"Unsaturated transition metal complexes are important in many stoichiometric and catalytic bond cleavage reactions. Therefore, low-coordinate transition metal complexes coordinated with sterically hindered ancillary ligands have been used for C-H activation, N2, and CO bond cleavage reactions. In this thesis, the coordination chemistry and reactivity of low-coordinate [beta]-diketiminate cobalt and iron complexes toward bond-breaking and bond-making reactions is explored and presented. In chapter 2, the unsaturated complex LtBuCo (LtBu = bulky [beta]-diketiminate ligand) is reported. The [beta]-diketiminate ligand in LtBuCo was ligated to cobalt in a slipped [kappa]N, [eta]6-arene mode. Addition of Lewis bases to LtBuCo yielded rapid and reversible conversion to the [kappa]2N, N' mode. The rate law of ligand binding to LtBuCo was first-order in both cobalt and substrate concentration. Therefore, ligand coordination was consistent with an associative or interchange mechanism that either preceded or occurred simultaneously to [beta]-diketiminate isomerization. In addition, LtBuCo cleaved Sn-F and aryl C-F bonds, and homolytic Sn-F bond cleavage yielded [LtBuCo([mu]-F)]2. Aryl C-F bond cleavage by LtBuCo yielded [LtBuCo([mu]-F)]2 and a cobalt(II) aryl complex in a 1:2 molar ratio. [LtBuCo([mu]-F)]2 reacted with triethylsilane (Et3SiH) to give pure hydride complex [LtBuCo([mu]-H)]2, which has different properties than previously reported. In chapter 3, treatment of LMeFeNNFeLMe with 4-tert-butylpyridine (tBupy) displaced the dinitrogen ligand to give LMeFe(tBupy)2 which is formally iron(I). However, LMeFe(tBupy)2 can be defined as high-spin iron(I) with a resonance form that is high-spin iron(II) antiferromagnetically coupled to a radical on the tBupy ligand. In contrast, treatment of LMeFeNNFeLMe with pyridine (py) resulted in the reductive coupling of pyridine via C-C bond formation to give {LMeFepy}2([mu]-C10H10N2), a complex with a bridging 4,4'-bis(hydridopyridyl) ligand. {LMeFepy}2([mu]-C10H10N2) was diiron(II) in the solid state, but C-C bond formation was rapidly reversible as the solution properties were consistent with LMeFe(py)2. Chapter 4 reports new synthetic routes to iron hydride complexes with higher purity than previously achieved. The binuclear oxidative addition of H2 to a transient iron(I) intermediate yielded [LtBuFe([mu]-H)]2. This method was adapted for the synthesis of [LMeFe([mu]-H)]2, and the deuterated isotopologues, [LtBuFe([mu]-D)]2 and [LMeFe([mu]-D)]2, were synthesized using D2. The H/D exchange of hydride ligands between isotopologues and H2/D2 was observed"--Page ix-x.

In this study, we describe a detailed investigation of cobalt complexes containing redox-active ligands. We have prepared an electronic series of the complex in three oxidation states: [CoIII(ap)2]-, CoIII(isq)(ap), and [CoIII(CH3CN)(isq)2]+. Characterization shows that the metal center remains cobalt(III) through the redox changes and indicates that the oxidation state changes occur with gain or loss of electrons from the ligand set. While CoIII(isq)(ap) reacts with halide radicals to form a new cobalt-halide bond in a single electron reaction, [CoIII(ap)2]- appears to be prone to multi-electron reactivity in reactions with sources of "Cl+". Both reactions occur with electrons derived from the ligand set. Mechanistic studies suggest a single, two electron step is responsible for the bond-formation. Similarly, [CoIII(ap)2]- reacts with alkyl halides to pseudo-oxidatively add the alkyl at the cobalt center. The product of the reaction can be isolated and fully characterized and was found to be best assigned as CoIII(alkyl)(isq)2. This assignment indicates that the reaction occurs, again, with the new bond formed with two electrons formally derived from the ligand set and with no change in oxidation state at the metal center. Mechanistic investigations of the pseudo-oxidative addition suggest the reaction is SN2-like. The reaction occurs with a wide scope of alkyl halides, including those containing beta-hydrogens. : The cross-coupling reaction of CoIII(alkyl)(isq)2 with RZnX forms a new carbon-carbon bond. Similarly, the two electron oxidized complex [CoIII(CH3CN)(isq)2]+ reacts with organozinc reagents to couple two carbon nucleophiles and form a new carbon-carbon bond. Both reactions are successful with both sp2 and sp3 carbons. When followed substoichiometrically, the homocoupling reaction can be observed to form CoIII(alkyl)(isq)2. This indicates that the homocoupling and cross-coupling reactions proceed by the same mechanism. However, both reactions have low yields. The yield of the reactions are decreased by steric bulk of the alkyl or aryl fragments or around the metal center created by substituents on the ligand. Also, while the steric congestion disfavors the addition of the first alkyl fragment, the addition of the second alkyl fragment and subsequent rapid elimination of the coupling product is almost completely inhibited. This result also implies that the coupling of the two alkyl fragments is entirely inner-sphere requiring installation of both for coupling.

