Synthese und Struktur von (tmeda)Ni(H₂C=CHCOOCH₃)₂

Ni(C2H4)3, Ni(cdt), or Ni(cod)2 react with tmeda and acrylic acid methyl ester, methyl vinyl ether, and acrylonitrile in ether to afford (tmeda)Ni(H2C=CHCOOCH3)2 (3, orange-red crystals), (tmeda)Ni(H2C=CHCOCH3)2 (4, red crystals), and (tmeda)Ni(H2C=CHCN)2 (5, yellow precipitate) in up to 90% yield. These complexes are also formed by interaction of the homoleptic bis(alkene) nickel(0) complexes Ni(H2C=CHCOOCH3)2, Ni(H2C=CHCOCH3)2, and Ni(H2C=CHCN)2 with tmeda, and by the reaction of (tmeda)Ni(CH3)2 with the alkenes in ether at −30°C or above, under reductive elimination of ethane. Solid 3 is stable to about 110°C, whereas 4 decomposes slowly at 20°C; decomposition of 5 occurs at 136°C. The IR, 1H and 13C NMR spectra are reported. In addition, 3 has been characterized by an X-ray diffraction study

Synthesis, structure, and properties of {(Me₃Si)₂CH}₂SnH(OH)

The Lappert stannylene SnR2, R = CH(SiMe3)2, adds water and methanol to yield the low-melting-point, crystalline hydroxy- and methoxydiorganostannanes R2SnH(OH) (1) and R2SnH(OMe) (2). The corresponding deuterated derivatives R2SnD(OD) (1-d2) and R2SnD(OCD3) (2-d4) have also been prepared. Compounds 1 and 2 react with D2O with retention of the Sn−H bond to give R2SnH(OD) (1-d(SnOD)). The reaction is thought to proceed by an SN2 type mechanism via a [R2SnH(OR‘)2]- (R‘ = H, D, or Me) intermediate or transition state. Consistent with this, 2-d4 is hydrolyzed to R2SnD(OH) (1-d(SnD)). A single-crystal X-ray structure analysis of 1 reveals that individual molecules form Ci-symmetrical dimers in the solid with short O−H···O* hydrogen bridges (O···O* = 2.854(2) Å). Reaction of 1 with (iPr2PC2H4PiPr2)Pd(C2H4) results in oxidative addition of the Sn−H bond to Pd0 to give the known (iPr2PC2H4PiPr2)Pd(H)−SnR2(OH) (3)

Preparation and Structural Characterization of the Pd⁰–Carbonyl Complexes (R₂PC₂H₄PR₂)Pd(CO)₂ and {(R₂PC₂H₄PR₂)Pd}₂(µ-CO)

(dippe)Pd(C2H4) and (dtbpe)Pd(C2H4) react with an excess of CO to afford the novel tetrahedral Pd0−dicarbonyl complexes (dippe)Pd(CO)2 (1) and (dtbpe)Pd(CO)2 (2). Partial CO elimination from 2 yields dinuclear {(dtbpe)Pd}2(μ-CO) (3), containing a single bridging carbonyl ligand. 3 represents the smallest cluster containing the Pd2(μ-CO) group and, as such, is also the simplest model compound for CO adsorption on a Pd(100) surface. Thermolysis of 3 produces a carbonyl-free product which is assumed to be the dinuclear species {(dtbpe)Pd}2 (4). The molecular structures of 2 and 3 have been determined, and the solids are furthermore characterized by CP-MAS NMR spectra

Synthesis, Structure, and Reactivity of (^tBu₂PC₂H₄P^tBu₂)Ni(CH₃)₂ and {(^tBu₂PC₂H₄P^tBu₂)Ni}₂(μ-H)₂

