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Article

Trinuclear and Tetranuclear Ruthenium Carbonyl Nitrosyls: Oxidation of a Carbonyl Ligand by an Adjacent Nitrosyl Ligand

by
Shengchun Chen
1,*,
Xuejun Feng
1,2,
Yaoming Xie
2,
R. Bruce King
2,* and
Henry F. Schaefer
2
1
School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China
2
Department of Chemistry and Center for Computational Quantum Chemistry, University of Georgia, Athens, GA 30602, USA
*
Authors to whom correspondence should be addressed.
Molecules 2024, 29(17), 4165; https://doi.org/10.3390/molecules29174165 (registering DOI)
Submission received: 23 July 2024 / Revised: 20 August 2024 / Accepted: 28 August 2024 / Published: 3 September 2024
(This article belongs to the Special Issue Multiconfigurational and DFT Methods Applied to Chemical Systems—2nd Edition)
Browse Figures
Versions Notes

Abstract

:
Trinuclear and tetranuclear ruthenium carbonyls of the types Ru3(CO)n(NO)2, Ru3(N)(CO)n(NO), Ru3(N)2(CO)n, Ru3(N)(CO)n(NCO), Ru3(CO)n(NCO)(NO), Ru4(N)(CO)n(NO), Ru4(N)(CO)n(NCO), and Ru4(N)2(CO)n related to species observed experimentally in the chemistry of Ru3(CO)10(µ-NO)2 have been investigated using density functional theory. In all cases, the experimentally observed structures have been found to be low-energy structures. The low-energy trinuclear structures typically have a central strongly bent Ru–Ru–Ru chain with terminal CO groups and bridging nitrosyl, isocyanate, and/or nitride ligands across the end of the chain. The low-energy tetranuclear structures typically have a central Ru4N unit with terminal CO groups and a non-bonded pair of ruthenium atoms bridged by a nitrosyl or isocyanate group.

Graphical Abstract

1. Introduction

A noteworthy feature of the chemistry of ruthenium is its propensity to form a variety of nitrosyl derivatives. The experimental approach to the subset of such ruthenium nitrosyl derivatives also containing carbonyl ligands starts with the trinuclear derivative Ru3(CO)10(µ-NO)2, itself obtained from the reaction of Ru3(CO)12 with nitric oxide in boiling benzene (Figure 1) [1]. The replacement of two terminal carbonyl groups in Ru3(CO)12 with two bridging nitrosyl groups in Ru3(CO)10(µ-NO)2, donating two “extra” electrons to the central Ru3 triangle, lengthens one of the three Ru–Ru bonds in the original equilateral triangle Ru3(CO)12 structure to a non-bonding distance of 3.18 Å. As a result, in the Ru3(CO)10(µ-NO)2 structure, the two nitrosyl groups, as bridges across the non-bonding Ru⋯Ru distance, contribute to holding together the isosceles Ru3 triangle. This weaker bonding in the Ru3 triangle in Ru3(CO)10(µ-NO)2 relative to that in the Ru3 triangle in Ru3(CO)12 makes the former Ru3 triangle more susceptible to rupture and rearrangement. Thus, the decomposition of Ru3(CO)10(µ-NO)2 at 110 °C in an atmosphere of CO leads to the disruption of the Ru3 triangle with rearrangement to the tetranuclear derivatives Ru4(µ4-N)(CO)12(µ-NO) and Ru4(µ4-N)(CO)12(µ-NCO), as well as the trinuclear derivative Ru3(CO)10(µ-NO)(µ-NCO) (Figure 2) [2].
The presence of nitride ligands bridging all four ruthenium atoms in the tetranuclear ruthenium carbonyl nitrosyl derivatives formed in the decomposition of Ru3(CO)10(µ-NO)2 (Figure 2) suggests that the reduction of an NO group by an adjacent CO group is occurring during the decomposition process. Our density functional theory studies of possible internal such redox processes in trinuclear Ru3(CO)10(µ-NO)2 leading to an N2O complex Ru3(CO)9(µ3-N2O) and finally a dinitrogen complex Ru3(CO)8(µ3-N2) were presented in a previous short communication [3]. Here, we present similar density functional theory studies on a wider range of trinuclear and tetranuclear ruthenium carbonyl structures also containing nitrosyl ligands including the reduction of nitrosyl ligands by adjacent CO groups to give, N2O and nitride ligands. These include examples of structures with five-electron donor bridging η2-µ3-NO ligands bonded to ruthenium atoms through both their nitrogen and oxygen atoms as well as structures containing the usual three-electron donor NO groups.

2. Results and Discussion

2.1. Trinuclear Ru3(NO)2(CO)n Derivatives

The optimized geometries are depicted in Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, Figure 13 and Figure 14 with all bond lengths in Å. All structures are in singlet state. The structures are designated by the labels x-A-y-z or x-A-y, where x is the number of ruthenium atoms, A is the nitrogen-containing group, y is the number of carbonyl groups, and z orders the isomeric structures (if any) by their relative energies. For example, the singlet global minimum of Ru3(CO)10(NO)2 is designated as 3-(NO)2-10-1.

