CONTRAST AGENTS FOR MAGNETIC RESONANCE IMAGING

Abstract

The present invention provides contrast agents of the formula [N(A.sub.1,A.sub.2,A.sub.3) M](counter ion(s)) for use in a diagnostic method practiced on the human or animal body. It also refers to the contrast agents, as well as pharmaceutical compositions containing same. Further, it relates to a method of in vitro medical imaging, especially of diagnostic imaging, comprising administering said compound to a sample.

Claims

1.-18. (canceled)

19. A method of diagnostic imaging which comprises administering a patient a contrast agent according claim 34.

20. The method according to claim 19, wherein the contrast agent is a magnetic resonance imaging contrast agent.

21. The method according to claim 19, wherein the transition metal is Fe.sup.II, Co.sup.II, Ni.sup.II, and Cu.sup.II.

22. The method according to claim 19, wherein the contrast agent is water-soluble.

23. The method according to claim 19, wherein A.sub.1, A.sub.2, and A.sub.3 are the same.

24. The method according to claim 19, wherein the diagnostic method is medical imaging.

25. The method according to claim 19, wherein A1, A2, and A3 are independently selected from the group consisting of: ##STR00006## ##STR00007##

26. The method according to claim 19, wherein the counter ion(s) is/are selected from the group consisting of acetate (OAc.sup.), chloride (Cl.sup.), iodide (I.sup.), bromide (Br.sup.), nitrate (NO.sub.3.sup.), triflate (OTf.sup.) and sulfate (SO.sub.4.sup.2).

27. The method according to claim 19, wherein R.sub.1, R.sub.4, R.sub.5, and R.sub.6 are independently selected from the group consisting of: H, F, Cl, Br, I methyl, OMe, OH, and CF.sub.3, wherein R.sub.2 is H, or OH; and R.sub.3H or CH.sub.3.

28. The method according to claim 19, wherein R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, and R.sub.6 are independently selected from the group consisting of: H, OH, SH, CF.sub.3, CN, halogen, optionally substituted C.sub.1-4 alkyl, C.sub.1-4 heteroalkyl, C.sub.3-7 cycloalkyl, C.sub.3-7 heterocycloalkyl, C.sub.4-12 aryl C(O)NH.sub.2 or C(O)OH and C.sub.4-12 heteroaryl groups. wherein R.sub.6 is H, OH, or an C.sub.1-4 alkyl; and wherein if R.sup.2 does not equal H, OH, SH, C(O)NH.sub.2, or C(O)OH, at least one of R.sub.1, R.sub.3, R.sub.4, and R.sub.5 must equal OH, SH, NH.sub.2, C(O)NH.sub.2, C(O)OH.

29. The method according to claim 19, wherein R.sub.1H, F, Cl, Br, I, CH.sub.3, OCH.sub.3, OH or CF.sub.3; R.sub.2, R.sub.4, R.sub.5, and R.sub.6H; and R.sub.3H or CH.sub.3.

30. The method according to claim 19, wherein at least one of R.sub.1OH; R.sub.2, R.sub.4, R.sub.5, and R.sub.6H; and R.sub.3H or CH.sub.3.

31. The method according to claim 19, wherein one or more of R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6 and each of R.sub.1 is coupled to a probe, or label, wherein the probe or label can be an antibody, peptide, or a dye, or .sup.19F-based probe.

32. The method according to claim 19, wherein A.sub.1, A.sub.2, and A.sub.3 are selected from ##STR00008##

33. The method according to claim 32, wherein the counter ion is trifluoromethanesulfonate or chloride and the metal is Fe or Co or Ni.

34. A contrast agent of the formula [N(A.sub.1,A.sub.2,A.sub.3) M](counter ion(s)) for use in a diagnostic method practiced on the human or animal body, wherein N is a nitrogen atom; M is a divalent metal ion selected from transition metals of the group: V.sup.II, Cr.sup.II, Fe.sup.II, Co.sup.II, Ni.sup.II, and Cu.sup.II; the counter ion(s) being pharmaceutically acceptable; and wherein A.sub.1, A.sub.2, and A.sub.3 are independently selected from the group of ligands consisting of: ##STR00009## ##STR00010## wherein denotes a single or a double bond, wherein R.sub.6 is absent if there is a double bond; and R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, and each of R.sub.1, are independently selected from the group consisting of: H, OH, SH, CF.sub.3, CN, C(O)NH.sub.2, C(O)H, C(O)OH, halogen, optionally substituted C.sub.1-4 alkyl; C.sub.1-4 heteroalkyl, C.sub.3-7 cycloalkyl, C.sub.3-7 heterocycloalkyl, C.sub.4-12 aryl and C.sub.4-12 heteroaryl groups, R.sup.k, SR.sup.k, S(O)R.sup.k, S(O).sub.2R.sup.k, S(O)OR.sup.k, S(O).sub.2OR.sup.k, OS(O)R.sup.k, OS(O).sub.2R.sup.k, OS(O)OR.sup.k, OS(O).sub.2R.sup.k, OR.sup.k, P(O)(OR.sup.k)(OR.sup.L), OP(O)(OR.sup.k)(OR.sup.L), SiR.sup.kR.sup.LR.sup.m, C(O)R.sup.k, C(O)OR.sup.k, C(O)N(R.sup.L)R.sup.k, OC(O)R.sup.k, OC(O)OR.sup.k, OC(O)N(R.sup.k)R.sup.L, wherein R, R.sup.k, R.sup.L, and R.sup.m are independently selected from the group consisting of H and optionally substituted C.sub.1-4 alkyl; C.sub.1-4 heteroalkyl, C.sub.3-7 cycloalkyl, C.sub.3-7 heterocycloalkyl, C.sub.4-12 aryl, or C.sub.4-12 heteroaryl groups, wherein two or more of R.sup.k, R.sup.L and R.sup.m may form, together with each other, one or more optionally substituted aliphatic or aromatic carbon cycles or heterocycles; and wherein one or more of R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6 and each of R.sub.1, can be coupled to a probe, or label; and wherein for ligands 1-6 the following conditions additionally apply: if R.sub.6 is one of H, OH, NH.sub.2, NHR, SH, C(O)NH.sub.2, C(O)NHR, or C(O)OH, then R.sub.1-R.sub.5 can be independently selected from the group as indicated above; if R.sub.6 is absent or not one of H, OH, NH.sub.2, NHR, SH, C(O)NH.sub.2, C(O)NHR, or C(O)OH, then R.sub.2 must be H, OH, NH.sub.2, NHR, SH, C(O)NH.sub.2, C(O)NHR, or C(O)OH and/or at least one of R.sub.1, R.sub.3, R.sub.4 and R.sub.5 must be OH, NH.sub.2, NHR, SH, C(O)NH.sub.2, C(O)NHR, or C(O)OH; and wherein for ligand 7 the following conditions additionally apply: if R.sub.6 is one of H, OH, NH.sub.2, NHR, SH, C(O)NH.sub.2, C(O)NHR, or C(O)OH, then R.sub.1, R.sub.3, R.sub.4 and R.sub.5 can be independently selected from the group as indicated above. if R.sub.6 is absent or R.sub.6 is not one of H, OH, NH.sub.2, NHR, SH, C(O)NH.sub.2, C(O)NHR, or C(O)OH then at least one of R.sub.1, R.sub.3, R.sub.4 and R.sub.5 must be OH, NH.sub.2, NHR, SH, C(O)NH.sub.2, C(O)NHR, or C(O)OH.

