Maria José Calhorda

Ring slippage in indenyl complexes: structure and bonding

Abstract A review of some structural and reactivity aspects of the coordinated indenyl ligand, dealing mainly with the systems theoretically studied by the authors is presented. In the first section, the structural characterization of η5 and η3 indenyl is attempted, noticing that the nodal properties of the π orbitals of the indenyl prevent a totally symmetric coordination in a η5-indenyl. The two bonds to the hinge carbon atoms are always longer, and the distance becomes longer than a M–C bond in the η3-indenyl derivatives. Some intermediate distances are found in [(Ind)2Ni] where, formally, the ligand is halfway between η5 and η3. The ring slippage occurs when two electrons are added to the system, occupying a metal–indenyl antibonding orbital, which becomes more stable upon folding. We reviewed electrochemical and ligand addition driven slippage, in the second section. A comparison with the behavior of the cyclopentadienyl ligand was attempted in the end.

[Re(η5-C5H5)(CO)3]+ Family of 17-Electron Compounds : Monomer/Dimer Equilibria and Other Reactions

The anodic electrochemical oxidations of ReCp(CO)3 (1, Cp = eta(5)-C5H5), Re(eta(5)-C5H4NH2)(CO)3 (2), and ReCp*(CO)3 (3, Cp* = eta(5)-C5Me5), have been studied in CH2Cl2 containing [NBu4][TFAB] (TFAB = [B(C6F5)4]-) as supporting electrolyte. One-electron oxidations were observed with E(1/2) = 1.16, 0.79, and 0.91 V vs ferrocene for 1-3, respectively. In each case, rapid dimerization of the radical cation gave the dimer dication, [Re2Cp(gamma)2(CO)6]2+ (where Cp(gamma) represents a generic cyclopentadienyl ligand), which may be itself reduced cathodically back to the original 18-electron neutral complex ReCp(gamma)(CO)3. DFT calculations show that the SOMO of 1+ is highly Re-based and hybridized to point away from the metal, thereby facilitating the dimerization process and other reactions of the Re(II) center. The dimers, isolated in all three cases, have long metal-metal bonds that are unsupported by bridging ligands, the bond lengths being calculated as 3.229 A for [Re2Cp2(CO)6]2+ (1(2)2+) and measured as 3.1097 A for [Re2(C5H4NH2)2(CO)6]2+ (2(2)2+) by X-ray crystallography on [Re2(C5H4NH2)2(CO)6][TFAB]2. The monomer/dimer equilibrium constants are between K(dim) = 10(5) M(-1) and 10(7) M(-1) for these systems, so that partial dissociation of the dimers gives a modest amount of the corresponding monomer that is free to undergo radical cation reactions. The radical 1+ slowly abstracts a chlorine atom from dichloromethane to give the 18-electron complex [ReCp(CO)3Cl]+ as a side product. The radical cation 1+ acts as a powerful one-electron oxidant capable of effectively driving outer-sphere electron-transfer reactions with reagents having potentials of up to 0.9 V vs ferrocene.

Weak hydrogen bonds: theoretical studies

Hydrogen bonds A–H⋯A′, where A and A′ are electronegative atoms have been widely discussed. Weak hydrogen bonds involving such different arrangements as X–H⋯A, where X can be C; X–H⋯π, with phenyl rings, CC bonds; X–H⋯M, where M is a transition metal; X–H⋯H–M and X–H⋯H–B, have also been described in recent years. While the first types are typical of organic and inorganic compounds, as well as biological molecules, those involving transition metal atoms are special to organometallic chemistry. Theoretical calculations of different kinds and at several levels have been performed for many systems, revealing that a similar geometrical arrangement can hide another type of interaction. This happens for N–H⋯M close contacts which can be agostic interactions or hydrogen bonds, not so easily distinguishable for 16-electron complexes. M–H⋯H–X interactions also exhibit a different behavior, depending on whether the complexes are neutral or ionic. The AIM approach, by analysing the topological properties of the charge density with the determination of critical points, provides another way of looking for bonds, as discussed in several examples.