2.1.1. Ru3(CO)10(NO)2

Two low-energy Ru3(CO)10(NO)2 singlet structures were found (Figure 3). The lowest-energy Ru3(CO)10(NO)2 structure, 3-(NO)2-10-1, is the experimental C2v structure with two bridging NO groups leading to coplanar Ru2NO units. The dihedral angles for the bending of the two Ru2N planes in the central Ru2(µ-NO)2 units in 3-(NO)2-10-1 are 156.6° (mPW1PW91) or 157.4° (BP86). The ν(NO) frequencies in 3-(NO)2-10-1 are 1557 and 1572 cm−1 (BP86) (Table S1 in Supporting Information) as compared with the experimental values [1] of 1500 and 1517 cm−1 and consistent with their bridging positions. The Ru–Ru distances of 3.178 Å (mPW1PW91) or 3.226 Å (BP86) indicate no bond between the two ruthenium atoms in the Ru2(µ-NO)2 units, consistent with the low WBI value of 0.12 (Table S42 in the Supporting Information). The other Ru–Ru distances of 2.888 Å (mPW1PW91) or 2.953 Å (BP86), with WBI values of ~0.3 in the typical range for formal single bonds between d-block metals, correspond to the formal single bond required to give each ruthenium atom the favored 18-electron configuration, since the NO ligands each donate three electrons to the central Ru2 unit.
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Figure 3. Two optimized Ru3(CO)10(NO)2 structures with bond distances in Å. The upper and lower distances are from the mPW1PW91 and BP86 methods, respectively.
Figure 3. Two optimized Ru3(CO)10(NO)2 structures with bond distances in Å. The upper and lower distances are from the mPW1PW91 and BP86 methods, respectively.
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The higher-energy singlet Ru3(CO)10(NO)2 structure 3-(NO)2-10-2, lying 18.4 kcal/mol (mPW1PW91) or 19.1 kcal/mol (BP86) of energy above 3-(NO)2-10-1, has one of the CO groups of its Ru(CO)4 unit bent over towards one of the bridging NO groups to form a C–O bond of length 1.392 Å (mPW1PW91) or 1.406 Å (BP86) (Figure 3). This leads to a bridging NOCO group well situated for CO2 elimination.3 The Ru2NO units remain coplanar in 3-(NO)2-10-2. In addition, the entire Ru2(µ-NO)2 unit in 3-(NO)2-10-2 is nearly coplanar as indicated by their dihedral angles of 177.3° (mPW1PW91) or 178.4° (BP86), close to the 180° for ideal planarity. The Ru⋯Ru distances of 3.306 Å (mPW1PW91) or 3.356 Å (BP86), with a low WBI value of 0.08, indicate no direct bond between the two ruthenium atoms in the Ru2(µ-NO)2 unit. The other Ru–Ru distances of 2.806 Å (mPW1PW91) or 2.853 Å (BP86), with WBI values of ~0.3, correspond to formal single bonds, thereby giving each ruthenium atom the favored 18-electron configuration. The low ν(NO) frequency of 974 cm−1 in the NOCO group of 3-(NO)2-10-2 is consistent with a single N–O bond rather than the multiple bonds in separate NO ligands. The other ν(NO) frequency in 3-(NO)2-10-2 of 1581 cm−1 is close to the ν(NO) frequencies of 3-(NO)2-10-1 and in a typical region for bridging ν(NO) groups.

2.1.2. Ru3(CO)n(NO)2 (n = 9, 8, 7)

The single low-energy Ru3(CO)9(NO)2 singlet structure 3-(NO)2-9 is a Cs structure with two bridging NO groups, leading to essentially coplanar Ru2(NO)2 units with dihedral angles for the bending of the two Ru2N2 planes of 175.8° (mPW1PW91) or 176.6° (BP86), close to the 180° indicative of coplanarity (Figure 4). One of the bridging NO groups in 3-(NO)2-9 is a five-electron donor η2-µ3-NO group bonding to two ruthenium atoms through its nitrogen atom with Ru–N distances of 2.036 Å (mPW1PW91) or 2.071 Å (BP86) and to the third ruthenium atom through its oxygen atom with a Ru–O distance of 2.216Å (mPW1PW91) or 2.261 Å (BP86). This η2-µ3-NO group has a relatively long N–O distance of 1.294 Å (mPW1PW91) or 1.310 Å (BP86), consistent with its very low ν(NO) frequency of 1200 cm−1. For comparison, the other NO group in 3-(NO)2-9 is a typical three-electron donor bridging µ-NO group with a more typical nitrosyl N–O distance of 1.204 Å (mPW1PW91) or 1.222 Å (BP86) and a more typical bridging ν(NO) frequency of 1571 cm−1. The Ru⋯Ru distance of 3.296 Å (mPW1PW91) or 3.353 Å (BP86) in 3-(NO)2-9 with a low WBI value of 0.08 similar to that in the Ru3(CO)10(µ-NO)2 structure 3-(NO)2-10-1 indicates no direct bond between the two ruthenium atoms in the Ru2(NO)2 unit. The other Ru–Ru distances of 2.780 Å (mPW1PW91) or 2.829 Å (BP86) with WBI values of ~0.35 correspond to formal single bonds. In 3-(NO)2-9, the combination of two rather than three Ru–Ru bonds in the Ru3 triangle, one three-electron donor µ-NO group, one five-electron donor η2-µ3-NO group, and the nine terminal CO groups give each of the ruthenium atoms the favored 18-electron configuration.
The single low-energy Ru3(CO)8(NO)2 octacarbonyl structure 3-(NO)2-8 is a singlet C2v structure with two µ3-NO groups bridging all three ruthenium atoms and two µ-CO groups, each bridging an Ru–Ru bonding edge of length 2.681 Å (mPW1PW91) or 2.716 Å (BP86) with a WBI value of 0.21 (Figure 4). The dihedral angles for the bending of the two Ru2N planes in the central Ru2(µ3-NO)2 units in 3-(NO)2-8 of 161.2° (mPW1PW91) or 160.8° (BP86) indicate significant deviations from non-planarity. The ν(NO) frequencies of 1435 and 1413 cm−1 for the two µ3-NO groups bridging all three ruthenium atoms in 3-(NO)2-8 are significantly lower than the ν(NO) frequencies of 1572 and 1557 cm−1 for the two µ-NO groups bridging Ru–Ru edges in the Ru3(CO)10(µ-NO)2 structure 3-(NO)2-10-1. The two edge-bridging µ-CO groups in 3-(NO)2-8 exhibit ν(CO) frequencies of 1907 and 1858 cm−1, which are significantly lower than the ν(CO) frequencies of the six terminal CO groups ranging from 2069 to 1983 cm−1. The Ru⋯Ru distance of 3.383 Å (mPW1PW91) or 3.490 Å (BP86) in 3-(NO)2-8 with a low WBI value of 0.07 indicates no bond between the two ruthenium atoms in the Ru2(µ3-NO)2 units. The other Ru–Ru distances of 2.681 Å (mPW1PW91) or 2.716 Å (BP86) in 3-(NO)2-8, with WBI values of 0.21, correspond to its formal single bonds. This configuration of the Ru–Ru bonds and the bonding of the CO and NO groups to the central Ru3 unit in 3-(NO)2-8 leads to the favored 18-electron configuration for the two ruthenium atoms in the Ru2(µ3-NO)2 unit, but only a 14-electron configuration for the unique third ruthenium atom. The latter ruthenium atom has a large vacancy in its coordination sphere consistent with its electronic configuration of four electrons less than the favorable 18-electron configuration.
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Figure 4. The optimized Ru3(CO)n(NO)2 (n = 9, 8, 7) structures with bond distances in Å.
Figure 4. The optimized Ru3(CO)n(NO)2 (n = 9, 8, 7) structures with bond distances in Å.
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The single low-energy Ru3(CO)7(NO)2 heptacarbonyl structure 3-(NO)2-7 is a singlet Cs structure with two bridging NO groups (Figure 4). One of the bridging NO groups in 3-(NO)2-7 is a five-electron donor η2-µ3-NO group bonding to two ruthenium atoms through its nitrogen atom with Ru–N distances of 1.950 Å (mPW1PW91) or 1.967 Å (BP86) and to the third ruthenium atom through its oxygen atom with a Ru–O distance of 2.234 Å (mPW1PW91) or 2.281 Å (BP86). This η2-µ3-NO group has a relatively long N–O distance of 1.288 Å (mPW1PW91) or 1.305 Å (BP86), consistent with its very low ν(NO) frequency of 1220 cm−1. The other NO group in 3-(NO)2-7 is a typical three-electron donor bridging µ-NO group with an N–O distance of 1.190 Å (mPW1PW91) or 1.288 Å (BP86) and a bridging ν(NO) frequency of 1627 cm−1. The dihedral angles for the bending of the two Ru2N planes in the central Ru2(µ-NO)2 units in 3-(NO)2-7 are 169.0° (mPW1PW91) or 167.2° (BP86) thereby representing a significant deviation from planarity. The Ru3(CO)7(µ-NO)(η2-µ3-NO) structure 3-(NO)2-7 can be derived from the Ru3(CO)9(µ-NO)(η2-µ3-NO) structure 3-(NO)2-9 by the removal of a terminal CO group from each of the ruthenium atoms in the Ru2(NO)2 unit with relatively little change otherwise in the structure’s geometry. In 3-(NO)2-7, the ruthenium atoms in the Ru2(NO)2 unit have only a 16-electron configuration, whereas the unique ruthenium atom retains the favorable 18-electron configuration.

