M. Angeles Alvarez

Activation of H-H and H-O bonds at phosphorus with diiron complexes bearing pyramidal phosphinidene ligands.

The complex [Fe(2)Cp(2)(μ-PMes*)(μ-CO)(CO)(2)] (Mes* = 2,4,6-C(6)H(2)(t)Bu(3)), which in the solid state displays a pyramidal phosphinidene bridge, reacted at room temperature with H(2) (ca. 4 atm) to give the known phosphine complex [Fe(2)Cp(2)(μ-CO)(2)(CO)(PH(2)Mes*)] as the major product, along with small amounts of other byproducts arising from the thermal degradation of the starting material, such as the phosphindole complex [Fe(2)Cp(2)(μ-CO)(2)(CO){PH(CH(2)CMe(2))C(6)H(2)(t)Bu(2)}], the dimer [Fe(2)Cp(2)(CO)(4)], and free phosphine PH(2)Mes*. During the course of the reaction, trace amounts of the mononuclear phosphide complex [FeCp(CO)(2)(PHMes*)] were also detected, a compound later found to be the major product in the carbonylation of the parent phosphinidene complex, with this reaction also yielding the dimer [Fe(2)Cp(2)(CO)(4)] and the known diphosphene Mes*P═PMes*. The outcome of the carbonylation reactions of the title complex could be rationalized by assuming the formation of an unstable tetracarbonyl intermediate [Fe(2)Cp(2)(μ-PMes*)(CO)(4)] (undetected) that would undergo a fast homolytic cleavage of a Fe-P bond, this being followed by subsequent evolution of the radical species so generated through either dimerization or reaction with trace amounts of water present in the reaction media. A more rational synthetic procedure for the phosphide complex was accomplished through deprotonation of the phosphine compound [FeCp(CO)(2)(PH(2)Mes*)](BF(4)) with Na(OH), the latter in turn being prepared via oxidation of [Fe(2)Cp(2)(CO)(4)] with [FeCp(2)](BF(4)) in the presence of PH(2)Mes*. To account for the hydrogenation of the parent phosphinidene complex it was assumed that, in solution, small amounts of an isomer displaying a terminal phosphinidene ligand would coexist with the more stable bridged form, a proposal supported by density functional theory (DFT) calculations of both isomers, with the latter also revealing that the frontier orbitals of the terminal isomer (only 5.7 kJ mol(-1) above of the bridged isomer, in toluene solution) have the right shapes to interact with the H(2) molecule. In contrast to the above behavior, the cyclohexylphosphinidene complex [Fe(2)Cp(2)(μ-PCy)(μ-CO)(CO)(2)] failed to react with H(2) under conditions comparable to those of its PMes* analogue. Instead, it slowly reacted with HOR (R = H, Et) to give the corresponding phosphinous acid (or ethyl phosphinite) complexes [Fe(2)Cp(2)(μ-CO)(2)(CO){PH(OR)Mes*}], a behavior not observed for the PMes* complex. The presence of BEt(3) increased significantly the rate of the above reaction, thus pointing to a pathway initiated with deprotonation of an O-H bond of the reagent by the basic P center of the phosphinidene complex, this being followed by the nucleophilic attack of the OR(-) anion at the P site of the transient cationic phosphide thus formed. The solid-state structure of the cis isomer of the ethanol derivative was determined through a single crystal X-ray diffraction study (Fe-Fe = 2.5112(8) Å, Fe-P = 2.149(1) Å).

Aurophilic Self-Assembly of a Mo4Au2 Phosphinidene Complex with an Unprecedented H-Shaped Planar Metal Core

The isomers [Mo2Cp2(mu-kappa(1):kappa(1),eta(6)-PR*)(CO)2] (1) and [Mo2Cp(mu-kappa(1):kappa(1),eta(5)-PC5H4)(CO)2(eta(6)-HR*)] (2) (Cp = eta(5)-C5H5; R* = 2,4,6-C6H2(t)Bu3) react with [AuCl(THT)] and with the cation [Au(THT)2](+) (THT = tetrahydrothiophene) to give phosphinidene-bridged Mo2Au complexes resulting from the addition of an AuCl or Au(THT)(+) electrophile to their multiple P-Mo bonds. Removal of the Cl(-) or THT ligand from these derivatives causes a dimerization of the trinuclear structures to give the cationic derivative [{AuMo2Cp(mu3-kappa(1):kappa(1):kappa(1),eta(5)-PC5H4)(CO)2(eta(6)-HR*)}2](2+), which displays a novel H-shaped metal core held by strong Mo-Au dative bonds [2.768(1) A] and an aurophilic interaction [Au-Au = 3.022(1) A].

