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The coordination chemistry of the title diphosphine–borane adduct at heterometallic MoRe centers was examined through its reactions with the hydride complex [MoReCp(μ-H)(μ-PCy2)(CO)5(NCMe)] (Cp = η5-C5H5). The latter reacted rapidly with stoichiometric amounts of dppm·BH3(dppm = Ph2PCH2PPh2) in refluxing toluene solution, with displacement of the nitrile ligand, to give [MoReCp(μ-H)(μ-PCy2)(CO)5(κ1P-dppm·BH3)], with a P-bound diphosphine–borane ligand arranged transto the PCy2group. Decarbonylation of the latter complex was accomplished rapidly upon irradiation with visible-UV light in toluene solution at 263 K, to give the agostic derivative [MoReCp(μ-H)(μ-PCy2)(CO)4(κ1P,η2-dppm·BH3)] as major product (Mo–Re = 3.2075(5) Å), along with small amounts of the diphosphine-bridged complex [MoReCp(μ-H)(μ-PCy2)(CO)4(μ-dppm)]. Extended photolysis of the agostic complex at 288 K promoted an unprecedented dehydrogenation process involving the borane group and the hydride ligand, to give the diphosphine–boryl complex [MoReCp(μ-η2:κ2P,B-H2B·dppm)(μ-PCy2)(CO)4] (Mo–Re = 3.075(1) Å). The latter displayed a boryl ligand in a novel bridging coordination mode, it being σ-bound to one of the metal atoms (B–Re = 2.38(2) Å) while interacting with the second metal atom via a strong side-ontricentric B–H–M interaction (B–Mo = 2.31(1); H–Mo = 1.9(1) Å). The overall dehydrogenation process was endergonic by 43 kJ/mol, according to density functional theory calculations.

The reactions of the title complex (1) with different p-block element (E) molecules was examined. Compound 1reacted with BH3·THF at room temperature to give the trihydride [W2Cp2(μ-H)H2(μ-PPh2)(NO)2], which formally results from hydrogenation of 1, a reaction that actually does not take place when neat dihydrogen is used. Clean E–H bond oxidative addition, however, took place when 1was reacted with HSnPh3, to give the related dihydride stannyl derivative [W2Cp2(μ-H)H(μ-PPh2)(NO)2(SnPh3)]. In contrast, the reaction of 1with HSPh involved H2elimination to give the thiolate-bridged complex [W2Cp2(μ-SPh)(μ-PPh2)(NO)2], while that with (p-tol)C(O)H resulted in insertion of the aldehyde to yield the related alkoxide complex [W2Cp2{μ-OCH2(p-tol)}(μ-PPh2)(NO)2]. Insertion also prevailed in the reactions of 1with CNtBu, which, however, involved the competitive formation of new C–H or N–H bonds, to give a mixture of formimidoyl and aminocarbyne derivatives, [W2Cp2(μ-κ1:η2-HCNtBu)(μ-PPh2)(NO)2] (W–W = 3.0177(2) Å) and [W2Cp2{μ-C(NHtBu)}(μ-PPh2)(NO)2] (W–W = 2.9010(4) Å), respectively, even though the latter was thermodynamically preferred, according to density functional theory calculations. The former represents the first structurally characterized complex displaying a formimidoyl or iminoacyl ligand in the alkenyl-like μ-κ1:η2coordination mode. The reaction of 1with diazomethane proceeded with N2elimination and C–H coupling to yield the agostic methyl-bridged complex [W2Cp2(μ-κ1:η2-CH3)(μ-PPh2)(NO)2] (calculated W–W = 2.923 Å), whereas the reaction with N2CH(SiMe3) proceeded with insertion of the diazoalkane to give the corresponding hydrazonide complex [W2Cp2{μ-NH(NCHSiMe3)}(μ-PPh2)(NO)2] (W–W = 2.8608(4) Å). The latter was converted under alkaline conditions to the methyldiazenide derivative [W2Cp2{μ-N(NMe)}(μ-PPh2)(NO)2] (W–W = 2.8730(2) Å), in a process involving hydrolysis of the C–Si bond coupled with a 1,3-H shift from N to C.

