C≡N and N≡O Bond Cleavages of Acetonitrile and Nitrosyl Ligands at a Dimolybdenum Center to Render Ethylidyne and Acetamidinate Ligands.

Extended reduction of [Mo2Cp2(µ-Cl)(µ-PtBu2)(NO)2] (1) with Na(Hg) in acetonitrile (MeCN) at room temperature resulted in an unprecedented full cleavage of the C≡N bond of a coordinated MeCN molecule to yield the vinylidene derivative Na[Mo2Cp2(µ-PtBu2)(µ-CCH2)(NO)2], which upon protonation with (NH4)PF6 gave the ethylidyne complex [Mo2Cp2(µ-PtBu2)(µ-CMe)(NO)2] [Mo1-Mo2 = 2.9218(2) Å] in a selective and reversible way. Controlled reduction of 1 at 273 K yielded instead, after protonation, the 30-electron acetamidinate complex [Mo2Cp2(µ-PtBu2)(µ-κN:κN'-HNCMeNH)(µ-NO)]PF6 [Mo1-Mo2 = 2.603(2) Å], in a process thought to stem from the paramagnetic MeCN-bridged intermediate [Mo2Cp2(µ-PtBu2)(µ-NCMe)(NO)2], followed by a complex sequence of elementary steps including cleavage of the N≡O bond of a nitrosyl ligand.

Reactions of Heterometallic Phosphinidene-Bridged MoMn and MoRe Complexes with Sulfur and Selenium: From Chalcogenophosphinidene- to Trithiophosphonate-Bridged Derivatives.

Reactions of [MoReCp(µ-PR*)(CO)6] with S8 were strongly dependent on experimental conditions (R* = 2,4,6-C6H2tBu3). When using 1 equiv of sulfur, complex [MoReCp(µ-η2:κ1S-SPR*)(CO)6] was slowly formed at 313 K, with a thiophosphinidene ligand unexpectedly bridging the dimetal center in the novel µ-κ1S:η2 coordination mode, as opposed to the µ-κ1P:η2 mode usually found in related complexes. The latter underwent fast decarbonylation at 363 K to give [MoReCp(µ-η2:η2-SPR*)(CO)5], with a six-electron donor thiophosphinidene ligand rearranged into the rare µ-η2:η2 coordination mode. Depending on reaction conditions, reactions with excess sulfur involved the addition of two or three S atoms to the phosphinidene ligand to give new complexes identified as the dithiophosphinidene-bridged complex [MoReCp(µ-η2:κ2S,S'-S2PR*)(CO)5], its dithiophosphonite-bridged isomer [MoReCp(µ-κ2S,S':κ2S,S'-S2PR*)(CO)5], or the trithiophosphonate-bridged derivative [MoReCp(µ-κ2S,S':κ2S,S'-S3PR*)(CO)5], all of them displaying novel coordination modes of their PRS2 and PRS3 ligands, as determined by X-ray diffraction studies. In contrast, the related MoMn complex yielded [MoMnCp(µ-η2:η2-SPR*)(CO)5] under most conditions. A similar output was obtained in reactions with gray selenium for either MoRe or MoMn phosphinidene complexes, which under different conditions only gave the pentacarbonyl complexes [MoMCp(µ-η2:η2-SePR*)(CO)5] (M = Re, Mn), these providing a new coordination mode for selenophosphinidene ligands.

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-PtBu2)(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-PtBu2)(CO)4] and involving the new complexes [W2Cp2(µ-PtBu2)(CO)4](BF4), [W2Cp2(µ-PtBu2)(CO)2(NO)2](BAr4), and [W2(µ-κ:η5-C5H4)Cp(µ-PtBu2)(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-PtBu2)(CO)(NO)2], and underwent N-O and W-W bond cleavages upon the addition of CNtBu to give the mononuclear phosphinoimido complex [WCp(NPtBu2)(CNtBu)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-OPtBu2)(NO)](BAr4), likely resulting from a N-O bond cleavage step following decarbonylation.

Electronic Structure and Donor Ability of an Unsaturated Triphosphorus-Bridged Dimolybdenum Complex.