Proton induced P–H and Mo–H bond activation at the phosphide bridged dimolybdenum complexes [Mo2Cp2(µ-H)(µ-PHR)(CO)4](R = Cy, 2,4,6-C6H2R′3; R′= H, Me, tBu)

The new hydride complexes [Mo2Cp2(mu-H)(mu-PHR)(CO)4] having bulky substituents (R = 2,4,6-C(6)H2tBu3= Mes*, R = 2,4,6-C6H2Me3= Mes) have been prepared in good yield by addition of Li[PHR] to the triply bonded [Mo2Cp2(CO)4] and further protonation of the resulting anionic phosphide complex [Mo2Cp2(mu-PHR)(CO)4]-. Protonation of the Mes* compound with either [H(OEt2)2][B{3,5-C6H3(CF3)2}4] or HBF4.OEt2 gives the cationic phosphinidene complex [Mo2Cp2(mu-H)(mu-PMes*)(CO)4]+ in high yield. In contrast, protonation of the analogous hydride compounds with Mes or Cy substituents on phosphorus give the corresponding unsaturated tetracarbonyls [Mo2Cp2(mu-PHR)(CO)4]+, which are unstable at room temperature and display a cis geometry. Decomposition of the latter give the electron-precise pentacarbonyls [Mo2Cp2(mu-PHR)(mu-CO)(CO)4]+, also displaying a cis arrangement of the metal fragments. In the presence of BF4- as external anion, fluoride abstraction competes with carbonylation to yield the neutral fluorophosphide hydrides [Mo2Cp2(mu-H)(mu-PFR)(CO)4]. Similar results were obtained in the protonation reactions of the hydride compounds having a Ph substituent on phosphorus. In that case, using HCl as protonation reagent gave the chloro-complex [Mo2ClCp2(mu-PHPh)(CO)4] in good yield. The structures and dynamic behaviour of the new compounds are analyzed on the basis of solution IR and 1H, 31P, 19F and 13C NMR data as well as the X-ray studies carried out on [Mo2Cp2(mu-H)(mu-PHMes)(CO)4](cis isomer), [Mo2Cp2(mu-H)(mu-PFMes)(CO)4](trans isomer), [Mo2Cp2(mu-PHCy)(mu-CO)(CO)4](BF4) and [Mo2ClCp2(mu-PHPh)(CO)4].

Mild P4 activation to give an anionic diphosphorus complex with a dual binding ability at a single P site.

The anion [Mo(2)Cp(2)(μ-PCy(2))(μ-CO)(2)](-) (1; Li(+) salt) reacts at 290 K with P(4) to give the diphosphorus-bridged complex [Mo(2)Cp(2)(μ-PCy(2))(CO)(2)(μ-κ(2):κ(2)-P(2))](-) (2). The latter reacts with MeI and ClSnPh(3) through a single P atom to give respectively diphosphenyl [Mo(2)Cp(2)(μ-PCy(2))(CO)(2)(μ-κ(2):κ(2)-P(2)Me)] (3) and stannyl [Mo(2)Cp(2)(μ-PCy(2))(CO)(2){μ-κ(2):κ(2)-P(2)(SnPh(3))}] (4) derivatives, with the P-P-Sn angle in 4 being unexpectedly acute [80.3(1)°]. According to density functional theory calculations, this novel nucleophilic behavior of 1 is derived from its anionic nature, thus enabling the P(2) ligand to act in a π-donor-like fashion.

Chemistry of the Oxophosphinidene Ligand. 1. Electronic Structure of the Anionic Complexes [MCp{P(O)R*}(CO)2]- (M = Mo, W; R* = 2,4,6-C6H2tBu3) and Their Reactions with H+ and C-Based Electrophiles

