Hypergolic materials, capable of spontaneous ignition upon contact with an external oxidizer, are of critical importance as fuels and propellants in aerospace applications (e.g., rockets and spacecraft). Currently used hypergolic fuels are highly energetic, toxic, and carcinogenic hydrazine derivatives, inspiring the search for cleaner and safer hypergols. Here, we demonstrate the first strategy to design hypergolic behavior within a metal-organic framework (MOF) platform, by using simple “trigger” functionalities to unlock the latent and generally not recognized energetic properties of zeolitic imidazolate frameworks, a popular class of MOFs. The herein presented six hypergolic MOFs, based on zinc, cobalt, and cadmium, illustrate a uniquely modular platform to develop hypergols free of highly energetic or carcinogenic components, in which varying the metal and linker components enables the modulation of ignition and combustion properties, resulting in excellent hypergolic response evident by ultrashort ignition delays as low as 2 ms.

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Pyridines with one or two substituents terminating in vinyl groups are prepared. Intramolecular ring-closing metatheses of trans-MCl 2 adducts and hydrogenations supply the title compounds. Williamson ether syntheses using the alcohols HO(CH 2 ) n CHCH 2 (n = 1 (a), 2 (b), 3 (c), 4 (d), 5 (e), 6 (f), 8 (h), 9 (i)) and appropriate halides give the pyridines 2-NC 5 H 4 (CH 2 O(CH 2 ) n CHCH 2 ) (1a,b), 3-NC 5 H 4 (CH 2 O(CH 2 ) n CHCH 2 ) (2a−e,h,i), and 2,6-NC 5 H 3 (CH 2 O-(CH 2 ) n CHCH 2 ) 2 (4a−d) in 92−45% yields. Reactions of 3,5-NC 5 H 3 (COCl) 2 and HO(CH 2 ) n CHCH 2 afford the diesters 3,5-NC 5 H 3 (COO(CH 2 ) n CHCH 2 ) 2 (5a−f,h, 90−41%). The reaction of 3,5-NC 5 H 3 (4-C 6 H 4 OH) 2 , Br(CH 2 ) 5 CHCH 2 , and Cs 2 CO 3 yields 3,5-NC 5 H 3 (4-C 6 H 4 O(CH 2 ) 5 CHCH 2 ) 2 (8; 32%). Reactions of PtCl 2 with 1a,b, 2a−e,h,i, 4a,b (but not 4c,d), 5a,c−f,h, and 8 afford the corresponding bis(pyridine) complexes trans-10a,b (40−12%), trans-12a−e,h,i (84−46%), trans-17a,b (88−22%), trans-19a,c−f,h (94−63%), and trans-22 (96%). Selected adducts are treated with Grubbs' catalyst and then H 2 (Pd/ C) to give trans-PtCl 2 [2,2′-(NC 5 H 4 (CH 2 O(CH 2 ) 2n+2 OCH 2 )H 4 C 5 N)] (trans-11a,b; 79−63%), trans-PtCl 2 [3,3′-(NC 5 H 4 (CH 2 O-(CH 2 ) 2n+2 OCH 2 )H 4 C 5 N)] (trans-13,d,h,i; 93−80%), trans-PtCl 2 [2,6,2′,6′-(NC 5 H 3 (CH 2 O(CH 2 ) 2n+2 OCH 2 ) 2 H 3 C 5 N)] (trans-18a,b; 22−10%), trans-PtCl 2 [3,5,3′,5′-(NC 5 H 3 (COO(CH 2 ) 2n+2 OCO) 2 H 3 C 5 N)] (trans-20d−f,h; 45−14%), and trans-PtCl 2 [3,5,3′,5′-(NC 5 H 3 (4-C 6 H 4 O(CH 2 ) 12 O-4-C 6 H 4 ) 2 H 3 C 5 N)] (40%). A previously reported ring-closing metathesis of trans-PdCl 2 [2,6-NC 5 H 3 (CH 2 CH 2 CHCH 2 ) 2 ] 2 is confirmed, and the new hydrogenation product trans-PdCl 2 [2,6,2′,6′-(NC 5 H 3 ((CH 2 ) 6 ) 2 H 3 C 5 N)] (trans-16; 62%) is isolated. Additions of CH 3 MgBr to 12b,h and 13d,h afford the corresponding PtClCH 3 species (94−41%), but analogous reactions fail with 2-substituted pyridine adducts. The reaction of trans-19c with PhCCH and CuI/i-Pr 2 NH gives the corresponding PtCl(CCPh) adduct (18%). The crystal structures of trans-17a, trans-11b, trans-13d, trans-13h•CH 2 Cl 2 , trans-16, trans-18a,b, and trans-20e•2CHCl 3 are determined. Steric effects in the preceding data, especially involving 2-substituents and the MCl 2 or MCl(X) rotators, are analyzed in detail.

The combination of an energetic imidazole bonded to a hypergolic trigger, such as certain boron hydride clusters, dramatically improves both the energy density and ignition delay of the resulting compounds while also introducing tunability via alkyl group substitution as demonstrated here with (Rim)BH (R = H, C, C, and C).

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