The triphosphorus complex [Mo2Cp2(µ-η3:η3-P3)(µ-PtBu2)] was prepared in 83% yield by reacting the methyl complex [Mo2Cp2(µ-κ1:η2-CH3)(µ-PtBu2)(µ-CO)] with P4 at 333 K, a process also giving small amounts of the methyldiphosphenyl complex [Mo2Cp2(µ-η2:η2-P2Me)(µ-PtBu2)(CO)2]. The latter could be better prepared by first reacting the anionic complex Na[Mo2Cp2(µ-PtBu2)(µ-CO)2] with P4 to give the diphosphorus derivative Na[Mo2Cp2(µ-η2:η2-P2)(µ-PtBu2)(CO)2] and further reaction of the latter with MeI. Density functional theory calculations on the title complex revealed that its triphosphorus group can be viewed as an allylic-like P3- ligand acting as a six-electron donor via the external P atoms, while coordination of the internal P atom involves donation from the π orbital of the ligand and back-donation to its π* orbital, both interactions having a weakening effect on the Mo-Mo and P-P connections. The reactivity of the title compound is dominated by the electron-donor ability associated with the lone pairs located at the P atoms. Its reaction with CF3SO3Me gave [Mo2Cp2(µ-η3:η3-P3Me)(µ-PtBu2)](CF3SO3) as a result of methylation at an external atom of the P3 ligand, while its reaction with [Fe2(CO)9] enabled the addition of one, two, or three Fe(CO)4 fragments at these P atoms, but only the diiron derivative [Mo2Fe2Cp2(µ-η3:η3:κ1:κ1-P3)(µ-PtBu2)(CO)8] could be isolated. This complex bears a Fe(CO)4 fragment at each of the external atoms of the P3 ligand, and the central P atom of the latter displays the lowest 31P chemical shift reported to date (δP -721.8 ppm). The related complexes [Mo2M2Cp2(µ-η3:η3:κ1:κ1-P3)(µ-PtBu2)(CO)10] (M = Mo, W) were prepared by reacting the title compound with the corresponding [M(CO)5(THF)] complexes in toluene, while reaction with [Mo(CO)4(THF)2] also enabled the formation of the heptanuclear derivative [Mo7Cp4(µ-η3:η3:κ1:κ1-P3)2(µ-PtBu2)2(CO)14]. The interatomic distances in the above compounds indicate that the central Mo2P3 skeleton of these molecules is little modified by the attachment of 16-electron M(CO)n fragments at the external atoms of the P3 ligand.

Efficient Synthesis and Multisite Reactivity of a Phosphinidene-Bridged Mo-Re Complex. A Platform Combining Nucleophilic and Electrophilic Features.

The heterometallic complex [MoReCp(µ-PR*)(CO)6] (3) was prepared in 60% overall yield from syn-[MoCp(PHR*)(CO)2] via a three-step procedure involving complexes syn-[MoCp(PClR*)(CO)2] and [MoReCp(µ-PR*)(CO)7] as intermediate species (R* = 2,4,6-C6H2tBu3). The PR* ligand in 3 displays a novel asymmetric interaction with the dimetal center, involving a double bond with one atom (Mo) and a dative single bond with the other one (Re). Compound 3 underwent thermal isomerization involving a C-H bond cleavage to yield the hydride [MoReCp(µ-H){µ-P(CH2CMe2)C6H2tBu2}(CO)6] and reacted with I2 to give [MoReCpI2(µ-PR*)(CO)6], which displays a symmetrical phosphinidene bridge. Its reaction with methyl propiolate at 293 K proceeded with [2 + 2] cycloaddition of the alkyne and decarbonylation to yield the phosphapropenylidene-bridged complex [MoReCp{µ-κ2P,C:η3-PR*CHC(CO2Me)}(CO)5] as the major product, whereas its reaction with excess CN(4-C6H4OMe) at 273 K proceeded with formal [2 + 1] cycloaddition of the isocyanide and further isocyanide addition at the Re site to yield the complex [MoReCp{µ-η2P,C:κ1P-PR*CN(4-C6H4OMe)}(CO)6{CN(4-C6H4OMe)}], which displays an azaphosphaallene ligand in a novel bridging coordination mode.

