Protonation studies of a mono-dinitrogen complex of chromium supported by a 12-membered phosphorus macrocycle containing pendant amines.
The reduction of fac-[CrCl3(P(Ph)3N(Bn)3)], (1(Cl3)), (P(Ph)3N(Bn)3 = 1,5,9-tribenzyl-3,7,11-triphenyl-1,5,9-triaza-3,7,11-triphosphacyclododecane) with Mg in the presence of dmpe (dmpe = 1,2-bis(dimethylphosphino)ethane) affords the first example of a monodinitrogen Cr(0) complex, Cr(N2)(dmpe)(P(Ph)3N(Bn)3), (2(N2)), containing a pentaphosphine coordination environment. 2(N2) is supported by a unique facially coordinating 12-membered phosphorus macrocycle containing pendant amine groups in the second coordination sphere. Treatment of 2(N2) at -78 °C with 1 equiv of [H(OEt2)2][B(C6F5)4] results in protonation of the metal center, generating the seven-coordinate Cr(II)-N2 hydride complex, [Cr(H)(N2)(dmpe)(P(Ph)3N(Bn)3)][B(C6F5)4], [2(H)(N2)](+). Treatment of 2((15)N2) with excess triflic acid at -50 °C afforded a trace amount of (15)NH4(+) from the reduction of the coordinated (15)N2 ligand (electrons originate from Cr). Electronic structure calculations were employed to evaluate the pKa values of three protonated sites of 2(N2) (metal center, pendant amine, and N2 ligand) and were used to predict the thermodynamically preferred Cr-NxHy intermediates in the N2 reduction pathway for 2(N2) and the recently published complex trans-[Cr(N2)2(P(Ph)4N(Bn)4)] upon the addition of protons and electrons.
‡Center for Molecular Electrocatalysis, Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
Michael T
MT
Mock
‡Center for Molecular Electrocatalysis, Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
Aaron W
AW
Pierpont
‡Center for Molecular Electrocatalysis, Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
Jonathan D
JD
Egbert
‡Center for Molecular Electrocatalysis, Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
Molly
M
O'Hagan
‡Center for Molecular Electrocatalysis, Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
Shentan
S
Chen
‡Center for Molecular Electrocatalysis, Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
R Morris
RM
Bullock
†Department of Chemistry, Villanova University, Villanova, Pennsylvania 19085, United States.
William G
WG
Dougherty
†Department of Chemistry, Villanova University, Villanova, Pennsylvania 19085, United States.
W Scott
WS
Kassel
‡Center for Molecular Electrocatalysis, Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
Roger
R
Rousseau
D000588:Amines / Q000737:chemistry; D001555:Benzene Derivatives / Q000737:chemistry; D002857:Chromium / Q000737:chemistry*; D056831:Coordination Complexes / Q000737:chemistry*; D003516:Cycloparaffins / Q000737:chemistry*; D008274:Magnesium / Q000737:chemistry; D009584:Nitrogen / Q000737:chemistry*; D010720:Phosphines / Q000737:chemistry; D011522:Protons; D013816:Thermodynamics
D000588:Amines; D001555:Benzene Derivatives; D056831:Coordination Complexes; D003516:Cycloparaffins; D010720:Phosphines; D011522:Protons; D002857:Chromium; D008274:Magnesium; D009584:Nitrogen