bonding has been realized experimentally in the heterometallic chromium-iron complex [tBu2C2P2 → Cr(CO)5]Fe (CO)3 [6] (Fig. 2). More interestingly, a metal sandwich compound with two 1,3-diphosphacyclobutadiene ligands can act as a bidentate chelating diphosphine to a second metal atom. In order to assess the effects of basicity of the ring phosphorus atom in 1,3-diphosphacyclobutadiene metal carbonyl derivatives we have used density functional theory to study the structures and thermochemistry of the binuclear metal carbonyl complexes (Me2C2P2)2M2(CO)n (M = Mn [7], Fe [8], Co [9]). Methyl rather than the bulkier tert-butyl substituents were used to facilitate the optimization of a large number of starting structures. The lowest energy structures of the carbonyl richest systems were found to have two mononuclear (Me2C2P2)M(CO)n units linked to each other solely through a single P → M dative bond from the C2P2 ring of one mononuclear moiety to the metal atom of the other moiety. Only for the (Me2C2P2)2Fe2(CO)3 and (Me2C2P2)2Mn2(CO)4 systems, isoelectronic with the experimentally known (η5-Me5C5)2Mn2(CO)3 [10] and (η4C4H4)2Cr2(CO)4 [11] systems, respectively, were low-energy structures found with exclusively terminal 1,3-diphosphacyclobutadiene ligands with no P → M dative bonds. These (Me2C2P2)2Fe2(CO)3 and (Me2C2P2)2Mn2(CO)4 structures have short M^M distances suggesting the formal triple bonds needed to give the metal atoms the favored 18-
E-mail address: [email protected] (R. Bruce King). https://doi.org/10.1016/j.ica.2019.119123 Received 24 July 2019; Received in revised form 26 August 2019; Accepted 2 September 2019 Available online 04 September 2019 0020-1693/ © 2019 Elsevier B.V. All rights reserved.
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R
:
P
R
Ni
R
:
P
:
P
R
( η4-R
2 C 2P 2 ) 2 M (M = Fe, Co, Ni)
Table 1 Harmonic ν(CO) vibrational frequencies (in cm−1) for the(Me2C2P2)2Cr2(CO)n (n = 6, 5, 4, 3) structures predicted with the BP86 method. Infrared intensities (in km/mol) are in parentheses. Those frequencies for the bridging CO groups are in bold face.
P
:
:
P
:
R
OC
P Fe C O
R CO
BP86 6S-1 5T-1 5T-2 5S-1 5S-2 5S-3 4T-1 4T-2 4T-3 4S-1 4S-2 4S-3 4S-4 3T-1 3T-2 3T-3 3T-4 3S-1
(η 4-R2C2P2)Fe(CO) 3
Fig. 1. Known mononuclear metal complexes of the 1,3-ditert-butyl-diphosphacyclobutadiene ligand (R = tBu).
1923(1126), 1939(808), 1945(268), 1945(716), 1971(1275), 2004(834) 1922(1315), 1931(1072), 1937(805), 1943(310), 2001(1185) 1915(525), 1925(1160), 1927(126), 1952(2296), 1979(408) 1873(152), 1894(714), 1932(876), 1953(535), 2004(1276) 1867(312), 1924(301), 1944(474), 1954(748), 1988(1304) 1883(408), 1905(143), 1923(715), 1951(732), 1978(429) 1918(1), 1925(1237), 1944(2116), 1964(67) 1916(859), 1924(386), 1936(1149), 1975(948) 1866(832), 1891(36), 1948(687), 1968(385) 1889(110), 1898(1096), 1936(469), 1971(1472) 1732(403), 1744(0), 1941(1310), 1951(0) 1729(399), 1743(0), 1940(348), 1968(1271) 1723(329), 1739(87), 1935(387), 1964(1211) 1830(331), 1923(1538), 1963(831) 1864(830), 1884(78), 1963(1234) 1731(352), 1869(748), 1901(112) 1851(849), 1882(220), 1943(334) 1860(768), 1897(742), 1929(1014)
hydrogen atoms. The Huzinaga-Dunning DZ basis sets augmented with one set of d functions with orbital exponents αd(C) = 0.75, αd(O) = 0.85, and αd(P) = 0.60 were used for the carbon, oxygen, and phosphorus atoms, respectively. The loosely contracted DZP basis set for Wachters primitive set [28] augmented by two sets of p functions and a set of d functions, contracted following Hood, Pitzer, and Schaefer [29], designated (14s11p6d/10s8p3d) was used for the chromium atom. 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 [30], exercising the default fine grid option (75 radial shells, 302 angular points) for evaluating integrals numerically [31].
Fig. 2. Binuclear metal complexes of 1,3-diallkyldiphosphacyclobutadiene ligands including the experimentally known [tBu2C2P2 → Cr(CO)5]Fe(CO)3 (R = tBu; alkyl groups attached to the phosphacyclobutadiene carbon atoms omitted for clarity.
electron configuration. We now report such studies on the binuclear 1,3-diphosphacyclobutadiene chromium complexes (Me2C2 P2)2Cr2(CO)n (n = 6, 5, 4, 3), again choosing methyl substituents to facilitate optimization of a variety of starting structures. These dichromium species are of interest because of the possibility of a (η4-Me2C2P2)2Cr(CO)n (n = 2, 1, 0) sandwich unit acting as a chelating ditertiary phosphine to replace two carbonyl groups in Cr(CO)6 to give [(η4-Me2C2P2)2Cr(CO)n]Cr(CO)4 derivatives in which one chromium atom is sandwiched between two C2P2 rings and the other chromium is octahedrally coordinated to two phosphorus atoms and four carbonyl groups (Fig. 2). If n = 2, then each chromium has the favored 18-electron configuration.
3. Results and discussion 3.1. Molecular structures
2. Theoretical methods
3.1.1. (Me2C2P2)2Cr2(CO)6 Only one energetically low-lying singlet (Me2C2P2)2Cr2(CO)6 structure 6S-1 was found using the BP86 and M06-L methods (Fig. 3 and Table 2). Other singlet and triplet (Me2C2P2)2Cr2(CO)6 structures lie at least 30 kcal/mol higher in energy than 6S-1 and thus are not likely to be chemically significant. Structure 6S-1 has two bridging η1, η4-Me2C2P2 rings, each of which is bonded through the ring π system to one chromium atom as a tetrahapto ligand and with the lone pair of a phosphorus atom to the other chromium atom. Thus 6S-1 can be
Electron correlation effects were considered by using density functional theory (DFT) methods, which have evolved as a practical and effective computational tool, especially for organometallic compounds [12–18]. Thus two DFT methods were used in the present study. The first method used was the M06-L functional of Zhao and Truhlar [19], which is suggested to be suitable for applications in transition metal chemistry. The other DFT method used was BP86, which combines Becke’s 1988 exchange functional with Perdew’s 1986 gradient corrected correlation functional [20,21]. In our study, the M06-L and BP86 results agree with each other fairly well in predicting the structural characteristics of the (Me2C2P2)2Cr2(CO)n derivatives. For the ν(CO) frequencies (Table 1), the BP86 method is known to give values that are closer to the experimental values without using any scaling factors [22–24]. This concurrence may be accidental, since the theoretical vibrational frequencies predicted by BP86 are harmonic frequencies, whereas the experimental fundamental frequencies are anharmonic. Since the DFT methods were reported to be less sensitive to the basis set size [25], all computations were performed using the double-ζ plus polarization (DZP) basis sets [26,27], which is consistent with our previous similar studies. The Huzinaga–Dunning DZ set augmented with a set of p polarization functions αp(H) = 0.75 was used for the
6S-1 (C2v) Fig. 3. Lowest energy (Me2C2P2)2Cr2(CO)6 structure. Distances are given in Ǻ. The upper numbers were obtained by the BP86 method, while the lower numbers were obtained by the M06-method. The structures in other figures have the same arrangement. 2
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chromium atom through its π system and a monohapto ligand to the other chromium atom through a P → Cr dative bond. Two terminal CO groups in 5T-2 are bonded to one chromium atom whereas three terminal CO groups are bonded to the other chromium atom. The long predicted Cr⋯Cr distance of 3.671 Å (BP86) or 3.679 Å (M06-L) suggests a lack of a direct bond between the two chromium atoms. The chromium atom in 5T-2 bonded to three terminal CO groups has the favored 18-electron configuration corresponding to its small spin density of 0.13. However, the chromium atom 5T-1 bonded to only two terminal CO groups has only a 16-electron configuration corresponding to a spin density of 1.77 and the two unpaired electrons of the triplet spin state. The singlet structure 5S-1, lying 7.2 kcal/mol (BP86) or 6.6 kcal/ mol (M06-L) in energy above 5T-1, has three terminal CO groups bonded to one chromium atom and the other two CO groups bridging the central Cr2 unit (Fig. 4 and Table 3). One η4-Me2C2P2 ring is bonded to the other chromium atom as a terminal tetrahapto ligand (the lower ring in Fig. 4). The other η1, η4-Me2C2P2 ring is bonded to the chromium atom bearing the three terminal carbonyl groups through a P → Cr dative bond and to the other chromium atom as a tetrahapto ligand (the upper ring in Fig. 4). The predicted Cr]Cr distance of 2.498 Å (BP86) or 2.486 Å (M06-L), suggests a formal double bond thereby giving each chromium atom the favored 18-electron configuration. The (Me2C2P2)2Cr2(CO)5 structure 5S-2, lying 8.2 kcal/mol (BP86) or 9.9 kcal/mol (M06-L) in energy above 5T-1, has two terminal CO groups bonded to each chromium atom and the fifth CO group bridging the central Cr2 unit (Fig. 4 and Table 3). A terminal η4-Me2C2P2 ring is bonded to one chromium atom, whereas the other η1, η4-Me2C2P2 ring bridges the central Cr2 unit as a tetrahapto ligand to one chromium atom and through a P → Cr dative bond to the other chromium atom. The Cr]Cr distance of 5S-2 of 2.472 Å (BP86) or 2.453 Å (M06-L) is close to that in 5S-2 and suggests the formal double bond required to give each chromium atom the favored 18-electron configuration. The (Me2C2P2)2Cr2(CO)5 structure 5S-3, lying 15.1 kcal/mol (BP86) or 19.8 kcal/mol (M06-L) in energy above 5T-1, has a terminal η4Me2C2P2 ring bonded to each chromium atom (Fig. 4 and Table 3). Two terminal CO groups in 5S-3 are bonded to one chromium atom and one terminal CO group is bonded to the other chromium atom leaving the