Reactivity of N-Phosphinoguanidines of the Formula (HNR)(Ph2PNR)C(NAr) toward Main Group Metal Alkyls: Facile Ligand Rearrangement from N-Phosphinoguanidinates to Phosphinimine-Amidinates.

We report the reactivity of N-phosphinoguanidines of the formula (HNR)(Ph2PNR)C(NAr) (R = iPr and Ar = 2,6-iPr2C6H3 [Dipp] for 1a, R = iPr and Ar = 2,4,6-Me3C6H2 [Mes] for 1b, and R = Cy and Ar = Dipp for 1c), prepared in high yields from the corresponding trisubstituted guanidines, toward main group metal alkyls AlMe3, ZnEt2, MgnBu2, and nBuLi to obtain novel phosphinoguanidinato and phosphinimine-amidinato compounds. Reactions of 1a-c with AlMe3 at room temperature led to the kinetic phosphinoguanidinato products [Al{κ2-N,N'-(NR)C(NAr)(NRPPh2)}Me2] (2a-c), whereas the mild heating (60-80 °C) of solutions of 2a-c give the thermodynamic phosphinimine-amidinato products [Al{κ2-N,N'-(NR)C(NAr)(PPh2NR)}Me2] (3a-c) after ligand rearrangement. The reactions of equimolar amounts of 1a-c and ZnEt2 initially give solutions containing unstable phosphinoguanidinato compounds [Zn{κ2-N,P-(NR)C(NAr)(NRPPh2)}Et] (4a-c), which rearrange upon mild heating to the phosphinimine-amidinato derivatives [Zn{κ2-N,N'-(NR)C(NAr)(PPh2NR)}Et] (6a-c). Bis(phosphinoguanidinato) compounds [Zn{κ2-N,P-(NR)C(NAr)(NRPPh2)}2] (5a-c) can be obtained under mild conditions (<45 °C) in THF, whereas bis(phosphinimine-amidinato) compounds [Zn{κ2-N,N'-(NR)C(NAr)(PPh2NR)}2] (7a-c) are also accessible under more forcing conditions (55-100 °C) from (i) ZnEt2 and 1b,c (2 equiv), (ii) 6a and 1a, or (iii) 5b,c. Equimolar mixtures of MgnBu2 and 1a-c in THF at room temperature give unstable phosphinimine-amidinato monoalkyl products [Mg{κ2-N,N'-(NR)C(NAr)(PPh2NR)}nBu(THF)2] (8a-c), whereas 2 equiv of 1a,b are required to reach the bischelate compounds [Mg{κ2-N,N'-(NiPr)C(NAr)(PPh2NiPr)}2] (9a,b). Finally, phosphinoguanidinato compounds [Li{κ2-N,P-(NR)C(NDipp)(NRPPh2)}(THF)2] (10a,c) were obtained in the reactions of 1a,c with nBuLi in THF under ambient conditions. The removal of the solvent from solutions of 10a,c under partial vacuum leads to the dinuclear compounds [Li2{µ-κ2-N,N':κ1-N-(NR)C(NDipp)(NRPPh2)}2(THF)2] (11a,c) after the decoordination of one of the THF molecules in 10a,c and dimerization. Heating solutions of 10a,c at 60 °C triggers ligand rearrangement to give phosphinimine-amidinato compounds [Li{κ2-N,N'-(NR)C(NDipp)(PPh2NR)}(THF)2] (12a,c). We also propose a mechanism for the ligand rearrangement reaction from 10a to give 12a, supported by DFT calculations, which fits nicely with our experimental results. It essentially involves a carbodiimide deinsertion reaction followed by a [3 + 2] cycloaddition between the resulting lithium phosphino-amide and the carbodiimide.

P-N and N-Mo Bond Formation Processes in the Reactions of a Pyramidal Phosphinidene-Bridged Dimolybdenum Complex with Diazoalkanes and Organic Azides.