35. A pharmaceutical composition comprising a contrast agent as defined in claim 34 and at least one pharmaceutically acceptable excipient.

36. A method of in vitro medical imaging, especially of diagnostic imaging, comprising administering a compound as defined in claim 34 to a sample.

37. A contrast agent as defined in claim 34, wherein the pharmaceutically acceptable counterion(s) is/are selected from the group consisting of acetate (OAc.sup.), chloride (Cl.sup.), iodide (I.sup.), bromide (Br.sup.), nitrate (NO.sub.3.sup.), triflate (OTf) and sulfate (SO.sub.4.sup.2).

Description

DESCRIPTION, OF THE FIGURES

[0039] FIG. 1. Tripodal Schiff Base ligands disclosed herein.

[0040] FIG. 2. .sup.1H NMR spectra [L.sup.1H3Fe](OTf).sub.2 in CD.sub.3CN under anaerobic and anhydrous conditions at 25 C.

[0041] FIG. 3. .sup.19F NMR spectra of [L.sup.1H3Fe](OTf).sub.2 in CD.sub.3CN under anaerobic and anhydrous conditions at 25 C.

[0042] FIG. 4. .sup.1H NMR spectra of [L.sup.1H3Fe](OTf).sub.2 in CD.sub.3OD under anaerobic and anhydrous conditions at 25 C.

[0043] FIG. 5. .sup.19F NMR spectra of [L.sup.1H3Fe](OTf).sub.2 in CD.sub.3OD under anaerobic and anhydrous conditions at 25 C.

[0044] FIG. 6. .sup.1H NMR spectra of [L.sup.1H3Fe](Cl).sub.2 in CD.sub.3OD under anaerobic and anhydrous conditions at 25 C.

[0045] FIG. 7. .sup.1H NMR spectra of [L.sup.1H3Co](Cl).sub.2 in CD.sub.3OD under anaerobic and anhydrous conditions at 25 C.

[0046] FIG. 8. .sup.1H NMR spectra of [L.sup.2H3Fe](OTf).sub.2 in CD.sub.3CN under anaerobic and anhydrous conditions.

[0047] FIG. 9. .sup.19F NMR spectra of [L.sup.2H3Fe](OTf).sub.2 in CD.sub.3CN under anaerobic and anhydrous conditions at 25 C.

[0048] FIG. 10. .sup.1H NMR spectra of [L.sup.2H3Fe](OTf).sub.2 in CD.sub.3OD under anaerobic and anhydrous conditions.

[0049] FIG. 11. .sup.19F NMR spectra of [L.sup.2H3Fe](OTf).sub.2 in CD.sub.3OD under anaerobic and anhydrous conditions.

[0050] FIG. 12. .sup.1H NMR spectra of [L.sup.3H3Fe](OTf).sub.2 in CD.sub.3CN under anaerobic and anhydrous conditions.

[0051] FIG. 13. .sup.19F NMR spectra of [L.sup.3H3Fe](OTf).sub.2 in CD.sub.3CN under anaerobic and anhydrous conditions at 25 C.

[0052] FIG. 14. .sup.1H NMR spectra of [L.sup.3H3Fe](OTf).sub.2 in CD.sub.3OD under anaerobic and anhydrous conditions.

[0053] FIG. 15. .sup.19F NMR spectra of [L.sup.3H3Fe](OTf).sub.2 in CD.sub.3OD under anaerobic and anhydrous conditions.

[0054] FIG. 16. .sup.1H NMR spectra of free ligand L.sup.1H3 in D.sub.2O. It should be noted that the resonance at 9.69 is not observed in the paramagnetic spectra of freshly prepared D.sub.2O solutions of [L.sup.1H3Fe](OTf).sub.2, [L.sup.1H3Fe](Cl).sub.2, or [L.sup.1H3Co](Cl).sub.2, thus providing a spectroscopic fingerprint to monitor complex stability in D.sub.2O over time.

[0055] FIG. 17. .sup.1H NMR spectra [L.sup.1H3Fe](OTf).sub.2 in D.sub.2O under anaerobic conditions at 25 C at 0 hr. Paramagnetic peaks are integrated against an internal capillary containing C.sub.6D.sub.6/C.sub.6H.sub.6 ( 6.81) (80:20).

[0056] FIG. 18. .sup.1H NMR spectra [L.sup.1H3Fe](OTf).sub.2 in D.sub.2O under anaerobic conditions at 25 C after 13 days. Paramagnetic peaks are integrated against an internal capillary containing C.sub.6D.sub.6/C.sub.6H.sub.6 ( 6.73), (80:20). Comparing the 0 hr/h spectra against the spectra collected after 13 days indicates less than 5% decomposition of the paramagnetic signals when integrated against the internal standard, while a analysis of the integration of the fingerprint resonance for L.sup.1H3 at 9.67 shows formation of approximately 1% free ligand in solution.

[0057] FIG. 19. .sup.1H NMR spectra [L.sup.1H3Co](Cl).sub.2 in D.sub.2O under anaerobic conditions at 25 C at 0 hr. Paramagnetic peaks are integrated against an internal capillary containing C.sub.6D.sub.6/C.sub.6H.sub.6 ( 6.62), (80:20).

[0058] FIG. 20. .sup.1H NMR spectra [L.sup.1H3Co](Cl).sub.2 in D.sub.2O under anaerobic conditions at 25 C at 12 days. Paramagnetic peaks are integrated against an internal capillary containing C.sub.6D.sub.6/C.sub.6H.sub.6 ( 6.51), (80:20). Comparing the 0 hr spectra against the spectra collected after 12 days indicates less than 5% decomposition of the paramagnetic signals when integrated against the internal standard, while an analysis of the fingerprint region for L.sup.1H3 at 9.67 indicates no observable free ligand L.sup.1H3 in solution.

[0059] FIG. 21. Z-Spectra of a 10 mM aqueous sample of [L.sup.1H3Fe](OTf).sub.2 in 100 mM NaCl and 20 mM HEPES buffer at pH 6.8, 7.0, and 7.4 at 37 C.

[0060] FIG. 22. Z-Spectra of a 10 mM aqueous sample of [L.sup.1H3Fe](Cl).sub.2 in 100 mM NaCl and 20 mM HEPES buffer at pH 6.8, 7.0, and 7.4 at 37 C.

[0061] FIG. 23. CEST spectra for 10 mM aqueous sample of [L.sup.1H3Co](Cl).sub.2 in 100 mM NaCl and 20 mM HEPES buffered at pH of 6.8, 7.0, and 7.4 collected at 37 C.

[0062] FIG. 24. Z-Spectra of a 10 mM aqueous sample of [L.sup.2H3Fe](OTf).sub.2 in 100 mM NaCl and 20 mM HEPES buffer at pH 6.8, 7.0, and 7.4 at 37 C.

