Binuclear Activation of Cumulenes:  Roles of the Adjacent Metals and the Cumulene Binding Mode in the Activation Process

Abstract

The binuclear complex [Ir2(CH3)(CO)(μ-CO)(dppm)2][CF3SO3] (1; dppm = μ-Ph2PCH2PPh2) reacts with allene and methylallene to ultimately yield the vinylcarbene products [Ir2H(CO)2(μ-η1:η3-HCC(CH3)C(H)R)(dppm)2][CF3SO3] (R = H (6), CH3 (7)). Monitoring the reactions by NMR spectroscopy (1H, 13C, 31P) between −78 °C and ambient temperature allows the observation of several intermediates in each of these transformations in which the allene moves from an η2 binding site on one metal, through an η1:η1-bridging geometry in which the cumulene is coordinated through the “H2CC” moiety, to an η1:η3-bridging geometry in which the central carbon of the cumulene is σ-bound to one metal, adjacent to the methyl ligand, while the three cumulene carbons are η3-bound to the adjacent metal. We propose that formation of the respective vinyl carbene products results from migration of the methyl ligand to the central cumulene carbon followed by activation of a cumulene C−H bond. 1,1-Dimethylallene reacts with 1 at −78 °C to yield a methylene hydride product containing an η2-bound cumulene on one metal, much as observed for the first products in the allene and methylallene reactions. Upon warming, this intermediate isomerizes to the final product containing a methyl ligand on one metal and an η2-bound cumulene on the other. No cumulene-bridged products are observed with this disubstituted allene. 1,1-Difluoroallene also yields a methylene hydride product at −78 °C, which is analogous to the first species observed in all cases noted above. In this case, warming results in movement of the cumulene to an η1:η1-bridging position in which this group binds to the metals via the “H2CC” moiety. Unlike the transformations observed with allene and methyl allene, difluoroallene undergoes no additional transformations as the temperature is raised. A rationalization of these transformations is presented together with a perspective on how the cumulene ligand moves over the dimetallic framework leading to the final products.

Coupling of Allenes with μ‐Alkylidyne Ligands in Diiron Complexes: Synthesis of Novel Bridging Thio‐ and Aminobutadienylidene Complexes

“…The latter can be described by using three different resonance forms (Scheme 4). The fact that all three C(13), C (14) and C (15) carbon atoms (corresponding to C α , C β and C γ , respectively) are coordinated to Fe [1.317(3) Å] is almost a pure double bond, whereas the C(13)-N(1) bond [1.395(2) Å] is quite elongated, which suggests a limited π-interaction. All these considerations point to the fact that the coordination of such an unsaturated ligand to a bimetallic frame results in a highly delocalized and complex electronic structure which cannot be described in an unequivocal way.…”

“…[13] Likewise, the reactions of the diiridium complexes [Ir 2 (CH 3 )(CO)(µ-CO)(dppm)][SO 3 CF 3 ] with allenes have been shown to occur by initial η 2 -allene coordination followed by intramolecular rearrangements. [14] In spite of these considerations, our attempts to detect and isolate possible η 2 -allene intermediates have been unsuccessful. Our efforts included: i) low temperature reaction conditions (-40°C), ii) the reaction of 1a with Me 2 C=C=CMe 2 , which does not contain removable hydrogen atoms and iii) the creation of coordinative unsaturation by chloride abstraction from [Fe 2 {µ-CN(Me)(Xyl)}(µ-CO)(CO)(Cl)(Cp) 2 ] upon treatment with AgSO 3 CF 3 .…”

“…Et)] can be described by resonance forms analogous to those depicted in Scheme 4, even though the different nature of the substituents present on C(13) and (14) [2.036(7) Å]; this indicates that the corresponding interactions can be more appropriately described as being nearly of σ-character for Fe(1)-C(15), whereas C(13) and C (14) are π-bonded in an olefinic fashion to Fe(1). Therefore, the bonding structure of cis-4b can be very well described by the 1-thio-1,3-butadienylidene form (C).…”

“…Non-H atoms were refined anisotropically, unless otherwise stated. H atoms were placed in calculated positions, except for H (14) for which the two disordered images are related by an inversion centre; in this case, an occupancy factor of 0.5 was assigned to the independent image of the molecule and then it was refined anisotropically. Similar U restraints were applied to all C and O atoms in cis-4b, all C atoms in 5a[CF 3 SO 3 ]·CH 2 Cl 2 and all C and F atoms in 7a[CF 3 SO 3 ] ·0.5CH 2 Cl 2 .…”

Fluoroarene Complexes with Small Bite Angle Bisphosphines: Routes to Amine–Borane and Aminoborylene Complexes

“…Interestingly this does not occur with the R 2 P(CH 2 ) 3 PR 2 ligands in I and II in which there is not an A‐frame motif . Similar geminal C–H activations of alkenes are effected by [{Ir(µ‐Et 2 PCH 2 PEt 2 )(CO)} 2 (µ‐H)(µ‐CO)] +[38] and [{Ir(µ‐Ph 2 PCH 2 PPh 2 )} 2 (µ‐CO)(CH 3 )(CO)] + , . Such C–H activations are proposed to proceed via a cooperative mechanism wherein π‐complexation of H 2 C=CRR′ to one metal enables σ‐CH complexation at the other metal and consequently C–H cleavage.…”