lecture 4 - Phosphines 1 Phosphine Donors Because of the...

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Phosphines 1 Phosphine Donors Because of the three R-groups on the phosphine ligand and the overall tetrahedral coordination geometry it is the most versatile of the neutral 2-electron donor ligands. Variation of the three R-groups can effect: large changes in the donor/acceptor properties of the phosphine (from excellent donor/poor -acceptor to poor donor/excellent -acceptor) large changes in the steric profile of the phosphine (from fairly small to enormous) generation of a large number of polydentate polyphosphines (bis-, tris-, tetra-, penta-, and hexaphosphine ligands are all known) that can adopt specific coordination geometries (cis-enforcing, facial tridentate, bridging, bridging and chelating, etc.) R P R 2 P R 2 R 2 P M M( 3 -tripod) facial coordinating M M P P R 2 P P R 2 R R racemic-M 2 (P4) binucleating phosphine able to bridge and chelate 2 metals
Phosphines 2 Tolman’s Cone Angle and Electronic Parameter In 1977 Chad Tolman (Dupont Chemicals) published a classic review article covering methods that he developed for ordering a wide variety of phosphine ligands in terms of their electron-donating ability and steric bulk ( Chemical Reviews , 1977 , vol 77 , pages 313-348). The electron-donating ability of a phosphine ligand was determined by reacting one equivalent of the phosphine (monodentate only) with Ni(CO) 4 to make a Ni(CO) 3 (phosphine) complex. He then measured the carbonyl CO IR stretching frequency (the very sharp a 1 high energy mode) of the Ni(CO) 3 (phosphine) complex. The more electron density the phosphine ligand donated to the metal center, the more -back- bonding occurred to the carbonyl ligands, weakening the C O triple bond, thus lowering the CO IR stretching frequency. C O C O C O R Lowest CO stretching frequency: most donating phosphine Highest CO stretching frequency: least donating phosphine (best -acceptor) The size or steric bulk of a phosphine ligand was determined from simple 3-D space-filling models of the phosphine ligands. Tolman coined the name cone angle ( ) to indicate the approximate amount of “space” that the ligand consumed about the metal c enter. M cone angle
Phosphines 3 Phosphine Donor Ability Ranked by Tolman’s Electronic Parameter (most donating to least) PR 3 mixed P(OR) 3 PX 3 , cm 1 P( t -Bu) 3 2056.1 PCy 3 2056.4 P( o -OMe-C 6 H 4 ) 3 2058.3 P( i -Pr) 3 2059.2 PBu 3 2060.3 PEt 3 2061.7 PEt 2 Ph 2063.7 PMe 3 2064.1 PMe 2 Ph 2065.3 P( p -OMe-C 6 H 4 ) 3 PPh 2 ( o -OMe-C 6 H 4 ) 2066.1 PBz 3 2066.4 P( o -Tol) 3 2066.6 P( p -Tol) 3 PEtPh 2 2066.7 PMePh 2 2067.0 P( m -Tol) 3 2067.2 PPh 2 (NMe 2 ) 2067.3 PPh 2 (2,4,6-Me-C 6 H 2 ) 2067.4 PPhBz 2 2067.6 PPh 2 ( p -OMe-C 6 H 4 ) 2068.2 PPh 2 Bz 2068.4 PPh 3 2068.9 PPh 2 (CH=CH 2 ) 2069.3 P(CH=CH 2 ) 3 PPh 2 ( p -F-C 6 H 4 ) 2069.5 PPh( p -F-C 6 H 4 ) 2 2070.0 P( p -F-C 6 H 4 ) 3 2071.3 PPh 2 (OEt) 2071.6 PPh 2 (OMe) 2072.0
Phosphines 4 PPh(O- i -Pr) 2 2072.2 P( p -Cl-C 6 H 4 ) 3 2072.8 PPh 2 H 2073.3 PPh(OBu) 2 2073.4 P( m -F-C 6 H 4 ) 3 2074.1 PPh(OEt) 2 2074.2 PPh 2 (OPh) 2074.6 PPh 2 (C 6 F 5 ) 2074.8 P(O- i -Pr) 3 2075.9 P(OEt) 3 2076.3 PPhH 2 2077.0 P(CH 2 CH 2 CN) 3 2077.9 P(OCH 2 CH 2 OMe) 3 2079.3 P(OMe) 3 2079.5 PPh(OPh) 2 2079.8 PPh 2 Cl 2080.7 PMe 2 CF 3 2080.9 P(O-2,4-Me-C 6 H 3 ) 3 PH 3 2083.2 P(OCH 2 CH 2 Cl) 3 2084.0 P(O-Tol) 3 2084.1 P(OPh) 3 2085.3 P(OCH 2 ) 3 CR 2086.8 P(OCH 2 CH 2 CN) 3 2087.6 P(C 6 F 5 ) 3 2090.9 PCl 3 2097.0 PF 3 2110.8 Problem:

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