[0063] FIG. 25. Z-Spectra of a 10 mM aqueous sample of [L.sup.3H3Fe](OTf).sub.2 in 100 mM NaCl and 20 mM HEPES buffer at pH 6.8, 7.0, and 7.4 at 37 C.

[0064] FIG. 26. Z-Spectra of a 10 mM aqueous sample of [L.sup.3H3Fe](OTf).sub.2 in 100 mM NaCl and 20 mM HEPES buffer at pH 6.8, 7.0, and 7.4 at 25 C.

[0065] FIG. 27. CEST spectra for 10 mM aqueous sample of [L.sup.1H3Fe](OTf).sub.2 (L1Fe) in 100 mM NaCl and 20 mM HEPES buffered at pH of 7.4, collected at 25 C. over the course of 7 days to gauge complex stability. No change in CEST efficacy was observed as indicated by the overlapping spectra.

[0066] FIG. 28. CEST spectra for 10 mM aqueous sample of [L.sup.2H3Fe](OTf).sub.2 (L2Fe) in 100 mM NaCl and 20 mM HEPES buffered at pH 7.4, collected at 25 C. over the course of 7 days to gauge complex stability. No change in CEST efficacy was observed as indicated by the overlapping spectra.

[0067] FIG. 29. CEST spectra for 10 mM aqueous sample of [L.sup.3H3Fe](OTf).sub.2 (L3Fe) in 100 mM NaCl and 20 mM HEPES buffered at pH of 6.8, collected at 25 C. over the course of 7 days to gauge complex stability. No change in CEST efficacy was observed as indicated by the overlapping spectra.

[0068] FIG. 30. Aldehyde and carbonyl appended heterocycles to be used to form tripodal Schiff base ligands to support first row transition metal ions to give coordination complexes to act as paraCEST contrast agents.

DETAILED DESCRIPTION

[0069] The present invention refers to the following embodiments:

1 Contrast agent of the formula [N(A.sub.1,A.sub.2,A.sub.3) M](counter ion(s)) for use in a diagnostic method practiced on the human or animal body, wherein
N is a nitrogen atom
M is a divalent metal ion selected from transition metals of the group: V.sup.II, Cr.sup.II, Fe.sup.II, Co.sup.II, and Cu.sup.II;
the counter ion(s) being pharmaceutically acceptable; and wherein
A.sub.1, A.sub.2, and A.sub.3 are independently selected from the group of ligands consisting of:

##STR00001## ##STR00002##

wherein denotes a single or a double bond, preferably a double bond, wherein R.sub.6 is absent if there is a double bond; and
R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, and each of R.sub.1, are independently selected from the group consisting of: H, OH, SH, CF.sub.3, CN, C(O)NH.sub.2, C(O)H, C(O)OH, halogen (in particular F, Cl, Br, I), optionally substituted C.sub.1-4 alkyl, preferably CH.sub.3, C.sub.1-4 heteroalkyl, cycloalkyl, C.sub.3-7 heterocycloalkyl, C.sub.4-12 aryl and C.sub.4-12 heteroaryl groups, R.sup.k, SR.sup.k, S(O)R.sup.k, S(O).sub.2R.sup.k, S(O)OR.sup.k, S(O).sub.2OR.sup.k, OS(O)R.sup.k, OS(O).sub.2R.sup.k, OS(O)OR.sup.k, OS(O).sub.2R.sup.k, OR.sup.k, P(O)(OR.sup.k)(OR.sup.L), OP(O)(OR.sup.k)(OR.sup.L), SiR.sup.kR.sup.LR.sup.m, C(O)R.sup.k, C(O)OR.sup.k, C(O)N(R.sup.L)R.sup.k, OC(O)R.sup.k, OC(O)OR.sup.k, OC(O)N(R.sup.k)R.sup.L,
wherein R, R.sup.k, R.sup.L, and R.sup.m are independently selected from the group consisting of H and optionally substituted C.sub.1-4 alkyl, preferably CH.sub.3; C.sub.1-4 heteroalkyl, C.sub.3-7 cycloalkyl, C.sub.3-7 heterocycloalkyl, C.sub.4-12 aryl, or C.sub.4-12 heteroaryl groups, wherein two or more of R.sup.k, R.sup.L and R.sup.m may form, together with each other, one or more optionally substituted aliphatic or aromatic carbon cycles or heterocycles; and wherein one or more of R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6 and each of R.sub.1, can be coupled to a probe, or label (R can e.g., be C.sub.1-C.sub.3 alkyl); and
wherein for ligands 1-6 the following conditions additionally apply: [0070] if R.sub.6 is one of H, OH, NH.sub.2, NHR, SH, C(O)NH.sub.2, C(O)NHR, or C(O)OH, then R.sub.1-R.sub.5 can be independently selected from the group as indicated for R.sub.1-R.sub.5 above; [0071] if R.sub.6 is absent or not one of H, OH, NH.sub.2, NHR, SH, C(O)NH.sub.2, C(O)NHR, or C(O)OH, then R.sub.2 must be H, OH, NH.sub.2, NHR, SH, C(O)NH.sub.2, C(O)NHR, or C(O)OH and/or at least one of R.sub.1, R.sub.3, R.sub.4 and R.sub.5 must be OH, NH.sub.2, NHR, SH, C(O)NH.sub.2, C(O)NHR, or C(O)OH; and
wherein for ligand 7 the following conditions additionally apply: [0072] if R.sub.6 is one of H, OH, NH.sub.2, NHR, SH, C(O)NH.sub.2, C(O)NHR, or C(O)OH, then R.sub.1, R.sub.3, R.sub.4 and R.sub.5 can be independently selected from the group as indicated for R.sub.1-R.sub.5 above. [0073] if R.sub.6 is absent or R.sub.6 is not one of H, OH, NH.sub.2, NHR, SH, C(O)NH.sub.2, C(O)NHR, or C(O)OH then at least one of R.sub.1, R.sub.3, R.sub.4 and R.sub.5 must be OH, NH.sub.2, NHR, SH, C(O)NH.sub.2, C(O)NHR, or C(O)OH.

[0074] In another embodiment, the following conditions apply:

for ligands 1-6 the following conditions additionally apply: [0075] if R.sub.6 is one of H, OH, NH.sub.2, SH, C(O)NH.sub.2, or C(O)OH, then R.sub.1-R.sub.5 can be independently selected from the group as indicated above; [0076] if R.sub.6 is absent or not one of H, OH, NH.sub.2, SH, C(O)NH.sub.2, or C(O)OH, then R.sub.2 must be H, OH, NH.sub.2, SH, C(O)NH.sub.2, or C(O)OH and/or at least one of R.sub.1, R.sub.3, R.sub.4 and R.sub.5 must be OH, NH.sub.2, SH, C(O)NH.sub.2, or C(O)OH; and
for ligand 7 the following conditions additionally apply: [0077] if R.sub.6 is one of H, OH, NH.sub.2, SH, C(O)NH.sub.2, or C(O)OH, then R.sub.1, R.sub.3, R.sub.4 and R.sub.5 can be independently selected from the group as indicated above. [0078] if R.sub.6 is absent or R.sub.6 is not one of H, OH, NH.sub.2, SH, C(O)NH.sub.2, or C(O)OH then at least one of R.sub.1, R.sub.3, R.sub.4 and R.sub.5 must be OH, NH.sub.2, SH, C(O)NH.sub.2, or C(O)OH.