Table 2 Total energies (E, in hartree) and the Cr-Cr distances (Å) for the structure of (Me2C2P2)2Cr2(CO)6. 6S-1 (C2v, 1A)
E Cr-Cr
BP86
M06-L
−4447.269484 4.053
−4447.593040 4.031
considered as a bidentate sandwich (η4-Me2C2P2)2Cr(CO)2 ligand chelating to a Cr(CO)4 unit through two P → Cr dative bonds (Fig. 2). In 6S1 each bridging η1, η4-Me2C2P2 ring is a six-electron donor to the central Cr2 unit. The long Cr⋯Cr distance of 4.053 Å (BP86) or 4.031 Å (M06-L) suggests no direct bond between the two chromium atoms. This gives each chromium atom in 6S-1 the favored 18-electron configuration. The infrared spectrum of 6S-1 is predicted to exhibit six ν(CO) frequencies ranging from 1923 to 2004, consistent with six terminal CO groups (Table 1). 3.1.2. (Me2C2P2)2Cr2(CO)5 Five (Me2C2P2)2Cr2(CO)5 low-energy isomers (two triplets and three singlets) were found (Fig. 4 and Table 3). The lowest energy (Me2C2P2)2Cr2(CO)5 structure 5T-1 has two bridging η1, η4-Me2C2P2 rings and five terminal CO groups. Structure 5T-1 can be derived from the (Me2C2P2)2Cr2(CO)6 structure by removing one CO group from the chromium atom sandwiched between the two P2C2 rings. The predicted Cr–Cr distance of 4.030 Å (BP86) or 4.008 Å (M06-L) suggests no direct bond between the two chromium atoms. This gives the chromium atom sandwiched between the two P2C2 rings in 5T-1 a 16-electron configuration whereas the other chromium atom has an 18-electron configuration. This is consistent with the triplet spin state and the spin density of 1.95 on the sandwiched chromium atom and only 0.03 on the other chromium atom. The (Me2C2P2)2Cr2(CO)5 structure 5T-2 lies 9.0 kcal/mol (BP86) or 11.9 kcal/mol (M06-L) in energy above 5T-1 with two bridging η1, η4Me2C2P2 rings and five terminal CO groups (Fig. 4 and Table 3). Each Me2C2P2 ligand in 5T-2 functions as a tetrahapto ligand to one
5T-1 (C2v)
5S-1 (C1)
5T-2 (Cs)
5S-2 (C1)
5S-3 (C1)
Fig. 4. The low-energy (Me2C2P2)2Cr2(CO)5 structures. Distances are given in Ǻ. 3
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Table 3 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), Cr-Cr distances (Å), and spin contamination values ( S 2 ) for the structures of (Me2C2P2)2Cr2(CO)5.
BP86 E ΔE Cr-Cr
S2
M06-L E ΔE Cr-Cr
S2
5T-1 (C2v, 3B1)
5T-2 (Cs, 3A)
5S-1 (C1, 1A)
5S-2 (C1, 1A)
5S-3 (C1, 1A)
−4333.860776 0.0 4.030 2.05
−4333.846387 9.0 3.671 2.03
−4333.849259 7.2 2.498 0
−4333.847687 8.2 2.472 0
−4333.836674 15.1 2.337 0
−4333.202745 0.0 4.008 2.13
−4333.183755 11.9 3.679 2.09
−4333.192188 6.6 2.486 0
−4333.186959 9.9 2.453 0
−4333.171260 19.8 2.335 0
remaining two CO groups as bridges across the central Cr2 unit. However, one of these bridging CO groups is actually only very weakly semibridging with a long Cr–C distance of ∼2.8 Å so it is almost like a terminal CO group. The predicted Cr^Cr distance in 5S-3 of 2.337 Å (BP86) or 2.335 Å (M06-L) is more than 0.1 Å shorter than the Cr]Cr distances in 5S-1 and 5S-2 thereby suggesting the formal triple bond required to give each chromium atom the favored 18-electron configuration. Structure 5S-3 resembles that of the experimentally known closely related Cp2V2(CO)5 [32–34] having a similar metal electronic configuration. The theoretical harmonic ν(CO) frequencies of these six (C2P2Me2)2Cr2(CO)5 structures are listed in Table 1. The ν(CO) vibrational frequencies for the bridging and semibridging CO groups ranging from 1867 to 1905 cm−1 in 5S-1, 5S-2, and 5S-3 are distinctly lower than the ν(CO) frequencies of the terminal CO groups ranging from 1915 to 2004 cm−1 in accord with expectation. However, the separation between the ν(CO) frequencies of the weakly semibridging CO groups of 1883 and 1905 cm−1 and the lowest terminal ν(CO) frequency of 1923 cm−1 in 5S-3 is relatively small.
the other chromium atom a 16-electron configuration. This interpretation is supported by spin densities of 2.43 and −0.27 on the chromium atoms with 16 and 18-electron configurations respectively. The lowest energy singlet (Me2C2P2)2Cr2(CO)4 structure 4S-1, lying 3.0 kcal/mol (BP86) or 5.3 kcal/mol (M06-L) in energy above 4T-1, has two η1, η4-Me2C2P2 bridging rings, three terminal CO groups, and one weakly semibridging CO group with a long Cr–C distance of 2.520 Å (BP86) or 2.439 (M06-L) (Fig. 5 and Table 4). Interpretation of the predicted Cr]Cr distance of 2.405 Å (BP86) or 2.391 Å (M06-L) in 4S-1 as a formal double bond gives each atom the favored 18-electron configuration. Structure 4S-1 can be dissected into a chelating diphosphine (η4-Me2C2P2)2Cr(CO) bonding as a bidentate ligand to a Cr(CO)3 moiety through two P → Cr dative bonds. The singlet (Me2C2P2)2Cr2(CO)4 structures 4S-2, 4S-3, and 4S-4, lying 8.9, 9.0, and 10.8 kcal/mol (BP86) or 14.9, 16.5, and 18.7 kcal/ mol (M06-L), respectively, in energy above 4T-1, are similar to each other. They each have two terminal η4-Me2C2P2 rings, two terminal CO groups, and two four–electron-donor bridging η2-µ-CO groups with short Cr–O distances of ∼2.35 Å (Fig. 5 and Table 4). These three structures differ in the orientation of the P2C2 rings. Their Cr]Cr distances, ranging from 2.569 to 2.608 Å, suggest formal double bonds, thereby giving each chromium atom the favored 18-electron configuration. The predicted harmonic ν(CO) frequencies of these seven (Me2C2P2)2Cr2(CO)4 structures are listed in Table 1. The ν(CO) vibrational frequencies for the two-electron donor bridging and semibridging CO groups in 4S-1 and 4T-3 range from 1866 to 1948 cm−1, which generally is significantly lower than the ν(CO) frequencies of the terminal CO groups in the same species. However, in 4T-3 the ν(CO) frequency of the most weakly semibridging CO at 1948 cm−1 is only 20 cm−1 below the terminal CO frequency. The four-electron donor bridging η2-µ-CO groups in 4S-2, 4S-3, and 4S-4 exhibit significantly lower ν(CO) frequencies in the narrow range from 1723 to 1744 cm−1 in accord with expectation.
3.1.3. (Me2C2P2)2Cr2(CO)4 Seven low-energy (Me2C2P2)2Cr2(CO)4 structures were found (three triplets and four singlets) (Fig. 5 and Table 4). The lowest energy (Me2C2P2)2Cr2(CO)4 structure 4T-1 is a triplet structure with a terminal η4-Me2C2P2 ring and two terminal CO groups on each chromium atom. The predicted Cr]Cr distance of 2.855 Å (BP86) or 2.669 Å (M06-L), suggests a formal double bond to give each Cr atom a 16-electron configuration. Considering the Cr]Cr double bond as a σ + 2/2 double bond with two orthogonal π bonds similar to the Fe]Fe double bond in the known triplet state organometallics (η5-R5C5)2Fe2(µ-CO)3 (R = H, Me) [35–37] can account for the triplet spin state. This interpretation is supported by equal spin densities of 0.95 on each chromium atom. The (Me2C2P2)2Cr2(CO)4 structure 4T-2, lying 6.6 kcal/mol (BP86) or 4.9 kcal/mol (M06-L) in energy above 4T-1, has one η3, η3-Me2C2P2 bridging ring and one η1, η4-Me2C2P2 bridging ring (Fig. 3 and Table 4). The terminal CO groups in 4T-2 are distributed unevenly with three terminal CO groups on one chromium atom, and only one terminal CO group on the other chromium atom. Interpreting the Cr]Cr distance of 2.958 Å (BP86) or 2.872 Å (M06-L) in 4T-2 as a formal double bond gives the chromium atom bearing three CO groups the favored 18electron configuration but the other chromium atom only a 16-electron configuration. The unsymmetrical distribution of spin densities with only 0.07 on one chromium atom but 1.91 on the other chromium atom suggest that the two unpaired electrons of the triplet spin state of 4T-2 reside on the chromium atom with a 16-electron configuration. The (Me2C2P2)2Cr2(CO)4 structure 4T-3, lying 10.8 kcal/mol (BP86) or 7.4 kcal/mol (M06-L) in energy above 4T-1, has one terminal CO group, three bridging CO groups, and two terminal η4-Me2C2P2 rings. (Fig. 5 and Table 4). The predicted Cr^Cr distance of 2.401 Å (BP86) or 2.451 Å (M06-L), suggests the formal triple bond required to give the chromium atom bearing the CO group an 18-electron configuration and
3.1.4. (Me2C2P2)2Cr2(CO)3 Five low-energy (Me2C2P2)2Cr2(CO)3 structures were found (one singlet and four triplets) (Fig. 6 and Table 5). In each of the four triplet structures both P2C2 rings are terminal ligands bonded to a single chromium atom. The lowest energy (Me2C2P2)2Cr2(CO)3 structure 3T-1 is one of these triplet structures with a terminal η4-Me2C2P2 ring and a terminal CO group bonded to each chromium atom. The third CO group in 3T-1 is primarily bonded to one of the chromium atoms with only a weak interaction with the other chromium atom through a long Cr–C bond of ∼2.4 Å. Interpreting the predicted Cr]Cr distance of 2.483 Å (BP86) or 2.526 Å (M06-L) in 3T-1 as a formal double bond and ignoring the weak Cr–C bond of length ∼2.4 Å in the semibridging CO group gives the chromium bearing two CO groups (including the weakly semibridging CO group) a 16-electron configuration and the other chromium atom only a 14-electron configuration. The spin density is unevenly distributed with 2.40 on the chromium atom with a 4
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4T-1 (C2)
4T-3(C1)
4T-2 (Cs)
4S-1 (Cs)
4S-2 (Ci)
4S-3 (C2)
4S-4 (C1)
Fig. 5. The lowest energyMe2C2P2)2Cr2(CO)4 structures. Distances are given in Ǻ.
