Synthesis of diamidopyrrolyl molybdenum complexes relevant to reduction of dinitrogen to ammonia

Abstract

A potentially useful trianionic ligand for the reduction of dinitrogen catalytically by molybdenum complexes is one in which one of the arms in a [(RNCH(2)CH(2))(3)N](3-) ligand is replaced by a 2-mesitylpyrrolyl-alpha-methyl arm, that is, [(RNCH(2)CH(2))(2)NCH(2)(2-MesitylPyrrolyl)](3-) (R = C(6)F(5), 3,5-Me(2)C(6)H(3), or 3,5-t-Bu(2)C(6)H(3)). Compounds have been prepared that contain the ligand in which R = C(6)F(5) ([C(6)F(5)N)(2)Pyr](3-)); they include [(C(6)F(5)N)(2)Pyr]Mo(NMe(2)), [(C(6)F(5)N)(2)Pyr]MoCl, [(C(6)F(5)N)(2)Pyr]MoOTf, and [(C(6)F(5)N)(2)Pyr]MoN. Compounds that contain the ligand in which R = 3,5-t-Bu(2)C(6)H(3) ([Ar(t-Bu)N)(2)Pyr](3-)) include {[(Ar(t-Bu)N)(2)Pyr]Mo(N(2))}Na(15-crown-5), {[(Ar(t-Bu)N)(2)Pyr]Mo(N(2))}[NBu(4)], [(Ar(t-Bu)N)(2)Pyr]Mo(N(2)) (nu(NN) = 2012 cm(-1) in C(6)D(6)), {[(Ar(t-Bu)N)(2)Pyr]Mo(NH(3))}BPh(4), and [(Ar(t-Bu)N)(2)Pyr]Mo(CO). X-ray studies are reported for [(C(6)F(5)N)(2)Pyr]Mo(NMe(2)), [(C(6)F(5)N)(2)Pyr]MoCl, and [(Ar(t-Bu)N)(2)Pyr]MoN. The [(Ar(t-Bu)N)(2)Pyr]Mo(N(2))(0/-) reversible couple is found at -1.96 V (in PhF versus Cp(2)Fe(+/0)), but the [(Ar(t-Bu)N)(2)Pyr]Mo(N(2))(+/0) couple is irreversible. Reduction of {[(Ar(t-Bu)N)(2)Pyr]Mo(NH(3))}BPh(4) under Ar at approximately -1.68 V at a scan rate of 900 mV/s is not reversible. Ammonia in [(Ar(t-Bu)N)(2)Pyr]Mo(NH(3)) can be substituted for dinitrogen in about 2 h if 10 equiv of BPh(3) are present to trap the ammonia that is released. [(Ar(t-Bu)N)(2)Pyr]Mo-N=NH is a key intermediate in the proposed catalytic reduction of dinitrogen that could not be prepared. Dinitrogen exchange studies in [(Ar(t-Bu)N)(2)Pyr]Mo(N(2)) suggest that steric hindrance by the ligand may be insufficient to protect decomposition of [(Ar(t-Bu)N)(2)Pyr]Mo-N=NH through a variety of pathways. Three attempts to reduce dinitrogen catalytically with [(Ar(t-Bu)N)(2)Pyr]Mo(N) as a "catalyst" yielded an average of 1.02 +/- 0.12 equiv of NH(3).

Figure 1

Proposed intermediates in the reduction of dinitrogen at a [HIPTN₃N]Mo (Mo) center (HIPT = hexaisopropylterphenyl) through stepwise addition of protons and electrons.

Figure 2

A “diamidopyrrolyl” complex.

Figure 3

Thermal ellipsoid drawing (50% probability) of the solid state structure of [(C₆F₅N)₂Pyr]Mo(NMe₂). H atoms are omitted for clarity. Selected bond distances (Å) and angles (°): N(5)-Mo(1) = 1.9383(12), Mo(1)-N(3) = 1.9738(12), Mo(1)-N(2) = 1.9885(12), Mo(1)-N(1) = 2.0807(12), Mo(1)-N(4) = 2.2630(12), C(21)-N(2)-Mo(1) = 129.36(10), C(31)-N(3)-Mo(1) = 128.3(3), N(1)-Mo(1)-N(4) = 77.26(5), N(2)-Mo(1)-N(4) = 78.61(5), N(3)-Mo(1)-N(4) = 77.51(5), N(5)-Mo(1)-N(4) = 176.92(5).

Figure 4

Thermal ellipsoid drawing (50% probability) of the solid state structure of [(C₆F₅N)₂Pyr]MoCl. H atoms are omitted for clarity. Selected bond distances (Å) and angles (°): Cl(2)-Mo(1) = 2.3583(4), Mo(1)-N(3) = 1.9539(12), Mo(1)-N(2) = 1.9688(12), Mo(1)-N(1) = 2.0184(12), Mo(1)-N(4) = 2.1737(12), C(21)-N(2)-Mo(1) = 125.99(10), C(31)-N(3)-Mo(1) = 123.41(10), N(1)-Mo(1)-N(4) = 79.05(5), N(2)-Mo(1)-N(4) = 79.49(5), N(3)-Mo(1)-N(4) = 79.98(5), N(4)-Mo(1)-Cl(2) = 174.98(3).

Figure 5

Thermal ellipsoid drawing (50% probability) of the solid state structure of [(Ar^t-BuN)₂Pyr]Mo(NMe₂). H atoms are omitted for clarity. Selected bond distances (Å) and angles (°): Mo(1)-N(5) = 1.6746(13), Mo(1)-N(4) = 2.4134(13), Mo(1)-N(1) = 2.0565(12), Mo(1)-N(2) = 1.9857(13), Mo(1)-N(3) = 1.9751(13), C(21)-N(2)-Mo(1) = 126.27(10), C(31)-N(3)-Mo(1) = 127.25(10), N(1)-Mo(1)-N(4) = 75.66(5), N(2)-Mo(1)-N(4) = 76.08(5), N(3)-Mo(1)-N(4) = 80.99(5), N(4)-Mo(1)-N(5) = 176.43(5).

Figure 6

Electrochemical behavior of {[(Ar^t-BuN)₂Pyr]Mo(N₂)}(n-Bu)₄N in 0.1M [NBu₄]BAr′₄ in PhF recorded at a glassy carbon electrode at 100mV/s to 900mV/s scan rates, referenced to Cp₂Fe^+/0.

Figure 7

IR spectrum of a C₆D₆ solution of [(Art-BuN)₂Pyr]Mo(N₂) and {[(Art-^BuN)₂Pyr]Mo(N₂)}Na(THF)x after exposure to ¹⁵N₂ for 2.5 h.

Figure 8

Electrochemical behavior of {[(Ar^t-BuN)₂Pyr]Mo(NH₃)}BPh₄ in 0.1M [NBu₄]BAr′₄ in PhF recorded at a glassy carbon electrode, referenced to Cp₂Fe^+/0.

Figure 9

Appearance of {[(Ar^t-BuN)₂Pyr]Mo(NH₃)}BPh₄ at scan rates of 10 and 50 mV/sec.