Ruthenium-mediated cyclotrimerization of alkynes utilizing the cationic complex [RuCp(CH3CN)3]PF6

The substitutionally labile cationic complex (RuCp(CH3CN)3) � (as the PFsalt) was tested with respect to its ability to catalyze the cyclotrimerization of terminal alkynes and diynes to afford benzene derivatives. Whereas (RuCp(CH3CN)3) � is in fact promoting the catalytic cyclotrimerization of alkynes, the formation of stable and inert sandwich complexes of the type (RuCp(h 6 - arene)) � deactivates the catalyst and thus quenches the catalytic cycle. All new sandwich complexes were isolated and characterized spectroscopically. A proposal for a plausible catalytic cycle including possible degradation pathways of the catalyst is presented based on DFT calculations. As key intermediates several novel carbene complexes have been identified including metallacyclo- pentatriene and metallaheptatetraene species. # 2003 Elsevier B.V. All rights reserved.

Synthesis, bonding and dynamic behavior of fac-[Mo(II)(CO)2(η3-allyl)] derivatives

Cationic complexes [Mo(η 3 -allyl)(CO) 2 (L–L)L′]PF 6 , (L–L=C 6 H 5 SCH 2 CH 2 SC 6 H 5 , L′=NCCH 3 ( 1 ); bipy, NCCH 3 ( 2 ); py, (NCCH 3 ) 2 ( 3 ); (NCCH 3 ) 3 ( 4 ); dppe, NCCH 3 ( 5 ) and the neutral analogues [Mo(η 3 -allyl)(CO) 2 (L–L)X] (L–L=phen ( 6 ); bipy ( 7 ); X=Br) were synthesized. Complexes 2 , 5 , 6 and 7 were characterized by single crystal X-ray diffraction. Depending on the chelating ligand, these pseudo-octahedral complexes undergo different dynamic processes in solution and NMR spectroscopic evidence was provided for those studies. The structural trends of the limiting structures depicted by these complexes as well as the pathways to their inter-conversion were analyzed by ab initio theoretical calculations. Both NMR data and the calculations showed that for complex 2 the equatorial species predominates at room temperature but that two forms differing only by the conformation of the allyl coexist. Lowering the temperature leads to the appearance of the equatorial–axial isomer.

Selective CC Bond Formation between Alkynes Mediated by the [RuCp(PR3)]+ Fragment Leading to Allyl, Butadienyl, and Allenyl Carbene Complexes—An Experimental and Theoretical Study

The reaction of alkynes with [RuCp(PR(3))(CH(3)CN)(2)]PF(6) (R=Me, Ph, Cy) affords, depending on the structure of the alkyne and the substituent of the phosphine ligand, allyl carbene or butadienyl carbene complexes. These reactions involve the migration of the phosphine ligand or a facile 1,2 hydrogen shift. Both reactions proceed via a metallacyclopentatriene complex. If no alpha C[bond]H bonds are accessible, allyl carbenes are formed, while in the presence of alpha C[bond]H bonds butadienyl carbenes are typically obtained. With diphenylacetylene, on the other hand, a cyclobutadiene complex is formed. A different reaction pathway is encountered with HC[triple bond]CSiMe(3), ethynylferrocene (HC[triple bond]CFc), and ethynylruthenocene (HC[triple bond]CRc). Whereas the reaction of [RuCp(PR(3))(CH(3)CN)(2)]PF(6) (R=Ph and Cy) with HC[triple bond]CSiMe(3) affords a vinylidene complex, with HC[triple bond]CFc and HC[triple bond]CRc this reaction does not stop at the vinylidene stage but subsequent cycloaddition yields allenyl carbene complexes. This latter C[bond]C bond formation is effected by strong electronic coupling of the metallocene moiety with the conjugated allenyl carbene unit, which facilitates transient vinylidene formation with subsequent alkyne insertion into the Ru[double bond]C bond. The vinylidene intermediate appears only in the presence of bulky substituents of the phosphine coligand. For the small R=Me, head-to-tail coupling between two alkyne molecules involving phosphine migration is preferred, giving the more usual allyl carbene complexes. X-ray structures of representative complexes are presented. A reasonable mechanism for the formation of both allyl and allenyl carbenes has been established by means of DFT calculations. During the formation of allyl and allenyl carbenes, metallacyclopentatriene and vinylidene complexes, respectively, are crucial intermediates.