single terminal CO group and −0.27 on the chromium atom bearing the two remaining CO groups. Structure 3T-2 has essentially the same energy as 3T-1, and it has similar structure to 3T-1 except for the arrangement of the CO groups. The (Me2C2P2)2Cr2(CO)3 structures 3T-3 and 3T-4 are similar to each other, with two terminal η4-Me2C2P2 rings and three semibridging CO groups (Fig. 6 and Table 5). These two structures differ in the arrangement of the semibridging CO groups. Thus in 3T-3, the shorter Cr–C bonds to the three semibridging CO groups come from the same chromium atom, while in 3T-4, two of the short Cr–C bonds come from
one chromium atom and the third Cr–C bond comes from the other chromium atom. Structure 3T-3 lies 9.4 kcal/mol (BP86) or 10.6 kcal/ mol (M06-L) above 3T-1. However, structure 3T-4 is a significantly higher energy structure lying 23.5 kcal/mol (BP86) or 27.0 kcal/mol (M06-L) in energy above 3T-1. Interpreting the Cr]Cr distances in 3T3 and 3T-4 ranging from 2.4 to 2.5 Å as formal double bonds gives each chromium atom a 15-electron configuration. However, the spin densities are unevenly distributed between the chromium atoms. The only low-energy singlet (Me2C2P2)2Cr2(CO)3 structure, namely 3S-1 lying 10.4 kcal/mol (BP86) or 20.0 kcal/mol (M06-L) in energy
Table 4 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), Cr-Cr distances (Å), and spin contamination values ( S 2 ) for the structures of (Me2C2P2)2Cr2(CO)4.
BP86 −(E + 4220) ΔE Cr-Cr
S2
M06-L −(E + 4219) ΔE Cr-Cr
S2
4T-1 (C2, 3B)
4T-2 (Cs, 3A)
4T-3 (C1, 3A)
4S-1 (Cs, 1A)
4S-2 (Ci, 1Ag)
4S-3 (C2, 1A)
4S-4 (C1, 1A)
0.467324 0.0 2.855 2.02
0.456869 6.6 2.958 2.05
0.450205 10.8 2.401 2.14
0.462627 3.0 2.405 0
0.453114 8.9 2.601 0
0.453078 9.0 2.608 0
0.450168 10.8 2.589 0
0.822187 0.0 2.669 2.07
0.814368 4.9 2.872 2.13
0.810350 7.4 2.451 2.45
0.813767 5.3 2.391 0
0.798370 14.9 2.578 0
0.795862 16.5 2.583 0
0.792432 18.7 2.569 0
5
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3T-1 (C1)
3T-2 (C1)
3T-4 (C1)
3T-3 (C1)
3S-1 (C1)
Fig. 6. The lowest energy (Me2C2P2)2Cr2(CO)3 structures. Distances are given in Ǻ.
above 3T-1, has two bridging η1, η4-Me2C2P2 rings and three terminal CO groups (Fig. 4 and Table 5). Structure 3S-1 can be derived from 4S to 1 by removal of a CO group from the chromium atom not sandwiched between the two C2P2 rings. Thus 3S-1 can be dissected into a chelating diphosphine (η4-Me2C2P2)2Cr(CO) bonding as a bidentate ligand to a Cr(CO)2 moiety through two P → Cr dative bonds. The predicted Cr^Cr distance of 2.320 Å (BP86) or 2.322 Å (M06-L) in 3S-1 is ∼0.1 Å shorter than that in 4S-1 and thus can be interpreted as the formal triple bond required to give each chromium atom the favored 18-electron configuration. This assumes a formal positive charge on the chromium atom sandwiched between the C2P2 rings and a formal negative charge on the chromium atom bearing the two terminal CO groups.
all sufficiently endothermic, from 38 to 51 kcal/mol (BP86) or from 28 to 46 kcal/mol (M06-L) thereby suggesting that the lowest energy (Me2C2P2)2Cr2(CO)n (n = 6, 5, 4) structures are viable towards CO loss. For comparison, the experimental dissociation energies for the wellknown simple metal carbonyls Ni(CO)4, Fe(CO)5, and Cr(CO)6 are 27, 41, and 37 kcal/mol, respectively. The monotonic increase of the CO dissociation energies of the (Me2C2P2)2Cr2(CO)n derivatives with increasing number of CO groups is consistent with the endothermic nature of the disproportionation processes 2(Me2C2P2)2Cr2(CO)n → (Me2C2P2)2Cr2(CO)n+1 + (Me2C2P2)2Cr2(CO)n−1 listed in Table 7. However, these endothermic disproportionation energies are rather small suggesting that conditions might be found where such disproportionation reactions could occur. Also of interest is the dissociation of the (Me2C2P2)2Cr2(CO)n derivatives into mononuclear (Me2C2P2)Cr(CO)m fragments. In order to obtain such energetic data, the structures of the mononuclear (Me2C2P2)Cr(CO)m were optimized by the same DFT methods as used to study the binuclear derivatives (Fig. 7 and Table S24). Using this information the dissociation energies of the binuclear metal carbonyl
3.2. Thermochemistry Table 6 shows the dissociation energies for the reactions (Me2C2P2)2Cr2(CO)n → (Me2C2P2)2Cr2(CO)n-1 + CO (n = 6, 5, 4) considering the lowest energy structures. The CO dissociation energies are
Table 5 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), Cr-Cr distances (Å), and spin contamination values ( S 2 ) for the structures of (Me2C2P2)2Cr2(CO)3.
BP86 E ΔE Cr-Cr
S2
M06-L E ΔE Cr-Cr
S2
3T-1 (C1, 3A)
3T-2 (C1, 3A)
3T-3 (C1, 3A)
3T-4 (C1, 3A)
3S-1 (C1, 1A)
−4107.079311 0.0 2.483 2.44
−4107.076673 1.7 2.404 2.63
−4107.064373 9.4 2.523 2.66
−4107.041851 23.5 2.368 2.16
−4107.062799 10.4 2.320 0
−4106.460363 0.0 2.526 2.97
−4106.459972 0.2 2.493 3.15
−4106.443413 10.6 2.575 3.07
−4106.417408 27.0 2.433 2.51
−4106.428433 20.0 2.322 0
6
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respectively. The other chromium atom in 6S-1, which is sandwiched between two η4-Me2C2P2 rings and bonded to two terminal CO groups, has an atomic charge of −0.84. Similarly, the chromium atoms in 5S-1, 4S-1, and 3S-1, also sandwiched between two η4-Me2C2P2 rings and fewer CO groups, have less negative atomic charges from −0.54 to −0.58. All of the chromium atoms in 4S-2, 4S-3, and 4S-4 are coordinated by two terminal η4-Me2C2P2 rings, two terminal CO groups, and two bridging CO groups and have atomic charges in the narrow range from −0.41 to −0.47. The two chromium atoms in 5S-3, which have similar ligands to those in 4S-2, 4S-3, and 4S-4 except for an additional semibridging CO group, have slightly more negative charges, i.e. −0.77 and −0.59. Compared with 5S-3, the chromium atoms in 5S-2 have one additional coordinated phosphorus atom leading to negative charges of −0.76 and −0.73, respectively, with that on one of the chromium atoms being more negative than that in 5S-3. The WBI values for the Cr-Cr bonds and the formal bond orders based on distance, spin state, and electron counting in the (Me2C2P2)2Cr2(CO)n derivatives are shown in Table 9. The number and nature of groups bridging the Cr-Cr bond are also listed in Table 9 since these can affect the WBI values listed. No examples of singlet (Me2C2P2)2Cr2(CO) n species with formal Cr–Cr single bonds are found in this work. The Cr]Cr WBIs for double bonds are found in a wide range from 0.40 to 0.72. The higher WBI values for the Cr]Cr double bonds ranging from 0.61 to 0.65 are found in the structures 4S-2, 4S-3, and 4S-4 which have four-electron donor η2-µ-CO groups bridging the Cr]Cr double bond. The apparently smaller predicted WBI value of 0.67 for the Cr^Cr triple bond in 5S-3 may be a consequence of the presence of only two semibridging CO groups. The WBI for the Cr^Cr triple bond in 3S-1 of 0.95 is clearly above the wide range observed for the WBIs of any of the Cr]Cr double bonds in the (Me2C2P2)2Cr2(CO)n species. The WBI values of these Cr-Cr multiple bonds in the (Me2C2P2)2Cr2(CO)n derivative are consistent with the general observation [39] that WBI values for single bonds between d-block transition metals typically range from 0.2 to 0.3 and thus are significantly lower than those reported here for Cr-Cr double and triple bonds. The very low WBI of 0.04 for the Cr⋯Cr interaction in the (Me2C2P2)2Cr2(CO)6 structure 6S-1 indicates the lack of a chromium–chromium direct bond consistent with the long Cr⋯Cr distance of 4.05 Å.