[0079] In one embodiment, which can be combined with all embodiments described herein, R.sup.k, R.sup.L, and R.sup.m are independently selected from the group consisting of H and optionally substituted C.sub.1-4 alkyl, preferably CH.sub.3.

[0080] In one embodiment, which can be combined with all embodiments described herein, R.sub.2, R.sub.3, R.sub.4, R.sub.5, and each of R.sub.1, are independently selected from the group consisting of: H, OH, SH, CF.sub.3, CN, C(O)NH.sub.2, C(O)H, C(O)OH, halogen (in particular F, Cl, Br, I), optionally substituted C.sub.1-4 alkyl, preferably CH.sub.3, C.sub.1-4 heteroalkyl, C.sub.3-7 cycloalkyl, C.sub.3-7 heterocycloalkyl, C.sub.4-12 aryl and C.sub.4-12 heteroaryl groups; wherein one or more of R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6 and each of R.sub.1, can be coupled to a probe, or label; and wherein the conditions for ligands 1-6 and ligand 7 as specified above additionally apply.

[0081] In one embodiment, which can be combined with all embodiments described herein, R.sub.2, R.sub.3, R.sub.4, R.sub.5, and each of R.sub.1, are independently selected from the group consisting of: H, OH, SH, CF.sub.3, CN, C(O)NH.sub.2, C(O)H, C(O)OH, halogen (in particular F, Cl, Br, I), optionally substituted C.sub.1-4 alkyl, preferably CH.sub.3; and C.sub.4-12 aryl; wherein one or more of R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6 and each of R.sub.1, can be coupled to a probe, or label; and wherein the conditions for ligands 1-6 and ligand 7 as specified above additionally apply.

[0082] In the paraCEST compounds of the present invention, the metal atom is in high spin and exhibit 3-fold symmetry in solution. Furthermore, said paraCEST compounds contain paramagnetically shifted labile OH, NH, or metal-bound H.sub.2O protons that undergo exchange with the protons of bulk water. Within the context of the present invention, halogen refers to F, Cl, Br, and I.

2. The contrast agent for use according to embodiment 1, which is a magnetic resonance imaging contrast agent, in particular a paraCEST (Chemical Exchange-dependent Saturation Transfer) contrast agent. Preferably, the complexes exhibit paraCEST efficacy at 37 C. of at least 10%, or at least 14%, preferably at least 30% and most preferably at least 45%.
3. The contrast agent for use according to embodiment 1 or 2, wherein the transition metal is Fe.sup.II, Co.sup.II, and Cu.sup.II, in particular Fe.sup.II or Co.sup.II.
4. The contrast agent for use according to any of the preceding embodiments, wherein the contrast agent is water-soluble. For example, a solution with a concentration of 10 millimolar is sufficient for these compounds 7.4 mg/ml.

[0083] The contrast agents also exhibit a high water stability. Stability can be monitored in D.sub.2O by .sup.1H NMR spectroscopy. Preferably, the contrast agents have a stability of at least 90% or at least 95% over the course of 12 days under anaerobic conditions against an internal standard consisting of a 80%/20% capillary of C.sub.6D.sub.6/C.sub.6H.sub.6.

5. The contrast agent for use according to any of the preceding embodiments, wherein A.sub.1, A.sub.2, and A.sub.3 are the same.
6. The contrast agent for use according to any of the preceding embodiments, wherein the diagnostic method is medical imaging, in particular diagnostic imaging.
7. The contrast agent for use according to any of the preceding embodiments, wherein A1, A2, and A3 are independently selected from the group consisting of:

##STR00003## ##STR00004##

8. The contrast agent for use according to any of the preceding embodiments, wherein the counter ion(s) is/are selected from the group consisting of acetate (OAc.sup.), chloride (Cl.sup.), iodide (I.sup.), bromide (Br.sup.), nitrate (NO.sub.3.sup.), triflate (OTf.sup.) and sulfate (SO.sub.4.sup.2). These counter ions are pharmaceutically acceptable.

[0084] In general, the pharmaceutically acceptable counteranions to each acid listed in the below table are suitable. Experimentally, the anions of strong acids such as (OTf) stabilize the compounds towards aerobic oxidation, while the anions from weaker acids (OAc) make these compounds more susceptible towards aerobic oxidation. Accordingly, it is preferred to use anions of strong acids, e.g., anions from strong acids with pKa's less than 4, which provide a higher stability against aerobic/O.sub.2 oxidation:

TABLE-US-00001 1-hydroxy-2-naphthoic acid glycolic acid 2,2-dichloroacetic acid hippuric acid 2-hydroxyethanesulfonic acid hydrobromic acid 2-oxoglutaric acid hydrochloric acid 4-acetamidobenzoic acid isobutyric acid 4-aminosalicylic acid lactic acid (DL) acetic acid lactobionic acid adipic acid lauric acid ascorbic acid (L) maleic acid aspartic acid (L) malic acid (L) benzenesulfonic acid malonic acid benzoic acid mandelic acid (DL) camphoric acid (+) methanesulfonic acid camphor-10-sulfonic acid (+) naphthalene-1,5-disulfonic acid capric acid (decanoic acid) naphthalene-2-sulfonic acid caproic acid (hexanoic acid) nicotinic acid caprylic acid (octanoic acid) nitric acid carbonic acid oleic acid cinnamic acid oxalic acid citric acid palmitic acid cyclamic acid pamoic acid dodecylsulfuric acid phosphoric acid ethane-1,2-disulfonic acid proprionic acid ethanesulfonic acid pyroglutamic acid (L) formic acid salicylic acid fumaric acid sebacic acid galactaric acid stearic acid gentisic acid succinic acid glucoheptonic acid (D) sulfuric acid gluconic acid (D) tartaric acid (+L) glucuronic acid (D) thiocyanic acid glutamic acid toluenesulfonic acid (p) glutaric acid undecylenic acid glycerophosphoric acid
9. The contrast agent for use according to any of the preceding embodiments, wherein R.sub.1, R.sub.4, R.sub.5, and R.sub.6 are independently selected from the group consisting of: H, F, Cl, Br, I methyl, OMe, OH, and CF.sub.3, wherein R.sub.2 is H, or OH; wherein preferably R.sub.2, R.sub.4, R.sub.5, and R.sub.6 are H, and R.sub.3H or CH.sub.3.
10. The contrast agent for use according to any of the preceding embodiments, wherein R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, and R.sub.6 are independently selected from the group consisting of: H, OH, SH, CF.sub.3, CN, halogen, optionally substituted C.sub.1-4 alkyl, C.sub.1-4 heteroalkyl, C.sub.3-7 cycloalkyl, C.sub.3-7 heterocycloalkyl, C.sub.4-12 aryl C(O)NH.sub.2 or C(O)OH and C.sub.4-12 heteroaryl groups.
wherein R.sub.6 is H, OH, or an C.sub.1-4 alkyl; and wherein if R.sup.2 does not equal H, OH, SH, C(O)NH.sub.2, or C(O)OH, at least one of R.sub.1, R.sub.3, R.sub.4, and R.sub.5 must equal OH, SH, NH.sub.2, C(O)NH.sub.2, C(O)OH.
11. The contrast agent for use according to any of the preceding embodiments, wherein R.sub.1H, F, Cl, Br, I, CH.sub.3, OCH.sub.3, OH or CF.sub.3; R.sub.2, R.sub.4, R.sub.5, and R.sub.6H; and R.sub.3H or CH.sub.3.
12. The contrast agent for use according to any of the preceding embodiments, wherein at least one of R.sub.1OH, preferably wherein one of R.sub.1OH and the remaining R.sub.1H; R.sub.2, R.sub.4, R.sub.5, and R.sub.6H; and R.sub.3H or CH.sub.3.
13. The contrast agent for use according to any of the preceding embodiments, wherein one or more of R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6 and each of R.sub.1, preferably one of R.sub.2, R.sub.3, R.sub.4, and R.sub.6 is coupled to a probe, or label, wherein the probe or label can be an antibody, peptide such as an oligopeptide, e.g., being comprised of 3-20 amino acids, or a dye, such as a fluorescent compound, and .sup.19F-based probe.
14. The contrast agent for use according to any of the preceding embodiments, wherein A.sub.1, A.sub.2, and A.sub.3 are selected from

