This dissertation describes the targeted attempts at the generation of transition metal species that function as precise electronic structure mimics to the well known spin triplet (S =1) metal carbonyls fragments Fe(CO)4 and CpCo(CO). These unsaturated fragments have been shown to display a wide range reactivity, and competency towards important reaction chemistry such as alkane and N2 binding, and E-H bond activation due to a unique interplay of a strong ligand field, formal dn count, and orbital symmetry, rendering these fragments primed for bond activation. Accordingly, ligand architectures that can accurately mimic the ligand field provided by CO to kinetically stabilize these fragments could provide new inroads to novel small molecule activation pathways. To this end, sterically encumbering m-terphenyl isocyanides serve as isolobal ligand surrogates for carbon monoxide (CO). Additionally isocyanides have the added benefit of providing kinetic stabilization by virtue of readily tunable isocyano-R (CN-R) group. The first section of this dissertation describes the synthesis and protonation of an encumbered tetra-isocyanide iron dianion, Na2[Fe(CNArMes2)4] (ArMes2 = 2,6-(2,4,6 --Me3C6H2)2C6H3), which serves as a platform for targeting species of the formulation Fe(CNArMes2)4. It is shown that the reactivity of the electronically unsaturated Fe(CNR)4 fragment upon protonation of Na2[Fe(CNArMes2)4] and subsequent alkylation of Na[HFe(CNArMes2)4], yields the dinitrogen stabilized species Fe(N2)(CNArMes2)4. Fe(N2)(CNArMes2)4 is shown to readily undergo intramolecular C-H activation of the ligand scaffold upon liberation N2 under ambient conditions purportedly through and insipient [Fe(CNArMes2)4] fragment. Further more, ability of Na2[Fe(CNArMes2)4] to facilitate the reductive disproportionation of CO2, in addition to CO2 capture with electrophilic silyl sources is presented culminating in a rare class of low valent Fe-aminocarbyne complexes. The second vignette of this dissertation focuses on the generation of species that mimic the formulation CpCo(L). It is shown that with less encumbering m-terphenyl isocyanides that aggregation akin to the unsaturated carbonyl congeners is realized. Use of encumbering m-terphenyl isocyanides provides access to the three memebered electron transfer series [([mu]2-CNArMes2)2[CpCo]2]n (n = 0,-1, -2). Notably, this series is the first of its kind to span all three ostensible electronic states (e.g. d8-d8, d8-d9, and d9-d9), previously unavailable with other [pi]-acidic ligand frameworks. Additionally this allows for a systematic reassessment of the metal-metal bonding within this class of dimeric species. Evidence is put forth in favor of no M-M bonding interactions occur within these systems and the integrity of the dimeric framework is in fact mitigated through a unique interplay of the metal d-manifold and the isocyanide [pi]*-system. Modulation of the steric profile of the m-terphenyl isocyanide and the Cp unit to Cp* so as to increase the steric pressure provides access to the first reported mono-nuclear Cp*Co(N2)L fragments. It is shown that these species function as viable sources of Cp*Co(CNR) for a number of bond activation processes including Si-H, H-H, and P-P bond scission. Moreover, the reactivity of these species culminates with the isolation of the second example of a structurally authenticated transition metal nitrous oxide (N2O) adduct, which exhibits an unprecedented [eta]2-(N,N) coordination mode to Co. Finally, the reduction of the encumbered Cp*Co(CNArTripp2) (CNArTripp2 2,6-(2,4,6-(i-Pr)3C6H3)2C6H3) fragment provide access to the unique dianion K2[Cp*Co≡CNArTripp2]. It is shown that the dianion K2[Cp*Co≡CNArTripp2] exhibits 3-fold bonding between Co and the isocyanide -Ciso through an extreme case of M-->(CN) [pi]*-back donation and gives rise to the first example of a Co-carbyne complex. The reactivity and electronic structure are presented for K2[Cp*Co≡CNArTripp2] and it is concluded that this reactive dianion behaves as a potent metal based nucleophile and source of [Cp*Co(CNR)]2- for a number of bond activation process.

This dissertation details the utilization of anionic, nucleophilic iron complexes supported by m-terphenyl isocyanide and carbonyl ligands to address longstanding questions in organometallic and inorganic chemistry. Chapter 1 offers a brief account of the development of low valent transition-metal chemistry with carbon monoxide (CO) and isocyanides in mononuclear and multinuclear systems. Metal carbonyl clusters gained popularity as molecular surrogates for reactive sites on heterogeneous catalyst surfaces, and some successes and shortcomings of this cluster-surface analogy are revisited in Chapter 2. The synthesis and reactivity of the tetra-iron nitrido cluster [Fe([mu]4-N)(CO)8(CNArMes2)4]- (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3) is contrasted with the less electron-rich all-carbonyl congener [Fe([mu]4-N)(CO)12]-. Ligand substitution is shown to impart nucleophilicity to the interstitial nitride, and this characteristic enables rational cluster expansion with main-group and transition-metal ions to yield unsaturated sites. The resulting clusters were found to display surface-like reactivity through coordination-sphere-dependent atom rearrangement and metal-metal cooperativity. The remaining three chapters stem from K2[Fe(CO)2(CNArTripp2)2] (ArTripp2 = 2,6-(2,4,6-(i-Pr)3C6H2)2C6H3). In Chapter 3, this metalate is used to generate Fe(BF)(CO)2(CNArTripp2)2, the first stable terminal fluoroborylene complex. Importantly, the isoelectronic species Fe(N2)(CO)2(CNArTripp2)2 and Fe(CO)3(CNArTripp2)2 are also described allowing for the direct comparison of neutral 10 valence-electron ligands. Single-crystal X-ray diffraction, nuclear magnetic resonance, infrared, and Mössbauer spectroscopic studies demonstrate that the terminal BF ligand possesses particularly strong s-donor and p-acceptor properties in accord with theoretical predictions. Density functional theory and electron-density topology calculations support this conclusion. The reactivity of Fe(BF)(CO)2(CNArTripp2)2 is discussed in Chapter 4. Like all terminal borylene ligands, coordinated BF is shown to be electrophilic at boron, forming Lewis acid-base adducts with various nucleophiles. However, the fluoroborylene ligand can be derivatized further than other borylenes and converted stepwise into aminoborylene and iminoboryl moieties. Additionally, BF can be transformed directly to the oxoboryl anion [BO]-. The last chapter presents efforts toward an analogue of the unsaturated binary metal carbonyl Fe(CO)4. Unusual solvent binding and bond activations suggest that Fe(N2)(CO)2(CNArTripp2)2 may indeed serve as a masked functional analogue of Fe(CO)4.