The reactivity of the complex [Mo2Cp(µ-κ1:κ1,η5-PC5H4)(CO)2(η6-HMes*)(PMe3)] (1) toward different diazoalkanes and organic azides was investigated. The pyramidal phosphinidene ligand in 1 displayed a strong nucleophilicity, enabling these reactions to proceed rapidly even below room temperature. Thus, 1 reacted rapidly at 253 K with different diazoalkanes N2CRR' (R,R' = H,H, Ph,Ph, H,CO2Et) to give the corresponding P:P-bridged phosphadiazadiene derivatives as major products which, however, could not be isolated. Reaction of the latter with [H(OEt2)2](BAr'4) yielded the corresponding cationic derivatives [Mo2Cp{µ-κ1P:κ1P,η5-P(NHNCRR')C5H4}(η6-HMes*)(CO)2(PMe3)](BAr'4), which were isolated in ca. 70% yield. The related species [Mo2Cp{µ-κ1P:κ1P,η5-P(NMeNCHCO2Et)C5H4}(η6-HMes*)(CO)2(PMe3)](BAr'4) was isolated upon reaction of the ethyl diazoacetate derivative with MeI and subsequent anion exchange with Na(BAr'4). Reaction of 1 with aryl azides (4-C6H4Me)N3 and (4-C6H4F)N3 proceeded rapidly at low temperature to give possibly the corresponding P:P-bridged phosphaimine derivatives as major products, which could be neither isolated. Protonation of these products with [H(OEt2)2](BAr'4) gave the corresponding aminophosphanyl complexes [Mo2Cp{µ-κ1P:κ1P,η5-P(NHR)C5H4}(η6-HMes*)(CO)2(PMe3)](BAr'4), isolated in ca. 75% yield. In contrast, the result of reactions of 1 with benzyl azide was strongly dependent on temperature, including the temperature in the subsequent methylation step that gave isolable cationic derivatives. By a careful choice of experimental conditions, different complexes having methylated phosphatriazadiene ligands were isolated, such as [Mo2Cp{µ-κ1P:κ1P,η5-P(NNNMeCH2Ph)C5H4}(η6-HMes*)(CO)2(PMe3)](BAr'4) and the metallacyclic derivatives syn- and anti-[Mo2Cp{µ-κ2P,N:κ1P,η5-P(NMeNNCH2Ph)C5H4}(η6-HMes*)(CO)(PMe3)](BAr'4). Density functional theory calculations, along with NMR monitoring experiments, revealed that the formation of the latter products stemmed from different and kinetically favored phosphatriazadiene intermediates, thermodynamically disfavored with respect to the denitrogenation process, otherwise yielding phosphaimine derivatives.

Coordination and Dehydrogenation of Diphosphine-Borane Ph2PCH2PPh2·BH3 at a Heterometallic MoRe Center to Give an Agostic Boryl-Bridged Derivative.

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 trans to the PCy2 group. 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-on tricentric 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.

Dehydrogenation, Methyl Elimination and Insertion Reactions of the Agostic Methyl-Bridged Complex [Mo2 Cp2 (µ-κ1 :η2 -CH3 )(µ-PtBu2 )(µ-CO)].

The high unsaturation of the title complex enabled it to react with a wide variety of molecules under mild conditions, whereby the agostic methyl ligand underwent unusual or unprecedented processes. Methane elimination occurred in the reactions with PPh2 H and SiPh2 H2 , this being followed in the latter case by Si-H bond oxidative addition to give the hydride silylene derivative [Mo2 Cp2 H(µ-PtBu2 )(µ-SiPh2 )(CO)]. Dehydrogenation, however, was the dominant process in the room temperature reaction with [Fe2 (CO)9 ], to give the unsaturated methylidyne cluster [Mo2 FeCp2 (µ3 -CH)(µ-PtBu2 )(CO)5 ] (Mo-Mo=2.6770(8) Å). In contrast, PMe elimination took place in the reaction with P4 , to give the unsaturated triphosphorus complex [Mo2 Cp2 (µ-η3 :η3 -P3 )(µ-PtBu2 )] (Mo-Mo=2.6221(3) Å). Yet a most remarkable reaction occurred with BH3 ⋅THF, involving insertion of two BH3 units and dehydrogenation to yield [Mo2 Cp2 (µ-B2 H4 Me)(µ-PtBu2 )(CO)], with the novel methyldiboranyl ligand acting as a 5-electron donor due to the presence of two 3-centre, 2-electron B-H-Mo interactions, according to spectroscopic data and DFT calculations (Mo-Mo ca. 2.65 Å).

