N-PYRROLYL PHOSPHINES - AN UNEXPLOITED CLASS OF PHOSPHINE-LIGANDS WITH EXCEPTIONAL PI-ACCEPTOR CHARACTER

The coordination chemistry of N-pyrrolyl phosphines (P-NC4H4) is described. These ligands are prepared in excellent yield from pyrrole, a phosphorus halide, and base, and this synthesis has been applied to the series PPh(x)(pyrrolyl)(3-x)(x = 0-2) and the chelate (pyrrolyl)(2)P(CH2)(2)P(pyrrolyl)(2). These ligands readily form coordination complexes, and the complexes trans-RhCl(CO)[PPh(x)(pyrrolyl)(3-x)](2) (n = 0-2) and Mo(CO)(4)[(pyrrolyl)(2)P(CH2)(2)P(pyrrolyl)(2)] are described. The carbonyl stretching frequencies of these complexes are shifted to significantly higher energy relative to ''traditional'' phosphine ligands, indicating that N-pyrrolyl phosphines are poor sigma-donors, exceeding phosphites and approaching fluoroalkylphosphines with respect to this property. For example, nu(CO) for trans-RhCl(CO)[P(pyrrolyl)(3)](2) exceeds that of the PPh(3) analogue by 59 cm(-1). That these ligands are pi-acceptors is suggested by the single crystal X-ray structure of trans-RhCl(CO)[P(pyrrolyl)(3)](2) which shows shortened Rh-P distances and a lengthened Rh-C distance, consistent with enhanced Rh to P back-bonding. The X-ray structure of trans-RhCl(CO)[P(pyrrolidinyl)(3)](2) is also reported; this complex possesses longer Rh-P distances which more closely resemble those found for other complexes of this type. The exceptional pi-acceptor character of these ligands is convincingly demonstrated by their substitution chemistry with electron rich [PPN][Rh(CO)(4)]. P(pyrrolyl)(3) is found to displace CO in a stepwise manner to give the entire series [PPN][Rh(CO)(4-x){P(pyrrolyl)(3)}(x)] (x = 1-4). Similar results are obtained with (pyrrolyl)(2)P(CH2)(2)P(pyrrolyl)(2), and the anions [PPN][Rh(CO)(x){(pyrrolyl)(2)P(CH2)(2)P(pyrrolyl)(2)}(y)] (x 2, y = 1; x = 0, y = 2) are reported. An X-ray structure analysis of [PPN][Rh(CO){P(pyrrolyl)(3)}(3)] shows that the Rh-P bonds in this tetrahedral anion are shorter than those found in the Rh(I) complex, consistent with significantly greater pi back-bonding in this more electron rich system. The infrared spectra of these anions again show a substantial shift in nu(CO) to higher frequency relative to other phosphine ligands. The structural results further indicate that PPh(x)(pyrrolyl)(3-x)(x = 0-2), PPh(3), and P(pyrrolidinyl)(3) possess nearly identical steric properties (cone angles). The wide range of electronic properties (pi-acceptor/sigma-donor) exhibited by this isosteric series, together with their ready availability, suggests that they, and N-pyrrolyl phosphines in particular, may find utility in physical inorganic and organometallic chemistry.