The title complex (1) reacted with [Mn2(CO)10] under visible–UV irradiation (toluene solution and quartz glassware) to give a mixture of the phosphinidene complex [MnMoCp(μ-κ1:κ1,η6-PMes*)(CO)4], the cluster [Mn2Mo2Cp2(μ-κ1:κ1,η6-PMes*)(μ3-S)(CO)8], and the thiophosphinidene complex [MnMoCp(μ-κ2:κ1,η4-SPMes*)(CO)5], in yields of ca. 60, 20, and 10% respectively (Mes* = 2,4,6-C6H2tBu3). The major product follows from formal replacement of the SMoCp(CO)2fragment in 1with a Mn(CO)4fragment, and displayed multiple bonding to phosphorus (Mn–P = 2.1414(8) Å); the tetranuclear cluster results from formal insertion of a Mn2(CO)6fragment in 1, with cleavage of P–S and P–Mo bonds, to render a μ3-S bridged Mn2Mo core bearing an exocyclic phosphinidene ligand involved in multiple bonding to one of the Mn atoms (Mn–P = 2.140(2) Å); the thiophosphinidene complex (Mn–P = 2.294(1) Å) formally results from addition of sulfur and carbon monoxide to the major MnMo product, a transformation which actually could be performed stepwise, via the MnMo thiophosphinidene complex [MnMoCp(μ-κ2:κ1,η6-SPMes*)(CO)4]. When the photolysis of 1and [Mn2(CO)10] was performed in tetrahydrofuran solution and using conventional glassware, then the V-shaped cluster [Mn2MoCp{μ-κ1:κ1:κ1,η5-P(C6H3tBu3)}(CO)8] was obtained selectively (Mo–Mn = 3.2910(5) Å, Mn–Mn = 2.9223(5) Å), which unexpectedly displays a cyclohexadienylidene-phosphinidene ligand resulting from H atom abstraction at the aryl ring of the precursor. Density functional theory calculations on the complexes [LnM(μ-κ1:κ1,η6-PMes*)MoCp] (MLn= MoCp(CO)2, Mn(CO)4, Co(CO)3) revealed that the degree of delocalization of the metal–phosphorus π-bonding interaction over the Mo–P–M chain is significantly conditioned by the heterometal fragment MLn, it being increased in the order Mn ≤ Mo < Co.

A high-yield synthetic route for the preparation of the unsaturated anion [Mo2Cp2(μ-PtBu2)(μ-CO)2]−(2) was implemented, via two-electron reduction of the chloride complex [Mo2Cp2(μ-Cl)(μ-PtBu2)(CO)2] (1). Reaction of 2with [NH4][PF6] led to the formation of the 30-electron complex [Mo2Cp2(H)(μ-PtBu2)(CO)2] (3), in which the hydride ligand adopts an uncommon terminal disposition. DFT analysis of the electronic structure of 3gave support to the presence of a MM triple bond in this complex following from a σ2δ2δ2configuration, a view also supported by the high electron density accumulated at the corresponding Mo–Mo bond critical point. In contrast, reactions of 2with IMe or ClCH2Ph gave the alkyl-bridged complexes [Mo2Cp2(μ-κ1:η2-CH2R)(μ-PtBu2)(CO)2] (R = H (4a), Ph (4b)), which in solution display agostic Mo–H–C interactions. Decarbonylation of 4atook place rapidly under photochemical conditions to give the 30-electron complex [Mo2Cp2(μ-κ1:η2-CH3)(μ-PtBu2)(μ-CO)] (7), with a stronger agostic coordination of its methyl ligand. In contrast, irradiation of 4bled to the formation of the benzylidyne derivative [Mo2Cp2(μ-CPh)(μ-PtBu2)(μ-CO)] (9), following from fast decarbonylation and dehydrogenation of the bridging benzyl ligand. Low-temperature photochemistry allowed for the NMR characterization of an intermediate preceding the hydrogen elimination, identified as the carbene hydride [Mo2Cp2(H)(μ-CHPh)(μ-PtBu2)(CO)] (10), a product which evolves slowly by H2elimination to the benzylidyne derivative. Analogous dehydrogenation of the methyl ligand in 7could be accomplished upon moderate heating, to yield the corresponding methylidyne derivative [Mo2Cp2(μ-CH)(μ-PtBu2)(μ-CO)] (9). A complete reaction mechanism accounting for these photochemical reactions was elaborated, based on the reaction intermediates identified experimentally and on extensive DFT calculations. Surprisingly, for both systems the C–H bond activation steps are relatively easy thermal processes occurring with modest activation energies after photochemical ejection of CO, with a rate-determining step involving the formation of agostic carbenes requiring also a strong structural reorganization of the central Mo2PC rings of these molecules.