The tridentate S3- donor ligand, phenyltris((tert-butylthio)methyl)borate, [PhTttBu]-, has been used to investigate the coordination chemistry of cobalt. A series of coordinatively and electronically unsaturated organocobalt(II) complexes [PhTttBu]Co(R) (R = CH3, CH2CH3, C6H5, CH2C6H5, C3H5) was synthesized through reaction of the starting material [PhTttBu]COCl, with the appropriate Grignard reagent. The resultant pseudotetrahedral complexes (allyl derivative is square pyramidal) proved to be both air and moisture sensitive. The complexes were characterized spectroscopically and crystal lographically. The small molecule reactivity of these complexes was assayed through reactions with CO and NO. The former reagent showed sensitivity to the identity of the organocobalt while the latter gave rise to the same product, regardless of the organic ligand. When R = CH3, CH2CH 3, C6H5, reaction of [PhTttBu]Co(R) with CO yielded red, five-coordinate [PhTttBu]Co(CO)(C(O)R). These square-pyramidal complexes contain apical thioether ligation with the remaining four ligands in the basal plane. Alternatively, when R = CH 2C6H5, C3H5, reaction with CO resulted in Co-C bond homolysis with formation of the brown Co(I) complex, [PhTtBu]Co(CO)2. In all cases, reaction of the organocobalt(II) complexes with NO resulted in the formation of the & kappa;2-dinitrosyl complex, [& kappa;2-PhTttBu]Co(NO)2. These results were confirmed both spectroscopically and crystallographically. A series of cobalt(I) phosphine complexes, [PhTttBu ]Co(PR3), PR3 = PMe3, PEt3, PMe2Ph, PMePh2, PPh3, P(OPh3), was prepared by reduction of [PhTttBu]COCl in the presence of the phosphine. These four-coordinate complexes were characterized spectroscopically and crystal lographically. The structures may be grouped into two classes, pseudo-tetrahedral and cis-divacant (i.e. an octahedron where two cis ligands have been removed, & alpha; = 54.7° for this complex; a & alpha; is the degree to which L lies off of the linear vector). The former class, for which PR 3 = PMe3, PEt3, PMe2Ph, the phosphine donor resides approximately on the B ... Co vector. The latter class, for which PR3 = PMePh2, PPh3, P(OPh) 3, the phosphine is significantly 'off-axis.' These complexes exhibited varying a angles allowing for insight into the structural choice of the complex based on the & sigma;-donating and & pi;-accepting properties of the phosphine donor. The cobalt and nickel isocyanide complexes, [PhTttBu]M(CNBu t) are readily alkylated with MeI, leading to the corresponding cationic iminoacyl complexes, [[PhTttBu]M(C(Me)NBut)]I.

This thesis is primarily concerned with the synthesis of low oxidation state alkaline earth metal complexes featuring novel [beta]-diketiminate ligands and their derivatives. The syntheses of a variety of different [beta]-diketiminates possessing extreme steric bulk, as well as those that have been electronically modified are detailed. These ligands have been used to produce a variety of different alkali metal, alkaline earth metal and lanthanide complexes.Chapter 1 provides a general introduction to low oxidation state alkaline earth metal chemistry. Particular attention is given to metal complexes featuring [beta]-diketiminates, given their central role in the research making up this thesis.Chapter 2 deals with the synthesis of a range of novel, sterically hindered [beta]-diketimines. These pro-ligands were targeted in the belief that their extreme bulk would allow the synthesis of currently unknown structural motifs.Chapter 3 is primary concerned with the exploitation of the developed sterically hindered [beta]-diketimines in the synthesis of a variety of alkali metal, alkaline earth metal and lanthanide complexes. A range of three coordinate magnesium complexes were produced using the developed ligands, most notably, the first crystallographically characterised [beta]-diketiminato magnesium hydride. In our quest for a calcium(I) dimer, a complex type which is currently unknown, a variety of [beta]-diketiminato calcium iodides were produced. The attempted reduction of these heteroleptic metal iodide complexes resulted not in the targeted low oxidation state complexes, but in two different calcium(II) dimers.In Chapter 4, the [beta]-diketimine ligand was adapted, resulting in two separate classes of pro-ligands. The first, a 1,3,5-triazapentadiene, has had the central carbon replaced by nitrogen. The second, instead of having the typical methyl groups on the ligand backbone, has dialkylamino groups. In each instance, it was envisaged that the ligands would donate additional electron density, and bind more strongly to the metal centre in coordination complexes, when compared to the standard [beta]-diketiminate ligand class. Six separate heteroleptic magnesium(II) iodide complexes have been developed from these ligands, the majority of which show promise as precursors to low oxidation state magnesium complexes.Chapter 5 details several new secondary amines which have been produced as pro-ligands for low oxidation state main group chemistry. In addition, preliminary studies have shown that these ligands have potential in coordination chemistry.