2.2. Trinuclear Ru3(N)(CO)n(NO) Derivatives Arising from CO2 Loss from Ru3(CO)n(NO)2 Derivatives

The lowest-energy structure 3-NNO-10 of the decacarbonyl is actually an Ru3(µ-CO)(CO)9(µ-N2O) structure with a bent N2O ligand bridging the ends of a bent Ru–Ru–Ru chain through Ru–N bonds to its terminal nitrogen atom (Figure 5). One of the carbonyl groups in 3-NNO-10 bridges a Ru–Ru bond in the Ru3 chain, exhibiting a ν(CO) frequency of 1929 cm−1, significantly lower than any of the terminal ν(CO) frequencies. The bridging bent µ-N2O ligand has a single-bond N–N distance of 1.378 Å (mPW1PW91) or 1.407 Å (BP86), a double-bond N=O distance of 1.208 Å (mPW1PW91) or 1.226 Å (BP86), and an N–N–O angle of 116.8°(mPW1PW91) or 117.0° (BP86). This bridging µ-N2O ligand functions as a four-electron donor using two lone pairs of the nitrogen atom in the Ru–N–Ru bridge. This gives the two end ruthenium atoms of the Ru–Ru–Ru chain in 3-NNO-10 the favored 18-electron configuration, but the central ruthenium atom only retains a 16-electron configuration.
The lowest-energy Ru3(N)(CO)9(NO) structure 3-NNO-9-1 of the nonacarbonyl, like that of the decacarbonyl, is a Cs symmetry structure with a bridging N2O group of a different type than that in 3-NNO-10 (Figure 5). Each nitrogen atom in the linear bridging η2-µ3-N2O group of 3-NNO-9-1 forms Ru–N bonds with all three ruthenium atoms. This linear bridging η2-µ3-N2O group has an elongated N–N distance of 2.417 Å (mPW1PW91) or 2.428 Å (BP86) and a double-bond N=O distance of 1.202 Å (mPW1PW91) or 1.220 Å (BP86), and exhibits a ν(NO) frequency of 1579 cm−1. The total of six Ru–N bonds formed by the η2-µ3-NO group allows it to become an 8-electron donor to the Ru3 chain, thereby giving each ruthenium atom in 3-NNO-9-1 the favored 18-electron configuration.
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Figure 5. The optimized Ru3(N)(CO)9(NO) (n = 10, 9) structures with bond distances in Å.
Figure 5. The optimized Ru3(N)(CO)9(NO) (n = 10, 9) structures with bond distances in Å.
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As we discussed in Section 2.1.1, CO2 elimination may occur from 3-NNO-10-2 to form a higher-energy Ru3(N)(CO)9(NO) structure 3-NNO-9-2, which has Cs symmetry with separate N and NO ligands lying 23.3 kcal/mol (mPW1PW91) or 25.4 kcal/mol (BP86) of energy above 3-NNO-9-1 (Figure 5). The NO ligand is a five-electron donor η2-µ3-NO group bridging all three ruthenium atoms by bonding to two ruthenium atoms through its nitrogen atom with Ru–N distances of 2.316 Å (mPW1PW91) or 2.336 Å (BP86) and to the third ruthenium atom with a Ru–N distance of 2.117 Å (mPW1PW91) or 2.140 Å (BP86) and a Ru–O distance of 2.253 Å (mPW1PW91) or 2.275 Å (BP86). This η2-µ3-NO group has a relatively long N–O distance of 1.274 Å (mPW1PW91) or 1.298 Å (BP86) consistent with its very low ν(NO) frequency of 1219 cm−1. The bridging nitride ligand in 3-NNO-9-2 functions as a three-electron donor by bridging all three nitrogen atoms with two Ru–N distances of 2.015 Å (mPW1PW91) or 2.038 Å (BP86) and one Ru–N distance of 2.316 Å (mPW1PW91) or 2.330 Å (BP86). The combination of a five-electron donor η2-µ3-NO group, a three-electron donor µ3-N nitride ligand, and two Ru–Ru bonds gives each ruthenium atom in 3-NNO-9-2 the favored 18-electron configuration. Compared with the lowest-energy 3-NNO-9-1 structure, 3-NNO-9-2 is relatively unstable towards the elimination of another CO2 molecule yielding 3-N2-8, since it has a bent NNO group with its oxygen atom close to the top ruthenium atom.
The lowest-energy octacarbonyl structure Ru3(N)(CO)8(NO), namely 3-NNO-8, has separate bridging nitrosyl and nitride ligands (Figure 6). The nitrosyl group bridging two ruthenium atoms with Ru–N distances of 2.100 Å (mPW1PW91) or 2.133 Å (BP86) and a N–O distance of 1.201 Å (mPW1PW91) or 1.218 Å (BP86) exhibits a ν(NO) frequency of 1582 cm−1 in a typical region for three-electron donor bridging µ-NO groups. The nitride ligand in 3-NNO-8 bridges all three ruthenium atoms with two Ru–N distances of 2.045 Å (mPW1PW91) or 2.057 Å (BP86) and one Ru–N distance of 1.812 Å (mPW1PW91) or 1.834 Å (BP86). Assigning the favorable 18-electron configuration for the two ruthenium atoms of the Ru2(µ3-N)(µ-NO) unit leaves a 14-electron configuration for the third ruthenium atom not bonded to the µ-NO group. This is consistent with the apparent gap in its coordination sphere.
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Figure 6. The optimized Ru3(N)(CO)8(NO) structure with bond distances in Å.
Figure 6. The optimized Ru3(N)(CO)8(NO) structure with bond distances in Å.
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2.3. Trinuclear Dinitrogen Complexes Ru3N2(CO)n (n = 10, 9, 8) Arising Formally by Double CO2 Loss from Ru3(CO)n(NO)2 Trinuclear Derivatives