Oxidative addition of CH3I to (dtbpe)Ni(C2H4) (dtbpe = tBu2PC2H4PtBu2) affords (dtbpe)Ni(I)CH3 (1). The reaction of (dtbpe)NiCl2 or 1 with the stoichiometric quantity of (tmeda)Mg(CH3)2 yields (dtbpe)Ni(CH3)2 (2). (dtbpe)Ni(I)CD3 (1-d3) and (dtbpe)Ni(CD3)2 (2-d6) have been prepared analogously. Thermolysis of 2 in benzene affords {(dtbpe)Ni}2(μ-η2:η2-C6H6) (4). The reaction of either 2 or 4 with hydrogen (H2, HD, D2) gives {(dtbpe)Ni}2(μ-H)2 (3) and the isotopomers {(dtbpe)Ni}3(μ-H)(μ-D) (3-d) and {(dtbpe)Ni}2(μ-D)2 (3-d2). According to the NMR spectra, the structure of 3 is dynamic in solution. The crystal structures of 2 and 3 have been determined by X-ray crystallography. Solution thermolysis of 2 or reduction of (dtbpe)NiCl2 with Mg* in the presence of alkanes probably involves σ-complex-type intermediates [(dtbpe)Ni(η2-R‘H)] (R‘ = e.g. C2H5, A). While the nonisolated [(dtbpe)Ni0] σ-complexes A are exceedingly reactive intermediates, isolated 3 and 4 represent easy to handle starting complexes for [(dtbpe)Ni0] reactions. Partial protolysis of 2 with CF3SO3H affords (dtbpe)Ni(CH3)(OSO2CF3) (5). Complex 5 reacts slowly with 2 equiv of ethene to give equimolar amounts of [(dtbpe)Ni(C2H5)]+(OSO2CF3-) (6) and propene. The reaction is thought to be initiated by an insertion of ethene into the Ni−CH3 bond of 5 to form the intermediate [(dtbpe)Ni(C3H7)(OSO2CF3)] (G), followed by elimination of propene to give the hydride intermediate [(dtbpe)Ni(H)(OSO2CF3)] (H), which on insertion of ethene into the Ni−H bond affords 6

Reversible Water and Methanol Activation at the PdSn Bond¹

Water, alkanol, and amine activation reactions of transition metals may lead to complexes in which a hydro substituent is paired with an hydroxy, alkoxy, or amide substituent, all of which are potential reactive sites. 2 Well-studied examples are the d6 Ir(III) cis hydro hydroxy complexes [(Me3P)4Ir(H)OH]+PF6- 3a (ionic; stable at 100 °C) and (R3P)3Ir(H)OH(Cl) (R = Me, Et; neutral; reversible water elimination at 20 °C). 3b Of the d10 metal complexes, {(c-C6H11)3P}2Pt oxidatively adds phenols (ArOH; Ar = C6H5, C6F5) at 20 °C to afford stable trans-{(c-C6H11)3P}2Pt(H)OAr. 4 Similarly, (iPr3P)2Pt reacts with H2O to give thermally unstable trans-(iPr3P)2Pt(H)OH. 5a,b In contrast, (iPr3P)2Pd does not react with water at 20 °C. 5c Furthermore, the stannylene SnR2 (R = CH(SiMe3)2) 6a is reported to decompose in water and alkanol. 6b,c As we have recently discovered, the adducts L2Pd(0)=SnR2 (L2 = chelating bidentate phosphane) 7 undergo reversible oxidative additions of water and methanol.

(R₂PC₂H₄PR₂)Pd⁰−1-Alkyne Complexes

Displacement of the ethene ligand in (dippe)Pd(C2H4) (dippe = iPr2PC2H4PiPr2) by 1-alkynes RC⋮CH affords the mononuclear complexes (dippe)Pd(RC⋮CH) (R = Me (2a), Ph (3a), CO2Me (4), SiMe3 (5)). The molecular structure of 3a has been determined by X-ray crystallography. Mononuclear 2a and 3a have been reacted with stoichiometric amounts of (dippe)Pd(η1-C3H5)2 as a source for [(dippe)Pd0] to yield the dinuclear derivatives {(dippe)Pd}2(μ-RC⋮CH) (R = Me (2b), Ph (3b)). By the reaction of (dippe)Pd(C2H4) with difunctional vinylacetylene the mononuclear complex (dippe)Pd{(1,2-η2)-RC⋮CH} (R = CHCH2 (6a)) is formed, which is in equilibrium with isomeric (dippe)Pd{(3,4-η2)-H2CCHC⋮CH} (6b). Addition of [(dippe)Pd0] to 6a,b yields dinuclear {(dippe)Pd}2(μ-RC⋮CH) (R = CHCH2 (6c)). Reaction of (dippe)Pd(C2H4) with butadiyne affords (dippe)Pd(η2-HC⋮CC⋮CH) (7c). From dippe, Pt(cod)2, and C4H2 the Pt homologue has also been synthesized and thus, together with the already known Ni derivative, the series (dippe)M(η2-HC⋮CC⋮CH) (M = Ni (7a), Pd (7c), Pt (7f)) is now complete. When 7c and [(dippe)Pd0] are combined, the dinuclear complex {(dippe)Pd}2(μ-RC⋮CH) (R = C⋮CH (7e)) is formed in solution, whereas isomeric {(dippe)Pd}2{μ-(1,2-η2):(3,4-η2)-HC⋮CC⋮CH} (7d) is present in the solid state. The preparation of the Pd0−1-alkyne complexes refutes the conventional wisdom that this type of compound is inherently unstable. By reaction of (dippe)Pd(C2H4) with internal alkynes C2R2 the complexes (dippe)Pd(RC⋮CR) (R = Me (8a), Ph (9), CO2Me (10), SiMe3 (11)) have also been prepared. Combining 8a with [(dippe)Pd0] affords dinuclear {(dippe)Pd}2(μ-MeC⋮CMe) (8b). Finally, solution thermolysis of 2b and 8b gives rise to dinuclear alkyne-free Pd2(dippe)2 (12)

