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The splitting of dinitrogen (1 atm, THF, 25°C) by Mo(N[R]Ar) 3 (R ) C(CD 3 ) 2 CH 3 , Ar ) 3,5-C 6 H 3 Me 2 ) giving 2 equiv of nitride NtMo(N[R]Ar) 3 is found to be accelerated in the presence of sodium amalgam. Careful control of the Mo(N[R]Ar) 3 concentration led to the isolation and characterization of the anionic dinitrogen complex, [(THF) x Na][(N 2 )Mo(N[R]Ar) 3 ], where x is from 0 to 3. Via electrochemical experiments and synthetic studies, [(THF) x Na][(N 2 )Mo(N[R]Ar) 3 ] is found to be a key intermediate in the acceleration of N 2 splitting by Mo(N[R]Ar) 3 in the presence of sodium amalgam. Accordingly, in the presence of an electron acceptor, [(THF) x Na][(N 2 )Mo(N[R]Ar) 3 ] reacts with Mo(N[R]Ar) 3 to give the neutral N 2 -bridged complex (µ-N 2 ){Mo(N[R]Ar) 3 } 2 , which in turn splits to 2 equiv of nitride NtMo(N[R]Ar) 3 . It is seen that the function of sodium amalgam in this system is as a redox catalyst, accelerating the conversion of Mo(N[R]Ar) 3 to (µ-N 2 ){Mo(N[R]Ar) 3 } 2 , a dinuclear dinitrogen complex that does not lose N 2 readily. Electrochemical or chemical outer-sphere oxidation of [(THF) x Na][(N 2 )Mo(N[R]Ar) 3 ] leads to rapid N 2 evolution with regeneration of Mo(N[R]Ar) 3 , presumably via the neutral mononuclear dinitrogen complex (N 2 )Mo(N[R]Ar) 3 . In situ generated [(THF) x Na][(N 2 )Mo(N[R]Ar) 3 ] was efficiently trapped by ClSiMe 3 to give (Me 3 SiNN)Mo(N[R]-Ar) 3 . This complex underwent reaction with methyl triflate to give the dimethyl hydrazido cationic species, [(Me 2 NN)Mo(N[R]Ar) 3 ][OTf]. The synthesis of the monomethyl complex (MeNN)Mo(N[R]Ar) 3 also was achieved. Experiments designed to trap the neutral mononuclear dinitrogen complex (N 2 )Mo(N[R]Ar) 3 gave rise to efficient syntheses of heterodinuclear dinitrogen complexes including (Ph[ t Bu]N) 3 Ti(µ-N 2 )Mo(N[R]-Ar) 3 , which also was synthesized in its 15 N 2 -labeled form. Synthesis and characterization data for the new N-adamantyl-substituted three-coordinate molybdenum(III) complex Mo(N[Ad]Ar) 3 (Ad ) 1-adamantyl, Ar ) 3,5-C 6 H 3 Me 2 ) are presented. The complex is found to react with dinitrogen (1 atm, THF, 25°C) in the presence of sodium amalgam to give the dinitrogen anion complex [(THF) x Na][(N 2 )Mo(N[Ad]Ar) 3 ]; the synthesis does not require careful regulation of the Mo(N[Ad]Ar) 3 concentration. Indeed, under no conditions has Mo-(N[Ad]Ar) 3 been observed to split dinitrogen or to give rise to a dinuclear µ-N 2 complex; this striking contrast with the reactivity of Mo(N[R]Ar) 3 (R ) C(CD 3 ) 2 CH 3 ) is attributed to the enhanced steric protection at Mo afforded by the 1-adamantyl substituents.

The air stable complex [(PNP)FeCl2] (1) (PNP = N[2-P(CHMe2)2-4-methylphenyl]2−), prepared from one-electron oxidation of [(PNP)FeCl] with ClCPh3, displays an unusual S = 3/2 to S = 5/2 transition above 80 K as inferred by the dc SQUID magnetic susceptibility measurement. The ac SQUID magnetization data, at zero field and between frequencies 10 and 1042 Hz, clearly reveals complex 1 to undergo a frequency dependent of the out-of-phase signal and thus be a single molecular magnet with a thermally activated barrier of Ueff = 32-36 cm−1 (47 - 52 K). Variable temperature Mössbauer data also corroborate a significant temperature dependence in δ and ΔEQ values for 1, which is in agreement with the system undergoing a change in spin state. Likewise, variable temperature X-band EPR spectra of 1 reveals the S = 3/2 to be likely the ground state with the S = 5/2 being close in energy. Multi-edge XAS absorption spectra suggest the electronic structure of 1 to be highly covalent with an effective iron oxidation state that is more reduced than the typical ferric complexes due to the significant interaction of the phosphine groups in PNP and Cl ligands with iron. A variable temperature single crystal X-ray diffraction study of 1 collected between 30-300 K also reveals elongation of the Fe–P bond lengths and increment in the Cl–Fe–Cl angle as the S = 5/2 state is populated. Theoretical studies show overall similar orbital pictures except for the d(z2) orbital which is the most sensitivity to change in the geometry and bonding where the quartet (4B) and the sextet (6A) states are close in energy.

A library of low-coordinate titanium and vanadium complexes containing terminal metal-ligand multiply bonded functionalities such as alkylidenes, alkylidynes, and imides have been prepared by one-electron oxidatively induced alpha-hydrogen abstraction reactions. In the case of the alkylidene motif, the nucleophilic nature of the M-C multiple bond permits subsequent reactions such as alpha-hydrogen migration to generate other rare functionalities such as phosphinidene-alkyl and imide-alkyls. Identifying and fine-tuning of the supporting ancillary ligand on the metal has allowed the isolation of kinetically stable titanium alkylidene and phosphinidene systems. The former is a key functionality to generate transient titanium alkylidynes, which readily engage in intermolecular C-H activation reactions of arenes and alkanes, and the ring-opening metathesis of aromatic substrates such as pyridines. In this Account, we describe several synthetic strategies to achieve reactive functionalities, functionalities that were previously portrayed as "incompatible" or "too kinetically reactive" with 3d early transition metals.

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