Ligands H 4 sal-dag (I) and H 4 Brsal-dag (II) derived from 1,3diaminoguanidine and salicylaldehyde or 5-bromosalicylaldehyde react with one or 2 mol equivalent of vanadium precursor to give two different series of vanadium complexes. Thus, complexes(2) were isolated by the reaction of an equimolar ratio of these ligands with [V IV O(acac) 2 ] in MeOH. In the presence of K + /Cs + ion and using aerially oxidized [V IV O(acac) 2 ], the above reaction gave complexes 6), which could also be isolated by direct aerial oxidation of complexes 1 and 2 in MeOH in the presence of K + /Cs + ion. 8) were isolated upon increasing the ligand-to-vanadium precursor molar ratio to 1:2 under an air atmosphere. When I and II were reacted with aerially oxidized [V IV O(acac) 2 ] in a 1:2 molar ratio in MeOH in the presence of K + /Cs + ion, they formed (12). The structures of complexes 3, 4, 5, and 9 determined by single-crystal X-ray diffraction study confirm the mono-, bi-, tri-, and tetra-anionic behaviors of the ligands. All complexes were found to be an effective catalyst for the oxidation of benzoin to benzil via oxygen atom transfer (OAT) between DMSO and benzoin. Under aerobic condition, this oxidation also proceeds effectively in the absence of DMSO. Electron paramagnetic resonance and 51 V NMR studies demonstrated the active role of a stable V(IV) intermediate during OAT between DMSO and benzoin.

Mono benzoxazine appended N‐capped amino bis(disubstitutedphenol) ligands [II(a–c)] upon reaction with VVO(OEt)3 in a 1 : 1 molar ratio in EtOH/MeOH give [{VVO}en(3,5‐dtbb)3] (1), [{VVO}en(3‐tb,5‐mb)3] (2) and [{VVO}en(3,5‐dmb)3] (3). During the reaction, the benzoxazine ring opens with the loss of methylene group and the newly formed ligands, N,N‐bis(2‐hydroxy‐3,5‐disubstitutedbenzyl)‐N’‐2‐hydroxy‐3,5‐disubstituted benzyledene‐1,2‐diaminoethane [III(a–c)], behave as tribasic pentadentate in these complexes. Under similar conditions, when [MVIO2(acac)2] (M=Mo or W; Hacac=acetylacetone) reacts with II(a–c), these ligands retain their identity and form cis‐[MVIO2] complexes, [{MoVIO2}{en(3,5‐dtbb)2(6,8‐dtbbenzox)}] (4), [{MoVIO2}{en(3‐tb,5‐mb)2(6‐tb,8‐mbbenzox)}] (5) and [{MoVIO2}{en(3,5‐dmb)2(6,8‐dmbenzox)}] (6), [{WVIO2}{en(3,5‐dtbb)2(6,8‐dtbbenzox)}] (7), and [{WVIO2}{en(3‐tb,5‐mb)2(6‐tb,8‐mbbenzox)}] (8). However, the benzoxazine ring ruptures in case of ligand IIc under these conditions and form [{WVIO2}{en(3,5‐dmb)3}] (10), similar to complexes 1–3. Complex [{WVIO2}{en(3,5‐dmb)2(6,8‐dmbenzox)}] (9), having structure similar to 4–8, could only be obtained when the reaction was carried out in toluene. Not only 9, even complexes 4–8 can be isolated in toluene. Rupturing of both benzoxazine rings has also been experienced when ligands 1,2‐bis(6,8‐disubstitutedbenzo[e][1,3]oxazin‐3(4H)‐yl)ethane [I(a–c)] react with [MVIO2(acac)2] (M=Mo or W) in MeOH and give salan type complexes [(MVIO2)en(3,5‐dtbb)2] [M=Mo (11), M=W (14)], [(MVIO2)en(3‐tb,5‐mb)4] [M=Mo (12), M=W (15)] and [(MVIO2)en(3,5‐dmb)4] [M=Mo (13), M=W (16)]. Complexes 1–9 have been used as catalyst for the multicomponent Biginelli reaction for the synthesis of 3,4‐dihydropyrimidin‐2(1H)‐ones (DHPMs) and oxidative bromination of phenol derivatives.

The reaction of [MoVIO2(acac)2] (acac=acetylacetonate anion) with N,N,N’,N’‐tetrakis(2‐hydroxy‐3,5‐disubstitutedbenzyl)‐1,2‐diaminoethane having different 3,5‐disubstituents on the phenyl ring in 1 : 1 molar ratio in MeOH results in the formation of two different types of substituent controlled mononuclear complexes. Ligands having 3,5‐di‐t‐butyl substituents [H4en(3,5‐dtbb)4, I] and 3‐t‐butyl‐5‐methyl substituents [H4en(3‐tb,5‐mb)4, II], result in the formation of complexes, [{MoVIO2(MeOH)}H2en(3,5‐dtbb)4] (1) and [{MoVIO2(MeOH)}H2en(3‐tb,5‐mb)4] (2), respectively. In these complexes ligands behave as a bis(dibasic ONO) donor and coordinate to only one side of the ONO functionalities. On the contrary, ligands having 3,5‐dimethyl substituents [H4en(3,5‐dmb)4, III] and 3,5‐dichloro substituents [H4en(3,5‐dcb)4, IV] facilitate mononuclear α‐cis, symmetric complexes, [{MoVIO2}H2en(3,5‐dmb)4] (3) and [{MoVIO2}H2en(3,5‐dcb)4] (4), respectively. Here ligands behave as a dibasic ONNO tetradentate. Further reaction of complexes 1 and 2 with equimolar amount of [MoVIO2(acac)2] in DMF‐MeOH gives homo‐binuclear complexes, [{MoVIO2(DMF)}2en(3,5‐dtbb)4] (5) and [{MoVIO2(DMF)}2en(3‐tb,5‐mb)4] (6), respectively. Reaction of VVO(OEt)3 with complexes 1–4 in refluxing EtOH caused the displacement of cis‐[MoVIO2] group by [VVO] along with the breaking of one of the arms of ligand and the formation of oxidovanadium(V) complexes, [{VVO}en(3,5‐dtbb)3] (7), [{VVO}en(3‐tb,5‐mb)3] (8), [{VVO}en(3,5‐dmb)3] (9), and [{VVO}en(3,5‐dcb)3] (10). This is an example of trans‐metalation. Complexes [{VVO}Hen(3,5‐dmb)4] (11) and [{VVO}Hen(3,5‐dcb)4] (12) upholding all four arms have also been obtained by direct reaction of ligands III and IV, respectively, with VVO(OEt)3. Biomimetic catalytic activity similar to enzymes haloperoxidases was performed with thymol and other phenol derivatives and all these complexes serve as excellent functional models.

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