The coordination chemistry of N-pynolyl phosphines (P-NC&) 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,(pyrrolyl)3-, (x = 0-2) and the chelate (pyrrolyl)zP(CHz)zP(pyrrolyl)~. These ligands readily form coordination complexes, and the complexes tran~-RhCl(CO)[PPh,(pynoly1)3-,]~ (x = 0-2) and Mo(C0)4[(pyrrolyl)2P(CH2)2-P(pyrrolyl)z] 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 a-donors, exceeding phosphites and approaching fluoroalkylphosphines with respect to this property. For example, vco for trans-RhCl(C0)-[P(pyrrolyl)3]~ exceeds that of the PPh3 analogue by 59 cm-I. That these ligands are n-acceptors is suggested by the single crystal X-ray structure of tran~-RhCl(CO)[P(pyrrolyl)3]:! 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(C0)-[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 macceptor 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-,{P(pyrr01yl)3}~] (x = 1-4). Similar results are obtained with (pyrrolyl)2P(CH~)~P(pyrrolyl)~, and the anions [PPNI[Rh(CO),{ (pyrrolyl)2P(CH~)2P(pyrrolyl)~}~] (x = 2, y = 1; x = 0, y = 2) are reported. An X-ray structure analysis of [
PPN][Rh(CO){P(pyrrolyl)3}3] shows thatthe Rh-P bonds in this tetrahedral anion are shorter than those found in the Rh(1) complex, consistent with significantly greater n back-bonding in this more electron rich system. The infrared spectra of these anions again show a substantial shift in YCO to higher frequency relative to other phosphine ligands. The structural results further indicate that PPhx(pyrrolyl)3-, (x = 0-2), PPh3, and P(pyrrolidiny1)s possess nearly identical steric properties (cone angles). The wide range of electronic properties (n-acceptorla-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.
IntroductionPhosphorus(II1) ligands of the type PZ3, where Z is a substituent such as hydrocarbyl, alkoxy, halo, etc., are ubiquitous in coordination chemistry, organometallic chemistry, and homogeneous catalysis.' The wide variety of Z groups available allows the systematic alteration of ligand steric and electronic (o-donor, n-acceptor) properties. This allows a great deal of leverage in fine tuning the reactivity of metal complexes of these
