Mirka Bergamo

Comments on the elusive crystal structure of 4-iodo-4′-nitrobiphenyl

The recently proposed crystal structure of 4-iodo-4′-nitrobiphenyl is here confirmed on the basis of experimental diffraction data on single crystals and powders of high crystallinity; a reinterpretation of the nature of the (faulted) material studied in the original paper is also presented.

Solid state and solution structure of the hetero-dimetallic complexes [Cp(CO)2Re{μ-C(OMe)(Ph)}Pt(COD)] and [Cp(CO)2Re{μ-CPh}Pt(COD)]+, containing bridging carbene and carbyne ligands

Abstract The reaction of the carbene complex [Cp(CO)2ReC(OMe)(Ph)] with [Pt(COD)2] affords in high yield the novel complex [Cp(CO)2Re{μ-C(OMe)(Ph)}Pt(COD)] (2), containing a bridging carbene ligand. Reaction of 2 with (Me3O)BF4 results in removal of the carbene-bound methoxy group, with formation of the cation [Cp(CO)2Re{μ-CPh}Pt(COD)]+ (3), in which a bridging carbyne is present. Both the complexes contain only ligands coordinated through carbon atoms and represent the highest members, as far as the Pt:Re ratio is concerned, of the small family of phosphorous-free Pt–Re mixed metal complexes containing PtRe bonds. Their full 1H and 13C NMR characterization in solution and their single-crystal solid state diffractometric analysis are reported and discussed. The X-ray analysis evidenced the substantially symmetric nature of the carbene bridge in the dimetalla-cyclopropane ring of complex 2 [RePt 2.7188(3), Re-(μ-C) 2.166(4) and Pt-(μ-C) 2.055(4) A]. In complex 3 the bond length pattern within the three-membered ring is suggestive of a significant double-bond character for the Re-(μ-C) interaction [RePt 2.707(1), Re-(μ-C) 1.904(9) and Pt-(μ-C) 1.981(8) A]. Both complexes show a semi-bridging CO ligand, with Pt⋯C distances of 2.452(4) and 2.299(9) A, in 2 and 3, respectively. The 13C NMR characterization showed that the Pt⋯CO interaction is also maintained in solution in both complexes. In cation 3 a libration of the ReCp(CO)2 fragment around the PtRe axis, fast down to 193 K, interchanges the two carbonyl locations observed in the solid state, as well as the environments of couples of COD carbon atoms, leading to an apparent Cs symmetry. The correlation between spectroscopic data and structural features (concerning bridging carbenes or carbynes and semi-bridging carbonyls) in the class of heterodimetallic M–Pt complexes is discussed.

[Re5(μ‐H)4(CO)20]− and [Re5(μ‐H)5(CO)20], Two Isolobal Analogues of Cyclopentane

The first five-membered rings of metal atoms connected by M-M or M-H-M bonds only have been obtained by a Re2 +Re3 condensation in which a polyhydride acts as a bridging bidentate ligand toward a coordinatively unsaturated fragment (see scheme below). In spite of the octahedral coordination of the Re centers, the Re5 rings display conformations (twisted and envelope) comparable with those observed for organic five-membered rings of tetrahedral carbon atoms.

Transition-metal hydrides and carbonyl anions as ligands toward a Pt centre in rhenium-platinum triangular clusters: A qualitative order of 'thermodynamic nucleophilicity'

Abstract Six novel mixed-metal spiked-triangle complexes [Re 2 Pt(μ-H) 2 (CO) 9 (X)] have been obtained. The metallo-ligands X bound to the Pt vertex are either transition-metal hydrides, such as HMn(CO) 5 , HRe(CO) 4 (PPh 3 ), HRe(CO) 3 (PPh 3 ) 2 , or carbonyl anions, such as [Mn(CO) 5 ] − , [WCp(CO) 3 ] − and [Co(CO) 4 ) − . Two main synthetic routes have been used to prepare these complexes: (i) replacement of the labile ligand 1,5-cyclooctadiene (COD) of [Re 2 Pt(μ-H) 2 (CO) 8 (COD)) with CO and X; (ii) substitution of the labile ‘ligand’ HRe(CO) 5 in [Re 2 Pt(μ-H) 2 (CO) 9 (HRe(CO) 5 )] by the ligands X. The neutral species [Re 2 PtMn(μ-H) 3 (CO) 14 ] has been obtained by protonation of [Re 2 Pt(μ-H) 2 (CO) 9 (Mn(CO) 5 )] − . Variable temperature 1 H NMR investigations showed that it exists in solution as two isomers a and b, likely differing in the location of one hydride, a having the structure [Re 2 Pt(μ-H) 2 (CO) 9 ( HMn(CO) 5 ]], and b are structure [Re 2 Pt(μ-H) 3 (CO) 9 (Mn(CO) 5 )]. The ligand HMn(CO) 5 Of a shows a high lability, being in fast exchange with free [HMn(CO) 5 ] even at 193 K. At higher temperature interconversion between the two isomers and exchange between the hydrides bound to Pt in b is observed. The treatment of [Re 2 Pt(μ-H) 2 (CO) 9 (WCp(CO) 3 )] − with strong acids failed to give the protonated derivative. Tlte strong nucleophile [FeCp(CO) 2 ] − reacted with [Re 2 Pt(μ-H) 2 (CO) 9 (HRe(CO) 5 ]] as a Broensted base rather than as a nucleophile, giving deprotonation instead of substitution of the labile HRe(CO) 5 ligand bound to Pt. The complex [Re 2 Pt(μ-H) 2 (CO) 9 (HRe(CO) 4 (PPh 3 )]] has been characterised by X-ray single crystal analysis. It crystallises in the monoclinic space group P2 1 c (No. 14) with a =9.229(3), b =30.700(8), c=12.915(3) A , β=98.05(2)°, V=3623(2) A 3 and Z=4. A series of displacement reactions of the type [Re 2 Pt(μ-H) 2 (CO) 9 [X{]+X′⇔ [Re 2 Pt(μ-H) 2 (CO) 9 [X′{] + X allowed the following qualitative order of affinity of the metallo-ligands X for the Pt atom (a scale of ‘thermodynamic nucleophilicity’) to be established: [Re(CO) 5 ] − > [HRe 2 (CO) 9 ] − ≈ [Mn(CO) 5 ] − > [WcP(CO) 3 ] in3 ] − > [Co(CO) 4 ] − > [HRe(CO) 4 (PPh 3 )] > [HRe(CO) 3 (PPh 3 ) 2 ] > [HRe(CO) 5 ] > [HMn(CO) 5 ] > [HWCp(CO) 3 ].