Table 6 Dissociation energies (kcal/mol) for the successive removal of carbonyl groups from the (Me2C2P2)2Cr2(CO)n derivatives.
(Me2C2P2)2Cr2(CO)6 (Me2C2P2)2Cr2(CO)6 (Me2C2P2)2Cr2(CO)5 (Me2C2P2)2Cr2(CO)4
(6S-1) → (Me2C2P2)2Cr2(CO)5 (5T-1) + CO (6S-1) → (Me2C2P2)2Cr2(CO)5 (5S-1) + CO (5T-1) → (Me2C2P2)2Cr2(CO)4 (4T-1) + CO (4T-1) → (Me2C2P2)2Cr2(CO)3 (3T-1) + CO
BP86
M06-L
51.1 58.3 41.6 38.1
46.3 52.9 40.2 28.5
Table 7 Disproportionation energies (kcal/mol) for the 2(Me2C2P2)2Cr2(CO)n → (Me2C2P2)2Cr2(CO)n+1 + (Me2C2P2)2Cr2(CO)n−1 reactions.
2(Me2C2P2)2Cr2(CO)5 (5T-1)→(Me2C2P2)2Cr2(CO)6 (6S-1) + (Me2C2P2)2Cr2(CO)4 (4T-1) 2(Me2C2P2)2Cr2(CO)4 (4T-1)→(Me2C2P2)2Cr2(CO)5 (5T-1) + (Me2C2P2)2Cr2(CO)3 (3T-1)
BP86
M06-L
9.6
6.1
3.4
11.8
derivatives into mononuclear fragments were predicted to be substantial, i.e., larger than 75 kcal/mol (Table 8). These data show that the lowest energy (Me2C2P2)2Cr2(CO)n structures are viable with respect to dissociation into mononuclear fragments. Also the dissociation energies of (Me2C2P2)2Cr2(CO)n into mononuclear fragments increase monotonically with decreasing n. 3.3. NBO analysis of the chromium-chromium bonding Table 9 lists the natural atomic charges for the chromium atoms and the Wiberg Bond Indices (WBIs) for the Cr-Cr bonds from NBO analysis [38] using the BP86 method. The Cr-Cr distances, the formal bond orders suggested by electron counting, and the types of 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. All of the natural charges on both 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 all of this thus acquired negative charge. The values of the chromium atomic natural charges depend on the numbers of coordinated CO groups and coordinated phosphorus lone pairs of the Me2C2P2 rings. In 6S-1, the chromium atom coordinated by two phosphorus lone pairs and four CO groups has the most negative atomic charge (−1.37). In 5S-1, the chromium atom coordinated by two phosphorus lone pairs, three terminal CO groups, and two semibridging CO groups has a slightly less negative atomic charge (−1.00). Similarly, the chromium atoms coordinated by two phosphorus lone pairs but even fewer CO groups in 4S-1 and 3S-1 have even less negative charges, i.e., −0.81 and −0.32,
(Me2C2P2)Cr(CO)
4. Conclusion The only low-energy structure for the hexacarbonyl (Me2C2P2)2Cr2(CO)6 consists of a bidentate (η4-Me2C2P2)2Cr(CO)2 sandwich diphosphine ligand bonded to a Cr(CO)4 unit with a long ∼4.0 Å Cr⋯Cr distance indicating lack of a direct metal–metal bond (6S-1 in Fig. 3). This unusually favorable singlet structure has an 18electron configuration for both chromium atoms and octahedral coordination of a P2Cr(CO)4 unit with four terminal CO groups and two P → Cr dative bonds. The lowest energy structure for the pentacarbonyl (Me2C2P2)2Cr2(CO)5 is a triplet structure derived from this singlet hexacarbonyl structure by loss of a CO group from the sandwiched chromium atom (5T-1 in Fig. 4). The bridging η1,4-Me2C2P2 ligands
(Me2C2P2)Cr(CO)2
(Me2C2P2)Cr(CO)3
Fig. 7. The optimized geometries for the mononuclear structures (Me2C2P2)2Cr2(CO)m (m = 3, 2, 1), which are the dissociation products in Table 8. 7
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X. Wan, et al.
Table 8 Energies for the dissociation of the binuclear (Me2C2P2)2Cr2(CO)n (n = 6, 5, 4, 3) derivatives into mononuclear fragments (kcal/ mol).
(Me2C2P2)2Cr2(CO)4(6S-1) → 2(Me2C2P2)Cr(CO)3 (Me2C2P2)2Cr2(CO)5(5T-1) → (Me2C2P2)Cr(CO)3 + (Me2C2P2)Cr(CO)2 (Me2C2P2)2Cr2(CO)4(4T-1) → 2(Me2C2P2)Cr(CO)2 (Me2C2P2)2Cr2(CO)3(3T-1) → (Me2C2P2)Cr(CO)2 + (Me2C2P2)Cr(CO)
Table 9 Atomic charges and Wiberg bond indices (WBIs) (MeC2P2)2Cr2(CO)n structures by the BP86 method.
for
the
singlet
Natural Charge on Cr/Cr
WBI
Cr-Cr (Å)
Bridges
Formal bond order
6S-1 5S-1
−0.84/−1.37 −0.58/−1.00
0.04 0.40
4.053 2.498
0 2
5S-2
−0.76/−0.73
0.55
2.472
5S-3 4S-1
−0.77/−0.59 −0.54/−0.81
0.67 0.72
2.337 2.405
4S-2 4S-3 4S-4 3S-1
−0.44/−0.46 −0.43/−0.43 −0.47/−0.41 −0.56/−0.32
0.61 0.65 0.62 0.95
2.602 2.608 2.589 2.320
2 Me2C2P2 2 Me2C2P2, 2 semi-CO Me2C2P2, semiCO 2 semi-CO 2 Me2C2P2, semi-CO 2 η2-µ-CO 2 η2-µ-CO 2 η2-µ-CO 2 Me2C2P2
[2] [3] [4] [5] [6] [7] [8] [9]
2 3 2
[10]
2 2 2 3
[11] [12] [13] [14] [15] [16]
found in these two structures, using the four π-electrons of the P2C2 ring to bond to one chromium atom and a phosphorus lone pair to bond to the other chromium atom but not sandwiching a chromium atom, are features of the next lowest energy triplet and singlet (Me2C2P2)2Cr2(CO)5 structures. In contrast to the hexa- and pentacarbonyls the lowest energy structures of the more highly unsaturated tetra- and tricarbonyls (Me2C2P2)2Cr2(CO)n (n = 4, 3) have terminal tetrahapto η4-Me2C2P2 rings not using their phosphorus lone pairs and chromium–chromium multiple bonds.
[17] [18] [19] [20] [21] [22] [23] [24]
Acknowledgment
[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38]
We are indebted to the New Century Excellent Talents in University (Grant No, NCET-10-0949) and the Scientific Research Fund of the Key Laboratory of the Education Department of Sichuan Province (Grant No. 10ZX012) in China 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.2019.119123. References
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M06-L
76.0 75.4 84.3 97.9
86.8 88.0 95.2 113.9
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1,3-Diphosphacyclobutadiene sandwich compounds as bidentate ligands in metal carbonyl chemistry: Binuclear chromium derivatives
T
Xin Wana, Xuefeng Wanga, Xiaohong Chena, Rong Jina, Quan Dua, Yaoming Xieb, R. Bruce Kingb a b
School of Sciences, Research Center for Advanced Computation, Xihua University, Chengdu 610039, China Department of Chemistry and Center for Computational Quantum Chemistry, University of Georgia, Athens, GA 30602, USA
ABSTRACT
The structures and thermochemistry of the binuclear 1,3-diphosphacyclobutadiene chromium carbonyl complexes (Me2C2P2)2Cr2(CO)n (n = 6, 5, 4, 3) have been examined by density functional theory. The only low-energy structure for the hexacarbonyl (Me2C2P2)2Cr2(CO)6 consists of a bidentate chelating (η4-Me2C2P2)2Cr (CO)2 sandwich diphosphine ligand bonded to a Cr(CO)4 unit with a long ∼4.0 Å Cr⋯Cr distance indicating lack of a direct metal-metal bond. The lowest energy structure for the pentacarbonyl (Me2C2P2)2Cr2(CO)5 is a triplet structure derived from this singlet hexacarbonyl structure by loss of a CO group from the sandwiched chromium atom. The bridging η1,4-Me2C2P2 ligands found in these two structures, using the four π-electrons of the P2C2 ring to bond to one chromium atom and a phosphorus lone pair to bond to the other chromium atom but not sandwiching a chromium atom, are features of the next lowest energy triplet and singlet (Me2C2P2)2Cr2(CO)5 structures. In contrast to the hexa- and pentacarbonyls, the lowest energy structures of the more highly unsaturated tetra- and tricarbonyls (Me2C2P2)2Cr2(CO)n (n = 4, 3) have terminal tetrahapto η4-Me2C2P2 rings and chromium-chromium multiple bonds.
