Electronic Structure and Reactivity of the Carbyne-Bridged Dimolybdenum Radical [Mo₂(η⁵-C₅H₅)₂(μ-CPh)(μ-PCy₂)(μ-CO)]⁺

Abstract

The unsaturated compound [Mo2Cp2(μ-CPh)(μ-PCy2)(μ-CO)] (1, Cp = η5-C5H5) reacts with trace amounts of water in the presence of [FeCp2]BF4 to give a mixture of the hydroxycarbyne complex [Mo2Cp2(μ-COH)(μ-CPh)(μ-PCy2)]BF4 (minor) and the hydroxo complex [Mo2Cp2(μ-CPh)(OH)(μ-PCy2)(CO)]BF4 (major product), with the latter rapidly rearranging to give the carbene isomer cis-[Mo2Cp2(μ-η1:η3-CHPh)(O)(μ-PCy2)(CO)]BF4 (Mo–Mo = 2.9435(3) Å). An analogous reaction takes place with phenol, to give selectively the related phenoxo complex [Mo2Cp2(μ-CPh)(OPh)(μ-PCy2)(CO)]BF4. In contrast, the reactions of 1 with H2SiPh2 or H3BNH2 t Bu in the presence of [FeCp2]BF4 result in the selective H transfer to the O atom of the carbonyl ligand, to give the mentioned hydroxycarbyne complex. All the above reactions can be rationalized by assuming the initial formation of the radical cation [Mo2Cp2(μ-CPh)(μ-PCy2)(μ-CO)]+ (2), a molecule displaying a somewhat weakened intermetallic bonding (Mo–Mo = 2.537 Å vs 2.493 Å in 1) and a linear semibridging carbonyl, with both the LUMO and most of the unpaired electron density being located at a single molybdenum atom, with a much smaller distribution over the oxygen atom of the carbonyl ligand, according to density functional theory calculations. As expected, the radical 2 adds rapidly a molecule of nitric oxide to give a diamagnetic product, but spontaneous decarbonylation also takes place to eventually give the 30-electron nitrosyl complex [Mo2Cp2(μ-CPh)(μ-PCy2)(μ-NO)]BF4. Deprotonation of cis-[Mo2Cp2(μ-η1:η3-CHPh)(O)(μ-PCy2)(CO)]BF4 gives the neutral carbyne complex cis-[Mo2Cp2(μ-CPh)(O)(μ-PCy2)(CO)] (Mo–Mo = 2.8024(5) Å), which upon protonation reverts to its carbene precursor, via the corresponding hydroxo complex. Related trans isomers can be prepared through protonation reactions of trans-[Mo2Cp2(μ-CPh)(O)(μ-PCy2)(CO)] (Mo–Mo = 2.8206(6) Å), a complex easily prepared by reacting the dicarbonyl [Mo2Cp2(μ-CPh)(μ-PCy2)(CO)2] with air.

Insertion, coupling and elimination processes in the reactions of the unsaturated alkyl-bridged complexes [Mo2(η5-C5H5)2(μ-CH2R)(μ-PCy2)(CO)2] (R = H, Ph) with isocyanides and secondary phosphines

“…Overall, these molecules would be isoelectronic with different dicarbonyls of the type [Mo 2 Cp 2 (μ-PCy 2 )(μ-X)(CO) 2 ] (X = 3-electron donor ligand) previously prepared by us. The latter, however, typically display poorly deshielded 31 P NMR resonances in the range 90-160 ppm, 15,22 whereas both isomers of 3 display much more deshielded resonances (around 230 ppm), a spectroscopic difference that we cannot explain at present.…”

“…Compounds 8b and 9, however, could be conveniently purified chromatographically, although dicarbonyl 9 turned out to be highly air-sensitive and partially transformed into the oxo derivative trans-[Mo 2 Cp 2 (μ-PCy 2 )(μ-PEt 2 )(O)(CO)] (10) upon manipulation, thus paralleling the well known behaviour of other unsaturated phosphide-bridged dicarbonyl complexes of the type [Mo 2 Cp 2 (μ-PCy 2 )(μ-X)(CO) 2 ] (X = 3-electron donor ligand). [29][30][31] Reaction of 1a with diphenylphosphine also proceeds smoothly at room temperature, to give an analogous mixture of products: the acetaldehyde complex [Mo 2 Cp 2 (μ-PCy 2 )-(μ-PPh 2 ){η 2 -C(O)HMe}(CO)] (11), the oxo complex [Mo 4 Cp 4 -(μ-PCy 2 ) 2 (μ-O) 4 ], and the corresponding dicarbonyl [Mo 2 Cp 2 (μ-PCy 2 )(μ-PPh 2 )(CO) 2 ], the latter being a compound previously described by us. 22a The relative ratio of products was now ca.…”

Hydride, gold(i) and related derivatives of the unsaturated ditungsten anion [W2Cp2(μ-PCy2)(μ-CO)2]−

“…Thus, isomer 3B displays two very different couplings (383 and 218 Hz), in agreement with a structure in which the coordination numbers of the two metal atoms differ by one unit. In contrast, the minor isomer 3T must display equivalent W centres, since just one set of satellite lines with a relatively low PW coupling (246 Hz 21 The major isomer 3B displays two distinct hydride resonances coupled to phosphorus, corresponding respectively to a terminal ligand cis to the PCy 2 ligand ( H 0.13 ppm; J HP = 32 Hz) and a bridging ligand ( H = 3.96 ppm; J HP = 3 Hz) strongly coupled to inequivalent tungsten atoms (J HW = 109, 107 Hz). In contrast, the equivalent hydrides in 3T give rise to a poorly shielded resonance ( H 0.19 ppm), retaining a high HP coupling (37 Hz) indicative of a cis disposition of these two H atoms with respect to the PCy 2 ligand.…”

“…We note that the chemical shifts of both resonances are much higher than those of its precursor 2, they being actually comparable to the values measured for several 30-electron dimolybdenum cations such as [Mo 2 Cp 2 (μ-COMe) 2 (μ-PCy 2 )] + (264.2 ppm), 10 and [Mo 2 Cp 2 -(μ-COH)(μ-CPh)(μ-PCy 2 )] + (254.8 ppm). 21 The major isomer 3B displays two distinct hydride resonances coupled to phosphorus, corresponding respectively to a terminal ligand cis to the PCy 2 ligand (δ H 0.13 ppm; J HP = 32 Hz) and a bridging ligand (δ H = −3.96 ppm; J HP = 3 Hz) strongly coupled to inequivalent tungsten atoms ( J HW = 109, 107 Hz). In contrast, the equivalent hydrides in 3T give rise to a poorly shielded resonance (δ H −0.19 ppm), retaining a high H-P coupling (37 Hz) indicative of a cis disposition of these two H atoms with respect to the PCy 2 ligand.…”

Chalcogenoacyl-bridged derivatives of the unsaturated carbyne complex [Mo 2 ( η 5 -C 5 H 5 ) 2 ( μ -CPh)( μ -PCy 2 )(CO) 2 ]

“…Among other reactions, compound 1 was shown to add selenium atoms at the Mo 2 C face upon reaction with grey selenium, to build a selenoacyl ligand symmetrically bridging the metal atoms in a - 2 : 2 fashion [1]. In contrast, we later found that reaction of 1 with elemental oxygen instead promotes decarbonylation, to give the corresponding oxoderivative trans-[Mo 2 Cp 2 (-CPh)(O)(-PCy 2 )CO)] (2) (Scheme 1), a reaction taking place with retention of the overall stereochemistry of the binuclear substrate [2].…”