The anionic phosphide-bridged complexes (H-DBU)[M(2)Cp(2)(μ-PHR*)(CO)(4)] (M = Mo, W; R* = 2,4,6-C(6)H(2)(t)Bu(3); Cp = η(5)-C(5)H(5), DBU = 1,8-diazabicyclo [5.4.0] undec-7-ene) react with molecular oxygen to give the corresponding oxophosphinidene complexes (H-DBU)[MCp{P(O)R*}(CO)(2)] as major products (Mo-P = 2.239(1) Å for the Mo complex). The latter anionic complexes are protonated by HBF(4)·OEt(2) to give the hydroxyphosphide derivatives [MCp{P(OH)R*}(CO)(2)]. In the presence of excess acid, the molybdenum complex yields the fluorophosphide complex [MoCp(PFR*)(CO)(2)] (Mo-P = 2.204(1) Å), while the tungsten compound reacts with excess HCl to give an unstable chlorophosphine complex [WCpCl(PHClR*)(CO)(2)] which is rapidly hydrolyzed to give [WCpCl{PH(OH)R*}(CO)(2)], having a complexed arylphosphinous acid (Mo-P = 2.460(2) Å). The molybdenum anion reacts with strong C-based electrophiles such as [Me(3)O]BF(4), Et(2)SO(4), C(2)H(3)C(O)Cl, and PhC(O)Cl to give the corresponding alkoxyphosphide derivatives [MoCp{P(OR)R*}(CO)(2)] (R = Me, Et, COC(2)H(3), COPh; Mo-P = 2.197(2) Å for the benzoyl compound), as a result of the attack of the electrophile at the O atom of the oxophosphinidene ligand. In contrast, the reactions with milder alkylating reagents such as the alkyl halides MeI, EtI, C(3)H(5)Br, and C(3)H(3)Br give selectively the corresponding κ(2)-phosphinite complexes [MoCp{κ(2)-OP(R)R*}(CO)(2)] [R = Me, Et, C(3)H(5), C(3)H(3); Mo-P = 2.3733(5) Å for the allyl compound] as a result of the attack of the electrophile at the P atom of the oxophosphinidene ligand. According to density functional theory (DFT) calculations, the oxygen atom of the phosphinidene ligand bears the highest negative charge in the molybdenum anion, while the highest occupied molecular orbital (HOMO) of this complex has substantial Mo-P π bonding character. Thus, it is concluded that the phosphinite complexes are formed under conditions of orbital control, while charge-controlled reactions tend to give alkoxyphosphide derivatives.

Reactivity of the anionic diphosphorus complex [Mo2Cp2(μ-PCy2)(CO)2(μ-κ2:κ2-P2)]- toward ER3X electrophiles (E = C to Pb): insights into the multisite donor ability and dynamics of the P2 ligand.

The Li(+) salt of the unsaturated anion [Mo(2)Cp(2)(μ-PCy(2))(μ-CO)(2)](-) reacted with P(4) in tetrahydrofuran at room temperature to give the title complex. This fluxional anion reacted with MeI and ClCH(2)Ph to give the diphosphenyl complexes [Mo(2)Cp(2)(μ-κ(2):κ(2)-P(2)CH(2)R)(μ-PCy(2))(CO)(2)] (R = H, Ph), with the incoming electrophile being attached at the basal P atom of the Mo(2)P(2) tetrahedron via the lone electron pair (P-P-CH(3) = 122.8(1)(o)). In contrast, reactions with ClER(3) (ER(3) = GePh(3), SnPh(3), PbMe(3), PbPh(3)) gave neutral complexes [Mo(2)Cp(2)(μ-κ(2):κ(2)-P(2)ER(3))(μ-PCy(2))(CO)(2)] having the incoming electrophile attached at the basal P atom but defining an acute P-P-E angle close to 90° with elongated P-P lengths of ca. 2.20 Å. These complexes undergo an easy fluxional process involving an exchange of the ER(3) group between the P atoms that could be properly modeled through DFT calculations, and some of them displayed minor isomers in solution. Their structure could be rationalized as derived from the interaction of the electrophile with high-energy orbitals of the anion having both σ(Mo-P) and π(P-P) bonding character. Reaction with BrSiMe(3) gave instead the agostic phosphenyl complex [Mo(2)Cp(2)(μ-κ(2):κ(1),η(2)-HP(2))(μ-PCy(2))(CO)(2)], formally derived from the attachment of a proton to a basal Mo-P edge of the anion (computed length 2.810 Å) and displaying an unusually low P-H coupling of 4 Hz. A similar structure, with the agostic H atom replaced with SnH(3), was found to be a satisfactory model for the minor isomer of the tin compound and represents a third and unprecedented coordination mode of the diphosphorus ligand. The agostic complex undergoes a fluxional process involving the intermediacy of the nonagostic isomer [Mo(2)Cp(2)(μ-κ(2):κ(2)-P(2)H)(μ-PCy(2))(CO)(2)], which was computed to display a geometry comparable to the major isomers of the ER(3) compounds (P-P = 2.183 Å; P-P-H = 81.7°).

Reactions of the Unsaturated Complex [Mo2(η5-C5H5)2(μ-PEt2)2(CO)2] with [Au(PR3)]+ Cations: Kinetic Preference of the Mo-P Bonds as the Site of Attack of the Gold(I) Electrophile