1. Introduction Dimerization of phosphaalkynes, RC^P:, on transition metal sites provides routes to metal complexes of substituted 1,3-diphosphacyclobutadienes [1]. Using this approach the sandwich compounds (η4-tBu2C2P2)2M (M = Fe [2], Co [3], Ni [4]) as well as the very stable iron carbonyl derivative (η4-tBu2C2P2)Fe(CO)3 [5] have been synthesized by reactions of tBuC^P: with [(η4-anthracene)2M]– (M = Co, Fe), (η2,2-C8H12)2Ni, and Fe(CO)5, respectively (Fig. 1). The analogy between a lone phosphorus atom and a CH moiety based on their valence isoelectronic relationship and similar electronegativities suggests a close similarity between mononuclear cyclobutadiene and diphosphacyclobutadiene metal complexes. However, in certain systems the diphosphacyclobutadiene metal complexers are more readily synthesized than the corresponding butadiene metal complexes. Thus the cyclobutadiene sandwich compounds (η4-R4C4)2M (M = Fe, Co) analogous to the 1,3-diphosphacyclobutadiene sandwich compounds (η4tBu2C2P2)2M remain unknown. The heterocyclic 1,3-diphosphacyclobutadiene and the homocyclic cyclobutadiene systems become distinctly different when binuclear metal complexes are considered. Complexation of a 1,3-diphosphacyclobutadiene ring to one metal atom as a tetrahapto ligand does not disturb the external phosphorus lone pairs which can coordinate to additional metal atoms. Therefore the chemistry of binuclear 1,3-diphosphacyclobutadiene metal complexes is likely to be significantly different from that of binuclear cyclobutadiene metal complexes
because of the accessibility of phosphorus lone pairs in the former systems. Thus a metal complex with a single 1,3-diphosphacyclobutadiene ligand can act as an electron pair donor to a second metal atom. This mode of bonding has been realized experimentally in the heterometallic chromium-iron complex [tBu2C2P2 → Cr(CO)5]Fe (CO)3 [6] (Fig. 2). More interestingly, a metal sandwich compound with two 1,3-diphosphacyclobutadiene ligands can act as a bidentate chelating diphosphine to a second metal atom. In order to assess the effects of basicity of the ring phosphorus atom in 1,3-diphosphacyclobutadiene metal carbonyl derivatives we have used density functional theory to study the structures and thermochemistry of the binuclear metal carbonyl complexes (Me2C2P2)2M2(CO)n (M = Mn [7], Fe [8], Co [9]). Methyl rather than the bulkier tert-butyl substituents were used to facilitate the optimization of a large number of starting structures. The lowest energy structures of the carbonyl richest systems were found to have two mononuclear (Me2C2P2)M(CO)n units linked to each other solely through a single P → M dative bond from the C2P2 ring of one mononuclear moiety to the metal atom of the other moiety. Only for the (Me2C2P2)2Fe2(CO)3 and (Me2C2P2)2Mn2(CO)4 systems, isoelectronic with the experimentally known (η5-Me5C5)2Mn2(CO)3 [10] and (η4C4H4)2Cr2(CO)4 [11] systems, respectively, were low-energy structures found with exclusively terminal 1,3-diphosphacyclobutadiene ligands with no P → M dative bonds. These (Me2C2P2)2Fe2(CO)3 and (Me2C2P2)2Mn2(CO)4 structures have short M^M distances suggesting the formal triple bonds needed to give the metal atoms the favored 18-
E-mail address: [email protected] (R. Bruce King). https://doi.org/10.1016/j.ica.2019.119123 Received 24 July 2019; Received in revised form 26 August 2019; Accepted 2 September 2019 Available online 04 September 2019 0020-1693/ © 2019 Elsevier B.V. All rights reserved.
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R
:
P
R
Ni
R
:
P
:
P
R
( η4-R
2 C 2P 2 ) 2 M (M = Fe, Co, Ni)
Table 1 Harmonic ν(CO) vibrational frequencies (in cm−1) for the(Me2C2P2)2Cr2(CO)n (n = 6, 5, 4, 3) structures predicted with the BP86 method. Infrared intensities (in km/mol) are in parentheses. Those frequencies for the bridging CO groups are in bold face.
P
:
:
P
:
R
OC
P Fe C O
R CO
BP86 6S-1 5T-1 5T-2 5S-1 5S-2 5S-3 4T-1 4T-2 4T-3 4S-1 4S-2 4S-3 4S-4 3T-1 3T-2 3T-3 3T-4 3S-1
(η 4-R2C2P2)Fe(CO) 3
Fig. 1. Known mononuclear metal complexes of the 1,3-ditert-butyl-diphosphacyclobutadiene ligand (R = tBu).
1923(1126), 1939(808), 1945(268), 1945(716), 1971(1275), 2004(834) 1922(1315), 1931(1072), 1937(805), 1943(310), 2001(1185) 1915(525), 1925(1160), 1927(126), 1952(2296), 1979(408) 1873(152), 1894(714), 1932(876), 1953(535), 2004(1276) 1867(312), 1924(301), 1944(474), 1954(748), 1988(1304) 1883(408), 1905(143), 1923(715), 1951(732), 1978(429) 1918(1), 1925(1237), 1944(2116), 1964(67) 1916(859), 1924(386), 1936(1149), 1975(948) 1866(832), 1891(36), 1948(687), 1968(385) 1889(110), 1898(1096), 1936(469), 1971(1472) 1732(403), 1744(0), 1941(1310), 1951(0) 1729(399), 1743(0), 1940(348), 1968(1271) 1723(329), 1739(87), 1935(387), 1964(1211) 1830(331), 1923(1538), 1963(831) 1864(830), 1884(78), 1963(1234) 1731(352), 1869(748), 1901(112) 1851(849), 1882(220), 1943(334) 1860(768), 1897(742), 1929(1014)
hydrogen atoms. The Huzinaga-Dunning DZ basis sets augmented with one set of d functions with orbital exponents αd(C) = 0.75, αd(O) = 0.85, and αd(P) = 0.60 were used for the carbon, oxygen, and phosphorus atoms, respectively. The loosely contracted DZP basis set for Wachters primitive set [28] augmented by two sets of p functions and a set of d functions, contracted following Hood, Pitzer, and Schaefer [29], designated (14s11p6d/10s8p3d) was used for the chromium atom. 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 [30], exercising the default fine grid option (75 radial shells, 302 angular points) for evaluating integrals numerically [31].
Fig. 2. Binuclear metal complexes of 1,3-diallkyldiphosphacyclobutadiene ligands including the experimentally known [tBu2C2P2 → Cr(CO)5]Fe(CO)3 (R = tBu; alkyl groups attached to the phosphacyclobutadiene carbon atoms omitted for clarity.
electron configuration. We now report such studies on the binuclear 1,3-diphosphacyclobutadiene chromium complexes (Me2C2 P2)2Cr2(CO)n (n = 6, 5, 4, 3), again choosing methyl substituents to facilitate optimization of a variety of starting structures. These dichromium species are of interest because of the possibility of a (η4-Me2C2P2)2Cr(CO)n (n = 2, 1, 0) sandwich unit acting as a chelating ditertiary phosphine to replace two carbonyl groups in Cr(CO)6 to give [(η4-Me2C2P2)2Cr(CO)n]Cr(CO)4 derivatives in which one chromium atom is sandwiched between two C2P2 rings and the other chromium is octahedrally coordinated to two phosphorus atoms and four carbonyl groups (Fig. 2). If n = 2, then each chromium has the favored 18-electron configuration.