The lowest-energy Ru3N2(CO)10 structure 3-NN-10 is a Cs structure having a central bent Ru3 unit with Ru–Ru distances of 2.911 Å (mPW1PW91) or 2.950 Å (BP86), corresponding to WBI values of 0.31 and thus a formal single bond and a Ru–Ru–Ru angle of 96.5° (mPW1PW91) or 98.9° (BP86) (Figure 7). Both Ru–Ru bonds are bridged by carbonyl groups exhibiting ν(CO) frequencies of 1864 and 1838 cm−1, significantly lower than the ν(CO) frequencies of the eight terminal CO groups in the range from 2066 to 1960 cm−1. The central ruthenium atom bears two terminal CO groups and a terminal dinitrogen ligand with a N≡N distance of 1.117 Å (mPW1PW91) or 1.139 Å (BP86), leading to the favored 18-electron configuration after considering the two Ru–Ru bonds, each bridged by a CO group. The terminal ruthenium atoms of the Ru–Ru–Ru chain each bear three terminal CO groups, thereby leading to 16-electron configurations.
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Figure 7. The lowest-energy Ru3(N)2(CO)n (n = 10, 9, 8) structures with bond distances in Å.
Figure 7. The lowest-energy Ru3(N)2(CO)n (n = 10, 9, 8) structures with bond distances in Å.
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The lowest-energy Ru3N2(CO)9 structure 3-NN-9 is a Cs symmetry structure in which the dinitrogen unit bridges all three ruthenium atoms (Figure 7). The N–N distance in 3-NN-9 of 1.417 Å (mPW1PW91) or 1.446 Å (BP86) suggests a formal single bond using only one valence electron from each nitrogen atom. This makes the other eight valence electrons of the N2 unit available for donation to the Ru3 unit, thereby giving each ruthenium atom in 3-NN-9 the favored 18-electron configuration. The lowest-energy Ru3N2(CO)8 structure 3-NN-8 is similar to that of 3-NN-9, except for one less CO group on the unique ruthenium atom, thereby giving that ruthenium atom only a 16-electron configuration.

