Reports: G3
48624-G3 Chelating Pyridyl Disulfide Ligands as Redox Modulators for Aerobic Oxidation
The PRF-G proposal concerned aerobic oxidation catalysis by complexes featuring novel redox-active ligands. However, progress on this project was slow and funds were diverted to another fast-progressing project detailing the synthesis and small molecule activation chemistry of complexes supported by m-terphenyl isocyanides. Such complexes have not previously been considered in detail, but are intended to serve as isolable models of the unstable unsaturated metal carbonyls. The high impact of this project is derived from uncovering new structural-types and reactivity patterns that have not been observed previously for transition metal complexes. Four publications have resulted during the Grant Period.
Copper complexes of the m-terphenyl isocyanide CNArMes2 (Inorganic Chemistry 2008, 47, 9010-9020): Our initial concern centered on defining the structural chemistry of complexes featuring m-terphenyl isocyanides. We elected to pursue the bis-mesityl m-terphenyl isocyanide, CNArMes2 because i) the encumbering ArMes2 unit has found application previously in transition metal systems and ii) the facile of synthesis of ArMes2 precursors on multi-gram scales. In addition, we chose Cu(I) centers as a suitable platform to assess the properties of CNArMes2 due to the fact that several XCu(CNR)n complexes have been prepared and characterized. Indeed, less encumbering isocyanides have been shown to readily form salts of the type [Cu(CNR)4]X. Thus utilization of this platform allows an assessment of the efficacy of m-terphenyl isocyanides at fostering low-coordination and controlling the extent of isocyanide ligation.
The key highlights of this paper include the formation of stable mono-, bis- and tris-CNArMes2 complexes that represent new structural types in the context of Cu(I)-isocyanide chemistry. Remarkably however, in an attempt to generate the solvent-free triflate complex [(OTf)Cu(CNArMes2)3], we isolated and the η1-benzene complex [(η1-C-C6H6)Cu(CNArMes2)3]OTf. Whereas [(η1-C-C6H6)Cu(CNArMes2)3]OTf attests to the potent Lewis acidity of the [Cu(CNArMes2)3]+ core, its structure provides instructive insights into the ability of the CNArMes2 ligand system to stabilize low-coordinate complexes, yet provide facile access to the metal center.
Preparation of Ni(CNArMes2)3 via thallium(I) coordination site protection (Angew. Chem. Int. Ed, 2009, 2009, 48, 3473-3477): With the ligation and structural properties of CNArMes2 established, we next targeted redox active metal centers. Accordingly, we focused on generating a tris-isocyanide complex of zero-valent nickel. A putative Ni(CNR)3 complex would be a clear analogue Ni(CO)3, which is the proposed active species in reactions mediated by Ni(CO)4.
In analogy to the Cu(I) chemistry, we sought to prepare the tris-isocyanide Ni(CNArMes2)3 by simple addition of CNArMes2 to a suitable Ni(0) source. Surprisingly however, treatment of Ni(COD)2 with three equiv of CNArMes2 led to an equimolar mixture of the bis-isocyanide complex Ni(COD)(CNArMes2)2 and the tetrakis-isocyanide Ni(CNArMes2)4. Thus, to prepare Ni(CNArMes2)3 in pure form, we sought a protecting group for the Ni center to preclude the association of a fourth CNArMes2 ligand.
Based on Catalano's finding that Tl(I) ions readily bind to zero-valent Pt centers, we reasoned that coordination of a Lewis acid to a suitable low-valent Ni-isocyanide fragment may adequately protect the Ni center from attack by a Lewis base. Accordingly, Ni(COD)(CNArMes2)2 was chosen for this purpose. Furthermore, we exploited the Tl(I) ion as a suitable protection agent because of its known metal-binding ability and because the general insolubility of Tl(I) halides would serve ideally for deprotection. Gratifyingly, this overall strategy was successful. Bis-isocyanide Ni(COD)(CNArMes2)2 was found to readily bind TlOTf, generating the complex [TlNi(COD)(CNArMes2)2]OTf. The latter reacted with an additional CNArMes2 ligand to selectively replace the COD unit, forming the four-coordinate salt [TlNi(CNArMes2)3]OTf. As hoped, deprotection of the Ni center was achieved by addition of one equiv of KI to [TlNi(CNArMes2)3]OTf, thereby generating the tris-isocyanide Ni(CNArMes2)3. Tris-isocyanide Ni(CNArMes2)3 was structurally characterized and a cursory investigation of its reactivity was presented.
Control of geometric isomerism utilizing the m-terphenyl isocyanide CNArDipp2 (Inorganic Chemistry, 2009, 48, 8362-8375): In order to control isocyanide binding without the use of protecting groups, we recently introduced the CNArDipp2 ligand (Dipp = 2,6-(i-Pr)2C6H3), which is a more encumbering variant of CNArMes2. Accordingly, relative to CNArMes2, the more encumbering nature of CNArDipp2 was immediately apparent when ligated to Cu(I) sources. For example, when studied with the [CuOTf] fragment, CNArDipp2 was incapable of forming a stable tris-isocyanide complex. Instead, the bis-isocyanide salt [(THF)2Cu(CNArDipp2)2]OTf was obtained irrespective of the reaction conditions.
Other highlights of work include: i) The observation of first well-defined conversion of a fac-Mo(CO)3(CNR)3 complex to its meridional isomer. Notably, the mer-orientations in d6 ML3L'3 are electronically disfavored to their fac counterparts. In this case, the steric properties of the m-terphenyl groups are sufficient to overcome the electronic preferences of metal centers. ii) This work also describes the first examples of trans-Mo(CO)4(CNR)2 complexes featuring non-fluorinated isocyanides.
A Monomeric Bis-isocyanide Complex of Pd(0) (J. Am. Chem. Soc. 2009, 131, 11318-11319): Our work with CNArDipp2-supported molybdenum complexes suggested that reactive two-coordinate bis-isocyanide complexes could be obtained. We therefore targeted a bis-isocyanide complex of zero-valent palladium to serve as an analogue of the binary carbonyl [Pd(CO)2]. As is the case for all binary carbonyl complexes of Pd(0), the dicarbonyl [Pd(CO)2] has only been observed in low-temperature matrices. Furthermore, the chemistry of binary isocyanide complexes of Pd(0) largely mimics that of the carbonyls in that stable [Pd(CNR)n] monomers have remained elusive.
We found that Mg reduction of divalent PdCl2(CNArDipp2)2 smoothly produces the bis-isocyanide monomer Pd(CNArDipp2)2. Structural characterization of Pd(CNArDipp2)2 revealed a near-linear geometry consistent with predictions for [Pd(CO)2]. As hoped, the low-coordinate nature of Pd(CNArDipp2)2 renders it highly reactive toward substrate molecules. The paper describes several bond activation and substrate addition processes accessible to Pd(CNArDipp2)2. Of note is the reaction between Pd(CNArDipp2)2 and two equiv of nitrosobenzene (PhNO). The square planar Pd geometry in the product, Pd(k1-N-PhNO)2(CNArDipp2)2, signifies the production of a di-valent metal center and thus a concomitant reduction of the ligated PhNO units. Indeed, DFT calculations on the model complex Pd(k1-N-PhNO)2(CNArPh2)2, revealed a delocalized p* system wherein the bond order of each NO unit has been reduced to 1.5. This formal reduction is unprecedented in the coordination chemistry of nitroso compounds and thus demonstrates that low-coordinate metal isocyanide complexes can elicit novel reactivity patterns.
