TY - JOUR

T1 - A Site-Specific Study of the Magnetic Field-Dependent Proton Spin Relaxation of an Iridium N-Heterocyclic Carbene Complex

AU - Pravdivtsev, Andrey N.

AU - Yurkovskaya, Alexandra V.

AU - Petrov, Pavel A.

AU - Ivanov, Konstantin L.

N1 - Publisher Copyright: © 2017 Walter de Gruyter GmbH, Berlin/Boston 2017.

PY - 2017/4/1

Y1 - 2017/4/1

N2 - We report a study of proton spin relaxation of an Iridium N-heterocyclic carbene complex [Ir(COD)(IMes)Cl] complex (where COD=1,5-cyclooctadiene, Imes=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene). This compound is a pre-catalyst of the most efficient complex allowing the signal amplification by reversible exchange (SABRE) effect, relevant for enhancing weak signals in nuclear magnetic resonance (NMR). An important feature of the study is a combination of relaxation measurements over a wide field range with high-resolution NMR detection. As a result, we are able to measure nuclear magnetic relaxation dispersion (NMRD) curves in the field range 0.1 mT-16.4 T (corresponding to the frequency range 4 kHz-700 MHz) for individual protons in the complex under study. This attractive possibility enables determination of the motional correlation times, τc, for the individual protons by analyzing the features in the NMRD curves (increase of the relaxation times) appearing at the magnetic fields where ωτc&-apcode;1 (here ω is the proton Larmor precession frequency at a given field strength). The following correlation times were determined: (1.3±0.1) ns for the protons of imidazol-2-ylidene, (0.96±0.1) ns for the ortho-protons of two phenyl moieties and (0.95±0.2) ns for the protons of methyl groups. Additionally, we report low-field features coming from "strong coupling" of the protons. One should note that such features must not be misinterpreted by associating them with motional features. From the low-field features we obtain consistent estimates for the proton spin-spin interactions. The analysis of motional correlation times is also of importance for interpretation of spin order transfer from parahydrogen to various substrates in transient organometallic complexes (termed the SABRE effect) at high magnetic field.

AB - We report a study of proton spin relaxation of an Iridium N-heterocyclic carbene complex [Ir(COD)(IMes)Cl] complex (where COD=1,5-cyclooctadiene, Imes=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene). This compound is a pre-catalyst of the most efficient complex allowing the signal amplification by reversible exchange (SABRE) effect, relevant for enhancing weak signals in nuclear magnetic resonance (NMR). An important feature of the study is a combination of relaxation measurements over a wide field range with high-resolution NMR detection. As a result, we are able to measure nuclear magnetic relaxation dispersion (NMRD) curves in the field range 0.1 mT-16.4 T (corresponding to the frequency range 4 kHz-700 MHz) for individual protons in the complex under study. This attractive possibility enables determination of the motional correlation times, τc, for the individual protons by analyzing the features in the NMRD curves (increase of the relaxation times) appearing at the magnetic fields where ωτc&-apcode;1 (here ω is the proton Larmor precession frequency at a given field strength). The following correlation times were determined: (1.3±0.1) ns for the protons of imidazol-2-ylidene, (0.96±0.1) ns for the ortho-protons of two phenyl moieties and (0.95±0.2) ns for the protons of methyl groups. Additionally, we report low-field features coming from "strong coupling" of the protons. One should note that such features must not be misinterpreted by associating them with motional features. From the low-field features we obtain consistent estimates for the proton spin-spin interactions. The analysis of motional correlation times is also of importance for interpretation of spin order transfer from parahydrogen to various substrates in transient organometallic complexes (termed the SABRE effect) at high magnetic field.

KW - correlation times

KW - NMRD

KW - spin relaxation

KW - strong coupling

KW - PARA-HYDROGEN

KW - DEVICE

KW - HYPERPOLARIZATION

KW - CYCLING NMR RELAXOMETRY

UR - http://www.scopus.com/inward/record.url?scp=85012286145&partnerID=8YFLogxK

U2 - 10.1515/zpch-2016-0849

DO - 10.1515/zpch-2016-0849

M3 - Article

AN - SCOPUS:85012286145

VL - 231

SP - 857

EP - 865

JO - Zeitschrift fur Physikalische Chemie

JF - Zeitschrift fur Physikalische Chemie

SN - 0942-9352

IS - 4

ER -