2.4. Trinuclear Ruthenium Carbonyl Isocyanates Ru3(N)(CO)n(NCO) and Ru3(CO)n(NCO)(NO)

A terminal isocyanate group NCO, considered artificially as a neutral pseudohalogen ligand, is a one-electron donor like the halogens themselves. However, a neutral isocyanate group bridging a pair of metal atoms through its nitrogen atom is a three-electron donor similar to a bridging nitrosyl group. Such bridging isocyanate ligands are predicted consistently in polynuclear ruthenium carbonyl derivatives to exhibit a low ν(CO) frequency in the narrow range of 1310 ± 4 cm−1 (BP86). In the chemistry of trinuclear and tetranuclear ruthenium carbonyl isocyanate derivatives, an isocyanate ligand can arise by the carbonylation of a nitride ligand.
Two structures of similar energies were found for the nonacarbonyl Ru3(N)(CO)9(NCO) (Figure 8). Both structures have bent Ru–Ru–Ru chains. The Ru3(µ3-N)(CO)9(µ-NCO) structure 3-NNCO-9-1 of Cs symmetry has its nitride ligand bridging all three ruthenium atoms with two Ru–N distances of 1.973 Å (mPW1PW91) or 1.994 Å (BP86) and one Ru–N distance of 1.885 Å (mPW1PW91) or 1.904 Å (BP86). The isocyanate ligand with an N=C distance of 1.217 Å (mPW1PW91) or 1.234 Å (BP86) bridges the two ruthenium atoms at each end of the chain with Ru–N distances of 2.188 Å (mPW1PW91) or 2.206 Å (BP86) and exhibits a ν(CO) frequency of 1389 cm−1. The nitride ligand, as a five-electron donor gives each ruthenium atom in 3-NNCO-9-1 the favored 18-electron configuration.
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Figure 8. Two isomeric Ru3(N)(CO)9(NCO) structures with bond distances in Å.
Figure 8. Two isomeric Ru3(N)(CO)9(NCO) structures with bond distances in Å.
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The other structure, 3-NNCO-9-2, of similar energy to 3-NNCO-9-1, has two isocyanate ligands. It may be regarded as Ru3(CO)8(µ-NCO)(η3-µ3-NCO), in which the nitride ligand in 3-NNCO-9-1 has been carbonylated to form a second isocyanate ligand that uses all three of its atoms to bond to all three ruthenium atoms in the cluster (Figure 8). This latter isocyanate group, formally considered as neutral, is a five-electron donor to the Ru3 system.
The lowest-energy Ru3(N)(CO)8(NCO) structure, namely the Cs symmetry structure 3-NNCO-8, has the isocyanate ligand bridging two ruthenium atoms and the nitride ligand bridging all three ruthenium atoms (Figure 9). Structure 3-NNCO-8 can be derived from the Ru3(µ3-N)(CO)9(µ-NCO) structure 3-NNCO-9-1 (Figure 8) by the loss of a CO group from the ruthenium atom not bridged by the isocyanate ligand. In 3-NNCO-8, this unique ruthenium atom has only a 16-electron configuration whereas its other two ruthenium atoms have the favored 18-electron configuration. Alternatively, 3-NNCO-8 can be derived from the Ru3(µ3-N)(CO)8(µ-NO) structure 3-NNO-8 (Figure 6) by replacing its three-electron donor bridging NO group with a bridging three-electron donor NCO group.
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Figure 9. The lowest-energy Ru3(N)(NCO)(CO)n (n = 8, 7) structures with bond distances in Å.
Figure 9. The lowest-energy Ru3(N)(NCO)(CO)n (n = 8, 7) structures with bond distances in Å.
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The lowest-energy Ru3(N)(CO)7(NCO) structure 3-NNCO-7 has a central bent Ru–Ru–Ru chain with the nitride ligand bridging all three ruthenium atoms (Figure 9). One of the edges of the Ru–Ru–Ru chain is bridged by the isocyanate group and the other edge by a CO group.
The lowest-energy Ru3(NO)(CO)10(NCO) structure 3-NONCO-10 is closely related to the experimental Ru3(CO)10(µ-NO)2 structure [1] by the replacement of one of the bridging µ-NO groups with a bridging µ-NCO group (Figure 10). The bridging ν(NO) frequency of 1564 cm−1 in 3-NONCO-10 is essentially the mean of the bridging ν(NO) frequencies of 1572 and 1557 cm−1 in the experimental Ru3(µ-NO)2(CO)10 structure 3-(NO)2-10-1. A Ru3(CO)10(NCO)(NO) derivative exhibiting a bridging µ(NO) frequency of 1507 cm−1 has been observed experimentally as a minor product from the decomposition of Ru3(CO)10(µ-NO)2 at 110 °C under 1 atm CO, but has not been structurally characterized [2].
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Figure 10. The lowest-energy Ru3(CO)n(NCO)(NO) (n = 10, 9) structures with bond distances in Å.
Figure 10. The lowest-energy Ru3(CO)n(NCO)(NO) (n = 10, 9) structures with bond distances in Å.
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The lowest-energy Ru3(CO)9(NCO)(NO) structure 3-NONCO-9 can be derived from the Ru3(CO)10(µ-NCO)(µ-NO) structure 3-NONCO-10 by the removal of one CO group from the Ru(CO)4 unit (Figure 10). The NO group in 3-NONCO-9 remains a three-electron donor, as reflected by its ν(NO) frequency of 1525 cm−1, but it bridges all three ruthenium atoms, thereby becoming a five-electron donor through two N→Ru dative bonds and one N–Ru single bond. In this way, each ruthenium atom in 3-NONCO-9 can retain the favored 18-electron configuration.