Acetonitrile Adduct [MoReCp(µ-H)(µ-PCy2)(CO)5(NCMe)]: A Surrogate of an Unsaturated Heterometallic Hydride Complex.

The title compound was prepared upon irradiation of acetonitrile solutions of the readily available hexacarbonyl [MoReCp(µ-H)(µ-PCy2)(CO)6]. The acetonitrile ligand in this compound could be replaced easily by donor molecules or displaced upon two-electron reduction. In most cases, the substitution step was followed by additional processes such as insertion into the M-H bonds, E-H bond cleavage, H2 elimination, and other transformations.

E-H Bond Activation and Insertion Processes in the Reactions of the Unsaturated Hydride [W2Cp2(µ-H)(µ-PPh2)(NO)2].

The reactions of the title complex (1) with different p-block element (E) molecules was examined. Compound 1 reacted 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 1 was reacted with HSnPh3, to give the related dihydride stannyl derivative [W2Cp2(µ-H)H(µ-PPh2)(NO)2(SnPh3)]. In contrast, the reaction of 1 with HSPh involved H2 elimination 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 1 with 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:η2 coordination mode. The reaction of 1 with diazomethane proceeded with N2 elimination 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.

Phosphinidene-Bridged MoMn Derivatives of the Thiophosphinidene Complex [Mo2Cp2(µ-κ2:κ1,η6-SPMes*)(CO)2] (Mes* = 2,4,6-C6H2tBu3).

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)2 fragment in 1 with a Mn(CO)4 fragment, and displayed multiple bonding to phosphorus (Mn-P = 2.1414(8) Å); the tetranuclear cluster results from formal insertion of a Mn2(CO)6 fragment 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 1 and [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.

N-O Bond Activation and Cleavage Reactions of the Nitrosyl-Bridged Complexes [M2Cp2(µ-PCy2)(µ-NO)(NO)2] (M = Mo, W).

The title complexes (1a,b) were prepared in two steps by first reacting the hydrides [M2Cp2(µ-H)(µ-PCy2)(CO)4] with [NO](BF4) in the presence of Na2CO3 to give dinitrosyls [M2Cp2(µ-PCy2)(CO)2(NO)2](BF4), which were then fully decarbonylated upon reaction with NaNO2 at 323 K. An isomer of the Mo2 complex having a cisoid arrangement of the terminal ligands ( cis-1a) was prepared upon irradiation of toluene solutions of 1a with visible-UV light at 288 K. The structure of these trinitrosyl complexes was investigated using X-ray diffraction and density functional theory (DFT) calculations, these revealing a genuine pyramidalization of the bridging NO that might be associated in part to an increase of charge at the N atom and anticipated a weakening of the N-O bond upon reaction with bases or reducing reagents. Complexes 1a,b reacted with [FeCp2](BF4) to give first the radicals [M2Cp2(µ-PCy2)(µ-NO)(NO)2](BF4) according to CV experiments, which then underwent H-abstraction to yield the nitroxyl-bridged complexes [M2Cp2(µ-PCy2)(µ-κ1:η2-HNO)(NO)2](BF4), alternatively prepared upon protonation with HBF4·OEt2. The novel coordination mode of the nitroxyl ligand in these products was thermodynamically favored over its tautomeric hydroximido form, according to DFT calculations, and similar nitrosomethane-bridged cations [M2Cp2(µ-PCy2)( µ-κ1:η2-MeNO)(NO)2]+ were prepared by reacting 1a,b with CF3SO3Me or [Me3O]BF4. Complexes 1 reacted with M(Hg) (M = Zn, Na) in tetrahydrofuran to give the amido-bridged derivatives [M2Cp2(µ-PCy2)(µ-NH2)(NO)2] with retention of stereochemistry, a transformation also induced by using mild O atom scavengers such as CO and phosphites in the presence of water. In the absence of water, phosphites accomplished a deoxygenation of the bridging NO of the Mo2 complexes to yield the phosphoraniminato-bridged derivatives [Mo2Cp2(µ-PCy2){µ-NP(OR)3}(NO)2] (R = Et, Ph), also with retention of stereochemistry.