3. Results and discussion 3.1. Molecular structures
2. Theoretical methods
3.1.1. (Me2C2P2)2Cr2(CO)6 Only one energetically low-lying singlet (Me2C2P2)2Cr2(CO)6 structure 6S-1 was found using the BP86 and M06-L methods (Fig. 3 and Table 2). Other singlet and triplet (Me2C2P2)2Cr2(CO)6 structures lie at least 30 kcal/mol higher in energy than 6S-1 and thus are not likely to be chemically significant. Structure 6S-1 has two bridging η1, η4-Me2C2P2 rings, each of which is bonded through the ring π system to one chromium atom as a tetrahapto ligand and with the lone pair of a phosphorus atom to the other chromium atom. Thus 6S-1 can be
Electron correlation effects were considered by using density functional theory (DFT) methods, which have evolved as a practical and effective computational tool, especially for organometallic compounds [12–18]. Thus two DFT methods were used in the present study. The first method used was the M06-L functional of Zhao and Truhlar [19], which is suggested to be suitable for applications in transition metal chemistry. The other DFT method used was BP86, which combines Becke’s 1988 exchange functional with Perdew’s 1986 gradient corrected correlation functional [20,21]. In our study, the M06-L and BP86 results agree with each other fairly well in predicting the structural characteristics of the (Me2C2P2)2Cr2(CO)n derivatives. For the ν(CO) frequencies (Table 1), the BP86 method is known to give values that are closer to the experimental values without using any scaling factors [22–24]. This concurrence may be accidental, since the theoretical vibrational frequencies predicted by BP86 are harmonic frequencies, whereas the experimental fundamental frequencies are anharmonic. Since the DFT methods were reported to be less sensitive to the basis set size [25], all computations were performed using the double-ζ plus polarization (DZP) basis sets [26,27], which is consistent with our previous similar studies. The Huzinaga–Dunning DZ set augmented with a set of p polarization functions αp(H) = 0.75 was used for the
6S-1 (C2v) Fig. 3. Lowest energy (Me2C2P2)2Cr2(CO)6 structure. Distances are given in Ǻ. The upper numbers were obtained by the BP86 method, while the lower numbers were obtained by the M06-method. The structures in other figures have the same arrangement. 2
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chromium atom through its π system and a monohapto ligand to the other chromium atom through a P → Cr dative bond. Two terminal CO groups in 5T-2 are bonded to one chromium atom whereas three terminal CO groups are bonded to the other chromium atom. The long predicted Cr⋯Cr distance of 3.671 Å (BP86) or 3.679 Å (M06-L) suggests a lack of a direct bond between the two chromium atoms. The chromium atom in 5T-2 bonded to three terminal CO groups has the favored 18-electron configuration corresponding to its small spin density of 0.13. However, the chromium atom 5T-1 bonded to only two terminal CO groups has only a 16-electron configuration corresponding to a spin density of 1.77 and the two unpaired electrons of the triplet spin state. The singlet structure 5S-1, lying 7.2 kcal/mol (BP86) or 6.6 kcal/ mol (M06-L) in energy above 5T-1, has three terminal CO groups bonded to one chromium atom and the other two CO groups bridging the central Cr2 unit (Fig. 4 and Table 3). One η4-Me2C2P2 ring is bonded to the other chromium atom as a terminal tetrahapto ligand (the lower ring in Fig. 4). The other η1, η4-Me2C2P2 ring is bonded to the chromium atom bearing the three terminal carbonyl groups through a P → Cr dative bond and to the other chromium atom as a tetrahapto ligand (the upper ring in Fig. 4). The predicted Cr]Cr distance of 2.498 Å (BP86) or 2.486 Å (M06-L), suggests a formal double bond thereby giving each chromium atom the favored 18-electron configuration. The (Me2C2P2)2Cr2(CO)5 structure 5S-2, lying 8.2 kcal/mol (BP86) or 9.9 kcal/mol (M06-L) in energy above 5T-1, has two terminal CO groups bonded to each chromium atom and the fifth CO group bridging the central Cr2 unit (Fig. 4 and Table 3). A terminal η4-Me2C2P2 ring is bonded to one chromium atom, whereas the other η1, η4-Me2C2P2 ring bridges the central Cr2 unit as a tetrahapto ligand to one chromium atom and through a P → Cr dative bond to the other chromium atom. The Cr]Cr distance of 5S-2 of 2.472 Å (BP86) or 2.453 Å (M06-L) is close to that in 5S-2 and suggests the formal double bond required to give each chromium atom the favored 18-electron configuration. The (Me2C2P2)2Cr2(CO)5 structure 5S-3, lying 15.1 kcal/mol (BP86) or 19.8 kcal/mol (M06-L) in energy above 5T-1, has a terminal η4Me2C2P2 ring bonded to each chromium atom (Fig. 4 and Table 3). Two terminal CO groups in 5S-3 are bonded to one chromium atom and one terminal CO group is bonded to the other chromium atom leaving the
Table 2 Total energies (E, in hartree) and the Cr-Cr distances (Å) for the structure of (Me2C2P2)2Cr2(CO)6. 6S-1 (C2v, 1A)
E Cr-Cr
BP86
M06-L
−4447.269484 4.053
−4447.593040 4.031
considered as a bidentate sandwich (η4-Me2C2P2)2Cr(CO)2 ligand chelating to a Cr(CO)4 unit through two P → Cr dative bonds (Fig. 2). In 6S1 each bridging η1, η4-Me2C2P2 ring is a six-electron donor to the central Cr2 unit. The long Cr⋯Cr distance of 4.053 Å (BP86) or 4.031 Å (M06-L) suggests no direct bond between the two chromium atoms. This gives each chromium atom in 6S-1 the favored 18-electron configuration. The infrared spectrum of 6S-1 is predicted to exhibit six ν(CO) frequencies ranging from 1923 to 2004, consistent with six terminal CO groups (Table 1). 3.1.2. (Me2C2P2)2Cr2(CO)5 Five (Me2C2P2)2Cr2(CO)5 low-energy isomers (two triplets and three singlets) were found (Fig. 4 and Table 3). The lowest energy (Me2C2P2)2Cr2(CO)5 structure 5T-1 has two bridging η1, η4-Me2C2P2 rings and five terminal CO groups. Structure 5T-1 can be derived from the (Me2C2P2)2Cr2(CO)6 structure by removing one CO group from the chromium atom sandwiched between the two P2C2 rings. The predicted Cr–Cr distance of 4.030 Å (BP86) or 4.008 Å (M06-L) suggests no direct bond between the two chromium atoms. This gives the chromium atom sandwiched between the two P2C2 rings in 5T-1 a 16-electron configuration whereas the other chromium atom has an 18-electron configuration. This is consistent with the triplet spin state and the spin density of 1.95 on the sandwiched chromium atom and only 0.03 on the other chromium atom. The (Me2C2P2)2Cr2(CO)5 structure 5T-2 lies 9.0 kcal/mol (BP86) or 11.9 kcal/mol (M06-L) in energy above 5T-1 with two bridging η1, η4Me2C2P2 rings and five terminal CO groups (Fig. 4 and Table 3). Each Me2C2P2 ligand in 5T-2 functions as a tetrahapto ligand to one
5T-1 (C2v)
5S-1 (C1)
5T-2 (Cs)
5S-2 (C1)
5S-3 (C1)
Fig. 4. The low-energy (Me2C2P2)2Cr2(CO)5 structures. Distances are given in Ǻ. 3
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Table 3 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), Cr-Cr distances (Å), and spin contamination values ( S 2 ) for the structures of (Me2C2P2)2Cr2(CO)5.
BP86 E ΔE Cr-Cr
S2
M06-L E ΔE Cr-Cr
S2
5T-1 (C2v, 3B1)
5T-2 (Cs, 3A)
5S-1 (C1, 1A)
5S-2 (C1, 1A)
5S-3 (C1, 1A)
−4333.860776 0.0 4.030 2.05
−4333.846387 9.0 3.671 2.03
−4333.849259 7.2 2.498 0
−4333.847687 8.2 2.472 0
−4333.836674 15.1 2.337 0
−4333.202745 0.0 4.008 2.13
−4333.183755 11.9 3.679 2.09
−4333.192188 6.6 2.486 0
−4333.186959 9.9 2.453 0
−4333.171260 19.8 2.335 0
remaining two CO groups as bridges across the central Cr2 unit. However, one of these bridging CO groups is actually only very weakly semibridging with a long Cr–C distance of ∼2.8 Å so it is almost like a terminal CO group. The predicted Cr^Cr distance in 5S-3 of 2.337 Å (BP86) or 2.335 Å (M06-L) is more than 0.1 Å shorter than the Cr]Cr distances in 5S-1 and 5S-2 thereby suggesting the formal triple bond required to give each chromium atom the favored 18-electron configuration. Structure 5S-3 resembles that of the experimentally known closely related Cp2V2(CO)5 [32–34] having a similar metal electronic configuration. The theoretical harmonic ν(CO) frequencies of these six (C2P2Me2)2Cr2(CO)5 structures are listed in Table 1. The ν(CO) vibrational frequencies for the bridging and semibridging CO groups ranging from 1867 to 1905 cm−1 in 5S-1, 5S-2, and 5S-3 are distinctly lower than the ν(CO) frequencies of the terminal CO groups ranging from 1915 to 2004 cm−1 in accord with expectation. However, the separation between the ν(CO) frequencies of the weakly semibridging CO groups of 1883 and 1905 cm−1 and the lowest terminal ν(CO) frequency of 1923 cm−1 in 5S-3 is relatively small.
the other chromium atom a 16-electron configuration. This interpretation is supported by spin densities of 2.43 and −0.27 on the chromium atoms with 16 and 18-electron configurations respectively. The lowest energy singlet (Me2C2P2)2Cr2(CO)4 structure 4S-1, lying 3.0 kcal/mol (BP86) or 5.3 kcal/mol (M06-L) in energy above 4T-1, has two η1, η4-Me2C2P2 bridging rings, three terminal CO groups, and one weakly semibridging CO group with a long Cr–C distance of 2.520 Å (BP86) or 2.439 (M06-L) (Fig. 5 and Table 4). Interpretation of the predicted Cr]Cr distance of 2.405 Å (BP86) or 2.391 Å (M06-L) in 4S-1 as a formal double bond gives each atom the favored 18-electron configuration. Structure 4S-1 can be dissected into a chelating diphosphine (η4-Me2C2P2)2Cr(CO) bonding as a bidentate ligand to a Cr(CO)3 moiety through two P → Cr dative bonds. The singlet (Me2C2P2)2Cr2(CO)4 structures 4S-2, 4S-3, and 4S-4, lying 8.9, 9.0, and 10.8 kcal/mol (BP86) or 14.9, 16.5, and 18.7 kcal/ mol (M06-L), respectively, in energy above 4T-1, are similar to each other. They each have two terminal η4-Me2C2P2 rings, two terminal CO groups, and two four–electron-donor bridging η2-µ-CO groups with short Cr–O distances of ∼2.35 Å (Fig. 5 and Table 4). These three structures differ in the orientation of the P2C2 rings. Their Cr]Cr distances, ranging from 2.569 to 2.608 Å, suggest formal double bonds, thereby giving each chromium atom the favored 18-electron configuration. The predicted harmonic ν(CO) frequencies of these seven (Me2C2P2)2Cr2(CO)4 structures are listed in Table 1. The ν(CO) vibrational frequencies for the two-electron donor bridging and semibridging CO groups in 4S-1 and 4T-3 range from 1866 to 1948 cm−1, which generally is significantly lower than the ν(CO) frequencies of the terminal CO groups in the same species. However, in 4T-3 the ν(CO) frequency of the most weakly semibridging CO at 1948 cm−1 is only 20 cm−1 below the terminal CO frequency. The four-electron donor bridging η2-µ-CO groups in 4S-2, 4S-3, and 4S-4 exhibit significantly lower ν(CO) frequencies in the narrow range from 1723 to 1744 cm−1 in accord with expectation.