2.5. Tetranuclear Derivatives with Central Ru4N Units

The decomposition of Ru3(CO)10(µ-NO)2 at 110 °C under a CO atmosphere yields two tetranuclear products, Ru4(µ4-N)(CO)12(µ-NO) and Ru4(µ4-N)(CO)12(µ-NCO), that have been structurally characterized by X-ray crystallography (Figure 11 and Figure 12) [2]. Both species are found to have a central Ru4 butterfly unit capped by the nitrogen atom bridging all four ruthenium atoms. The nitrosyl or isocyanate ligand bridges the body of the butterfly and each ruthenium atom bears three CO terminal groups. Considering the bridging µ4-N nitride ligand as a donor of all five of its valence electrons and the bridging η2-NO or η2-NCO group as a three-electron donor, all four ruthenium atoms have the favored 18-electron configuration in these Ru4(µ4-N)(CO)12(µ-X) derivatives (X = NO, NCO).
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Figure 11. Optimized low-energy Ru4(N)(CO)n(NO) (n = 12, 11, 10) structures with bond distances in Å.
Figure 11. Optimized low-energy Ru4(N)(CO)n(NO) (n = 12, 11, 10) structures with bond distances in Å.
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Figure 12. Optimized Ru4(N)(CO)n(NCO) (n = 12, 11,10) structures with bond distances in Å.
Figure 12. Optimized Ru4(N)(CO)n(NCO) (n = 12, 11,10) structures with bond distances in Å.
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Low-energy structures very close to the experimental structures [2] are found for the Ru4(N)(CO)12(X) derivatives (X = NO, NCO). For the Ru4(µ4-N)(CO)12(µ-NO) structure 4-NNO-12 (Figure 11), the calculated Ru–Ru distances of 2.837 Å (mPW1PW91) or 2.878 Å (BP86) with WBI values of ~0.3 are reasonably close to the experimental distances averaging ~2.82 Å. In addition, the two calculated Ru–N distances of 2.045 and 2.171 Å (mPW1PW91) or 2.012 and 2.188 Å (BP86) for the wingtip ruthenium atoms and 1.901 Å (mPWPW91) or 1.922 Å (BP86) for the body ruthenium atoms are reasonably close to the experimental values of 2.16 Å and 1.90 Å, respectively. Similarly, for the Ru4(µ4-N)(CO)12(µ-NCO) structure 4-NNCO-12 (Figure 12), the calculated Ru–Ru distances of 2.833 Å (mPW1PW91) or 2.875 Å (BP86), likewise with WBI values of ~0.3, are close to the experimental distances averaging ~2.82 Å. In addition, the two calculated Ru–N distances of 2.145 and 1.905 Å (mPW1PW91) or 2.168 and 1.927 Å (BP86) are close to the experimental distances of 2.13 and 1.90 Å, respectively.
The decarbonylation of the Ru4(µ4-N)(CO)12(µ-X) (X = NO, NCO) derivatives preserves the central capped butterfly Ru4(µ4-N) unit as well as the bridging X group in the low-energy structures (Figure 11 and Figure 12). The electronic configurations of the ruthenium atoms in the original Ru(CO)3 moieties in 4-NNO-12 and 4-NNCO-12 losing CO groups go from 18 to 16 in this process.
The low-energy structures of the tetranuclear ruthenium dinitride carbonyls Ru4(N)4(CO)n are significantly different from those with nitrosyl or isocyanate groups discussed above. Thus the low-energy Ru4(N)2(CO)12 structure 4-NN-12 has a central Ru4 tetragon with one nitride ligand bridging all four ruthenium atoms and the other nitride ligand bridging only three ruthenium atoms (Figure 13). Each ruthenium atom bears three terminal carbonyl groups. If the µ4-N ligand contributes all five valence electrons and the µ3-N ligand contributes only three of its five valence electrons to the central Ru4 network, then each ruthenium atom in 4-NN-12 has the favored 18-electron configuration.
The decarbonylation of the Ru4(µ4-N)(µ3-N)(CO)12 structure 4-NN-12 to a Ru4(N)2(CO)11 structure has the effect of forming a new Ru–N bond to the µ3-N nitride to give a structure 4-NN-11 in which both nitride ligands bridge the entire Ru4 central tetragon (Figure 13). If both µ4-N nitride ligands in 4-NN-11 are five-electron donors, then each of the four ruthenium atoms has the favored 18-electron configuration. The central Ru4N2 unit in 4-NN-11 is a distorted octahedron, with each ruthenium vertex forming two Ru–Ru bonds and two Ru–N bonds and each nitrogen vertex forming four Ru–N bonds.
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Figure 13. The optimized Ru4(N)2(CO)n (n = 12, 11) structures with bond distances in Å.
Figure 13. The optimized Ru4(N)2(CO)n (n = 12, 11) structures with bond distances in Å.
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Further decarbonylation of Ru4(N)2(CO)11 gives two isomeric decacarbonyls Ru4(N)2(CO)10 having essentially the same energy within the error limits of the calculations (Figure 14). Structure 4-NN-10-1 retains the Ru4N2 distorted octahedron of 4-NN-11 with both nitride ligands bridging the entire Ru4 tetragon. Structure 4-NN-10-2 is a bi-capped tetrahedral Ru4N2 structure with a central Ru3N tetrahedron having a Ru2N face capped by the fourth ruthenium atom and a Ru3 face capped by the second nitrogen atom. Thus, 4-NN-10-2 has five Ru–Ru bonds, one nitrogen atom bonded to four ruthenium atoms, and the other nitrogen atom bonded to only three ruthenium atoms, whereas 4-NN-10-1 has only four Ru–Ru bonds and both nitrogens bonded to all four ruthenium atoms.
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Figure 14. The two Ru4(N)2(CO)10 structures of similar energies with bond distances in Å.
Figure 14. The two Ru4(N)2(CO)10 structures of similar energies with bond distances in Å.
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2.6. Thermochemistry

Table 1 shows the carbonyl dissociation energies of the Ru3(CO)n(NO)2 (n = 10, 9, 8) derivatives. The dissociation of a carbonyl ligand is highly endothermic for Ru3(CO)n(NO)2 (n = 10, 9) but only mildly endothermic for Ru2(CO)8(NO)2. This suggests the viability of the Ru3(CO)n(µ-NO)2 (n = 10, 9) structures for CO dissociation and the possibility of easy CO dissociation from the Ru3(µ-CO)2(CO)6(µ3-NO)2 structure.
Table 2 shows the energies for other types of reactions involving the trinuclear ruthenium carbonyl nitrosyls, including their dissociation into smaller fragments. In general, such processes appear to be highly endothermic, suggesting the viability of the indicated trinuclear species. The one exception is possibly the same Ru3(CO)8(NO)2 (3-(NO)2-8), being only slightly endothermic toward CO dissociation (Table 1) and also nearly being thermoneutral for dissociation into Ru2(CO)6 and Ru2(CO)2(NO)2 fragments, with the ruthenium atom in the latter species having the favored 18-electron configuration.

