A compound of formula I: wherein: each of R 1 and R 2 is independently R, -OR, -SR, -N 2, -OC R, -SOR, -SO 2 R,-SO 2 N 2, -C N 2, -NRC R, or -NRSO 2 R; R 3 is -OR; R 4 is -N 2; each R is independently hydrogen or an optionally substituted group selected from C 1-20 aliphatic, C 1-20 heteroaliphatic having 1-3 heteroatoms independently selected from nitrogen, oxygen, or sulfur, phenyl, ferrocene, a 3-7 membered saturated or partially unsaturated carbocyclic ring, an 8-10 membered bicyclic saturated, partially unsaturated or aryl ring, a 5-6 membered monocyclic heteroaryl ring having 1-4 heteroatoms independently selected from nitrogen, oxygen, or sulfur, a 4-7 membered saturated or partially unsaturated heterocyclic ring having 1-3 heteroatoms independently selected from nitrogen, oxygen, or sulfur, a 7-10 membered bicyclic saturated or partially unsaturated heterocyclic ring having 1-5 heteroatoms independently selected from nitrogen, oxygen, or sulfur, or an 8-10 membered bicyclic heteroaryl ring having 1-5 heteroatoms independently selected from nitrogen, oxygen, or sulfur, or: two R groups on the same nitrogen atom are taken together with the nitrogen to form an optionally substituted 3-8 membered saturated, partially unsaturated, or aryl ring having 0-3 additional heteroatoms not including the same nitrogen atom independently selected from nitrogen, oxygen, or sulfur; n is 0, 1, or 2; each R 5 is independently a monodentate ligand, or two R 5 are taken together with their intervening atoms to form an optionally substituted bidentate group. each of R 1 and R 2 is independently R, -OR, -SR, -N 2, -OC R, -SOR, -SO 2 R,-SO 2 N 2, -C N 2, -NRC R, or -NRSO 2 R; R 3 is -OR; R 4 is -N 2; each R is independently hydrogen or an optionally substituted group selected from C 1-20 aliphatic, C 1-20 heteroaliphatic having 1-3 heteroatoms independently selected from nitrogen, oxygen, or sulfur, phenyl, ferrocene, a 3-7 membered saturated or partially unsaturated carbocyclic ring, an 8-10 membered bicyclic saturated, partially unsaturated or aryl ring, a 5-6 membered monocyclic heteroaryl ring having 1-4 heteroatoms independently selected from nitrogen, oxygen, or sulfur, a 4-7 membered saturated or partially unsaturated heterocyclic ring having 1-3 heteroatoms independently selected from nitrogen, oxygen, or sulfur, a 7-10 membered bicyclic saturated or partially unsaturated heterocyclic ring having 1-5 heteroatoms independently selected from nitrogen, oxygen, or sulfur, or an 8-10 membered bicyclic heteroaryl ring having 1-5 heteroatoms independently selected from nitrogen, oxygen, or sulfur, or: two R groups on the same nitrogen atom are taken together with the nitrogen to form an optionally substituted 3-8 membered saturated, partially unsaturated, or aryl ring having 0-3 additional heteroatoms not including the same nitrogen atom independently selected from nitrogen, oxygen, or sulfur; n is 0, 1, or 2; each R 5 is independently a monodentate ligand, or two R 5 are taken together with their intervening atoms to form an optionally substituted bidentate group. two R groups on the same nitrogen atom are taken together with the nitrogen to form an optionally substituted 3-8 membered saturated, partially unsaturated, or aryl ring having 0-3 additional heteroatoms not including the same nitrogen atom independently selected from nitrogen, oxygen, or sulfur; n is 0, 1, or 2; each R 5 is independently a monodentate ligand, or two R 5 are taken together with their intervening atoms to form an optionally substituted bidentate group.