Selective Three-Component Coupling for CO2 Chemical Fixation to Boron Guanidinato Compounds.

A selective three-component coupling was employed to fix carbon dioxide to boron guanidinato compounds. The one-pot reaction of carbon dioxide, carbodiimides, and borylamines (ArNH)BC8H14 afforded the corresponding 1,2-adducts {R(H)N}C{N(Ar)}(NR)(CO2)BC8H14. Alternatively, the reaction with p-MeOC6H4NC or 2,6-Me2C6H3NC gave the corresponding isocyanide 1,1-adducts { i-PrHN}C{N(p-Me-C6H4)}(N i-Pr){CNAr}BC8H14. The molecular structures of products (2,6- i-Pr2C6H3NH)BC8H14 7, { i-Pr(H)N}C{N(p-MeC6H4)}(N i-Pr)(CO2)BC8H14 9, {Cy(H)N}C{N( p-MeC6H4)}(Cy)(CO2)BC8H14 13, and { i-PrHN}C{N( p-MeC6H4)}(N i-Pr){CNR″}BC8H14 (R″ = p-MeOC6H4, 2,6-Me2C6H3) 14 and 15 were established by X-ray diffraction. Density functional theory calculations at the M05-2X level of theory revealed that CO2 fixation and formation of the corresponding adduct is exothermic and proceeds via a nonchelate boron guanidinato intermediate.

C-C and C-N Couplings Following Hydride Addition on Isocyanide Cyclopolyenyl Dimolybdenum Complexes to Give Tethered Aldimine and Aminocarbene Derivatives.

Reaction of [Mo2 Cp2 (µ-κ1 :κ1 ,η6 -PMes*)(CO)2 ] with S or Se followed by protonation with [H(OEt2 )2 ](BAr'4 ) gave the cationic derivatives [Mo2 Cp2 {µ-κ2P,E :κ1P ,η5 -EP(C6 H3 tBu3 )}(CNR)(CO)2 ](BAr'4 ) (E=S; R=tBu, iPr, Ph, 4-C6 H4 OMe, Xyl; or E=Se; R=tBu; Ar'=3,5-C6 H3 (CF3 )2 ). Reaction of the latter with K[BHsBu3 ] yielded the aldimine complexes [Mo2 Cp2 {µ-κ2P,E :κ2P,N ,η4 -SP(C6 H3 tBu3 (CHNR))}(CO)2 ] and their aminocarbene isomers [Mo2 Cp2 {µ-κ2P,E :κ2P,C ,η4 -SP(C6 H3 tBu3 (NRCH))}(CO)2 ] (R ≠ Xyl), following C-C and C-N couplings, respectively. Monitoring of these reactions revealed that the initial H- attack takes place at a Cp ligand to give cyclopentadiene intermediates [Mo2 Cp{µ-κ2P,S :κ1P ,η5 -SP(C6 H3 tBu3 )}(η4 -C5 H6 )(CNR)(CO)2 ], which then undergo C-H oxidative addition to give the hydride isomers [Mo2 Cp2 {µ-κ2P,S :κ1P ,η3 -SP(C6 H3 tBu3 )}(H)(CNR)(CO)2 ]. In turn, the latter rearrange to give the aldimine and aminocarbene complexes. DFT calculations revealed that the hydride intermediates first undergo migratory insertion of the isocyanide ligand into the Mo-H bond to give unobservable formimidoyl intermediates, which then evolve either by nucleophilic attack of the N atom on the C6 ring (C-N coupling) or by migratory insertion of the formimidoyl ligand into the C6 ring (C-C coupling). Our data suggest that increasing the size of the substituent R at the isocyanide ligand destabilizes the aldimine isomer to a greater extent, thus favoring formation of the aminocarbene complex.

