Chloro- and Trifluoromethyl-Substituted Flanking-Ring m-Terphenyl Isocyanides: η⁶-Arene Binding to Zero-Valent Molybdenum Centers and Comparison to Alkyl-Substituted Derivatives

Abstract

Presented herein are synthetic and structural studies exploring the propensity of m-terphenyl isocyanide ligands to provide flanking-ring η(6)-arene interactions to zerovalent molybdenum centers. The alkyl-substituted m-terphenyl isocyanides CNAr(Mes2) and CNAr(Dipp2) (Ar(Mes2) = 2,6-(2,4,6-Me3C6H2)2C6H3; Ar(Dipp2) = 2,6-(2,6-(i-Pr)2C6H3)2C6H3) react with Mo(η(6)-napthalene)2 in a 3:1 ratio to form tris-isocyanide η(6)-arene Mo complexes, in which a flanking mesityl or 2,6-diisopropylphenyl group, respectively, of one isocyanide ligand is bound to the zerovalent molybdenum center. Thermal stability and reactivity studies show that these flanking ring η(6)-arene interactions are particularly robust. To weaken or prevent formation of a flanking-ring η(6)-arene interaction, and to potentially provide access to the coordinatively unsaturated [Mo(CNAr(R))3] fragment, the new halo-substituted m-terphenyl isocyanides CNAr(Clips2) and CNAr(DArF2) (Ar(Clips) = 2,6-(2,6-Cl2C6H3)2(4-t-Bu)C6H2; Ar(DArF2) = 2,6-(3,5-(CF3)2C6H3)2C6H3) have been prepared. Relative to their alkyl-substituted counterparts, synthetic and structural studies demonstrate that the flanking aryl rings of CNAr(Clips2) and CNAr(DArF2) display a lower tendency toward η(6)-binding. In the case of CNAr(DArF2), it is shown that an η(6)-bound 3,5-bis(trifluoromethyl)phenyl group can be displaced from a zerovalent molybdenum center by three molecules of acetonitrile. This observation suggests that the CNAr(DArF2) ligand effectively masks low-valent metal centers in a fashion that provides access to low-coordinate isocyano targets such as [Mo(CNAr(R))3]. A series of Mo(CO)3(CNAr(R))3 complexes were also prepared to compare the relative π-acidities of CNAr(Mes2), CNAr(Clips2), and CNAr(DArF2). It is found that CNAr(DArF2) shows increased π-acidity relative to CNAr(Mes2) and CNAr(Clips2), despite the fact that its electron-withdrawing CF3 groups are fairly distal to the terminal isocyano unit.

Oxidorhenium(V) and Rhenium(III) Complexes with m‐Terphenyl Isocyanides

“…But the example of the Re(III) complex also shows that the corresponding IR frequencies are sensitive to other structural features such as the trans ligand (here Cl – vs. PPh 3 ). Similar findings have been reported earlier for various complexes with m ‐terphenylisocyanides . Thus, the absolute values of these stretches should be regarded with suspicion in the discussion of the bonding situation in such compounds.…”

“…This dual property allows isocyanides to bind well to transition metals in a range of formal oxidation states. It has been previously reported that m ‐terphenyl isocyanides can stabilize unusual low‐valent complexes of both manganese and molybdenum,, but also some medium to high valent m ‐terphenyl isocyanide complexes of Mn and Mo have been isolated via chemical oxidation of low‐valent precursors,, thereby demonstrating the utility of these encumbering ligands across a range of metal‐based redox states. Very recently, we studied reactions of m ‐terphenyl isocyanides with technetium complexes of various oxidation states and found that even technetium(V) complexes with these ligands are stabilized when strong donors such as nitrido or phenylimido ligands are present .…”

“…m ‐Terphenyl isocyanides have been shown to be useful ancillary ligands for the isolation of unusual and highly reactive transition metal complexes . These encumbering ligands stabilize transition metal centers featuring high degrees of coordinative and electronic unsaturation.…”

Fully Solvated, Monomeric Re(II) Complexes: Insights into the Chemistry of [Re(NCCH3)6]2+

“…In the past few years, we developed synthetic strategies towards new [Re( 6 -C6R6)2] + and [ 99(m) Tc( 6 -C6R6)2] + sandwich complexes. [14][15][16][17][18] Similar to other [M( 6 -C10H8)2] bis-naphthalene complexes (M = Cr, Mo), [19][20][21] coordination of the naphthalene ligands to the Re I center is comparably weak in [Re( 6…”

Di- and Trinuclear Mixed-Valence Copper Amidinate Complexes from Reduction of Iodine

“…Reports of I 3 − as a terminal 64–70 and bridging ligand have been described but with one metal bound η 1 to both terminal iodine atoms. 71,72 The absorption at 530 nm is attributed to a metal to ligand charge transfer band.…”