To Split or Not to Split: [AsCCAs]-Coordinated Mo, W, and Re Complexes and Their Reactivity toward Molecular Dinitrogen.

Molybdenum, tungsten, and rhenium halides bearing a 2,2'-( i Pr 2 As) 2 -substituted diphenylacetylene ([AsCCAs], 1-As ) were prepared and reduced under an atmosphere of dinitrogen in order to activate the latter substrate. In the case of molybdenum, a diiodo ( 2-As ) and a triiodo molybdenum precursor ( 5 ) were equally suited for reductive N 2 splitting, which led to the isolation of [AsCCAs]Mo≡N(I) ( 3-As ) in each case. For tungsten, [AsCCAs]WCl 3 ( 6 ) was reduced under N 2 to afford {[AsCCAs]WCl 2 } 2 (N 2 ) ( 7 ), which is best described as a dinuclear π 8 δ 4 -configured μ-(η 1 : η 1 )-N 2 -bridged dimer. Attempts to reductively cleave the N 2 unit in 7 did not lead to the expected tungsten nitride ( 8 ), which had to be prepared independently via the treatment of 7 with sodium azide. To arrive at a π 10 δ 4 -configured N 2 -bridged dimer in a tetragonally distorted ligand environment, [AsCCAs]ReCl 3 ( 9 ) was reduced in the presence of N 2 . As expected, a μ-(η 1 : η 1 )-N 2 -bridged dirhenium species, namely, {[AsCCAs]ReCl 2 } 2 (N 2 ) ( 10 ), was formed, but found to very quickly decompose (presumably via loss of N 2 ), not only under reduced pressure, but also upon irradiation or heating. Hence, an alternative synthetic route to the originally envisioned nitride, [AsCCAs]Re≡N(Cl) 2 ( 11 ), was developed. While all the aforementioned nitrides ( 3-As , 8, and 11 ) were found to be fairly robust, significantly different stabilities were noticed for {[AsCCAs]MCl 2 } 2 (N 2 ) ( 7 for M = W, 10 for M = Re), which is ascribed to the electronically different MN 2 M cores (π 8 δ 4 for 7 vs π 10 δ 4 for 10 ) in these μ-(η 1 : η 1 )-N 2 -bridged dimers.