Bonding and structural preferences of indenyl complexes: MInd2Ln (n = 0-3)

Abstract A search of bis(indenyl) derivatives available in the Cambridge Crystallographic Data Centre was performed and the two main families, MInd2 and MInd2Ln (n=1–3), were structurally analyzed in detail. DFT calculations were performed for some relevant compounds in order to understand their electronic structure and interpret the experimental data. For MInd2 complexes, the rotation angles between the rings show a wide range of values, depending both on the electron count and on the steric effects of the ring substituents. Hapticity change toward η3 is induced by extra electrons, but a perfect η3 coordination is never observed. For the electron deficient Cr(II) complexes, two isomers having two and four unpaired electrons are known for different substituents, and the calculated energies in models are very close, as expected. The MInd2L2 family is the largest one and examples of η3 coordination can be found. Both electronic and steric effects play a major role in determining the structural parameters of these species, but in the absence of bulky ring substituents, the rings are fluxional.

Heteropolynuclear Gold Complexes with Metallophilic Interactions: Modulation of the Luminescent Properties

Metalloligands of stoichiometry [AuCl(P-N)] have been obtained by the reaction of the heterofunctional phosphines P-N = PPh(2)py, PPh(2)CH(2)CH(2)py, or PPhpy(2) with [AuCl(tht)] (tht = tetrahydrothiophene). Reactions of these metalloligands with several metal compounds have afforded heteropolynuclear species which exhibit luminescent properties. The stoichiometries depend on the molar ratio and the heterometal. Thus, the reaction with [Cu(NCMe)(4)](+) in a molar ratio 2:1 gives the trinuclear compounds [Au(2)CuCl(2)(P-N)(2)](+), in which the structure and Au···Cu interactions depend on the phosphine ligand. With rhodium and iridium derivatives the reactivity is different leading to complexes of the type [AuMCl(2)(cod)(P-N)] for P-N = PPh(2)py, PPhpy(2), and [Au(2)M(2)Cl(cod)(2)(P-N)(2)]Cl with PPh(2)CH(2)CH(2)py. Using [MCl(2)(NCPh)(2)] (M = Pd, Pt) in a 2:1 molar ratio yields [Au(2)MCl(4)(P-N)(2)] and in a 1:1 molar ratio [AuPdCl(3)(μ(3)-PPhpy(2))]. Several compounds have been characterized by X-ray diffraction showing in many cases short Au···M distances. The luminescence of these derivatives has been studied. The metalloligands display bands assigned to intraligand (IL) transitions. For the bimetallic (Au/M) systems the luminescence depends on the heterometal present and on the metallophilic interactions. The most important excitations in the relevant energy range were assigned essentially a MMLCT character (from Rh/Ir and Au to ligands) based on density functional theory (DFT) calculations in selected complexes. The luminescence behavior in Rh/Ir [AuMCl(2)(cod)(PPh(2)py)] complexes was interpreted on the basis of the different nature of the half occupied orbitals in the triplet state.

Nitrile complexes of dicyclopentadienyl-molybdenum and -tungsten: preparation and reactivity. The structure of di-η5-cyclopentadienyliodoacetonitrile-molybdenum(IV) hexafluorophosphate, [Mo(η5-C5H5)2I(NCCH3)][PF6]

Abstract New complexes of the type [MCp2X(NCR)][PF6] (M = Mo, W; X = SR, halides; R = Me, Et, Ph) have been prepared from [MCp2X2] and TlPF6, NOPF6 or [FeCp2][PF6], in nitrile solvents. Some reactions of these cations with nucleophiles [L] have been studied. With L = PR3 or CO, the complexes [MCp2(SR)L]+ (R = Me, Ph) are formed. [MoCp2(SPh)(NCMe)]+ reacts with NaBH4 to give [MoCp2(SPh)H] and with acetone to give [MoCp2{S(Ph)C(Me)2O}]+. The W complexes [WCp2X(NCR)]+ add NHR′2 to give amidine complexes [WCp2X{HNC(R)Nr′2}]+ (X = SPh, Br). The mode of coordination of the nitriles to the [MoCp2Cl]+ fragment has been studied by EHMO calculations. The molecular structure of [MoCp2I(NCMe)][PF6] has been determined. The crystals are triclinic, space group P 1 , a 7.7989(6), b 10.3044(8), c 10.5565(5) A, α 96.218(4), β 94.466(4), γ 102.697(5)°, V 818.21 A3, Z = 2. The cation has the usual bent bis-metallocene structure, and within this family of complexes is the first reported example of a complex containing a MoI bond.

