Richard D. Ernst

Pentadienyl Ligands: Their Properties, Potential, and Contributions to Inorganic and Organometallic Chemistry

Abstract Pentadienyl ligands have been found to possess many unique and potentially useful characteristics, including the ability to adopt a wide variety of η1, η3, and η5 bonding modes, a propensity for bonding to metals in low oxidation states, a tendency toward forming sterically crowded complexes, and the ability to engage in a wide variety of coupling reactions, even while being more strongly bound to a metal center than the ubiquitous cyclopentadienyl ligand. The combination of these properties has led to a rich body of chemistry for these ligands, and the promise that much more remains to be gained.

Metal complexes of anionic 3-borane-1-alkylimidazol-2-ylidene derivatives

Addition of BH 3 ·thf to 1-alkylimidazoles (alkyl=methyl, butyl) and 1-methylbenzimidazole leads to BH 3 adducts, which are deprotonated by BuLi to yield the organolithium compounds (L)Li + ( 1b – d ) − . In the solid state (thf)Li + 1b − is dimeric. The acyl–iron complexes (thf) 3 Li + ( 3b , d ) − are formed from (thf)Li + ( 1b , d ) − and Fe(CO) 5 . (L)Li + ( 1a – c ) − react with [CpFe(CO) 2 X], however, the only complex obtained is [CpFe(CO) 2 1a ] (5a ). The analogous reaction of (L)Li + 1a − with the pentadienyl complex [(C 7 H 11 )Fe(CO) 2 Br] yields the corresponding iron compound 6a . Their compositions follow from spectroscopic data. Treatment of Cp 2 TiCl with (L)Li + 1a − leads to [Cp 2 Ti 1a ] ( 7a ), which could not be oxidized with PbCl 2 to give the corresponding Ti(IV) complex. The compounds [Li(py) 4 ] + 9a − and [Li(L) 4 ] + ( 10b – d ) − are obtained when (L)Li + 1 − are reacted with VCl 3 and ScCl 3 . The X-ray structure analysis of the vanadium complex reveals a distorted tetrahedron of the anion [V( 1a ) 4 ] − with two smaller and four larger CVC angles. The scandium compound [Li(dme) 2 + 10c − ] has a different structure: the distorted tetrahedron of the anion [Sc( 1c ) 4 ] − contains two larger (140.2 and 142.9°) and four smaller CScC angles (93.9–98.7°). This arrangement allows the formation of four bridging BHSc 3c,2e bonds to give an eight-fold coordination. The anion 10c − is formally a 16e complex.

The preparation of the 2,4-di(t-butyl)pentadienyl anion and its Ti(II), Cr(II), and Zn(II) complexes

The potassium of the 2,4-di(t-butyl)pentadienyl anion may be prepared by metallation of the corresponding 1,3-diene. This anion reacts readily with titanium, chromium, and zinc dichloride complexes to yield the appropriate M[2.4-(t-C4H9)2C5H5]2 species, which display significant differences relative to their 2,4-dimethylpentadienyl analogs.

Charge Density Analysis of the (C−C)→Ti Agostic Interactions in a Titanacyclobutane Complex

The experimental electron density study of Ti(C(5)H(4)Me)(2)[(CH(2))(2)CMe(2)] provides direct evidence for the presence of (C-C)-->Ti agostic interactions. In accord with the model of Scherer and McGrady, the C(alpha)-C(beta) bond densities no longer show cylindrical symmetry in the vicinity of the Ti atom and differ markedly from those of the other C-C bonds. At the points along the C(alpha)-C(beta) bond where the deviation is maximal the electron density is elongated toward the metal center. The distortion is supported by parallel theoretical calculations. A calculation on an Mo complex in which the agostic interaction is absent supports the Scherer and McGrady criterion for agostic interactions. Despite the formal d(0) electron configuration for this Ti(IV) species, a significant nonzero population is observed for the d orbitals, the d orbital population is largest for the d(xy) orbital, the lobes of which point toward the two C(alpha) atoms. Of the three different basis sets for the Ti atom used in theoretical calculations with the B3LYP functional, only the 6-311++G** set for Ti agrees well with the experimental charge density distribution in the Ti-(C(alpha)-C(beta))(2) plane.

Investigation of the reactivity of the phosphorushydrogen bond in Cp′RuL1L2Cl complexes with diphenylphosphine ligands