##STR00005##

15. The contrast agent for use according to embodiment 14, wherein the counter ion is trifluoromethanesulfonate or chloride and the metal is Fe or Co or Ni, in particular, the contrast agent is selected from the group consisting of [L.sup.1H3 Co](counter ion(s)), [L.sup.2H3 Co](counter ion(s)), [L.sup.3H3 Co](counter ion(s)), [L.sup.1H3 Fe](counter ion(s)), [L.sup.2H3 Fe](counter ion(s)), [L.sup.3H3 Fe](counter ion(s)), [L.sup.1H3 Ni](counter ion(s)), [L.sup.2H3 Ni](counter ion(s)), and [L.sup.3H3 Ni](counter ion(s)), further preferred [L.sup.1H3 Co](OTf).sub.2, [L.sup.2H3 Co](OTf).sub.2, [L.sup.3H3 Co](OTf).sub.2, [L.sup.1H3 Fe](OTf).sub.2, [L.sup.2H3 Fe](OTf).sub.2, [L.sup.3H3 Fe](OTf).sub.2, [L.sup.1H3 Ni](OTf).sub.2, [L.sup.2H3 Ni](OTf).sub.2, [L.sup.3H3 Ni](OTf).sub.2, [L.sup.1H3 Co](Cl).sub.2, [L.sup.2H3 Co](Cl).sub.2, [L.sup.3H3 Co](Cl).sub.2, [L.sup.1H3 Fe](Cl).sub.2, [L.sup.2H3 Fe](Cl).sub.2, [L.sup.3H3 Fe](Cl).sub.2, [L.sup.1H3 Ni](Cl).sub.2, [L.sup.2H3 Ni](Cl).sub.2, and [L.sup.3H3 Ni](Cl).sub.2.
16. A contrast agent as defined in any of embodiments 1-17, preferably wherein the pharmaceutically acceptable counterion(s) is/are as defined in embodiment 8.
17. A pharmaceutical composition comprising a contrast agent as defined in any of embodiments 1 to 16 and at least one pharmaceutically acceptable excipient.
18. A pharmaceutical composition as defined in embodiment 17 for use as a medicament.
19. A method of in vitro medical imaging, especially of diagnostic imaging, comprising administering a compound as defined in any of embodiments 1 to 16 to a sample.

[0085] Ligands L.sup.1H3, L.sup.2H3, and L.sup.3H3 (FIG. 1) were prepared as described in the example section.

[0086] Ligand L.sup.1H3 corresponds A.sub.1/A.sub.2/A.sub.3 as defined in the claims, wherein ligand 1 is used, and wherein R.sub.2, R.sub.3, R.sub.4, R.sub.5, and each of R.sub.1H.

[0087] Ligand L.sup.2H3 corresponds A.sub.1/A.sub.2/A.sub.3 as defined in the claims, wherein ligand 2 is used, and wherein R.sub.2, R.sub.3, R.sub.4, R.sub.5, and each of R.sub.1H.

[0088] Ligand L.sup.3H3 corresponds A.sub.1/A.sub.2/A.sub.3 as defined in the claims, wherein ligand 3 is used, and wherein R.sub.2, R.sub.3, R.sub.4, R.sub.5, and each of R.sub.1H.

[0089] The metalation of ligands L.sup.1H3, L.sup.2H3, and L.sup.3H3 was straightforward, providing dicationic coordination complexes when combined with M.sup.II ion (M=Fe.sup.II(OTf).sub.2, Fe.sup.IICl.sub.2, or Co.sup.IICl.sub.2). The method is broadly applicable giving the coordination complexes [L.sup.1H3Fe.sup.II](OTf).sub.2, [L.sup.1H3Fe.sup.II](Cl).sub.2, [L.sup.1H3Co.sup.II](Cl).sub.2, [L.sup.2H3Fe.sup.II](OTf).sub.2, and [L.sup.3H3Fe.sup.II](OTf).sub.2.

[0090] The neutral six-coordinate tripodal Schiff base ligands L.sup.1H3 L.sup.2H3, and L.sup.3H3 (FIG. 1) were previously reported to support high-spin Fe.sup.II as perchlorate salts with the general formulation [LFe].sup.2+(ClO.sub.4).sub.2. (cf. Brewer, C.; Brewer, G.; Luckett, C.; Marbury, G. S.; Viragh, C.; Beatty, A. M.; Scheidt, W. R. Inorg Chem 2004, 43 (7), 2402-2415; Hardie, M. J.; Kilner, C. A.; Halcrow, M. A. Acta Cryst (2004). C60, 177-179 [doi:10.1107/S010827010400407X] 2004, 1-10).

[0091] The ligands L.sup.1H3, L.sup.2H3, and L.sup.3H3 have been examined for their ability to form analogous water-soluble dicationic first row transition metal complexes as triflate (OTf) or chloride (co salts with the metals Fe and Co. In specific, the complexes [L.sup.1H3Fe](OTf).sub.2, [L.sup.1H3Fe](Cl).sub.2, [L.sup.1H3Co](Cl).sub.2, [L.sup.2H3Fe](OTf).sub.2, and [L.sup.3H3Fe](OTf).sub.2 have been synthesized, characterized in the solution-state, and studied in relation to their paraCEST efficacy through the compilation of Z-spectra. Solution-state .sup.1H and .sup.19F NMR spectra for many of the complexes are presented here for the first time. The reported family of isostructural six-coordinate tripodal Schiff base ligands (L.sup.1H3, L.sup.2H3, and L.sup.3H3) supports Fe.sup.n as dicationic triflate and chloride salts and Co.sup.II as a chloride salt. All complexes exhibit 3-fold symmetry in solution, and .sup.19F NMR reveal non-coordinating triflate anions for [L.sup.1H3Fe](OTf).sub.2, [L.sup.2H3Fe](OTf).sub.2, and [L.sup.3H3Fe](OTf).sub.2 in CD.sub.3CN and CD.sub.3OD.

