Reactions of metal-to-metal multiple bonds. 4. .mu.-Acetylene-bis(cyclopentadienyl)tetracarbonyldimolybdenum compounds. Preparations, properties, structural characterizations, and dynamical solution behavior

Abstract

Cp2Mo2(CO)4, which contains a Mo-Mo triple bond, reacts with acetylenes RC=CR' in hydrocarbon solvents to form 1:1 adducts Cp2Mo2(CO)4(RCCR') in which the acetylenic moiety RCCR', acting as a four-electron donor ligand, bridges two molybdenum atoms that are also joined by a Mo-Mo single bond. The reactivity order HC=CH > MeC=CMe ~PhC=CH > CF3C=CH > CF3C=CCF3 ~PhC=CPh has been found for 1:1 adduct formation. Three of the Cp2Mo2(CO)4(RCCR) compounds, where R = H, Et, and Ph, have been structurally characterized by X-ray studies. There is a common Cp2Mo2(CO)4C2 moiety having (1) a quasi-tetrahedral M02C2 core with Mo-Mo distances of 2.96-2.98 Á and(2) an asymmetric 2 2(€0)4 unit, in which one molybdenum atom has two terminal carbonyl ligands while the other has one terminal and one semibridging carbonyl ligand. The occurrence of the semibridging carbonyl group is attributed to internal crowding. A number of these acetylene adducts have been characterized by infrared, NMR ( , 13C, and 13C[!H]), and mass spectrometry. The dynamical solution behavior of Cp2Mo2(CO)4(RCCR') compounds can be interpreted on the following assumptions. (1) A low-energy process interconverts Mo(l) and Mo(2), Cp(l) and Cp(2), and the carbonyl ligands a s d, b ^c, This process allows the Cp2Mo2(CO)4 moiety to adopt a time-averaged structure having C2 symmetry. (2) A higher energy process effects rotation about the Mo-Mo bond.(3) The central M02C2 moiety is rigid on the NMR time scale up to 90 °C. ( 4) Carbonyl ligand scrambling between molybdenum atoms is not observed on the NMR time scale at temperatures up to 90 °C. ( 5) For certain acetylene adducts, e.g., R = R' = Et, an isomer different from that found in the crystalline state is also present In equilibrium in solution. In Cp2Mo2(CO)4(HCCH) the acetylene is not labile toward ligand displacement reactions (with CO and MeC=CMe) or hydrogenation. 1.989 (3) C(l,3)-C(l,4) 1.395 (8) -C(3) 38.72 (8) -C(3) 2.902 (4) C(l,4)-C(l,5) 1.415 (7) -C(5) 45.34 (9) -C(5) 2.190(3) C(l,5)-C(l,l) 1.408 (7) -C(6) 47.50 (9) -C(6) 2.172 (3) C(1,1) 111.1 (1) -C(l,l) 2.308 (4) C(2,l)-C(2,2) 1.386 (9) C( 1,2) 93.0(1) -C( 1,2) 2.364 (4) C(2,2)-C(2,3) 1.41 (1) C( 1,3) 108.8 (2) -C( 1,3) 2.363 (4) C(2,3)-C(2,4) 1.30 (1) C( 1,4) 143.4 (2) -C( 1,4) 2.322 (4) C(2,4)-C(2,5) 1.36(1) C( 1,5) 146.6 (1) -C( 1,5) 2.310 (4) C(2,5)-C(2,l) 1.39(1)Mo(l )-Mo(2)-C(3) 68.5 (1) C( 5)-H(5)

The crystal and molecular structure of μ-carbonyl-bis(η-cyclopentadienyl)-μ-η-(hexafluoro-2-butyne)-dirhodium (Rh–Rh)

“…The lengthening of the C-C bond indicates some reduction in bond order, and this is consistent with the observed decrease of v(C-C) from 1642 to 1592 cm -1 in the infrared spectra of (CsHs)2Rh2(CO)2(CF3C2CF 3) and (CsHs)2Rh2(CO)(CF3C 2CF3), respectively. The average C-C-CF 3 angle of 137.1 (6) is similar to the corresponding angle in related alkyne-dimetal complexes, including 141.3 for Rh2(PF3)4(PPh3)2(PhC2Ph) (Bennett, Johnson, Robertson, Turney & Whimp, 1976) and 134.2 (6) ° for (CsHs)2Mo2(CO)4(EtC2Et) (Bailey, Chisholm, Cotton & Rankel, 1978). The corresponding angle in (CsHs)2RhE(CO)2(CF3C2CF3) is 128.5 (1.0) ° which is closer to the normal olefinic angle of 120 °.…”

Delocalized bonding in tetrahedral organo-transition metal clusters: an EHMO study of metal vertex rotations in Co2Fe(CO)9S, CpMoCo2(CO)8CR, and in Cp2Mo2(CO)4(RC≡CR)

“…The calculations of the energy barriers for these two processes revealed that (i) the most favored rotamer does indeed possess the C, geometry found crystallographically for Cp,Mo,(CO),As, by Sullivan and Rheingold (30), and (ii) the proposed C,, structure 16, is almost certainly not attainable since it would bring the hydrogens of the cyclopentadienyl groups into direct contact. As Cotton and co-workers have noted (26), these steric constraints are the probable cause of the low symmetry observed even in the parent cluster Cp,Mo,(CO),(HC=CH).…”

“…The lower energy process involved a pairwise interchange of a terminal carbonyl ligand environment on one molybdenum with a semi-bridging CO on the other metal. The second, and higher energy, process equilibrated the environments of the two cyclopentadienyl groups by a mechanism that must involve some degree of CpMo(CO), vertex rotation (26). It is this latter phenomenon that we wish to address.…”

“…This point X must be positioned above the midpoint of the Mo-Mo bond along the pseudo-C, axis of the molecule; however, establishing its location was a nontrivial exercise and required a Biirgi-Dunitz analysis of the trajectory of the ring-centroid of the cyclopentadienyl fragment. Fortunately, since Cotton's original report (26) of the structure and fluxionality of [(C,H,)Mo(CO),],(H=H), 4, there have been more than 20 crystallographically characterized tetrahedral clusters of the general type Cp,M,(CO),(A=B), and selected parameters for a number of these clusters are collected in Table 1.…”

A variable-temperature 13C NMR study of (η5-C5H5)2Mo2(CO)4 complexes of chiral alkynes derived from a terpene or a hormonal steroid: interconversion of diastereomers via exchange of terminal and semi-bridging carbonyls

“…The molecular structures and fluxional behavior of clusters of the type C~,(CO),M,(R~~%CR~), where M = Mo or W, were first described by Cotton and co-workers (15). In the solid state, three of the carbonyl ligands are terminally bonded while the fourth is in a semi-bridging environment.…”