A synthetic and structural investigation of cobalt complexes stabilized by m-terphenyl isocyanides was conducted. These species were studied as isolable mimics to unsaturated cobalt carbonyls and as platforms to access coordination complexes with unusual dn/valence combinations. Through these studies, a critical assessment of isocyanide for carbonyl substitution is presented. This assessment includes a comparative study on the electronic influence of isocyanides (C[equivalence]N--R), as a function of R-group identity, in Cr(CNR)(CO)5 complexes. Structural mimics to all three conformational isomers of Co2(CO)8 were prepared using the m-terphenyl isocyanides CNArMes2 (ArMes2 = 2,6-(2,4,6-Me3C6H2)C6H3) and CNArDipp2 (ArDipp2 = 2,6-(2,6-i-Pr2C6H3)C6H3). Solid-state X-ray diffraction data showed that 1,1-Co2(CO)6 (CNArMes2)2 adopts a "D2d-type" conformation, analogous to the elusive D2d isomer of Co2(CO)8. The cobalt hydride HCo(CNArMes2)4 was synthesized and structurally characterized. As a potential analogue to HCo(CO)4, a broad survey of its reactivity was undertaken. In the course of this study, HCo(CNArMes2)4 was found to mediate 1,1-hydrogenation and 1,1-hydrosilylation of CNArMes2. Complementing this work, a complete series of mixed carbonyl-isocyanide cobalt metallates, [Co(CO)4-n(CNArMes2)n]-, and hydrides, HCo(CO)4-n(CNArMes2)n, were prepared. The synthesis and solution-phase properties of a zwitterionic cobalt metallate is presented. 2D EXSY NMR studies and isotopic labeling experiments demonstrate that [Eta]2-PPNCo(CNArMes2)3, functions as a masked source of [Co(CNArMes2)3]1-, a 16 valence electron isocyano metallate. Electrophillic functionalization of [Eta]2-PPNCo(CNArMes2)3, with TMSCl, provided access to Co(SiMe3)(CNArMes2)3. This complex was structurally characterized by X-ray diffraction. The previously reported ability of Co(SiMe3)(CNArMes2)3 to form a [alpha]-complex with n-hexane at low-temperature in the solid state is contextualized with variable temperature IR and NMR studies. Reduction of Co(SiMe3)(CNArMes2)3 generated the highly reduced isocyano metallate K2[Co(SiMe3)(CNArMes2)3]. This dissertation concludes with the synthesis and characterization of the first examples of terminal transition metal antimony trifluoride (SbF3) and antimony difluoride (SbF2) complexes.

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Sustained interest in the unsaturated mononuclear cobalt carbonyls arises from their presumed role as reactive intermediates in industrial hydroformylation and carbonylation processes, yet their observations have been limited to low temperature matrix isolations. Herein, we report a full library of isolobal analogues of unsaturated cobalt carbonyl complexes by using sterically encumbering m-terphenyl isocyanides. A detailed mechanistic study on both the reactivity of Co(CNArMes2)4 along with the unique bond activation processes of (SiMe3)Co(CNArMes2)3 with N2O are presented. For the latter system, a demonstration of the catalytic capability of these cobalt complexes for the production of organoisocyanates is presented, which provides an exciting opportunity to produce value-added products using N2O as a terminal oxidant. Furthermore, a dispersion forced solid-state host-guest interaction with n-hexane is discussed with support of EDA (Energy Decomposition Analysis) calculation. A solution-phase persistence weakly coordinated 1,6-diaminohexane dimer complex, ([mu]2-N-(N2C6H18)[Co(SiMe3)(CNArMes2)3]2, was made and the CH/[pi] interactions was monitored by VT-NMR (Variable Temperature Nuclear Magnetic Resonance), providing new insight into the existence of [sigma]-alkane adducts of Co(SiMe3)(CNArMes2)3 forced by van der Waals (CH/[pi]) interactions in the solid-state. Solution phase 2D-IR (Ultrafast Two-Dimensional Infrared Spectroscopy) data and DFT (Density Functional Theory) calculations presented to provide the direct observation of rapid isomerization of Co(CNArMes2)4 between the C3v and D2d isomers. Furthermore, for the first time, a controlled, stepwise cobalt phosphide cluster synthesis using cobalt metallates as building blocks is achieved.

"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.