[0092] ParaCEST efficacy experiments were conducted and demonstrate that these agents exhibit paramagnetic chemical exchange saturation transfer in aqueous solution buffered to physiologically relevant pH and ionic strength under anaerobic conditions (FIGS. 21 to 26). These results demonstrate proof-of-principle that these types of complexes can act as paraCEST contrast agents in a biological setting. Surprisingly, Z-spectra collected at the acidic pH of diseased tissue (pH 6.8) provide greater contrast (FIG. 21-26) than when at the neutral or the slightly alkaline pH of healthy tissue (pH 7.0, 7.4), thus providing paraCEST agents that are activated by the acidic pH associated with diseased tissue (FIGS. 21-26). Furthermore, these complexes maintain their contrast efficacy in anaerobic aqueous solution for at least a week in solutions of physiologically relevant pH and ionic strength (selected stability experiments presented in FIGS. 27, 28 and 29). Additionally, the stability of complexes [L.sup.1H3Fe](OTf).sub.2 and [L.sup.1H3Co](Cl).sub.2 were monitored in D.sub.2O by .sup.1H NMR spectroscopy over the course of 12 days under anaerobic conditions against an internal standard consisting of a 80%/20% capillary of C.sub.6D.sub.6/C.sub.6H.sub.8, revealing very little decomposition (less than 5%) or formation of free ligand over the course of 12 days (FIGS. 16-20).

[0093] These results lead to a number of rational conclusions. 1) The use of heterocycles appended with aldehyde or carbonyl functionalities (see FIG. 30, R.sup.2H, Me), when combined with 0.33 molar equivalents of tris-(2-aminoethyl)amine will allow the synthesis of hexacoordinate tripodal Schiff-base ligands that are anticipated to act in an analogous fashion to L.sup.1H3, L.sup.2H3, and L.sup.3H3 to provide ligands that will coordinate transition metals and give complexes that exhibit paraCEST efficacy. 2) Tripodal Schiff-base ligands formed from heterocycles appended with aldehydes or carbonyl functionalities, that are further functionalized at the heterocycle with electron donating or withdrawing substituents (FIG. 30, R.sup.2H, Me; R.sup.1H, F, Cl, Br, I, CH.sub.3, OCH.sub.3, CF.sub.3) will alter the electronics of the ligand, and the stability of the metal complexes towards oxidative degradation (FIG. 30). 3) Tripodal Schiff-base ligands formed from heterocycles appended with aldehydes or carbonyl functionalities, that are further functionalized at the heterocycle with electron donating or withdrawing substituents (FIG. 30, R.sup.2H, Me; R.sup.1H, F, Cl, Br, I, CH.sub.3, OCH.sub.3, CF.sub.3) will support first-row transition metals to provide M.sup.II complexes (M=Fe.sup.II, Co.sup.II, and Ni.sup.II) that act to provide contrast in MRI through paramagnetic chemical exchange saturation transfer. 4) Tripodal Schiff-base ligands formed from heterocycles appended with aldehydes or carbonyl functionalities, that are further functionalized at the heterocycle with electron donating or withdrawing substituents (FIG. 30, R.sup.2H, Me; R.sup.1H, F, Cl, Br, I, CH.sub.3, OCH.sub.3, CF.sub.3) will be able to support the first row transition metals Fe.sup.II, Co.sup.II, and Ni.sup.II as acetate (OAc.sup.), bromide (Br.sup.), chloride (Cl.sup.), iodide (I.sup.), nitrate (NO.sub.3.sup.), triflate (OTf.sup.), and sulfate (SO.sub.4.sup.2) salts. These complexes can be anticipated to exhibit varying degrees of solubility in H.sub.2O, CH.sub.3OH, and CH.sub.3CN, allowing their solution-state structures to be determined and paraCEST efficacy studied.

[0094] Methods

[0095] General Experimental Procedures

[0096] All reactions and subsequent manipulations were performed under anaerobic and anhydrous conditions under an atmosphere of nitrogen or argon in an MBraun glovebox or using Schlenk techniques unless otherwise noted. Diethyl ether was dried over sodium benzophenone ketyl and distilled under a 1.2 atm dynamic argon flow directly into solvent transfer flasks that had undergone three vacuum-argon-purge cycles on a high-vacuum Schlenk line prior to solvent transfer, thus ensuring transfer of anhydrous and anaerobic solvent for glovebox use. Likewise MeCN, and MeOH were dried over CaH.sub.2 and distilled under a 1.2 atm dynamic argon flow directly into solvent transfer flasks as outlined above for glovebox use. All glovebox solvents were stored over 10% by mass activated 3 molecular sieves for a minimum of 24 h before use.

[0097] CD.sub.3CN and CD.sub.3OD were transferred into the glovebox as received and stored over 10% mass of 3 molecular sieves for 48 h prior to use. All other reagents were purchased from commercial suppliers and used as received. All NMR experiments were conducted at 25 C. unless otherwise noted. .sup.1H NMR and .sup.19F NMR spectra were recorded on a Bruker AV 401 and Bruker AV301 instrument, respectively. .sup.1H and .sup.19F{.sup.1H} NMR spectra are referenced to external SiMe.sub.4 and CFCl.sub.3 using the unified xi scale (.sup.19F freq CFCl.sub.3/1H freq TMS=0.9409011). Ligands L.sup.1H3, L.sup.2H3, and L.sup.3H3 were synthesized and purified based upon previously reported procedures.

[0098] CEST Spectroscopy (Z-spectra) CEST experiments were conducted on a 9.4 T NMR spectrometer through a presaturation experiment plotted as normalized water signal intensity (M.sub.z/M.sub.0%) against frequency offset (ppm) in 0.5 ppm increments. A presaturation pulse power (B.sub.1) of 18.7 T was applied for 2 seconds at either 25 C. or 37 C. The 2-D array of spectra are analyzed with the MestReNova software package and an integral table compiled containing the integration of the H.sub.2O resonance centered at 4.79 ppm from 5.4 to 4.2 ppm as a function of presaturation frequency. This data is in-turn used to compile a plot of the signal intensity of bulk water M.sub.z/M.sub.0 as a function of presaturation frequency, which indicates the paramagnetic presaturation frequency that results in the greatest reduction in signal arising from bulk water. These experiments are conducted by preparing a 10 mM solution of the appropriate complex in an anaerobic aqueous stock solution buffered to relevant physiological pH (6.8, 7.0, 7.4) and a physiologically relevant ionic strength using 20 mM HEPES buffer and 100 mM NaCl in a J-Young NMR tube. A D.sub.2O capillary is inserted into the J-Young NMR tube to provide a deuterium lock signal.

EXAMPLES

[0099] The following examples describe the present invention in detail, but they are not to be construed to be in any way limiting for the present invention.

Example 1: Synthesis of Ligands L.SUP.1H3., L.SUP.2H3., and L.SUP.3H3 .(FIG. 1)

[0100] Ligands L.sup.1H3, L.sup.2H3, and L.sup.3H3 were synthesized and purified based upon previously reported procedures.

