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Phosphine and Phosphonite Complexes of a Ruthenium(II) Porphyrin. 1. Synthesis, Structure, and Solution State Studies

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posted on 12.09.2002, 00:00 by Eugen Stulz, Michael Maue, Neil Feeder, Simon J. Teat, Yiu-Fai Ng, Andrew D. Bond, Scott L. Darling, Jeremy K. M. Sanders
We have investigated the effect of complexation of different phosphorus ligands on the stability, solid state structure, and spectroscopic properties (NMR, IR, UV−vis) of a 5,15-diphenyl-substituted ruthenium porphyrin, (MeOH)RuII(CO)(DPP) 2 [DPP = 5,15-bis(3‘,5‘-di-tert-butyl)phenyl-2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin]. The ligands used are PPh3, diphenyl(phenylacetenyl)phosphine (DPAP), bis(diphenylphosphino)acetylene (DPPA), tris(phenylacetenyl)phosphine [(PA)3P], and diethyl (phenylacetenyl)phosphonite [PAP(OEt)2]. The mono-phosphine complexes (PR3)RuII(CO)(DPP) are readily formed in solution in quantitative yields. The complexes display association constants ranging from 1.2 × 104 M-1 for PPh3 to 4.8 × 106 M-1 for PAP(OEt)2. The weak association of PPh3 does not correlate with its pKa, δ(31P), or cone angle value and is attributed to steric effects. Due to their kinetic lability, which is shown by 2D NMR spectroscopy, and the weakening of the carbonyl ligand via a trans effect, the mono-phosphine complexes could not be isolated. IR spectroscopy gives the relative order of π-acceptor strength as PPh3 < DPAP, DPPA < (PA)3P < PAP(OEt)2, whereas the relative order of the σ-donor strength is PPh3 < (PA)3P < DPAP, DPPA < PAP(OEt)2, based on the calculated pKa values and on the 31P{1H} NMR chemical shifts of the ligands. The chemical shift differences in the 31P{1H} NMR spectra upon ligand binding display a linear correlation with the calculated pKa values of the protonated ligands HPR3+; we propose that the pKa, and probably other electronic properties, of a specific phosphorus ligand can be estimated on the basis of the chemical shift difference Δδ(31P) upon complexation to a metalloporphyrin. The bis-phosphine complexes can be isolated in pure form by crystallization from CHCl3−MeOH solutions using excess ligand. Association of the second ligand is in the same order of magnitude as the first binding for the phosphines, but the second phosphonite binding is decreased by a factor of about 100. The solid state structures show only marginal differences in the geometrical parameters. The calculated and the crystallographic cone angles of the ligands generally do not match, apart from the values obtained for PAP(OEt)2.