You will not need to collect and re-check your luggage in Toronto. Entering your flight information into the Connecting Guide on YYZ's webpage yields the following (the italics are mine):
Determine your gate number: Once you're in the terminal, look for your departing flight on the flight information screens and confirm you are departing from a "F" Gate. Follow the purple "Connections", "Gates" or "F" Gates signs.
Follow "F" Gate Signage to security screening: Go through security screening. For more information regarding security screening restrictions, please refer to the Canadian Air Transport Security Authority (CATSA) website.
Proceed to the USA Connection Area: This area includes a 3 Step process that eliminates the need to collect your baggage. Airline representatives are available to help guide you through this process and for questions.
Continue to U.S. Customs and Border Protection (US CBP):
Upon direction follow the step 3 signage and enter U.S. Customs and Border Protection (US CBP). For more information on US CBP, please visit their website.
Proceed to your departure gate: Use the flight information screens to confirm your departure gate number. Follow the signage to your "F" gate.
I fly between the US and Canada frequently via YYZ, though not internationally. Over the past 5–10 years, YYZ has greatly redesigned its traffic flow to make the connections process easier, allowing many connecting passengers to stay in a secure area (even for US & International flights) and eliminating much of the need for claiming and re-checking your baggage. Before this, a 1h45m connection time would have made me nervous too; but now I'd tend to trust the guidelines a bit more.
Finally, if the whole thing is booked as one ticket, and delays in the flights and/or immigration cause you to miss your original connection, Air Canada will get you to St. Louis eventually; they'll just re-book you on the next available flight from Toronto to St. Louis. This may involve staying in Toronto overnight, so I would pack a toothbrush and a change of clothes in your carry-on if you can. If this happens, seek out an Air Canada customer service representative in the airport; there are customer service desks inside the secure area, including in the waiting area after US pre-clearance.