3.1.3. (Me2C2P2)2Cr2(CO)4 Seven low-energy (Me2C2P2)2Cr2(CO)4 structures were found (three triplets and four singlets) (Fig. 5 and Table 4). The lowest energy (Me2C2P2)2Cr2(CO)4 structure 4T-1 is a triplet structure with a terminal η4-Me2C2P2 ring and two terminal CO groups on each chromium atom. The predicted Cr]Cr distance of 2.855 Å (BP86) or 2.669 Å (M06-L), suggests a formal double bond to give each Cr atom a 16-electron configuration. Considering the Cr]Cr double bond as a σ + 2/2 double bond with two orthogonal π bonds similar to the Fe]Fe double bond in the known triplet state organometallics (η5-R5C5)2Fe2(µ-CO)3 (R = H, Me) [35–37] can account for the triplet spin state. This interpretation is supported by equal spin densities of 0.95 on each chromium atom. The (Me2C2P2)2Cr2(CO)4 structure 4T-2, lying 6.6 kcal/mol (BP86) or 4.9 kcal/mol (M06-L) in energy above 4T-1, has one η3, η3-Me2C2P2 bridging ring and one η1, η4-Me2C2P2 bridging ring (Fig. 3 and Table 4). The terminal CO groups in 4T-2 are distributed unevenly with three terminal CO groups on one chromium atom, and only one terminal CO group on the other chromium atom. Interpreting the Cr]Cr distance of 2.958 Å (BP86) or 2.872 Å (M06-L) in 4T-2 as a formal double bond gives the chromium atom bearing three CO groups the favored 18electron configuration but the other chromium atom only a 16-electron configuration. The unsymmetrical distribution of spin densities with only 0.07 on one chromium atom but 1.91 on the other chromium atom suggest that the two unpaired electrons of the triplet spin state of 4T-2 reside on the chromium atom with a 16-electron configuration. The (Me2C2P2)2Cr2(CO)4 structure 4T-3, lying 10.8 kcal/mol (BP86) or 7.4 kcal/mol (M06-L) in energy above 4T-1, has one terminal CO group, three bridging CO groups, and two terminal η4-Me2C2P2 rings. (Fig. 5 and Table 4). The predicted Cr^Cr distance of 2.401 Å (BP86) or 2.451 Å (M06-L), suggests the formal triple bond required to give the chromium atom bearing the CO group an 18-electron configuration and
3.1.4. (Me2C2P2)2Cr2(CO)3 Five low-energy (Me2C2P2)2Cr2(CO)3 structures were found (one singlet and four triplets) (Fig. 6 and Table 5). In each of the four triplet structures both P2C2 rings are terminal ligands bonded to a single chromium atom. The lowest energy (Me2C2P2)2Cr2(CO)3 structure 3T-1 is one of these triplet structures with a terminal η4-Me2C2P2 ring and a terminal CO group bonded to each chromium atom. The third CO group in 3T-1 is primarily bonded to one of the chromium atoms with only a weak interaction with the other chromium atom through a long Cr–C bond of ∼2.4 Å. Interpreting the predicted Cr]Cr distance of 2.483 Å (BP86) or 2.526 Å (M06-L) in 3T-1 as a formal double bond and ignoring the weak Cr–C bond of length ∼2.4 Å in the semibridging CO group gives the chromium bearing two CO groups (including the weakly semibridging CO group) a 16-electron configuration and the other chromium atom only a 14-electron configuration. The spin density is unevenly distributed with 2.40 on the chromium atom with a 4
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4T-1 (C2)
4T-3(C1)
4T-2 (Cs)
4S-1 (Cs)
4S-2 (Ci)
4S-3 (C2)
4S-4 (C1)
Fig. 5. The lowest energyMe2C2P2)2Cr2(CO)4 structures. Distances are given in Ǻ.
single terminal CO group and −0.27 on the chromium atom bearing the two remaining CO groups. Structure 3T-2 has essentially the same energy as 3T-1, and it has similar structure to 3T-1 except for the arrangement of the CO groups. The (Me2C2P2)2Cr2(CO)3 structures 3T-3 and 3T-4 are similar to each other, with two terminal η4-Me2C2P2 rings and three semibridging CO groups (Fig. 6 and Table 5). These two structures differ in the arrangement of the semibridging CO groups. Thus in 3T-3, the shorter Cr–C bonds to the three semibridging CO groups come from the same chromium atom, while in 3T-4, two of the short Cr–C bonds come from
one chromium atom and the third Cr–C bond comes from the other chromium atom. Structure 3T-3 lies 9.4 kcal/mol (BP86) or 10.6 kcal/ mol (M06-L) above 3T-1. However, structure 3T-4 is a significantly higher energy structure lying 23.5 kcal/mol (BP86) or 27.0 kcal/mol (M06-L) in energy above 3T-1. Interpreting the Cr]Cr distances in 3T3 and 3T-4 ranging from 2.4 to 2.5 Å as formal double bonds gives each chromium atom a 15-electron configuration. However, the spin densities are unevenly distributed between the chromium atoms. The only low-energy singlet (Me2C2P2)2Cr2(CO)3 structure, namely 3S-1 lying 10.4 kcal/mol (BP86) or 20.0 kcal/mol (M06-L) in energy
Table 4 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), Cr-Cr distances (Å), and spin contamination values ( S 2 ) for the structures of (Me2C2P2)2Cr2(CO)4.
BP86 −(E + 4220) ΔE Cr-Cr
S2
M06-L −(E + 4219) ΔE Cr-Cr
S2
4T-1 (C2, 3B)
4T-2 (Cs, 3A)
4T-3 (C1, 3A)
4S-1 (Cs, 1A)
4S-2 (Ci, 1Ag)
4S-3 (C2, 1A)
4S-4 (C1, 1A)
0.467324 0.0 2.855 2.02
0.456869 6.6 2.958 2.05
0.450205 10.8 2.401 2.14
0.462627 3.0 2.405 0
0.453114 8.9 2.601 0
0.453078 9.0 2.608 0
0.450168 10.8 2.589 0
0.822187 0.0 2.669 2.07
0.814368 4.9 2.872 2.13
0.810350 7.4 2.451 2.45
0.813767 5.3 2.391 0
0.798370 14.9 2.578 0
0.795862 16.5 2.583 0
0.792432 18.7 2.569 0
5
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3T-1 (C1)
3T-2 (C1)
3T-4 (C1)
3T-3 (C1)
3S-1 (C1)
Fig. 6. The lowest energy (Me2C2P2)2Cr2(CO)3 structures. Distances are given in Ǻ.
above 3T-1, has two bridging η1, η4-Me2C2P2 rings and three terminal CO groups (Fig. 4 and Table 5). Structure 3S-1 can be derived from 4S to 1 by removal of a CO group from the chromium atom not sandwiched between the two C2P2 rings. Thus 3S-1 can be dissected into a chelating diphosphine (η4-Me2C2P2)2Cr(CO) bonding as a bidentate ligand to a Cr(CO)2 moiety through two P → Cr dative bonds. The predicted Cr^Cr distance of 2.320 Å (BP86) or 2.322 Å (M06-L) in 3S-1 is ∼0.1 Å shorter than that in 4S-1 and thus can be interpreted as the formal triple bond required to give each chromium atom the favored 18-electron configuration. This assumes a formal positive charge on the chromium atom sandwiched between the C2P2 rings and a formal negative charge on the chromium atom bearing the two terminal CO groups.
all sufficiently endothermic, from 38 to 51 kcal/mol (BP86) or from 28 to 46 kcal/mol (M06-L) thereby suggesting that the lowest energy (Me2C2P2)2Cr2(CO)n (n = 6, 5, 4) structures are viable towards CO loss. For comparison, the experimental dissociation energies for the wellknown simple metal carbonyls Ni(CO)4, Fe(CO)5, and Cr(CO)6 are 27, 41, and 37 kcal/mol, respectively. The monotonic increase of the CO dissociation energies of the (Me2C2P2)2Cr2(CO)n derivatives with increasing number of CO groups is consistent with the endothermic nature of the disproportionation processes 2(Me2C2P2)2Cr2(CO)n → (Me2C2P2)2Cr2(CO)n+1 + (Me2C2P2)2Cr2(CO)n−1 listed in Table 7. However, these endothermic disproportionation energies are rather small suggesting that conditions might be found where such disproportionation reactions could occur. Also of interest is the dissociation of the (Me2C2P2)2Cr2(CO)n derivatives into mononuclear (Me2C2P2)Cr(CO)m fragments. In order to obtain such energetic data, the structures of the mononuclear (Me2C2P2)Cr(CO)m were optimized by the same DFT methods as used to study the binuclear derivatives (Fig. 7 and Table S24). Using this information the dissociation energies of the binuclear metal carbonyl
3.2. Thermochemistry Table 6 shows the dissociation energies for the reactions (Me2C2P2)2Cr2(CO)n → (Me2C2P2)2Cr2(CO)n-1 + CO (n = 6, 5, 4) considering the lowest energy structures. The CO dissociation energies are
Table 5 Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), Cr-Cr distances (Å), and spin contamination values ( S 2 ) for the structures of (Me2C2P2)2Cr2(CO)3.
