Reactivity Patterns of Transition Metal Hydrides and Alkyls

Author: Jones, William Davidson, II

Year: 1979

Degree: Dissertation (Ph.D.)

Advisor: Bergman, Robert G.

Committee Member: Unknown, Unknown

Option: Chemistry

DOI: 10.7907/yj3c-pw50

Abstract

The complex PPN+CpV(CO)3H- (Cp=n5-C5H5) has been prepared in 70% yield by sodium reduction of CpV(CO)4 followed by protonation of the resulting dianion [CpV(CO)3]2- with water and cation exchange with PPN+Cl-. The physical properties and chemical reactions of PPN+CpV(CO)3H- have been investigated. The sodium salt of CpV(CO)3H- is contact ion-paired in the solid state and in THF; dissociation of solvent-separated pairs occurs on conversion to the PPN+ salt or dissolution in polar solvents such as HMPA. PPN+CpV(CO)3H- reacts with a wide range of organic halides, resulting in substitution of the halogen atom by the hydrogen of CpV(CO)3H-. The organometallic products of these reactions are the vanadium halides PPN+[CpV(CO)3X]-. In some cases a second organometallic product is observed; this material is the binuclear bridging hydride PPN+ [CpV(CO)3]2 H-, and it is formed by reaction of the kinetic product CpV(CO)3X- with starting CpV(CO)3H- present in the reduction solutions. Irradiation of CpV(CO)4 in the presence of CpV(CO)3H- provides an alternate route to bridging hydride [CpV(CO)3]2 H-. Competition experiments against PPh3 have shown that CpV(CO)3H- reacts more rapidly than the phosphine with transient, coordinatively unsaturated CpV(CO)3, but thermodynamically PPh3 is the better ligand. The borohydride salt PPN+[CpV(CO)3BH4]- has also been prepared, by treating CpV(CO)4 with NaBH4 and by treating CpV(CO)3H- with BH3THF. The mechanism of the reaction between CpV(CO)3H- and organic halides has been investigated in detail, and compared in several cases with halide reductions carried out using tri-n-butyltin hydride. Relative reactivity, stereochemistry, cyclization and trapping studies demonstrate that in almost all cases, the reduction reaction proceeds via free radical intermediates. As in the R3SnH case, these intermediates are generated in a chain process, and are trapped by hydrogen transfer from CpV(CO)3H-. The absolute rate constant for this transfer step can be estimated to be ca. 2 X 107 M-1 sec-1, nearly an order of magnitude larger than the rate constant for hydrogen transfer from tri-n-butyltin hydride. Reduction of cyclopropylcarbinyl bromide appears to be exceptional, and probably proceeds by a two-electron mechanism,

Sodium amalgam reduction of CpRh(CO)2 or a mixture of CpRh(CO)2 and CpCo(CO)2 affords two new anions, PPN+ [Cp2Rh3(CO)4]- and PPN+[Cp2RhCo(CO)2]-. The latter compound appears to be physically and chemically similar to the known PPN+[Cp2Co2(CO2]-, whereas the former posesses the same substructure bridged by a Rh(Co)2 unit. Both anions react with CH3I producing acetone via CpRh(CO)(CH3)2 or CpCo(CO)(CH3)2. In addition, the compounds Cp2RhCo(CO2R2 (R=CH3, CH2CF3) are formed upon alkylation of [Cp2RhCo(CO)]+ and are observed to decompose to CpRh(CO)(CH2CF3)2 and CpCo(CO)(CH2CF3)2.

CpMo(CO)3H has been found to react with CpMo(CO)3R (R=CH3, C2H5, CH2C6H5; 2a, 2b, 2c) at 25-50°C to quantitatively produce aldehyde RCHO and the dimers [CpMo(CO)3]2 and [CpMo(CO)3]2. Evidence is presented indicating that aldehyde formation occurs by insertion of CO into the Mo-R bond followed by hydride transfer by 1 forming an intermediate acyl hydride. The intermediate acyl CpMo(CO)2(COR) also reacts with ethylene and CpMo(CO)3H producing ketones R-C-C2H5. A mechanism is proposed for these reactions and a comparison with the cobalt analogues in the hydroformylation process discussed.

The reactions of PPN+CpV(CO)3H- with some transition metal carbonyls, alkyl and acyls are studied. In almost all cases substitution of hydride (H-) for CO is observed. The chemistry of CpMo(CO)3H with similar compounds is also examined and compared with that of CpV(CO)3H-. In general, CpV(CO)3H- appears to transfer a hydrogen atom to the metal radical anion formed in an electron transfer process 5 whereas CpMo(CO)3H- transfers hydride in a 2-electron process to a vacant coordination site. The chemical consequences are that CpV(CO)3H- generally reacts with metal alkyls to give alkanes via. intermediate alkyl hydride species whereas CpMo(CO)3H reacts with metal alkyls to produce aldehyde, via an intermediate acyl hydride species.

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