Metallocene Antitumor Agents. Solution and Solid-State Molybdenocene Coordination Chemistry of DNA Constituents

This contribution reports a solution and solid-state structural study of the aqueous nucleobase and nucleotide coordination chemistry of the organometallic antineoplastic agent, Cp₂MoCl₂ (1, Cp = ·⁵−C₅H₅). In neutral aqueous solution, Cp₂MoCl₂ undergoes essentially complete chloride aquation within 60 min to yield what is formulated as Cp₂Mo(H₂O)OH⁺, while under the same conditions, the Mo-Cp bonds are hydrolytically stable. Reaction of aqueous Cp₂MoCl₂ with the alkylated nucleobases, 9-methyladenine and 1-methylcytosine yields two isomeric [Cp₂Mo(9-methyladenyl)] [PF₆] complexes (3a and 3b) and a single isomer of [Cp₂Mo(l-methylcytosyl)] [PF₆] (4). On the basis of solution NMR spectroscopy, 3a and 3b are assigned HN6−/N1 and HN6⁻/N7 chelation modes, respectively, while 4 involves an HN4⁻/N3 chelation mode. Complex 3a crystallizes in the triclinic space group Pl with a = 10.682 (3) Å= 11.619 (5) Å, c = 7.701 (5) Å, β 106.93 (4)°, β = 96.32 (3)°, γ = 86.98 (3)°, V = 908.64 ų, Z = 2; R(F) = 0.045 for 3614 independent reflections having I < 3σ(I). The Cp₂Mo²⁺ fragment is in a bent sandwich geometry with an average Mo-C distance of 2.309 (4) Å, a Mo-N1 distance of 2.173 (3) Å, a Mo-N6 distance of 2.145 (3) Å, a NI-M0-N6 angle of 60.9 (1)°, and a ring centroid-Mo-ring centroid angle of 135.3°. The 9-methyladenyl ligand in 3a lies in the plane which bisects the ring centroid-Mo-ring centroid angle. Chelation constricts the N6-C6-N1 angle to 108.5 (3)°. Complex 4 crystallizes in the monoclinic space group (P21,/c) with a = 11.703 (1) Å, b = 10.794 (2) Å, c = 14.416 (2) Å, β = 111.28 (1)°, Z = 4, V= 1696.8 ų; R(F) = 0.047 for 3007 independent reflections having I > 3σ(I). The Cp₂Mo²⁺ fragment is also in a bent sandwich geometry with an average Mo—C distance of 2.294 (7) Å, a Mo-N4 distance of 2.140 (5) Å, a Mo-N3 distance of 2.130 (5) Å, a NI-M0-N6 angle of 59.9 (1)°, and a ring centroid-Mo-ring centroid angle of 136.5°. The 1-methyicytosyl ligand in 4 lies in the plane which bisects the ring centroid-Mo-ring centroid angle, and chelation constricts the N4-C4-N3 angle to 106.8 (5)°. On the NMR time scale and in the absence of other competing ligands, complex 1 forms 1:1 complexes with the 2′-deoxyribonucleotide-5′-monophosphates of guanosine (5′-dGMP), adenosine (5′-dAMP), cytosine (5′-dCMP), and thymidine (5′-dTMP). There is little selectivity in the complexation, and nucleotide-nucleotide exchange processes are detectable. Although nucleotide complexation is observed, there is no NMR evidence that Cp₂MoCl₂(aq) disrupts Watson—Crick base pairing in 5′-dGMP/5′-dCMP or 5′-dAMP/5′-dTMP dimers. The Cp₂Mo²⁺ adduct of 5′-dGMP (5) crystallizes in the triclinic space group Pl with a = 10.690 (3) Å, b = 14.567 (5) Å, c = 9.298 (3) Å, α = 107.20 (2)°, β = 99.22 (3)°, γ = 77.62 (3)°, Z = 1, V= 1344 (2) ų; R(F) = 0.045 for 5491 independent reflections having I > 3σ(I). The crystal structure of complex 5 consists of dimeric [Cp₂Mo(5′-dGMP)]₂ units interconnected by water bridges. Each Cp₂Mo²⁺ unit of the dimer is in a bent sandwich geometry and is coordinated to N7 and O(phosphate) of different 5′-dGMP moieties. Metrical parameters for 5 are as follows: Mo-C distance(av), 2.307 (9) Å; Mo-N7 distance, 2.20 (1) Å; Mo-O(phosphat e) distance, 2.096 (9) Å; N7-Mo-O(phosphate) angle, 77.8 (2)°; and ring centroid-Mo-ring centroid angle, 133.8 (6)°. The 5′-dGMP unit has β^gg and γ^gt torsional conformers and exhibits an unusual syn glycosidic and C3′-endo sugar puckering conformation. Compound 1 forms a monomeric complex with 5′-dAMP (6) in aqueous solution via Mo-N7 and Mo-O(phosphate) chelation, two complexes with 5′-dCMP that both involve O(phosphate) coordination, and a single complex with 5′-dTMP which involves O(phosphate) and N3 coordination. These results place significant ligational restrictions on the mode(s) by which Cp₂MX₂(aq) species might bind to DNA and, together with a molecular graphics investigation of Cp₂Mo⁺² coordination to a model oligonucleotide duplex, argue against cisplatinlike complexation motifs.