BP86 E ΔE Cr-Cr
S2
M06-L E ΔE Cr-Cr
S2
3T-1 (C1, 3A)
3T-2 (C1, 3A)
3T-3 (C1, 3A)
3T-4 (C1, 3A)
3S-1 (C1, 1A)
−4107.079311 0.0 2.483 2.44
−4107.076673 1.7 2.404 2.63
−4107.064373 9.4 2.523 2.66
−4107.041851 23.5 2.368 2.16
−4107.062799 10.4 2.320 0
−4106.460363 0.0 2.526 2.97
−4106.459972 0.2 2.493 3.15
−4106.443413 10.6 2.575 3.07
−4106.417408 27.0 2.433 2.51
−4106.428433 20.0 2.322 0
6
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respectively. The other chromium atom in 6S-1, which is sandwiched between two η4-Me2C2P2 rings and bonded to two terminal CO groups, has an atomic charge of −0.84. Similarly, the chromium atoms in 5S-1, 4S-1, and 3S-1, also sandwiched between two η4-Me2C2P2 rings and fewer CO groups, have less negative atomic charges from −0.54 to −0.58. All of the chromium atoms in 4S-2, 4S-3, and 4S-4 are coordinated by two terminal η4-Me2C2P2 rings, two terminal CO groups, and two bridging CO groups and have atomic charges in the narrow range from −0.41 to −0.47. The two chromium atoms in 5S-3, which have similar ligands to those in 4S-2, 4S-3, and 4S-4 except for an additional semibridging CO group, have slightly more negative charges, i.e. −0.77 and −0.59. Compared with 5S-3, the chromium atoms in 5S-2 have one additional coordinated phosphorus atom leading to negative charges of −0.76 and −0.73, respectively, with that on one of the chromium atoms being more negative than that in 5S-3. The WBI values for the Cr-Cr bonds and the formal bond orders based on distance, spin state, and electron counting in the (Me2C2P2)2Cr2(CO)n derivatives are shown in Table 9. The number and nature of groups bridging the Cr-Cr bond are also listed in Table 9 since these can affect the WBI values listed. No examples of singlet (Me2C2P2)2Cr2(CO) n species with formal Cr–Cr single bonds are found in this work. The Cr]Cr WBIs for double bonds are found in a wide range from 0.40 to 0.72. The higher WBI values for the Cr]Cr double bonds ranging from 0.61 to 0.65 are found in the structures 4S-2, 4S-3, and 4S-4 which have four-electron donor η2-µ-CO groups bridging the Cr]Cr double bond. The apparently smaller predicted WBI value of 0.67 for the Cr^Cr triple bond in 5S-3 may be a consequence of the presence of only two semibridging CO groups. The WBI for the Cr^Cr triple bond in 3S-1 of 0.95 is clearly above the wide range observed for the WBIs of any of the Cr]Cr double bonds in the (Me2C2P2)2Cr2(CO)n species. The WBI values of these Cr-Cr multiple bonds in the (Me2C2P2)2Cr2(CO)n derivative are consistent with the general observation [39] that WBI values for single bonds between d-block transition metals typically range from 0.2 to 0.3 and thus are significantly lower than those reported here for Cr-Cr double and triple bonds. The very low WBI of 0.04 for the Cr⋯Cr interaction in the (Me2C2P2)2Cr2(CO)6 structure 6S-1 indicates the lack of a chromium–chromium direct bond consistent with the long Cr⋯Cr distance of 4.05 Å.
Table 6 Dissociation energies (kcal/mol) for the successive removal of carbonyl groups from the (Me2C2P2)2Cr2(CO)n derivatives.
(Me2C2P2)2Cr2(CO)6 (Me2C2P2)2Cr2(CO)6 (Me2C2P2)2Cr2(CO)5 (Me2C2P2)2Cr2(CO)4
(6S-1) → (Me2C2P2)2Cr2(CO)5 (5T-1) + CO (6S-1) → (Me2C2P2)2Cr2(CO)5 (5S-1) + CO (5T-1) → (Me2C2P2)2Cr2(CO)4 (4T-1) + CO (4T-1) → (Me2C2P2)2Cr2(CO)3 (3T-1) + CO
BP86
M06-L
51.1 58.3 41.6 38.1
46.3 52.9 40.2 28.5
Table 7 Disproportionation energies (kcal/mol) for the 2(Me2C2P2)2Cr2(CO)n → (Me2C2P2)2Cr2(CO)n+1 + (Me2C2P2)2Cr2(CO)n−1 reactions.
2(Me2C2P2)2Cr2(CO)5 (5T-1)→(Me2C2P2)2Cr2(CO)6 (6S-1) + (Me2C2P2)2Cr2(CO)4 (4T-1) 2(Me2C2P2)2Cr2(CO)4 (4T-1)→(Me2C2P2)2Cr2(CO)5 (5T-1) + (Me2C2P2)2Cr2(CO)3 (3T-1)
BP86
M06-L
9.6
6.1
3.4
11.8
derivatives into mononuclear fragments were predicted to be substantial, i.e., larger than 75 kcal/mol (Table 8). These data show that the lowest energy (Me2C2P2)2Cr2(CO)n structures are viable with respect to dissociation into mononuclear fragments. Also the dissociation energies of (Me2C2P2)2Cr2(CO)n into mononuclear fragments increase monotonically with decreasing n. 3.3. NBO analysis of the chromium-chromium bonding Table 9 lists the natural atomic charges for the chromium atoms and the Wiberg Bond Indices (WBIs) for the Cr-Cr bonds from NBO analysis [38] using the BP86 method. The Cr-Cr distances, the formal bond orders suggested by electron counting, and the types of 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. All of the natural charges on both 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 all of this thus acquired negative charge. The values of the chromium atomic natural charges depend on the numbers of coordinated CO groups and coordinated phosphorus lone pairs of the Me2C2P2 rings. In 6S-1, the chromium atom coordinated by two phosphorus lone pairs and four CO groups has the most negative atomic charge (−1.37). In 5S-1, the chromium atom coordinated by two phosphorus lone pairs, three terminal CO groups, and two semibridging CO groups has a slightly less negative atomic charge (−1.00). Similarly, the chromium atoms coordinated by two phosphorus lone pairs but even fewer CO groups in 4S-1 and 3S-1 have even less negative charges, i.e., −0.81 and −0.32,
(Me2C2P2)Cr(CO)
4. Conclusion The only low-energy structure for the hexacarbonyl (Me2C2P2)2Cr2(CO)6 consists of a bidentate (η4-Me2C2P2)2Cr(CO)2 sandwich diphosphine ligand bonded to a Cr(CO)4 unit with a long ∼4.0 Å Cr⋯Cr distance indicating lack of a direct metal–metal bond (6S-1 in Fig. 3). This unusually favorable singlet structure has an 18electron configuration for both chromium atoms and octahedral coordination of a P2Cr(CO)4 unit with four terminal CO groups and two P → Cr dative bonds. The lowest energy structure for the pentacarbonyl (Me2C2P2)2Cr2(CO)5 is a triplet structure derived from this singlet hexacarbonyl structure by loss of a CO group from the sandwiched chromium atom (5T-1 in Fig. 4). The bridging η1,4-Me2C2P2 ligands
(Me2C2P2)Cr(CO)2
(Me2C2P2)Cr(CO)3
Fig. 7. The optimized geometries for the mononuclear structures (Me2C2P2)2Cr2(CO)m (m = 3, 2, 1), which are the dissociation products in Table 8. 7
Inorganica Chimica Acta 498 (2019) 119123
X. Wan, et al.
Table 8 Energies for the dissociation of the binuclear (Me2C2P2)2Cr2(CO)n (n = 6, 5, 4, 3) derivatives into mononuclear fragments (kcal/ mol).
(Me2C2P2)2Cr2(CO)4(6S-1) → 2(Me2C2P2)Cr(CO)3 (Me2C2P2)2Cr2(CO)5(5T-1) → (Me2C2P2)Cr(CO)3 + (Me2C2P2)Cr(CO)2 (Me2C2P2)2Cr2(CO)4(4T-1) → 2(Me2C2P2)Cr(CO)2 (Me2C2P2)2Cr2(CO)3(3T-1) → (Me2C2P2)Cr(CO)2 + (Me2C2P2)Cr(CO)
Table 9 Atomic charges and Wiberg bond indices (WBIs) (MeC2P2)2Cr2(CO)n structures by the BP86 method.
for
the
singlet
Natural Charge on Cr/Cr
WBI
Cr-Cr (Å)
Bridges
Formal bond order
6S-1 5S-1
−0.84/−1.37 −0.58/−1.00
0.04 0.40
4.053 2.498
0 2
5S-2
−0.76/−0.73
0.55
2.472
5S-3 4S-1
−0.77/−0.59 −0.54/−0.81
0.67 0.72
2.337 2.405
4S-2 4S-3 4S-4 3S-1
−0.44/−0.46 −0.43/−0.43 −0.47/−0.41 −0.56/−0.32
0.61 0.65 0.62 0.95
2.602 2.608 2.589 2.320
2 Me2C2P2 2 Me2C2P2, 2 semi-CO Me2C2P2, semiCO 2 semi-CO 2 Me2C2P2, semi-CO 2 η2-µ-CO 2 η2-µ-CO 2 η2-µ-CO 2 Me2C2P2
[2] [3] [4] [5] [6] [7] [8] [9]
2 3 2
[10]
2 2 2 3
[11] [12] [13] [14] [15] [16]
found in these two structures, using the four π-electrons of the P2C2 ring to bond to one chromium atom and a phosphorus lone pair to bond to the other chromium atom but not sandwiching a chromium atom, are features of the next lowest energy triplet and singlet (Me2C2P2)2Cr2(CO)5 structures. In contrast to the hexa- and pentacarbonyls the lowest energy structures of the more highly unsaturated tetra- and tricarbonyls (Me2C2P2)2Cr2(CO)n (n = 4, 3) have terminal tetrahapto η4-Me2C2P2 rings not using their phosphorus lone pairs and chromium–chromium multiple bonds.
[17] [18] [19] [20] [21] [22] [23] [24]
Acknowledgment
[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38]
We are indebted to the New Century Excellent Talents in University (Grant No, NCET-10-0949) and the Scientific Research Fund of the Key Laboratory of the Education Department of Sichuan Province (Grant No. 10ZX012) in China 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.2019.119123. References
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76.0 75.4 84.3 97.9
86.8 88.0 95.2 113.9
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