[0101] Ligands L.sup.1H3, L.sup.2H3, and L.sup.3H3 were prepared by the condensation of one equivalent of tris-(2-aminoethyl)amine with 3.05 equivalents of the appropriate imidazole or pyrazole-bearing aldehydes in refluxing anhydrous methanol under an aerobic atmosphere in an adaptation of previously described procedures (cf. Brewer, C.; Brewer, G.; Luckett, C.; Marbury, G. S.; Viragh, C.; Beatty, A. M.; Scheidt, W. R. Inorg Chem 2004, 43 (7), 2402-2415; Hardie, M. J.; Kilner, C. A.; Halcrow, M. A. Acta Cryst (2004). C60, 177-179 [doi:10.1107/S010827010400407X] 2004, 1-10). Reduction of solvent volume, followed by trituration with ethyl acetate resulted in precipitation of the off-white-to-yellow solids L.sup.1H3, L.sup.2H3, and L.sup.3H3, which were isolated by vacuum filtration and dried under vacuum at 100 C. to afford nearly quantitative yields. Metalation of ligands L.sup.1H3, L.sup.2H3, and L.sup.3H3 was straightforward, providing dicationic coordination complexes when an equivalent of M.sup.II ion (M=Fe.sup.II(OTf).sub.2, Fe.sup.IICl.sub.2, or Co.sup.IICl.sub.2) was combined with an equivalent of ligand in anhydrous methanol under anaerobic conditions, and refluxed for an hour. This method was broadly applicable giving the coordination complexes [L.sup.1H3Fe.sup.II](OTf).sub.2, [L.sup.1H3Fe.sup.II](Cl).sub.2, [L.sup.1H3Co.sup.II](Cl).sub.2, [L.sup.2H3Fe.sup.II](OTf).sub.2, and [L.sup.3H3Fe.sup.II](OTf).sub.2. Nearly quantitative yields of a fine powder precipitate can be isolated by layering a methanol solution of a dicationic complex with diethyl ether and storing for 48 h in a glovebox freezer (35 C.), followed by removal of solvent by decantation, and drying in vacuo. The solution state structures of compounds [L.sup.1H3Fe.sup.II](OTf).sub.2, [L.sup.1H3Fe.sup.II](Cl).sub.2, [L.sup.1H3Co.sup.II](Cl).sub.2, [L.sup.2H3Fe.sup.II](OTf).sub.2, and [L.sup.3H3Fe.sup.II](OTf).sub.2 are all consistent with 3-fold symmetry in solution, but as is the case with most paramagnetic .sup.1H NMR spectra individual .sup.1H resonances can not be definitively assigned without costly isotopic labeling experiments.

[0102] The ligands L.sup.1H3, L.sup.2H3, and L.sup.3H3 have been examined for their ability to form analogous water-soluble dicationic first row transition metal complexes as triflate (OTf) or chloride (co salts with the metals Fe and Co. In specific, the complexes [L.sup.1H3Fe.sup.II](OTf).sub.2, [L.sup.1H3Fe.sup.II](Cl).sub.2, [L.sup.1H3Co.sup.II](Cl).sub.2, [L.sup.2H3Fe.sup.II](OTf).sub.2, and [L.sup.3H3Fe.sup.II](OTf).sub.2 have been synthesized, characterized in the solution-state, and studied in relation to their paraCEST efficacy through the compilation of Z-spectra. Solution-state .sup.1H and .sup.19F NMR spectra for many of the complexes are presented here for the first time. The reported family of isostructural six-coordinate tripodal Schiff base ligands (L.sup.1H3, L.sup.2H3, and L.sup.3H3) supports Fe.sup.II as dicationic triflate and chloride salts and Co.sup.II as a chloride salt. All complexes exhibit 3-fold symmetry in solution, and .sup.19F NMR reveal non-coordinating triflate anions for [L.sup.1H3Fe](OTf).sub.2, [L.sup.2H3Fe](OTf).sub.2, and [L.sup.3H3Fe](OTf).sub.2 in CD.sub.3CN and CD.sub.3OD.

[0103] The paraCEST efficacy experiments were conducted at concentrations of 10 mM of metal complex and demonstrate that these agents exhibit paramagnetic chemical exchange saturation transfer in aqueous solution buffered to physiologically relevant pH and ionic strength under anaerobic conditions (FIGS. 21 to 26). These results demonstrate proof-of-principle that these types of complexes can act as paraCEST contrast agents in a biological setting.

[0104] Interestingly, Z-spectra collected at the acidic pH of diseased tissue (pH 6.8) provide greater contrast (FIGS. 21-26) than when at the neutral or the slightly alkaline pH of healthy tissue (pH 7.0, 7.4), thus providing paraCEST agents that are activated by the acidic pH associated with diseased tissue (FIGS. 21-26). Furthermore, these complexes maintain their contrast efficacy in anaerobic aqueous solution for at least a week in solutions of physiologically relevant pH and ionic strength (selected stability experiments presented in FIGS. 27-29). Additionally, the stability of complexes [L.sup.1H3Fe](OTf).sub.2 and [L.sup.1H3Co](Cl).sub.2 were monitored in D.sub.2O by .sup.1H NMR spectroscopy over the course of 12 days under anaerobic conditions against an internal standard consisting of a 80%/20% capillary of C.sub.6D.sub.6/C.sub.6H.sub.6, revealing very little decomposition (less than 5%) or formation of free ligand over the course of 12 days (FIGS. 16-20).

Example 2: Synthesis of [L.SUP.1H3.Fe](OTf).SUB.2

[0105] Fe(OTf).sub.2 (524 mg, 1.46 mmol) and L.sup.1H3 (557 mg, 1.46 mmol) were combined in a 100 ml Schlenk tube in anhydrous MeOH (10 ml) under anaerobic conditions in a glovebox, sealed and refluxed for 1 h before removal of solvent in vacuo using Schlenk techniques. The resulting orange solid was dissolved in minimal anhydrous MeOH inside a glovebox, passed through a filter with celite pad and subsequently layered with Et.sub.2O and stored at 35 C. for 48 h, providing a fine orange powder in nearly quantitative yields (1.05 g, 1.4 mmol, 98%). .sup.1H NMR (400 MHz, 25 C., CD.sub.3CN): 184.7 (s, 4H); 153.6 (s, 3H); 129.8 (s, 3H); 95.7 (s, 3H); 80.7 (s, 3H); 39.3 (s, 3H); 36.3 (s, 3H); 28.8 (s, 3H). .sup.19F NMR (400 MHz, 25 C., CD.sub.3CN): 78.2. .sup.1H NMR (400 MHz, 25 C., CD.sub.3OD): 181.1 (s, 4H); 153.7 (s, 3H); 124.5 (s, 3H); 80.1 (s, 3H); 39.0 (s, 3H); 37.1 (s, 31-1); 28.2 (s, 3H). .sup.19F NMR (300 MHz, 25 C., CD.sub.3CN): 79.6.