Synthesis and reactivity of molybdenocene isocyanide complexes; crystal structure of (η5-C5H5)2MoCNtBu

Abstract Synthetic, structural and reactivity studies on Mo IV and Mo II alkyl isocyanide complexes of the types [Cp 2 Mo(X)CNR]Y and Cp 2 MoCNR (X = H, Me, Et, Cl, I; R = Me, Et, t Bu; Y = I, BF 4 , PF 6 ) are reported. Reaction of Cp 2 Mo(H)I ( 2 ) with EtNC gives the hydrido- isocyanide complex [Cp 2 Mo(H)CNEt]I ( 3 ) in high yield. Complex 3 is converted by CHI 3 into the corresponding iodo derivative [Cp 2 Mo(I)CNEt]I ( 4 ). An alternative route to halo-isocyanide complexes of molybdenocene involves halide abstraction from Cp 2 MoX 2 ( 5 : X = Cl; 6 : X = I) by TIPF 6 in the presence of RNC (R = Et, t Bu); this affords the complexes [Cp 2 Mo(X)CNR]PF 6 ( 7a – 8b ) ( 7 : X = Cl, 8 : X = I; a : R = Et, b : R = t Bu) in high yield. Reduction of 7a – 8b by sodium amalgam in THF results in the formation of the Mo II , isocyanide complexes Cp 2 MoCNR ( 9a : R = Et, 9b : R = t Bu). An alternative high yield route to these compounds involves reaction of the acetonitrile complex Cp 2 Mo(η 2 -MeCN) ( 10 ) with RNC. Alkylation of 9a with MeI or Et 3 OBF 4 occurs exclusively at the metal centre to yield the Mo IV alkyl complexes [Cp 2 Mo(Me)CNEt]I ( 11 ) and [Cp 2 Mo(Et)CNEt]BF 4 ( 12 ), respectively. Similarly, complex 9a reacts with AuPPh 3 Cl to give the heterobimetallic compound [Cp 2 mo (AuPPh 3 )CNEt]Cl ( 13 ). By contrast, a carbonyl/isocyanide exchange reaction occurs between 9a and Re(CO) 5 Br, to give Cp 2 MoCO ( 14 ) and cis -Re(CO) 4 (CNEt)Br ( 15 ). The alkyl complexes 11 and 12 , when heated in CH 2 Cl 2 undergo a clean isocyanide insertion to give the Mo IV iminoacyl complexes [Cp 2 Mo[η 2 -C(NEt)Me]I ( 16 ) and [Cp 2 Mo[η 2 -C(NEt)Et]BF 4 ( 17 ), respectively. Similarly the alkyl complexes [Cp 2 Mo(R)CNMe]I ( 18 : R = Me; 19 : R = Et), which are obtained from Cp 2 MoCNMe ( 9c ) and RI, rearrange in refluxing CH 2 Cl 2 to the η 2 -iminoacyl complexes Cp 2 Mo[η 2 -C(NMe)R]I ( 20 : R = Me; 21 : R = Et), whereas the tert-butyl isocyanide derivative [Cp 2 Mo(Me)CN t Bu]I ( 22 ), obtained from 9b and MeI, is stable even in refluxing acetonitrile. In the solid-state 9b consists of a bent molybdenocene fragment with an ‘end on’ bound tert-butyl isocyanide ligand. The isocyanide ligand lies within the mirror plane of the molecule, which is perpendicular to the Cp-Mo-Cp direction. A short bond from molybdenum to C α of the isocyanide ligand, a long Cα-N bond, and extensive bending of the isocyanide ligand at the nitrogen atom are observed. On the basis of these structural features and the ‘carbene like’ character of the 16e Cp 2 Mo fragment, complexes 9a – 9c can be described as organometallic analogues of ketene imines. High-yield synthetic routes to Mo IV and Mo II isocyanide complexes of the type [Cp 2 Mo(X)CNR]Y and Cp 2 MoCNR (X = H, Me, Et, Cl, I; Y = I, BF 4 , PF 6 ; R = Me, Et, t Bu) have been developed, allowing detailed studies of the reactivity of these rare compounds. Reactions of the electron-rich molybdenocene isocyanide complexes Cp 2 MoCNR with organic and inorganic electrophiles have been shown to be frontier-orbital controlled, i.e. the entering electrophiles accepts electron density from the high-lying metal-localized 1 a 1 HOMO orbital, converting the organometallic substrate to a Mo IV isocyanide complex. The latter displays interesting reactivity patterns, as shown for example by the clean isocyanide insertion rearrangement of the alkyl complexes [Cp 2 Mo(R)CNR′]Y (R, R′ = Me, Et; Y = I, BF 4 ) to give η 2 -iminoacyl compounds.