The title complex reacts with [Au(PR(3))](+) cations (PF(6)(-) salts, R = p-tol, Me) in dichloromethane solutions to give first the corresponding agostic-like products [AuMo(2)Cp(2)(mu-PEt(2))(mu(3)-PEt(2))(CO)(2)(PR(3))]PF(6), which then partially rearrange to reach an equilibrium with the hydride-like isomers [AuMo(2)Cp(2)(mu-PEt(2))(2)(CO)(2)(PR(3))]PF(6), the latter being characterized through an X-ray study (R = p-tol, Mo-Mo = 2.8244(2) A). These unsaturated complexes react smoothly with CO (1 atm) to give the corresponding electron-precise derivatives [AuMo(2)Cp(2)(mu-PEt(2))(2)(CO)(3)(PR(3))](+) (Mo-Mo = 3.0438(6) A when R = Me), this implying the rearrangement of the mu(3)-PEt(2) ligand to a more common mu(2)-coordination mode. Density functional theory (DFT) calculations on the dimolybdenum complexes [Mo(2)Cp(2)(mu-PR(2))(2)(CO)(2)] (R = Cy, Et) reveal the presence of a framework M-P bonding orbital high in energy and with the right shape to act as a donor to H(+) and [Au(PR(3))](+) cations, thus explaining the formation of agostic and agostic-like products respectively in these reactions. The unusually high energy of this donor orbital can be related to the close approach of the metal centers in these unsaturated molecules. The carbyne complex [Mo(2)Cp(2)(mu-COMe)(mu-PCy(2))(CO)(2)] reacts with [Au{P(p-tol)(3)}](+) to give the tricarbonyl [AuMo(2)Cp(2)(mu-COMe)(mu-PCy(2))(CO)(3){P(p-tol)(3)}](+) (Mo-Mo = 2.986(1) A), a process most likely initiated by the binding of the gold cation to one of the Mo-P bonds in the carbyne complex.

Synthesis and decarbonylation reactions of the triiron phosphinidene complex [Fe3Cp3(μ-H)(μ3-PPh)(CO)4]: easy cleavage and formation of P-H and Fe-Fe bonds.

The binuclear phosphine complex [Fe(2)Cp(2)(μ-CO)(2)(CO)(PH(2)Ph)] (Cp = η(5)-C(5)H(5)) reacted with the acetonitrile adduct [Fe(2)Cp(2)(μ-CO)(2)(CO)(NCMe)] in dichloromethane at 293 K to give the trinuclear hydride-phosphinidene derivative [Fe(3)Cp(3)(μ-H)(μ(3)-PPh)(CO)(4)] as a mixture of cis,anti and trans isomers (Fe-Fe = 2.7217(6) Å for the cis,anti isomer). In contrast, photochemical treatment of the phosphine complex with [Fe(2)Cp(2)(CO)(4)] gave the phosphide-bridged complex trans-[Fe(3)Cp(3)(μ-PHPh)(μ-CO)(2)(CO)(3)] as the major product, along with small amounts of the binuclear hydride-phosphide complexes [Fe(2)Cp(2)(μ-H)(μ-PHPh)(CO)(2)] (cis and trans isomers), which are more selectively prepared from [Fe(2)Cp(2)(CO)(4)] and PH(2)Ph at 388 K. The photochemical decarbonylation of either of the mentioned triiron compounds led reversibly to three different products depending on the reaction conditions: (a) the 48-electron phosphinidene cluster [Fe(3)Cp(3)(μ-H)(μ(3)-PPh)(μ-CO)(2)] (Fe-Fe = 2.592(2)-2.718(2) Å); (b) the 50-electron complex [Fe(3)Cp(3)(μ-H)(μ(3)-PPh)(μ-CO)(CO)(2)], also having carbonyl- and hydride-bridged metal-metal bonds (Fe-Fe = 2.6177(3) and 2.7611(4) Å, respectively); and (c) the 48-electron phosphide cluster [Fe(3)Cp(3)(μ-PHPh)(μ(3)-CO)(μ-CO)(2)], an isomer of the latter phosphinidene complex now having three intermetallic bonds (Fe-Fe = 2.5332(8)-2.6158(8) Å).

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.

From coordinated oxophosphinidene to O,O,P-bound arylhypophosphite(2-) to build a singular Mo2Sn2O4P2 metallocycle.

P-bound derivatives of the hypophosphorous acids can be prepared from complexed phosphinidene oxides. The anion [MoCp(CO) 2{P(O)R*}] (-)[(H-DBU) (+) salt (R* = 2,4,6-C 6H 2 (t)Bu 3; Cp = eta (5)-C 5H 5) reacts with SnCl 2Ph 2 to give the neutral oxophosphinidene complex [MoCp{P(O)R*}(CO) 2(SnPh 2Cl)] in high yield. Basic hydrolysis of the latter complex occurs readily to yield the novel Mo-Sn metallocyclic derivatives [MoSn{mu- O, P-OP(OH)R*}CpPh 2(CO) 2] and [MoSn{mu- O, O, P-O 2PR*}CpPh(CO) 2] 2. These products display respectively hydrogenhypophosphite(1-) and hypophosphite(2-) anions coordinated through all of their O (to Sn) and P (to Mo) atoms.