Structural and Chemical Effects of the PtBu2 Bridge at Unsaturated Dimolybdenum Complexes Having Hydride and Hydrocarbyl Ligands.

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 2 with [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 3 gave support to the presence of a M≡M triple bond in this complex following from a σ2δ2δ2 configuration, a view also supported by the high electron density accumulated at the corresponding Mo-Mo bond critical point. In contrast, reactions of 2 with 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 4a took 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 4b led 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 H2 elimination to the benzylidyne derivative. Analogous dehydrogenation of the methyl ligand in 7 could 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.

Synthesis and DFT Study of a Diphenylsilanone-Bridged Dimolybdenum Complex.

Reaction of the 30-electron benzylidyne complex [Mo2 Cp2 (µ-CPh)(µ-PCy2 )(µ-CO)] with excess Ph2 SiH2 under visible-UV irradiation yields the silylene-bridged complex [Mo2 Cp2 (µ-CPh)(µ-PCy2 )(µ-SiPh2 )]. This compound undergoes selective oxidation with O2 to give the unsaturated complex [Mo2 Cp2 (µ-CPh)(µ-PCy2 )(µ-κ(1) :κ(1) -OSiPh2 )], which contains an unprecedented bridging diphenylsilanone ligand, as confirmed by X-ray diffraction analysis and DFT calculations. The bonding within the central Mo2 SiO ring of this complex approaches the extreme description of a dimetallacyclosiloxane according to the relevant solid-state bond lengths and theoretical calculations.

Cycloaddition Reactions of the Phosphinidene-Bridged Complex [Mo2Cp(µ-κ1:κ1,η5-PC5H4)(CO)2(η6-HMes*)] with Diazoalkanes and Other Heterocumulenes.

The title phosphinidene complex reacted at room temperature with CS2 and SCNPh to give the phosphanyl derivatives [Mo2Cp{µ-κ2P,S:κ1S',η5-P(CS2)C5H4}(CO)2(η6-HMes*)] and [Mo2Cp{µ-κ2P,S:κ1P,η5-P(C(NPh)S)C5H4}(CO)2(η6-HMes*)], respectively (Mes* = 2,4,6-C6H2tBu3), which result from a [2 + 2] cycloaddition of a C═S bond in the organic reagent to the Mo═P bond of the phosphinidene complex, with further insertion of S into the remaining Mo-P bond, in the CS2 reaction. The title complex also reacted with diazoalkanes N2CRR' at room temperature to give the corresponding phosphaalkene derivatives [Mo2Cp{µ-η2:κ1P,η5-P(CRR')C5H4}(CO)2(η6-HMes*)] (CRR' = CH2, CPh2, CH(SiMe3)). These products follow from a formal [2 + 1] cycloaddition of the carbene CRR' fragment to the Mo═P bond of the parent compound but were shown to proceed through a [3 + 2] cycloaddition of the diazoalkane molecule, followed by N2 elimination. The diazomethane reaction allowed the identification at low temperature of a stabilized form of the intermediate product, the phosphanyl complex [Mo2Cp{µ-κ2P,N:κ1P,η5-P(CHN2H)C5H4}(CO)2(η6-HMes*)], which follows from a reversible 1,3-shift of a methylenic H atom from C to N. It was concluded that all of the above cycloaddition reactions are initiated by heteroatom coordination of the unsaturated organic molecule to the MoCp(CO)2 fragment in the parent phosphinidene complex, this triggering the P-C bond formation step which leads to the products eventually isolated. The structures of the new complexes were determined by spectroscopic, diffractometric, and (in some cases) density functional theory methods.

Mild N-O Bond Cleavage Reactions of a Pyramidalized Nitrosyl Ligand Bridging a Dimolybdenum Center.