Example 3: Synthesis of L.SUP.1H3.FeCl.SUB.2

[0106] FeCl.sub.2THF.sub.1.5 (57 mg, 0.242 mmol) and L.sup.1H3 (90 mg, 0.242 mmol) were combined in a 100 ml Schlenk tube in anhydrous MeOH (10 ml) under anaerobic conditions in a glovebox, sealed and refluxed for 1 h before removal of solvent in vacuo using Schlenk techniques. The resulting orange solid was dissolved in minimal anhydrous MeOH inside a glovebox, passed through a filter with celite pad and subsequently layered with Et.sub.2O and stored at 35 C. for 48 h, providing a fine orange solid in good yields (0.105 g, 0.207 mmol, 85%). .sup.1H NMR (400 MHz, 25 C., CD.sub.3OD): ): 181.1 (s, 3H); 153.6 (s, 3H); 124.6 (s, 3H); 80.2 (s, 3H); 39.0 (s, 3H); 37.1 (s, 3H); 28.2 (s, 3H). L.sup.1H3FeCl.sub.2 was found to be insoluble in CD.sub.3CN.

Example 4: Synthesis of [L.SUP.1H3.Co](Cl).SUB.2

[0107] CoCl.sub.2 (130 mg, 1.00 mmol) and L.sup.1H3 (380 mg, 1.00 mmol) were combined in a 100 ml Schlenk tube in anhydrous MeOH (10 ml) under anaerobic conditions in a glovebox, sealed and refluxed for 1 h before removal of solvent in vacuo using Schlenk techniques. The resulting beige solid was dissolved in minimal anhydrous MeOH inside a glovebox, passed through a filter with celite pad and subsequently layered with Et.sub.2O and stored at 35 C. for 48 h, providing a fine beige powder in nearly quantitative yields (0.505 g, 0.989 mmol, 98%). .sup.1H NMR (400 MHz, 25 C., CD.sub.3OD: 170.8 (s, 3H); 81.5 (s, 3H); 43.6 (s, 3H); 36.9 (s, 3H); 26.5 (s, 3H); 3.35 (s, 3H); 22.7 (s, 3H). L.sup.1H3CoCl.sub.2 was found to be insoluble in CD.sub.3CN.

Example 5: Synthesis of [L.SUP.2H3.Fe](OTf).SUB.2

[0108] Fe(OTf).sub.2 (850 mg, 2.23 mmol) and L.sup.2H3 (800 mg, 2.23 mmol) were combined in a 100 ml Schlenk tube in anhydrous MeOH (10 ml) under anaerobic conditions in a glovebox, sealed and refluxed for 1 h before removal of solvent in vacuo using Schlenk techniques. The resulting orange solid was dissolved in minimal anhydrous MeOH inside a glovebox, passed through a filter with celite pad and subsequently layered with Et.sub.2O and stored at 35 C. for 48 h, providing a fine orange powder in nearly quantitative yields (1.58 g, 2.15 mmol, 96%). .sup.1H NMR (400 MHz, 25 C., CD.sub.3CN): 226.8 (s, 3H); 159.7 (s, 3H); 154.5 (s, 3H); 79.9 (s, 3H); 74.5 (s, 3H); 38.6 (s, 3H); 36.5 (s, 3H); 27.2 (s, 3H). .sup.19F NMR (300 MHz, 25 C., CD.sub.3CN): 77.0 (s, 3F).sup.1H NMR (400 MHz, 25 C., CD.sub.3OD): 218.2 (s, 3H); 153.5 (s, 3H); 145.8 (s, 3H); 78.0 (s, 3H); 37.6 (s, 6H); 26.3 (s, 3H). .sup.19F NMR (300 MHz, 25 C., CD.sub.3OD): 79.7 (s, 3F).

Example 6: Synthesis of [L.SUP.3H3.Fe](OTf).SUB.2

[0109] Fe(OTf).sub.2 (531 mg, 1.5 mmol) and L.sup.3H3 (570 mg, 1.5 mmol) were combined in a 100 ml Schlenk tube in anhydrous MeOH (10 ml) under anaerobic conditions in a glovebox, sealed and refluxed for 1 h before removal of solvent in vacuo using Schlenk techniques. The resulting purple solid was dissolved in minimal anhydrous MeOH inside a glovebox, passed through a filter with celite pad and subsequently layered with Et.sub.2O and stored at 35 C. for 48 h, providing a fine purple powder in nearly quantitative yields (1.05 g, 1.42 mmol, 94%). .sup.1H NMR (400 MHz, 25 C., CD.sub.3CN): 165.5 (s, 3H); 151.8 (s, 3H); 137.2 (s, 3H); 94.9 (s, 3H); 72.7 (s, 3H); 51.9 (s, 3H); 39.6 (s, 3H); 35.5 (s, 3H). .sup.19F NMR (300 MHz, 25 C., CD.sub.3CN): 79.5 (s, 3F).sup.1H NMR (400 MHz, 25 C., CD.sub.3OD): 161.0 (s, 3H); 149.8 (5, 3H); 132.2 (s, 3H); 95.0 (s, 3H); 71.3 (s, 3H); 39.6 (s, 3H); 36.1 (s, 3H). .sup.19F NMR (300 MHz, 25 C., CD.sub.3OD): 80.0 (s, 3F).

CITED LITERATURE

[0110] Du, K.; Harris, T. D. J. Am. Chem. Soc. 2016, 138 (25), 7804-7807. [0111] Jeon, I.-R.; Park, J. G.; Haney, C. R.; Harris, T. D. Chem. Sci. 2014, 5 (6), 2461-2465. [0112] Brewer, C.; Brewer, G.; Luckett, C.; Marbury, G. S.; Viragh, C.; Beatty, A. M.; Scheidt, W. R. Inorg Chem 2004, 43 (7), 2402-2415 [0113] Hardie, M. J.; Kilner, C. A.; Halcrow, M. A. Acta Cryst (2004). C60, 177-179 [doi:10.1107/S010827010400407X] 2004, 1-10. [0114] Acta Crystallographica 2004, C60, m177-m179, Hardie et al.; {Tris[4-(1H-pyrazol-3-yl)-3-azabut-3-enyl]amine}iron(II) diperchlorate monohydrate discloses ligands, however not in the context of MRI. [0115] Brewer et al., Dalton Transactions March 2006, 5617-5629; A DFT computational study of spin crossover in iron (III) and iron (II) tripodal imidazole complexes. A comparison of . . . . [0116] Brewer et al., Dalton Transactions February 2006; 28(8), 1009-1019; [0117] Synthesis and characterization of manganese(II) and iron(III) d.sup.5 tripodal imidazole complexes. Effect of oxidation state, protonation state and ligand conformation on coordination number and spin state. [0118] Brewer et al., Inorg Chem. 2004, Apr. 5; 43(7), 2402-2415 Proton Contol of Oxidation and Spin State in a Series of Iron Tripodal Imidazole Complexes. [0119] U.S. Pat. No. 5,820,851A. [0120] U.S. Pat. No. 5,869,026 [0121] WO 2006/114739A2. [0122] WO2009/072079A2 [0123] WO 2009/126289A2 [0124] WO 2012/155076A2 [0125] WO2012/155085A1 [0126] JP2000119247 [0127] JPH01272568