3. Theoretical Methods

This study uses two different DFT methods. The first DFT method is the BP86 method, which combines Becke’s 1988 exchange functional (B) with Perdew’s 1986 gradient-corrected correlation functional method (BP86) [4,5]. The second method uses a newer generation functional, mPW1PW91, which combines the modified Perdew–Wang exchange functional with Perdew–Wang’s 91 gradient correlation functional [6]. This functional has been shown to be better for second- and third-row transition-metal compounds [7].
The geometries of all structures considered were fully optimized using both the MPW1PW91 and BP86 methods. The vibrational frequencies and the corresponding infrared intensities were determined analytically at the same levels. All of the predicted ν(CO) and ν(NO) frequencies discussed in this paper were obtained from the BP86 method, which were found to be close to the experimental results without scaling factors for the compounds containing transition metals [8]. This concurrence may be accidental, since the theoretical vibrational frequencies predicted by BP86 are harmonic frequencies, whereas the experimental fundamental frequencies are anharmonic. All vibrational frequencies are given in the Supporting Information. The NBO analysis used the same DFT methods to provide information on the WBI values for the Ru–Ru interactions discussed in the manuscript (Table S42 of the Supporting Information) [9].
The Stuttgart–Dresden double-ζ (SDD) basis set with an effective core potential (ECP) was used for the ruthenium atoms [10,11]. In this basis set, the 28 core electrons for the ruthenium atoms are replaced by ECP. Such an effective core approximation includes scalar relativistic contributions, which become significant for the heavy transition metal atoms. For the ruthenium atoms, our loosely contracted DZP basis set (14s11p6d/10s8p3d) uses the Wachters primitive set augmented by two sets of p functions and one set of d functions contracted following Hood et al. [12,13].
The all-electron double-ζ plus polarization (DZP) basis sets, namely, (9s5p1d/4s2p1d), are used for the carbon, oxygen, and nitrogen atoms. The basis sets are Huzinaga and Dunning’s contracted double-ζ contraction sets [14,15] plus a set of spherical harmonic d polarization functions with the orbital exponents αd(C) = 0.75, αd(O) = 0.85, and αd(N) = 0.80. All of the computations were carried out with the Gaussian 09 program [16], in which the fine grid (75, 302) is the default for the numerical evaluation of the integrals.

4. Summary

The experimental Ru3(CO)10(µ-NO)2 structure is shown to be a low-energy structure. The decarbonylation of Ru3(CO)10(µ-NO) is predicted to convert one of the three-electron donor µ-NO groups into a five-electron donor η2-µ3-NO group bridging all three ruthenium atoms. Further decarbonylation leads to a low-energy Ru3(µ-CO)2(CO)6(µ3-NO)2 structure with two three-electron donor µ3-NO groups bridging all three ruthenium atoms and two µ-CO groups bridging the two Ru–Ru bonds.
The lowest-energy Ru3(N)(CO)9(NO) structure obtained by the loss of CO2 from Ru3(µ-NO)2(CO)10 has a bridging η3-µ3-N2O ligand but with an elongated N–N bond. A higher-energy Ru3(µ3-N)(CO)9(η2-µ-NO) isomer has a nitride ligand bridging all three ruthenium atoms and a five-electron donor η2-µ-NO group. The decarbonylation of these structures leads to low-energy Ru3(µ3N)(CO)n(µ-NO) (n = 8, 7) structures in which the nitride ligand bridges all three ruthenium atoms and the three-electron donor µ-NO ligand bridges only two of the ruthenium atoms.
The loss of two CO2 units from Ru3(CO)10(µ-NO)2 leads to a low-energy Ru3(CO)8(η2-µ3-N2) species in which a dinitrogen ligand with an elongated N–N distance bridges all three ruthenium atoms. This type of bridging dinitrogen ligand is also found in the low-energy carbonyl-richer structure Ru3(CO)9(η2-µ3-N2).
The carbonylation of a nitride ligand leads to an isocyanate ligand. An isocyanate ligand bridging two metal atoms through its nitrogen atom is a three-electron donor similar to an NO ligand bridging two metal atoms. The experimentally observed Ru3(CO)10(µ-NCO)(µ-NO), as a minor product from the decomposition of Ru3(µ-NO)2(CO)10, is found to be a low-energy structure. The decarbonylation of Ru3(CO)10(µ-NCO)(µ-NO) to give Ru3(CO)9(µ-NCO)(µ-NO) is predicted to preserve the central Ru2(µ-NCO)(µ-NO) unit.
The decomposition of Ru3(CO)10(µ-NO)2 is also found to produce the tetranuclear derivatives Ru4(µ4-N)(CO)12(µ-NO) and Ru4(µ4-N)(CO)12(µ-NCO) [2]. These are found to be low-energy structures. A central Ru4(µ4-N) unit is also found to be the key feature in other low-energy tetranuclear ruthenium carbonyl structures of the type Ru4(µ4-N)(CO)n(µ-X) (X = NO, NCO; n = 12, 11, 10). The low-energy structure of the tetranuclear ruthenium carbonyl dinitride Ru4(µ4-N)(µ3-N)(CO)12 has one nitride ligand bonded to all four ruthenium atoms and the other nitride ligand bonded to only three ruthenium atoms. The decarbonylation of this species gives low-energy Ru4(µ4-N)2(CO)n (n = 11, 10) structures in each of which both nitrogen atoms are bonded to all four ruthenium atoms, leading to a central Ru4N2 octahedron.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/molecules29174165/s1, Table S1: All CO and NO vibrational stretching frequencies (in cm−1) and their infrared intensities (in km/mol) for the Ru3(CO)n(NO)2 (n = 10, 9, 8, 7) structures; Table S2: All CO and NO vibrational stretching frequencies (in cm−1) and their infrared intensities (in km/mol) for the Ru3(N)(CO)n(NO) (n = 10, 9, 8) structures; Table S3: All CO vibrational stretching frequencies (in cm−1) and their infrared intensities (in km/mol) for the Ru3(N)2(CO)n (n = 10, 9, 8) structures; Table S4: All CO and NO vibrational stretching frequencies (in cm−1) and their infrared intensities (in km/mol) for the Ru3(N)(CO)n(NCO) (n = 9, 8, 7) structures; Table S5: All CO and NO vibrational stretching frequencies (in cm−1) and their infrared intensities (in km/mol) for the Ru3(CO)n(NCO)(NO) (n = 10, 9) structures; Table S6: All CO and NO vibrational stretching frequencies (in cm−1) and their infrared intensities (in km/mol) for the Ru4(N)(CO)n(NO) (n = 12, 11, 10) structures; Table S7: All CO and NO vibrational stretching frequencies (in cm−1) and their infrared intensities (in km/mol) for the Ru4(N)(CO)n(NCO) (n = 12, 11, 10) structures; Table S8: All CO vibrational stretching frequencies (in cm−1) and their infrared intensities (in km/mol) for the Ru4(N)2(CO)n (n = 12, 11, 10) structures; Tables S9–S13: Optimized geometries for the Ru3(CO)n(NO)2 (n = 10, 9, 8, 7) structures; Tables S14–S17: Optimized geometries for the Ru3(N)(CO)n(NO) (n = 10, 9, 8) structures; Tables S18–S20: Optimized geometries for the Ru3(N)2(CO)n (n = 10, 9, 8) structures; Tables S21–S24: Optimized geometries for the Ru3(N)(CO)n(NCO) (n = 9, 8, 7) structures; Tables S25 and S26: Optimized geometries for the Ru3(CO)n(NCO)(NO)n (n = 10, 9) structures; Tables S27–S29: Optimized geometries for the Ru4(N)(CO)n(NO) (n = 12, 11, 10) structures; Tables S30–S32: Optimized geometries for the Ru4(N)(CO)n(NCO) (n = 12, 11, 10) structures; Tables S33–S36: Optimized geometries for the Ru4(N)2(CO)n (n = 12, 11, 10) structures; complete Gaussian 09 reference [16]; Tables S37–S41: Total energies with ZPVE correction (E in Hartree), Total free energies with ZPVE correction (G in Hartree), numbers of imaginary vibrational frequencies (Nimg) for all the structures; Table S42: Wiberg bond indices (WBI) for the Ru–Ru bonds and natural charges on the ruthenium atoms for all of the structures; complete Gaussian 09 reference [16].