A ketone complex by alkylation of an acyl anion. Synthesis, crystal structure and spectroscopic characterization of [Cp(CO)2Re{OC(Me)Ph}]

Abstract Reaction of Li[Cp(CO) 2 Re(COPh)] ( 1 ) with CF 3 SO 3 Me afforded, besides the expected carbene complex [Cp(CO) 2 ReC(OMe)(Ph)] ( 2 ) and the alkyl–acyl derivative [Cp(CO) 2 Re(Me)(COPh)] ( 3 ), a third structural isomer [Cp(CO) 2 Re{OC(Me)Ph}] ( 4 ), which contains an acetophenone molecule coordinated to the metal center. The X-ray analysis showed that in solid acetophenone is bound to ‘CpRe(CO) 2 ’ exclusively through an oxygen σ-donor interaction, while in solution an equilibrium between σ-bound (η 1 ) and π-bound (η 2 ) forms occurs, as judged by IR data and 1 H and 13 C variable temperature NMR spectra (π/σ ratio 2.87 at 183 K and 1.16 at 263 K in CD 2 Cl 2 , Δ H °=−4.5 kJ mol −1 for the σ⇔π reaction, E a 58(1) kJ mol −1 ). In solvents different from Et 2 O ( n -hexane, THF, acetone) and with alkylating agent different from CF 3 SO 3 Me (MeI, Me 3 OBF 4 ) the formation of 4 was negligible. It has been demonstrated that 4 does not originate by acetophenone reductive elimination from 3 . No evidence of the involvement of radicals has been obtained.

[Re6(μ-H)5(CO)24]−: The First Carbonyl Cluster with a Cyclohexane-Like Structure

A chair conformation comparable to that observed for six-membered rings composed of tetrahedral carbon atoms is found for the cluster anion [Re6(μ-H)5(CO)24]− (see picture; black spheres: Re, white spheres: μ-H; CO ligands omitted for clarity) in spite of the octahedral coordination at the Re centers. This is the first example of a carbonyl cluster exhibiting a cyclohexane-like geometry of the metallic framework.

Hydrido-Carbonyl Rhenium Clusters with a Square Geometry of the Metal Core. Synthesis and X-Ray Characterization of the Novel [Re~4(mu-H)~3(CO)~1~6]^- Anion

Three and tetranuclear ring clusters have been obtained by treatment of [Re2(CO)8(THF)2] with carbonyl-rhenates containing two terminal hydrides. The reaction with [ReH2(CO)4]- provided a selective route to the previously known [Re3(μ-H)2(CO)12]- triangular cluster anion 1. The reaction with [Re2H2(μ-H)(CO)8]- gave the novel [Re4(μ-H)3(CO)16]- anion 2, containing a rare example of a puckered-square metal cluster. Protonation of 1 is known to afford the neutral [Re3(μ-H)3(CO)12] species 3. Analogously the reaction of 2 with a strong acid afforded the previously known square metal clusters [Re4(μ-H)4(CO)16] 4. The reaction could not be reversed by treatment with bases. Photolysis of 4 gave the unsaturated complex [Re2(μ-H)2(CO)8] 5: this is the reverse of the dimerization reaction, that in THF at room temperature produces 4 from 5. Thermal treatment (reflux in cyclohexane for 24 h) left 4 almost unchanged. A single crystal X-ray analysis of [NEt4]2 showed a s/e/s/s (e=eclipsed, s=staggered) conformation of the Re(CO)4 units, leading to a puckered geometry of the ring, at variance with the square-planar geometry of 4 (all eclipsed). Two of the three hydrides of 2 have been located as bridging the Re–Re edges from inside the metal ring, as previously observed in 4. Density functional computations indicated a puckered conformation as the most stable for both 2 and 4, with very low activation energies for ring inversion (6.6 and 2.2 kcal·mol-1, respectively), but ruled out solid state fluxionality for 4, whose observed planar geometry must be attributed to packing stabilization.

[Re5(μ-H)4(CO)20]− und [Re5(μ-H)5(CO)20], zwei isolobale Analoga von Cyclopentan

Die ersten funfgliedrigen Ringe aus Metallatomen, die nur durch M-M- oder M-H-M-Bindungen verknupft sind, wurden durch eine Re2+Re3-Kondensation erhalten, bei der ein Polyhydrid gegenuber einem koordinativ ungesattigten Fragment als verbruckender zweizahniger Ligand auftritt (siehe unten). Trotz der oktaedrischen Koordination der Re-Zentren liegen die Re5-Ringe in Konformationen vor (Twist- und Briefumschlag-Konformation), die denen organischer funfgliedriger Ringe aus tetraedrisch umgebenen Kohlenstoffatomen ahneln.