Bis(N-1,2,4-diazaphosphol-1-yl)methanes and the related copper and cobalt complexes
Graphical abstract
Bis(N-1,2,4-diazaphosphol-1-yl)methane (bdpm) and the related complexes [Cu(bdpm)2(H2O)2]2+·2ClO4−, [(bdpm)Cu(μ-Cl)Cl]2, and [(bdpm)CoCl2] were synthesized. The EPR and magnetic study showed that [Cu(bdpm)2(H2O)2]2+·2ClO4− with a Cu(II) ion is paramagnetic.
Introduction
Heterocycles containing a low-coordinated phosphorus center [1] have found widespread applications ranging from ligands in metal complexes [2], devices in material science [3] to fundamental importance in theoretical and experimental research [4], [5], [6], [7]. 1H-1,2,4-Diazaphospholes H[3,5-R2dp] (R = H, iPr, Ph, tBu), exhibiting electrochemical and coordinating properties endowed by the low-coordinated P(σ2λ3) atom, represent a class of unique aromatic five-membered heterocyclic system [8], [6], and may be viewed as hybrid molecules of the corresponding phospholes [9] and pyrazoles [10] (P-doping pyrazoles) [5], or as 1H-1,2,4-trizole analogue in which 4-nitrogen atom is replaced by a phosphorus atom (σ2λ3) [7]. Recent work in our group and others has shown that the deprotonated 1H-1,2,4-diazaphospholes ([3,5-R2dp]−) are highly versatile ligands for the metals across the periodic table [11], [12]. Due to the unique electronic requirement, the resulting complexes of the 1,2,4-diazaphospholides have presented novel molecular structure characters with the varied coordination of the types η1(N), η1(N1):η1(N2), and η2(N1,N2) via the nitrogen atom(s), of the η5 type via the π-electron system [12]. Further studies have evidenced that several 1,2,4-diazaphospholide complexes are excellent catalysts in the organic transformations [12h] or/and have an unusual redox activity toward a persistent dianionic radical species [12f], and that 1,2,4-diazaphospholide ligands are non-innocent [12i].
It has been know that an incorporation of different heterocycles into the ligand framework, with their differing basicities and π-orbital energies, has a substantial effect on the electron richness, kinetic liability and luminescent properties of the resultant metal complexes. For example, derivatives of bis(N-pyrazol-1-yl)methanes (bpm) [13a], bis(N-triazol-1-yl)methanes (btm) [13](b), [13](c), [13](d), arising from an incorporation of pyrazoles, triazoles into methane, have been used as ligands. In this respect, a few of transition metal bpm [13](e), [13](f) and btm [13](b), [13](c), [13](d) complexes with three-dimensional MOFs structures have been investigated [13g].
Impetus was provided by the recent results [12], [14], [15] that 1H-1,2,4-diazaphospholes may incorporate with methane to give the derivatives of bis(N-1,2,4-diazaphosphol-1-yl)methane (bdpm) that may be used as ligands for metal ions. Due to the extraordinary electronic structure and redox-active behavior of the 1,2,4-diazaphopholide moiety [12], [14], the resultant bdpm complexes may provide a unique molecular environment with low-valent phosphorus (σ2λ3) on the molecule [1], [2], [3], [4], [5], [12](g). Such complexes are expected to be fundamental interesting in potential coordinated catalysis [12h] or in distinct magnetic and luminescent properties due to the doping low-valent phosphorus atom (σ2λ3) in the molecules [4]. Herein, we describe the synthetic approach to the several new bis(N-1,2,4-diazaphosphol-1-yl)methane ligands (1–4) as well as the first bdpm metal complexes [Cu(bdpm)2(H2O)2]2+·2ClO4− (5), [(bdpm)Cu(μ-Cl)Cl]2 (6), and [(bdpm)CoCl2] (7).
Section snippets
General considerations and spectroscopic measurements
All manipulations were carried out in an argon atmosphere under anaerobic conditions using standard Schlenk-line, vacuum-line, and glovebox techniques. The solvents were thoroughly dried, deoxygenated, and distilled in an argon atmosphere prior to use. DMSO-d6 was degassed and dried over molecular sieves for 24 h before use. CDCl3 was dried with CaH2 and distilled before use. The 1H NMR, 13C{1H} NMR, and 31P{1H} NMR spectra were recorded with a Bruker DRX-600 spectrometer. IR measurements were
Synthetic comments for compounds 1–7
As 1,2,4-diazaphosphole is valence-isoelectronic with pyrazole and triazole [6], bis(N-1,2,4-diazaphosphol-1-yl)methanes may be prepared following a procedure similar to that reported for the corresponding bismethylated analogue of bis(N-pyrazol-1-yl)methanes (bpzm).[17] The synthetic methods for various bpzm ligands were first reported by Trofimenko [18] and modified by Elguero et al. [19]. Referring to the reported methods, the reaction of pyrazole with dicholormethane in an autoclave was
Conclusions
In conclusion, we present the synthesis of several novel bis(N-1,2,4-diazaphosphol-1-yl)methanes (1–4) and structural characterization of bis(N-1,2,4-diazaphosphol-1-yl)metanes (1, 2, 4) and the first bdpm copper(II) and cobalt(II) complexes (5, 6, and 7), These results may allow to a variation of metal centers toward other transition metal bdpm complexes to be prepared, which might be further advantageous in terms of organic catalysts [12](g), [12](h), magnetic materials [13], and redox
Acknowledgment
W.Z. is thankful to the Program for National Natural Science Foundation of China (NSFC; Grant No. 21272143), the Program for Changjiang Scholar and Innovative Research Team in University (IRT1156).
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