Vanadium and chromium complexes supported by sterically demanding ligands : studies relevant to the reduction of dinitrogen to ammonia

Abstract

Chapter 1. Using the [HIPTN3N]3- ligand ([HIPTN3N]3- = [(HIPTNCH2CH2)3N]3-; HIPT = 3,5-(2,4,6-i-Pr3C6H2)2C6H3 = HexaIsoPropylTerphenyl), green paramagnetic [HIPTN3N]V(THF) (1) can be prepared from VCl3(THF)3. Reduction of 1 with potassium graphite in ethereal solvents yields a highly sensitive red solution identified as containing paramagnetic {[HIPTN3N]VN2}-(2) via infrared spectroscopy. 1 also reacts with ammonia to form bright green paramagnetic [HIPTN3N]V(NH3) (5), 2-methylaziridine to form red diamagnetic [HIPTN3N]V=NH (7), azidotrimethylsilane to form orange diamagnetic [HIPTN3N]V=N(SiMe3) (9), propylene oxide to form purple diamagnetic [HIPTN3N]V=O (11), elemental sulfur to form dark green diamagnetic [HIPTN3N]V=S (10), and carbon monoxide to form red-gold [HIPTN3N]V(CO) (12). X-Ray crystal structures were obtained for 1, 5, and the decomposition product [HIPTN3N]VH. 5 could be converted to 7 by oxidation/deprotonation using [FeCp2]OTf/(Me3Si)2NLi. The anionic nitride "{[HIPTN3N]V=N}-" could not be obtained through deprotonation of 7, removal of the -SiMe3 group from 9, or the reaction of 1 with azides. Addition of potassium graphite to 5 resulted in decomposition rather than the formation of 2. Under the catalytic conditions used for the [HIPTN3N]Mo system, 1 produced no ammonia, while 2 and 7 yielded 0.2 and 0.78 equivalents respectively.

(cont.) Chapter 2. Four dianionic diamidoamine-donor based ligands were synthesized. Two were based on a pyridine donor arm 2-(C5H4N)CH2N(CH2CH2NHIPT)2]2- ([Pyrl]2-) and 2-(6-MeC5H3N)CH2N(CH2CH2NHIPT)2]2- ([Myrl]2-), one was based on an oxygen donor arm (3,5-Me2C6H3)OCH2CH2N(CH2CH2NHIPT)2]2- ([NNO]2-), and one was based on a sulfur donor arm [(3,5-Me2C6H3)SCH2CH2N(CH2CH2NHIPT)2]2- ([NNS]2-) where HIPT = 3,5-(2,4,6-i-Pr3C6H2)2C6H3. The reaction between H2[Pyrl] with VCl3(THF)3 yielded an unstable orange product believed to be dimeric. Using H2[Myrl] yielded a stable product that on reduction with potassium graphite generated what is believed to be a dimeric, bridging dinitrogen complex, which was not catalytically active under the conditions used for the [HIPTN3N]Mo system. The reaction between VCl3(THF)3 and H2[NNO] yielded {[NNO]VCl}2 (32), which underwent ligand decomposition upon reduction with potassium graphite (33) or attempted alkylation with di-neopentyl magnesium (34). Ligand decomposition was also observed in the reaction of H2[NNS] with VCl3(THF)3 (35). X-Ray crystal structures were obtained for 32, 33, 34, and 35.

(cont.) Chapter 3. Red-black [HIPTN3N]Cr (1) can be prepared from CrCl3 ([HIPTN3N]3- = [(HIPTNCH2CH2)3N]3-, while green-black [HIPTN3N]Cr(THF) (2) can be prepared from CrCl3(THF)3 where HIPT = 3,5-(2,4,6-i-Pr3C6H2)2C6H3 = exaIsoPropylTerphenyl). Reduction of {1-2} (which means either 1 or 2) with potassium graphite in diethyl ether at room temperature yields [HIPTN3N]CrK (3) as a yellow-orange powder. There is no evidence that dinitrogen is incorporated into 1, 2, or 3. Compounds that can be prepared readily from {1-2} include red [HIPTN3N]CrCO (4), blood-red [HIPTN3N]CrNO (6), and purple [HIPTN3N]CrCl (7, upon oxidation of {1-2} with AgCl). The dichroic (purple/green) Cr(VI) nitride, [HIPTN3N]CrN (8) was prepared from Bu4NN3 and 7. X-ray studies have been carried out on 4, 6, and 7, and on two co-crystallized compounds, 7 and [HIPTN3N]CrN3 (65:35) and[HIPTN3N]CrN3 and 8 (50:50). Exposure of a degassed solution of {1-2} to an atmosphere of ammonia does not yield "[HIPTN3N]Cr(NH3)" as a stable and well-behaved species analogousto [HIPTN3N](NH3). An attempt to reduce dinitrogen under conditions described for thecatalytic reduction of dinitrogen by [HIPTN3N]Mo compounds with 8 yielded a substoichiometric amount (0.8 equiv) of ammonia, which suggests that some ammonia is formedfrom the nitride, but none is formed from dinitrogen.

(cont.) Chapter 4. TRAP (P(CH2CH2NH2)3), a phosphine containing analogue to TREN (N(CH2CH2NH2)3) was synthesized from PH3, but suitable conditions for arylation of the amine arms could not be found. Using the "pre-arylated" arm BrCH2CH2N(BOC)HIPT (BOC = tert-butoxycarbonyl; HIPT = 3,5-(2,4,6-i-Pr3C6H2)2C6H3) in the reaction with PH3 resulted in the isolation of H2PCH2CH2N(BOC)HIPT, but further substitution was unsuccessful. HIPTBr could be converted to HIPTNH2 using benzophenone imine and a rac-BINAP (2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) supported Pd catalyst in preparation for DCC (dicyclohexylcarbodiimide) mediated coupling to tris-(3-propylcarboxylicacid)phosphine.