Complex [Mo2Cp2(µ-PCy2)(µ-NO)(NO)2] (1) was prepared by reacting [Mo2Cp2(µ-H)(µ-PCy2)(CO)4] with 2 equiv of [NO]BF4 and then treating the resulting product [Mo2Cp2(µ-PCy2)(CO)2(NO)2](BF4) with NaNO2 at 323 K, and it was shown to display a bridging nitrosyl ligand with significant pyramidalization at the N atom, a circumstance related to an unusual behavior concerning degradation of the bridging nitrosyl. Indeed, complex 1 reacts with HBF4·OEt2 to give the nitroxyl-bridged derivative [Mo2Cp2(µ-PCy2)(µ-κ(1):η(2)-HNO)(NO)2](BF4), is reduced by Zn(Hg) in the presence of trace H2O to give the amido complex [Mo2Cp2(µ-PCy2)(µ-NH2)(NO)2], and reacts with excess P(OPh)3 to give the phosphoraniminato-bridged derivative [Mo2Cp2(µ-PCy2){µ-NP(OPh)3}(NO)2].

Electronic Structure and Multisite Basicity of the Pyramidal Phosphinidene-Bridged Dimolybdenum Complex [Mo2(η(5)-C5H5)(µ-κ(1):κ(1),η(5)-PC5H4)(η(6)-C6H3(t)Bu3)(CO)2(PMe3)].

The title phosphinidene complex could be sequentially protonated with HBF4·OEt2 or [H(OEt2)2](BAr'4) to give the phosphido-bridged derivatives [Mo2Cp(µ-κ(1):κ(1),η(5)-HPC5H4)(η(6)-HMes*)(CO)2(PMe3)]X and then the hydrides [Mo2Cp(H)(µ-κ(1):κ(1),η(5)-HPC5H4)(η(6)-HMes*)(CO)2(PMe3)]X2 (X = BF4, BAr'4; Ar' = 3,5-C6H3(CF3)2; Mes* = 2,4,6-C6H2(t)Bu3). Density functional theory (DFT) calculations revealed that the most favored site for initial electrophilic attack is the metallocene Mo atom, but attachment of the electrophile to the phosphinidene P atom gives more stable products. This was in agreement with all other reactions investigated, which invariably involved the attachment of the added electrophile at the P site. Thus, the title compound reacted with S8 at 223 K to give the thiophosphinidene-bridged complex [Mo2Cp{µ-κ(1):κ(1),η(5)-P(S)C5H4}(η(6)-HMes*)(CO)2(PMe3)], a poorly stable molecule which reacted with MeI at room temperature to give the corresponding thiolatophosphido derivative, isolated as [Mo2Cp{µ-κ(1):κ(1),η(5)-P(SMe)C5H4}(η(6)-HMes*)(CO)2(PMe3)](BAr'4) (P-S = 2.128(4) Å) after anion exchange with Na(BAr'4). Reaction of the title compound with MeI proceeded smoothly to give the corresponding methylphosphido derivative, isolated analogously as [Mo2Cp{µ-κ(1):κ(1),η(5)-P(Me)C5H4}(η(6)-HMes*)(CO)2(PMe3)](BAr'4). The related complex [Mo2Cp{µ-κ(1):κ(1),η(5)-P(Me)C5H4}(η(6)-HMes*)(CO)2(PMe2Ph)](BAr'4) (P-C(Me) = 1.841(5) Å) could be prepared analogously from the neutral precursor [Mo2Cp{µ-κ(1):κ(1),η(5)-PC5H4}(η(6)-HMes*)(CO)2(PMe2Ph)]. In contrast, reaction of the title complex with ethylene sulfide involved opening of the C2S ring and formation of new P-C and Mo-S bonds (1.886(7) and 2.493(2) Å, respectively), with displacement of the PMe3 ligand, to give the phosphido-thiolato complex [Mo2Cp{µ-κ(2)(P,S):κ(1)P,η(5)-P(C2H4S)C5H4}(η(6)-HMes*)(CO)2]. All these derivatives of the title complex displayed an unusual trigonal pyramidal-like environment around the bridging P atom, with the added electrophile placed in the Mo2P plane as a result of the directionality of the relevant frontier orbital of the phosphinidene complex, according to DFT calculations.