Reactivity of P4 butterfly complexes towards NHCs - generation of a metal-bridged P2 dumbbell complex.

The reactivity of the P4 butterfly complexes [{Cp'''Fe(CO)2}2(µ,η1:1-P4)] (A, Cp''' = C5H2tBu3) and [{Cp*Cr(CO)3}2(µ,η1:1-P4)] (B, Cp* = C5(CH3)5) towards the N-heterocyclic carbene IMe (1,3,4,5-tetramethyl-imidazol-2-ylidene) is reported. The reaction of A affords [P(IMe)2][Fe(CO)2Cp'''] (1) or [P(IMe)2][{Cp'''Fe}2(µ,η3:3-P3)] (2), the latter possessing a P3-allylic moiety. In contrast, the reaction of B yields [P(IMe)2][Cr(CO)3Cp*] (3) and [{Cp*Cr(CO)2}(η2-P2IMe2)][Cr(CO)3Cp*] (4), featuring a novel metal-bridged P2 dumbbell.

The Butterfly Complex [{Cp*Cr(CO)3 }2 (µ,η1:1 -P4 )] as a Versatile Ligand and Its Unexpected P1 /P3 Fragmentation.

The versatile coordination behavior of the P4 butterfly complex [{Cp*Cr(CO)3 }2 (µ,η1:1 -P4 )] (1) towards Lewis acidic pentacarbonyl compounds of Cr, Mo and W is reported. The reaction of 1 with [W(CO)4 (nbd)] (nbd=norbornadiene) yields the complex [{Cp*Cr(CO)3 }2 (µ3 ,η1:1:1:1 -P4 ){W(CO)4 }] (2) in which 1 serves as a chelating P4 butterfly ligand. In contrast, reactions of 1 with [M(CO)4 (nbd)] (M=Cr (a), Mo (b)) result in the step-wise formation of [{Cp*Cr(CO)2 }2 (µ3 ,η3:1:1 -P4 ){M(CO)5 }] (3 a,b) and [{Cp*Cr(CO)2 }2 -(µ4 ,η3:1:1:1 -P4 ){M(CO)5 }2 ] (4 a,b) which contain a folded cyclo-P4 unit. Complex 4 a undergoes an unprecedented P1 /P3 -fragmentation yielding the cyclo-P3 complex [Cp*Cr(CO)2 (η3 -P3 )] (5) and the as yet unknown phosphinidene complex [Cp*Cr(CO)2 {Cr(CO)5 }2 (µ3 -P)] (6). The identity of 6 is confirmed by spectroscopic methods and by the in situ formation of [{Cp*Cr(CO)2 (tBuNC)}P{Cr(CO)5 }2 (tBuNC)] (7). DFT calculations throw light on the bonding situation of the reported products.

Isomerism and dynamic behavior of bridging phosphaalkynes bound to a dicopper complex.

A dicopper complex featuring a symmetrically bridging nitrile ligand and supported by a binucleating naphthyridine-based ligand, [Cu2(µ-η 1 :η 1 -MeCN)DPFN](NTf2)2, was treated with phosphaalkynes (RC[triple bond, length as m-dash]P, isoelectronic analogues of nitriles) to yield dicopper complexes that exhibit phosphaalkynes in rare µ-η 2:η 2 binding coordination modes. X-ray crystallography revealed that these unusual "tilted" structures exist in two isomeric forms (R "up" vs. R "sideways"), depending on the steric profile of the phosphaalkyne's alkyl group (R = Me, Ad, or t Bu). Only one isomer is observed in both solution and the solid state for R = Me (sideways) and t Bu (up). With intermediate steric bulk (R = Ad), the energy difference between the two geometries is small enough that both are observed in solution, and NMR spectroscopy and computations indicate that the solid-state structure corresponds to the minor isomer observed in solution. Meanwhile, treatment of [Cu2(µ-η 1:η 1-MeCN)DPFN](NTf2)2 with 2-butyne affords [Cu2(µ-η 2:η 2-(MeC[triple bond, length as m-dash]CMe))DPFN](NTf2)2: its similar ligand geometry demonstrates that the tilted µ-η 2:η 2 binding mode is not limited to phosphaalkynes but reflects a more general trend, which can be rationalized via an NBO analysis showing maximization of π-backbonding.