The principal goal of our research program is the development of organometallic nitrosyl complexes as specific reactants or selective catalysts for chemical transformations of practical significance. Typically, the attainment of this goal involves a three-step process. First, methods for synthesizing new complexes are developed. Then the newly prepared compounds are isolated and characterized fully by conventional spectroscopic means (including NMR and IR spectra and single-crystal X-ray crystallography of representative compounds). Finally, their characteristic chemical properties are established, and those that are unique to them are productively exploited. Two families of compounds that we have investigated in this regard recently are described in the following paragraphs.
Cp*M(NO)(alkyl)2 [M = Mo, W; Cp* = η5-C5Me5] Complexes. These compounds may be synthesized from Cp*M(NO)Cl2precursors via metatheses reactions, and they are rare examples of electronically and coordinatively unsaturated transition-metal alkyls that can be isolated under ambient conditions. The solid-state molecular structure of one of these remarkable complexes, namely Cp*W(NO)(CH2CMe3)2, has been established by a neutron-diffraction analysis, and it is shown in Figure 1.
During our investigations of the characteristic chemistry of these complexes, we have discovered that they react with a variety of small molecules, often in an unprecedented manner. We have also found that their chemical properties are often dependent on the metal and the nature of the alkyl group. Two particularly interesting aspects of their chemistry are illustrated below.
(1) Treatment of Cp*M(NO)(alkyl)2 with dihydrogen generates transient Cp*M(NO)(alkyl)(H) intermediates that exhibit a rich derivative chemistry. This is illustrated in Scheme 1 for Cp*W(NO)(CH2SiMe3)2.
(1) Furthermore, thermal activation of the Cp*M(NO)(alkyl)2 complexes in neat hydrocarbon solutions generates highly reactive alkylidene complexes that can be trapped by PMe3. This is illustrated for Cp*W(NO)(CH2CMe3)2 in Scheme 2, and the neopentylidene complex thus formed can effect both single (Scheme 2) and multiple (Scheme 3) activations of solvent C-H bonds even in the presence of excess PMe3.
Cp*W(NO)(η3-allyl)(CH2CMe3) Complexes. These compounds may also be synthesized from Cp*M(NO)Cl2 precursors via metatheses reactions. Gentle thermolyses of these complexes result in the loss of neopentane and the transient formation of the 16-electron intermediate species, Cp*W(NO)(η2-allene) and/or Cp*W(NO)(η2-diene). These transient intermediates first effect the selective single activation of hydrocarbon C-H bonds intermolecularly via the reverse of the transformations by which they were generated and form new Cp*W(NO)(η3-allyl)(η1-hydrocarbyl) complexes. This thermal chemistry is illustrated for Cp*W(NO)(η3-CH2CHCHMe)(CH2CMe3) in Schemes 4 and 5.
The solid-state molecular structure of the PMe3-trapped diene complex is shown in Figure 2.
The solid-state molecular structure of one of the chloroalkyl complexes is shown in Figure 3.
Most interestingly, exposure of the newly formed Cp*W(NO)(η3-allyl)(η1-hydrocarbyl) product complexes to pressures of CO gas generally results in the migratory insertion of the CO into the metal-alkyl linkages to form acyl compounds, as illustrated for a representative reactant in Scheme 6
We are currently investigating the chemical properties of the new Cp*W(NO)(η3-allyl)(η1-acyl) compounds resulting from these carbonylation reactions, with particular emphasis being placed on those reactions that result in the selective conversion of the original hydrocarbon substrate into oxygen-containing organics that can be released from the metal’s coordination sphere. These types of transformations mediated by our tungsten complexes are of considerable current interest, primarily due to their potential for eventual conversion of readily available hydrocarbons into a plethora of value-added products.
These studies in metal-nitrosyl chemistry are excellent for the training of highly qualified personnel since they require the individuals involved to become proficient with concepts in inorganic, organometallic, organic, and physical chemistry. During the course of her/his work, each individual in our group develops into a proficient synthetic chemist, fully familiar with the latest methodologies for the manipulation, isolation, and characterization of reactive compounds. Furthermore, mechanistic investigations train the individual to think critically during experimental design and the evaluation and interpretation of experimental data. Finally, every member of our research group is required to be familiar with the relevant chemical literature so that the oral and written presentations of their research results are always at the cutting edge of the field.