Terminal vs. bridging coordination of CO and NO ligands after decarbonylation of [W2Cp2(μ-PR2)(CO)3(NO)] complexes (R = Ph, Cy). An experimental and computational study

Low-Valent Transition Metalate Anions in Synthesis, Small Molecule Activation, and Catalysis

This review surveys the synthesis and reactivity of low-oxidation state metalate anions of the d-block elements, with an emphasis on contributions reported between 2006 and 2022. Although the field has a long and rich history, the chemistry of transition metalate anions has been greatly enhanced in the last 15 years by the application of advanced concepts in complex synthesis and ligand design. In recent years, the potential of highly reactive metalate complexes in the fields of small molecule activation and homogeneous catalysis has become increasingly evident. Consequently, exciting applications in small molecule activation have been developed, including in catalytic transformations. This article intends to guide the reader through the fascinating world of low-valent transition metalates. The first part of the review describes the synthesis and reactivity of d-block metalates stabilized by an assortment of ligand frameworks, including carbonyls, isocyanides, alkenes and polyarenes, phosphines and phosphorus heterocycles, amides, and redox-active nitrogen-based ligands. Thereby, the reader will be familiarized with the impact of different ligand types on the physical and chemical properties of metalates. In addition, ion-pairing interactions and metal-metal bonding may have a dramatic influence on metalate structures and reactivities. The complex ramifications of these effects are examined in a separate section. The second part of the review is devoted to the reactivity of the metalates toward small inorganic molecules such as H2, N2, CO, CO2, P4 and related species. It is shown that the use of highly electron-rich and reactive metalates in small molecule activation translates into impressive catalytic properties in the hydrogenation of organic molecules and the reduction of N2, CO, and CO2. The results discussed in this review illustrate that the potential of transition metalate anions is increasingly being tapped for challenging catalytic processes with relevance to organic synthesis and energy conversion. Therefore, it is hoped that this review will serve as a useful resource to inspire further developments in this dynamic research field.

Chemistry of a Nitrosyl Ligand κ:η-Bridging a Ditungsten Center: Rearrangement and N–O Bond Cleavage Reactions

The novel nitrosyl-bridged complex [W₂Cp₂(μ-P^tBu₂)(μ-κ:η-NO)(CO)(NO)](BAr₄) [Ar = 3,5-C₆H₃(CF₃)₂] was prepared in a multistep procedure starting from the hydride [W₂Cp₂(μ-H)(μ-P^tBu₂)(CO)₄] and involving the new complexes [W₂Cp₂(μ-P^tBu₂)(CO)₄](BF₄), [W₂Cp₂(μ-P^tBu₂)(CO)₂(NO)₂](BAr₄), and [W₂(μ-κ:η⁵-C₅H₄)Cp(μ-P^tBu₂)(CO)(NO)₂] as intermediates, which follow from reactions with HBF₄·OEt₂, NO, and Me₃NO·2H₂O, respectively. The nitrosyl-bridged cation easily added chloride upon reaction with [N(PPh₃)₂]Cl, with concomitant NO rearrangement into the terminal coordination mode, to give [W₂ClCp₂(μ-P^tBu₂)(CO)(NO)₂], and underwent N–O and W–W bond cleavages upon the addition of CN^tBu to give the mononuclear phosphinoimido complex [WCp(NP^tBu₂)(CN^tBu)₂](BAr₄). Another N–O bond cleavage was induced upon photochemical decarbonylation at 243 K, which gave the oxo- and phosphinito-bridged nitrido complex [W₂Cp₂(N)(μ-O)(μ-OP^tBu₂)(NO)](BAr₄), likely resulting from a N–O bond cleavage step following decarbonylation.

The π binding of the NO ligand in the cation [W₂Cp₂(μ-P^tBu₂)(μ-κ:η-NO)(CO)(NO)]⁺ facilitates the addition of ligands with concomitant rearrangement of the bridging nitrosyl into the terminal coordination mode and also facilitates cleavage of the N−O bond of that ligand at low temperature possibly in two different ways: either through the oxidative addition of this ligand to the dimetal center or through deoxygenation by another ligand.

Structure and dynamics of heterometallic clusters derived from addition of metal carbonyl fragments to the unsaturated hydride [W2Cp2(μ-H)(μ-PPh2)(NO)2]

The title complex reacted with [Fe₂(CO)₉] to give the trinuclear derivative [FeW₂Cp₂(μ-H)(μ-PPh₂)(CO)₄(NO)₂] (W-W = 3.044(1) Å) as a result of full insertion of the 16-electron Fe(CO)₄ fragment into the tricentric W-H-W bond of the parent substrate. In contrast, the reactions with the THF adducts [M(CO)₅(THF)] (M = W, Mo) and [MnCp'(CO)₂(THF)] (Cp' = C₅H₄Me) yielded the μ₃-hydride derivatives [MW₂Cp₂(μ₃-H)(μ-PPh₂)(CO)₅(NO)₂] (W-W = 3.006(1) to 3.164(1) Å for the W₃ compound) and [MnW₂Cp₂Cp'(μ₃-H)(μ-PPh₂)(CO)₂(NO)₂] respectively, all of them resulting from addition (rather than insertion) of the corresponding 16-electron fragment to the W₂H moiety of the parent compound. Density Functional Theory calculations revealed that edge- and face-bridged hydride clusters were of similar energy in the W₂Fe system, while the face-bridged structure was significantly more stable (by more than ca. 40 kJ mol^-1) for the W₃ system. Both clusters displayed fast rearrangement in solution involving a flapping movement of the puckered PW₂M core of these molecules. This was combined, in the W₂Fe cluster, with fast exchange between the almost isoenergetic edge- and face-bridged hydride isomers. The reactions of the title compound with several carbonyl dimers were also examined as an additional synthetic approach to the rational synthesis of heterometallic clusters, but were unsuccessful except in the case of [Co₂(CO)₈], which reacted at 253 K in the dark to give a mixture of the binuclear complex [CoWCp(μ-PPh₂)(CO)₄(NO)] (Co-W = 2.8623(6) Å) and the trinuclear cluster [CoW₂Cp₂(μ-PPh₂)(CO)₄(NO)₂] (W-W = 3.1654(4) Å; W-Co = 2.638(1), 2.829(1) Å), the latter resulting from formal replacement of the hydride ligand with the 17-electron fragment Co(CO)₄, which displayed an asymmetric binding to the W₂ centre.