Carboxylate- and Phosphate Ester-Bridged Dimagnesium(II), Dizinc(II), and Dicalcium(II) Complexes. Models for Intermediates in Biological Phosphate Ester Hydrolysis

Carboxylate-bridged dimagnesium(II) complexes were synthesized and characterized by employing the dinucleating ligand XDK, where H 2 XDK is m -xylylenediamine bis(Kemp's triacid imide). The reaction of 1 or 2 equiv of sodium diphenyl phosphate with [Mg 2 (XDK)(CH 3 OH) 4 (H 2 O) 2 (NO 3 )](NO 3 ), 1 (NO 3 ), afforded [Mg 2 (XDK){μ-η 2 -(PhO) 2 PO 2 }(CH 3 OH) 3 (H 2 O)(NO 3 )]·3CH 3 OH ( 2 ·3CH 3 OH) and [Mg 2 (XDK){μ-η 2 -(PhO) 2 PO 2 }{η 1 -(PhO) 2 PO 2 }(CH 3 OH) 3 (H 2 O)]·CH 3 OH ( 3 ·CH 3 OH), respectively. These are the first structurally characterized phosphate ester-bridged dimagnesium(II) complexes. The reaction of 1 with 1 equiv of bis(4-nitrophenyl) hydrogen phosphate resulted in protonation of one of the carboxylate ligands and liberation of one magnesium(II) ion to give [Mg(HXDK) 2 (H 2 O) 2 ] ( 4 ), an octahedral complex containing two short, low barrier intramolecular OH···O hydrogen bonds. The phosphate ester exchange rate of free and bound diphenyl phosphate in 3 in methanol solution was measured by variable-temperature 31 P{ 1 H} NMR spectroscopy and compared to that of structurally analogous dizinc(II), [Zn 2 (XDK){μ-η 2 -(PhO) 2 PO 2 }{η 1 -(PhO) 2 PO 2 }(CH 3 OH) 2 (H 2 O)] ( 5 ), and dicalcium(II), [Ca 2 (XDK){μ-η 2 -(PhO) 2 PO 2 }{η 1 -(PhO) 2 PO 2 }(CH 3 OH) 3 (H 2 O)]·CH 3 OH, ( 6 ·CH 3 OH), complexes. The synthesis and structural characterization of 6 is presented, along with a discussion of the differences between the carboxylate- and phosphate ester-bridged dimagnesium(II), dizinc(II), and dicalcium(II) centers. Crystallographic data are as follows. 1 (NO 3 ): monoclinic, P 2 1 / c, a = 11.240(3) Å, b = 13.019(2) Å, c = 30.208(7) Å, β = 99.11(1)°, V = 4365(2) Å 3, Z = 4, R = 0.045, and R w = 0.054 for 5021 independent reflections with I > 3σ( I ). 2 ·3CH 3 OH: monoclinic, P 2 1 / c, a = 16.611(5) Å, b = 16.059(6) Å, c = 21.930(9) Å, β = 93.34(6)°, V = 5840(4) Å 3, Z = 4, R = 0.069, and R w = 0.085 for 3759 independent reflections with I > 2σ( I ). 3 . CH 3 OH: monoclinic, P 2 1 / n, a = 18.912(4) Å, b = 16.254(2) Å, c = 21.646(5) Å, β = 112.26(2)°, V = 6158(2) Å 3, Z = 4, R = 0.060, and R w = 0.072 for 5184 independent reflections with I > 3σ( I ). 4: monoclinic, P 2 1 / n, a = 15.210(5) Å, b = 15.772(3) Å, c = 13.093(3) Å, β = 96.35(3)°, V = 3122(1) Å 3, Z = 2, R = 0.069, and R w = 0.070 for 1840 independent reflections with I > 2σ( I ). 6 . CH 3 OH: monoclinic, P 2 1 / n, a = 16.5471(3) Å, b = 24.3415(6) Å, c = 16.5865(3) Å, β = 104.2530(10)°, V = 6475.1(2) Å 3, Z = 4, R = 0.053, and wR 2 = 0.142 for 7858 independent reflections with I > 2σ( I ).