1,6-Diene Complexes of Palladium(0) and Platinum(0): Highly Reactive Sources for the Naked Metals and [L−M⁰] Fragments

The complexes (cod)MCl2 (M = Pd, Pt; cod = cis,cis-1,5-cyclooctadiene) react with Li2(cot) (cot = cyclooctatetraene) in a 1,6-diene/diethyl ether mixture (1,6-diene = hepta-1,6-diene, diallyl ether, dvds (1,3-divinyl-1,1,3,3-tetramethyldisiloxane)) to afford the isolated homoleptic dinuclear Pd0 and Pt0 compounds Pd2(C7H12)3 (1), Pd2(C6H10O)3·C6H10O (2‘; 2:  Pd2(C6H10O)3), Pd2(dvds)3 (3), and Pt2(C7H12)3 (4). When 1−4 are treated with additional 1,6-diene the equally homoleptic but mononuclear derivatives of type M(1,6-diene)2 (5−8) and with ethene the mixed alkene complexes (C2H4)M(1,6-diene) (9−12) are obtained in solution. Complexes 1−12 react with donor ligands such as phosphanes, phosphites, or tBuNC to give isolated complexes of types L−M(1,6-diene) (13−41), which have also been prepared by other routes. In all complexes the metal centers are TP-3 coordinated:  complexes 1−4 contain chelating and bridging 1,6-diene ligands, whereas the other complexes contain a chelating 1,6-diene ligand and an η2-alkene (5−12) or η1-donor ligand (13−41). Of the studied 1,6-diene complexes the hepta-1,6-diene derivatives are most reactive, while the diallyl ether complexes are often more convenient to handle. The readily isolable dinuclear hepta-1,6-diene and diallyl ether complexes 1, 2‘, and 4, and their mononuclear pure olefin derivatives are among the most reactive sources for naked Pd0 and Pt0. The corresponding L−M(1,6-diene) complexes are equally reactive precursor compounds for the generation of [L−M0] fragments in solution, which for M = Pd are available otherwise only with difficulty. The results are significant for the operation of naked Pd0 and L−Pd0 catalysts in homogeneous catalysis

