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SUPPLEMENTARY MATERIAL FOR ONLINE VERSIONTable S1. List of structurally characterized heptamolybdate compounds. No Compound Space Group Binding mode of {Mo 7 O 24 } -6 Ref (H 2 DABCO) 3 [Mo 7 O 24 ]•4H 2 O Cc Counterion 26 (2-ampH) 4 [Co(H 2 O) 5 Mo 7 O 24 ]•9H 2 O Pna2 1 Monodentate 9 [3-ampH] 4 [{Zn(3-ampy)(H 2 O) 4 }Mo 7 O 24 ]•4H 2 O Cc Monodentate 15 [3-ampH] 4 [{Co(3-ampy)(H 2 O) 4 }Mo 7 O 24 ]•4H 2 O Cc Monodentate 15 [2-ampH] 4 [{Zn(H 2 O) 5 }Mo 7 O 24 ] •9H 2 O Pnma Monodentate 15 Na(NH 4 )[bppH 2 ] 2 [Mo 7 O 24 ]•8H 2 O P2 1 /m Tridentate 8 (NH 4 ) 4 [Ru(DMSO) 3 Mo 7 O 24 ]•6.5H 2 O C2/c Tridentate 14 (NH 4 ) 4 [Os(DMSO) 3 Mo 7 O 24 ]•4.5H 2 O C2/c Tridentate 14 (ImH) 4 [Ca(H 2 O) 6 (µ-O) 2 {Mo 7 O 24 }]•2 (Im)•3H 2 O C2/m µ 2 -bidentate 17 (NH 4 ) 6 H 2 [Cu(C 2 O 4 ) 2 (Mo 7 O 24 )]•9H 2 O P2 1 /m µ 2 -bidentate 18 (hmtH) 2 [{Mg(H 2 O) 5 } 2 {Mo 7 O 24 }]•3H 2 O C2/c µ 2 -bidentate 10 (hmtH) 2 [{(Zn(H 2 O) 5 }{Zn(H 2 O) 4 }{Mo 7 O 24 }]•2H 2 O C2/c µ 2 -tridentate 11 (hmtH) 2 [Mn 2 (H 2 O) 9 Mo 7 O 24 ]•2H 2 O C2/c µ 2 -tridentate 12 (hmtH) 2 [Fe 2 (H 2 O) 9 Mo 7 O 24 ]•2H 2 O C2/c µ 2 -tridentate 12 (NH 4 )[Cu(en) 2 ][Na(en)Cu(en) 2 (H 2 O)(Mo 7 O 24 )]•4H 2 O P1 µ 3 -35 (NH 4 ) 6 [Mo 7 O 24 ]•4H 2 O P2 1 /c Counterion 22, 23 36 (2-ampH) 6

The synthesis, crystal structure, redox characteristics and photochemistry of a new heptamolybdate supported magnesium-aqua coordination complex viz. (hmtH) 2 [{Mg(H 2 O) 5 } 2 {Mo 7 O 24 }]•3H 2 O 1 (hmt = hexamethylenetetramine) is reported. The cyclic voltammogram reveals quasireversible redox behaviour while the degradation of rhodamine B (RhB) by 1 under sunlight irradiation indicates its photocatalytic activity. In the crystal structure of 1,t w o[ M g (H 2 O) 5 ] 2+ units are linked by a µ 2-bridging bidentate heptamolybdate ligand resulting in a heptamolybdate supported magnesium-aqua coordinated dianionic complex [{Mg(H 2 O) 5 } 2 {Mo 7 O 24 }] 2− charge balanced by two (hmtH) + cations. The cations, anions and the lattice water molecules are linked by several hydrogen bonding interactions.

The synthesis, crystal structure, spectral characterization, photochemistry, electrochemical and thermal studies of the hexakis(butan-1-aminium) heptamolybdate(VI) tetrahydrate (1) are reported. Dissolution of a mixed mono-hepta compound (BuNH 3) 8 [(Mo 7 O 24)(MoO 4)]•3H 2 O in water results in its transformation to the title compound viz., (BuNH 3) 6 [Mo 7 O 24 ]•4H 2 O 1 (BuNH 3 = butan-1-aminium). The structure of the title compound consists of two crystallographically unique [Mo 7 O 24 ] 6− anions, twelve independent (BuNH 3) + cations and eight unique lattice water molecules, all of which are interlinked with the aid of three varieties of Hbonding interactions. Solar irradiation of 1 results in the formation of a bis(μ 2-oxo) bridged diheptamolybdate product. Electrochemical studies reveal the role of 1 in the photodimerization process. Thermal decomposition of 1 results in the formation of crystalline α-MoO 3 .

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