Abstract Products resulting from the oxidation, chlorination, hydrolysis and ethanolysis of Cp*Ru(PHPh2)2Cl (1) are described. Chlorination of the PH bond in 1 allowed isolation of Cp*Ru(PClPh2)2Cl (6) and there is spectroscopic evidence for the presence of Cp*Ru(PClPh2)(PHPh2)Cl (5). Hydrolysis of compounds 1 and 5 gave a dihydride complex [Cp*Ru(H)2(OPPh2)(POHPh2)] (13) and cationic species [Cp*Ru(PHPh2)3]Cl (14) and [Cp*RuPOHPh2(PHPh2)2]Cl (15). Formation of 14 was confirmed by the synthesis of [Cp*Ru(PHPh2)3]OTf (14′) through intermediate [Cp*Ru(MeCN)(PHPh2)2]OTf (16) which was obtained from the addition of AgOTf to 1 in MeCN solution. Reaction of (Cp*RuCl2)2 (12) in EtOH with two equivalents of PHPh2 gave compound 1 and ethanolysis products Cp*Ru(POEtPh2)(PHPh2)Cl (18) and Cp*Ru(POEtPh2)2Cl (11), while reaction of 12 with diphenylphosphine oxide (OPHPh2) led to the formation of Cp*Ru(POHPh2)2Cl (7), which can either be converted to hydride compound Cp*Ru(H)(Cl)[(OPPh2)(POHPh2)] (10) or an aromatic ring of its coordinated phosphine will bind to the electrophilic [Cp*Ru]+ fragment to give compound [Cp*Ru[μ-(η6-C6H5)POHPh]Cp*Ru(POHPh2)Cl]Cl (8). Based on 1H- and 31P-NMR spectroscopic data, compounds with two different P-donor ligands have also been prepared, including Cp*Ru(POHPh2)(PHPh2)Cl (9), [Cp*Ru(POHPh2)2(PHPh2)]OTf (17) and Cp*Ru(POHPh2)(POEtPh2)Cl (19). According to NMR observations, a free radical mechanism is proposed for the chlorination and oxidation reactions. Crystal structures are provided for complexes 6, 13 and 18, and a related cationic Cp complex [CpRu(POHPh2)(PHPh2)2]Cl (20).

Pentadienyl Complexes of the Group 4 Transition Metals

Publisher Summary This chapter focuses on the advances made in the pentadienyl chemistry of three elements—namely, titanium, zirconium, and hafnium. As group 4 transition metals, these elements generate a rich coupling chemistry, and as their metallocenes also they have proven to be remarkably unique. The chapter describes some of the most commonly used pentadienyl ligands as illustrated in a chart. In general, a numbering scheme is utilized in which the dienyl termini are defined as C1 and C5, with C2–C4 appearing between C1 and C5. Formally, any charge delocalized over the dienyl fragment can be shared by C1, C3, and C5, leaving C2 and C4 uncharged. The metals themselves play a further role by promoting coupling reactions between unsaturated organic molecules and pentadienyl ligands, much as had already been observed for early metal diene complexes. A number of open and half-open titanocenes, zirconocenes, and hafnocenes have been reported to yield catalysts for the hydrogenation of unsaturated polymers such as polybutadiene. The hydrogenations are carried out in solution phase, and generally are very efficient (84–89%). In addition, the chapter discusses reactions in which pentadienyl ligands are altered, as may occur via a coupling reaction, and applications of metal pentadienyl compounds in materials and catalytic processes.

Zr(C5H5)(6,6-dmch)(PMe3)2, an edge-bridged half-open zirconocene-synthesis, structure, and its reaction with C6H5C2SiMe3

Abstract The synthesis and characterization of an edge-bridged half-open zirconocene, Zr(C5H5)(dmch)(PMe3)2 (dmch=dimethylcyclohexadienyl), are described. As expected, the ZrC bond distances for the dmch ligand are significantly shorter than those for the C5H5 ligand. The compound readily incorporates 2 equiv. of alkyne, with loss of the PMe3 ligands, leading to a zirconacyclopentadiene complex, so that the dmch ligand has remained intact.

Carbonyl and trifluorophosphine adducts of bis(2,4-dimethylpentadienyl)titanium and bis(2,4-dimethylpentadienyl)vanadium

The syntheses of mono(carbonyl) and mono(trifluorophosphine) adducts of bis(2,4-dimethylpentadienyl)titanium and bis(2,4-dimethylpentadienyl)vanadium are described. Characterization has been achieved by routine spectral and analytical procedures. Detailed 1H and 13C NMR studies, as well as electron paramagnetic resonance and comparative infrared studies have been carried out in order to gain further understanding of the electronic nature of pentadienylmetal compounds, particularly compared to cyclopentadienylmetal compounds. Thus, infrared and EPR studies of V(C5H5)2(CO) and V(2,4-C7H11)2(CO) suggest that the withdrawal of electron density by the 2,4-dimethylpentadienyl ligand is considerably greater than that by the cyclopentadienyl ligand

Synthesis, characterization, and solid-state structures of the 14-electron open metallocenes M[1,5-(Me3Si)2C5H5]2 (M= Ti or Zr)☆

Abstract Reactions of titanium(II) or zirconium(IV) chlorides with two or four equivalents of K[1,5-(Me 3 Si) 2 C 5 H 5 ], respectively, lead to the diamagnetic 14-electron M[1,5-(Me 3 Si) 2 C 5 H 5 ] 2 complexes. Both have been characterized analytically, spectroscopically, and through single crystal X-ray diffraction studies.

The synthesis and characterization of tris(2,4-dimethylpentadienyl)uranium,U(2,4-C7H11)3

Abstract The reaction of “UCl3·nTHF” with three equivalents of the 2,4-dimethylpentadienyl anion leads to the formation of pyrophoric tris(2,4-dimethylpentadienyl)uranium, a rare example of a homoleptic uranium(III) organometallic compound. The compound has been characterized by infrared and NMR spectroscopy, elemental analysis, and magnetic susceptibility measurements.