A Palladium-Catalyzed Stannole Synthesis

A palladium-catalyzed (2 + 2 + 1) cycloaddition reaction of two C2H2 and one SnR2 to form C-unsubstituted stannoles (C4H4)SnR2 [R = CH(SiMe3)2 2a, R2 = {C(SiMe3)2CH2}2 2c] is described. Catalysts are (R‘2PC2H4PR‘2)Pd complexes (slow reaction) and (R‘3P)2Pd complexes (fast reaction). The mechanism of the catalysis has been elucidated in detail from stoichiometric reactions based on R = CH(SiMe3)2. For the [(R‘2PC2H4PR‘2)Pd]-catalyzed system, the starting Pd(0)−ethene complexes (R‘2PC2H4PR‘2)Pd(C2H4) (R‘ = iPr (3), tBu (4)) react both with ethyne to give the Pd(0)−ethyne derivatives (R‘2PC2H4PR‘2)Pd(C2H2) (R‘ = iPr (5), tBu (6)) and with SnR2 to yield the Pd(0)−Sn(II) adducts (R‘2PC2H4PR‘2)PdSnR2 (R‘ = iPr (7), tBu (8)). The Pd−Sn bond [2.481(2) Å] of 7 is very short, indicative of partial multiple bonding. Subsequent reactions of the Pd(0)−ethyne complexes 5 and 6 with SnR2 or of the Pd(0)−Sn(II) complexes 7 and 8 with ethyne afford the 1,2-palladastannete complexes (R‘2PC2H4PR‘2)Pd(CHCH)SnR2 (Pd−Sn) (R‘ = iPr (10), tBu (11)). The derivative with R‘ = Me (9) has also been synthesized. In 10 a Pd−Sn single bond [2.670(1) Å] is present. Complexes 10 and 11 (as well as 7 and 8 but not 9) react slowly with additional ethyne at 20 °C to reform the Pd(0)−ethyne complexes 5 and 6 with concomitant generation of the stannole (C4H4)SnR2 (2a). Likely intermediates of this reaction are the Pd(0)−η2-stannole complexes (R‘2PC2H4PR‘2)Pd(η2-C4H4SnR2) (R‘ = iPr (12), tBu (13)), which have been synthesized independently. The stannole ligand in 12, 13 is easily displaced by ethyne to yield 5 or 6 or by SnR2 to yield 7 or 8. Thus, the isolated complexes 5−8 and 10−13 are conceivable intermediates of the catalytic stannole formation, and from their stoichiometric reactions the catalysis cycle can be assembled. For the [(R‘3P)2Pd]-catalyzed system, the corresponding intermediates (Me3P)2Pd(C2H2) (15), (iPr3P)2Pd(C2H2) (17), (Me3P)2PdSnR2 (18), (iPr3P)2PdSnR2 (20), and (Me3P)2Pd(CHCH)SnR2 (Pd−Sn) (19) have been isolated or detected by NMR, and (iPr3P)2Pd(CHCH)SnR2 (Pd−Sn) (21) is postulated as an intermediate. The [(Me3P)2Pd] system (stannole formation above 0 °C) is catalytically more active than any of the [(R‘2PC2H4PR‘2)Pd] systems (slow stannole formation for R‘ = tBu at 20 °C). Most active is the [(iPr3P)2Pd] system, allowing a catalytic synthesis of the stannole 2a from SnR2 and ethyne at −30 °C [1% of 17; yield 2a:  87%; TON (turnover number):  87]. By carrying out the catalysis in pentane at 20 °C (0.04% of 17), the TON is increased to 1074 but the yield of 2a is diminished to 43% due to uncatalyzed thermal side reactions

Synthesis, Structure, and Properties of {(^tBu₂PC₂H₄P^tBu₂)Ni}₂(μ-η²:η²-C₆H₆) and (^tBu₂PC₂H₄P^tBu₂)Ni(η²-C₆F₆)¹

Thermolysis of (tBu2PC2H4PtBu2)NiMe2 in benzene or reduction of (tBu2PC2H4PtBu2)NiCl2 with Mg* in THF/benzene affords a solution of mononuclear (tBu2PC2H4PtBu2)Ni(η2-C6H6) (1) and dinuclear {(tBu2PC2H4PtBu2)Ni}2(μ-η2:η2-C6H6) (2) in equilibrium. Complex 2 has been isolated; the X-ray structure analysis reveals an antifacial coordination of the [(tBu2PC2H4PtBu2)Ni0] moieties to adjacent CC bonds of a formal cyclohexatriene ligand. According to solid-state and solution NMR the structure of 2 is static in the solid and fluxional in solution. Displacement of the benzene ligand in 1 or 2 by C6F6 affords mononuclear (tBu2PC2H4PtBu2)Ni(η2-C6F6) (5) for which the molecular structure is also reported

Novel Ni(0)-COT Complexes, Displaying Semiaromatic Planar COT Ligands with Alternating C−C and C=C Bonds

Reaction of (R2PC2H4PR2)Ni(C2H4) with COT gives the mononuclear complexes (R2PC2H4PR2)Ni(η2-C8H8) (R = iPr 1a, tBu 1b). The COT ligand in 1a,b is planar with alternating C−C and C=C bonds, corresponding to a formal semiaromatic [C8H8]- ligand. Reactions of 1a with (iPr2PC2H4PiPr2)Ni(η2,η2-C6H10) and of 1b with stoichiometric amounts of {(tBu2PC2H4PtBu2)Ni}2(μ-C6H6) or lithium afford the dinuclear complexes {(iPr2PC2H4PiPr2)Ni}2{μ-η4(1,2,5,6):η4(3,4,7,8)-C8H8} (2a) and {(tBu2PC2H4PtBu2)Ni}2(μ-η2:η2-C8H8) (2b; two isomers). The COT ligand in 2a is tub-shaped and olefinic, whereas in 2b (as in 1a,b) it is planar and semiaromatic. The products are characterized by IR, solution and solid-state NMR spectroscopy, and by X-ray structure analysis