Author Contributions

X.F. performed the calculations to generate the data; S.C. supervised the group at Changzhou University and provided an initial summary of the data; Y.X. reviewed the data and various drafts of the manuscript; R.B.K. suggested the original project, wrote the Introduction, and edited the remainder of the manuscript; H.F.S. reviewed and edited the final version of the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University and Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Changzhou University in China. Research at the University of Georgia was supported by the U.S. Department of Energy, Basic Energy Sciences, Division of Chemistry, Computational and Theoretical Chemistry (CTC) Program under Contract DE-SC0018412.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Figure 1. Comparison of the structures of Ru3(CO)12 and Ru3(CO)10(µ-NO)2.
Figure 1. Comparison of the structures of Ru3(CO)12 and Ru3(CO)10(µ-NO)2.
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Figure 2. Structure of the decomposition products of Ru3(CO)10(µ-NO)2.
Figure 2. Structure of the decomposition products of Ru3(CO)10(µ-NO)2.
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Table 1. Energies (ΔE) and free energies (ΔG, at 298 K) in kcal/mol for carbonyl dissociation of Ru3(CO)n(NO)2 structures. Both ∆E and ∆G include the zero-point vibrational energy (ZPVE) corrections.
Table 1. Energies (ΔE) and free energies (ΔG, at 298 K) in kcal/mol for carbonyl dissociation of Ru3(CO)n(NO)2 structures. Both ∆E and ∆G include the zero-point vibrational energy (ZPVE) corrections.
Dissociation ReactionmPW1PW91BP86
ΔEΔGΔEΔG
Ru3(CO)10(NO)2→Ru3(CO)9(NO)2 + CO29.218.729.519.1
Ru3(CO)9(NO)2→Ru3(CO)8(NO)2 + CO73.463.064.355.3
Ru3(CO)8(NO)2→Ru3(CO)7(NO)2 + CO6.4−4.413.62.4
Table 2. Energies (ΔE) and free energies (ΔG, at 298K) in kcal/mol for disproportionation and fragmentation processes of the trinuclear ruthenium carbonyl nitrosyls. Both ∆E and ∆G include zero-point vibrational energy (ZPVE) corrections. The Ru3(CO)n(NO)2 structures considered in Table 2 are the same as those in Table 1.
Table 2. Energies (ΔE) and free energies (ΔG, at 298K) in kcal/mol for disproportionation and fragmentation processes of the trinuclear ruthenium carbonyl nitrosyls. Both ∆E and ∆G include zero-point vibrational energy (ZPVE) corrections. The Ru3(CO)n(NO)2 structures considered in Table 2 are the same as those in Table 1.
Dissociation ReactionmPW1PW91BP86
ΔEΔGΔEΔG
2Ru3(CO)9(NO)2→Ru3(CO)10(NO)2 + Ru3(CO)8(NO)244.244.334.936.2
Ru3(CO)10(NO)2→Ru(NO)2(CO)2 + Ru2(CO)845.628.325.99.4
Ru3(CO)9(NO)2→Ru(NO)2(CO)2 +Ru2(CO)744.828.721.45.5
Ru3(CO)8(NO)2→Ru(CO)2(NO)2 +Ru2(CO)612.8-3.84.4−11.6
Ru3(CO)10(NO)2→Ru2(CO)5(NO)2 + Ru(CO)540.925.523.28.3
Ru3(CO)10(NO)2→Ru2(CO)6(NO)2 + Ru(CO)435.820.427.312.5
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MDPI and ACS Style

Chen, S.; Feng, X.; Xie, Y.; King, R.B.; Schaefer, H.F. Trinuclear and Tetranuclear Ruthenium Carbonyl Nitrosyls: Oxidation of a Carbonyl Ligand by an Adjacent Nitrosyl Ligand. Molecules 2024, 29, 4165. https://doi.org/10.3390/molecules29174165

AMA Style

Chen S, Feng X, Xie Y, King RB, Schaefer HF. Trinuclear and Tetranuclear Ruthenium Carbonyl Nitrosyls: Oxidation of a Carbonyl Ligand by an Adjacent Nitrosyl Ligand. Molecules. 2024; 29(17):4165. https://doi.org/10.3390/molecules29174165

Chicago/Turabian Style

Chen, Shengchun, Xuejun Feng, Yaoming Xie, R. Bruce King, and Henry F. Schaefer. 2024. "Trinuclear and Tetranuclear Ruthenium Carbonyl Nitrosyls: Oxidation of a Carbonyl Ligand by an Adjacent Nitrosyl Ligand" Molecules 29, no. 17: 4165. https://doi.org/10.3390/molecules29174165

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